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Earthquake magnitudes are widely relied upon measures of earthquake size. Although moment magnitude (Mw) has become the established standard for moderate and large earthquakes, difficulty in reliably measuring seismic moments for small (generally Mw

Differences in the seismic coda of neighboring events can be used to investigate source location offsets and medium change with coda wave interferometry (CWI). We employ CWI to infer the known relative location between two chemical explosions in Phase I of the Source Physics Experiment (SPE). The inferred displacement between the first, SPE-1, and second, SPE-2, chemical explosion is between 6 and 18 m, with an expectation of 9.2 m, where the known separation is close to 9.4 m. We also employ CWI to find any velocity perturbation due to damage from SPE-2, by comparing its coda with the collocated third SPE chemical explosion, SPE-3. We find that damage due to SPE-2 must be confined to a spherical region with radius less than 10 m and velocity perturbation less than 25%.

We demonstrate successful crustal imaging via teleseismic P-wave coda autocorrelation, using data recorded on a 261 station array of vertical-component high-frequency geophones in the area of the Bighorn Mountains, Wyoming, U.S.A. We autocorrelate the P-wave coda of 30 teleseismic events and use phase-weighted stacking to yield seismic profiles comparable to low-passed versions of those produced via controlled-source vertical seismic reflection. Our process recovers reflections from the bottoms of the Bighorn and Powder River basins that flank the Bighorn Mountains. We also identify a mid-crustal reflector that aligns with a region of increased reflectivity, previously interpreted as a Precambrian province boundary. Our results demonstrate the utility of crustal imaging with teleseismic P-wave coda energy using modern large-array seismic data, and they corroborate previous interpretations of crustal structures in the study area.

Seismologists distinguish underground nuclear explosions from more commonly occurring earthquakes using moment tensor inversion, high-frequency P/S amplitude ratios, mb:Ms comparisons, and P-pP differential travel times. These methods are generally successful for large seismic events (M>3–4) well recorded at regional-to-teleseismic distances (>150 km); however, it is unclear whether they can be modified to work for small events (M

Two M 5 earthquakes struck the southeastern Korean Peninsula in September 2016 and November 2017, causing damage near the epicentral areas. We analyze the stress-drop scaling of these two earthquake sequences using coda-based methods and Bayesian inversion. The 2016 Gyeongju earthquake sequence is a typical earthquake sequence generated by tectonic processes. In contrast, the 2017 Pohang earthquake sequence is believed to be related to fluid injections conducted for the development of enhanced geothermal systems. As the two sequences occurred in the same tectonic regime, our study provides a good opportunity to compare the stress-drop scaling between a tectonic earthquake sequence and an earthquake sequence influenced by fluid injections. We found that the stress drops of events in the Pohang sequence are lower than those of the Gyeongju sequence with similar magnitude. Although it is likely that this difference results from focal depth variations, a reduction of stress drop due to fluid injections cannot be ruled out.

Author(s): Yang, C; Niu, F; Daley, TM; Taira, T | Abstract: In situ stress measurement at seismogenic depth is critically important for deciphering fault zone processes. In this study, we conducted a second active-source crosswell field experiment at the Parkfield San Andreas Fault Observatory at Depth (SAFOD) drill site to investigate the detectability of stress-induced seismic velocity changes at the top part of the seismogenic zone. We employed the same configuration of our previous experiments, which deployed a piezoelectric source and a three-component (3C) accelerometer at 1 km deep inside the pilot and main holes, respectively. We also added a hydrophone, which is attached to the source, to monitor the repeatability of the source waveforms. Over a 40-day recording period, we confirmed an ∼0:04% travel-time variation in S wave and coda that roughly follows the fluctuation of barometric pressure. We attributed this correlation to stress sensitivity of seismic velocity and the stress sensitivity is estimated to be 2:0 × 10 −7 Pa −1 , which is approximately two orders of magnitude higher than those measured in laboratory with dry rock samples, but is consistent with our previous results. Our results confirm the hypothesis that substantial cracks and/or pore spaces exist at seismogenic depths and thus may be used to monitor the subsurface stress field with active-source crosswell seismic.

A comprehensive earthquake catalog was compiled for Iraq and neighboring areas as part of a broader probabilistic seismic‐hazard assessment project. The Iraq Seismic Network (ISN) was established in 1976 and became operational in the early 1980s. However, recording and reporting of seismic data has been intermittent in Iraq. Hence, events were collected from various sources, including the ISN when available, International Seismological Centre (ISC), European‐Mediterranean Seismological Centre, U.S. Geological Survey Centennial Catalog, Global Centroid Moment Tensor solutions, and Ambraseys’ extensive work on cataloging of instrumental era earthquakes in the Middle East (e.g., Ambraseys, 1978, 2001, 2009). We supplement these with new direct moment magnitude calculations based on coda calibration technique. For many of the larger events in the catalog, more than one magnitude is available. Directly calculated moment magnitudes ( M w) were favored, followed by body‐wave magnitude ( m b) obtained from the ISC. Where no directly calculated M w was available, other magnitude scales were converted to M w using relationships compatible with the local catalog. The resulting earthquake catalog spans from the year 1900 until the end of 2009, covers the region bounded by 26°–40° N latitudes and 36°–51° E longitudes, and includes more than 18,000 earthquakes, of which roughly 4000 are M w 4.0 or larger.

We present an analysis of aftershocks with M L >3 from the 2011 Prague, Oklahoma, earthquake sequence, an intraplate sequence of moderate‐size earthquakes within the North American craton. We apply waveform analysis to seismograms from temporary local seismograph networks to relocate the aftershocks’ hypocenters. The relocations show that the hypocenters are confined to the Precambrian basement. The character of the recorded seismograms waveforms are reconstructed by finite‐difference forward modeling of seismic sources buried at different depths in 2D crustal models. The numerical modeling indicates that the observed complex waveforms, and particularly the coda, are created by waves scattering off numerous shallow crustal velocity anomalies. We infer that these anomalies correspond to paleoriver channels that are embedded in the Pennsylvanian limestone formation. The modeling indicates that the amount of energy within the coda depends on the depth of the source. Our waveform modeling of the observed seismograms is consistent with our relocations of the aftershocks to depths of 5–10 km, within the Precambrian basement. Electronic Supplement:Empirical and synthetic data, model parameters, and geological figures.

The relationship between the moment magnitude scale and other magnitude scales is a subject of continuing research ever since the moment magnitude ( M w ) was first proposed. Empirical results show that in western North America, the moment magnitude is greater than, equal to, and less than the Richter ( M L ) and Nuttli magnitudes ( m b Lg ), whereas over 97% of the earthquakes in eastern North America have an M w value that is less than M L , m b Lg , or M e . To explain the large differences that exist between the two regions, first it is shown theoretically that the magnitudes that are based on a peak amplitude measurement are approximately equal to the energy magnitude ( M e ) that is based on the square root of the observed seismic trace Lg coda energy. If we define p as the ratio M w / M e and α as the seismic moment scaling constant, then it is shown theoretically that p =(4/9) α . Most of the major fault‐parameter relations such as stress drop, corner frequency, fault area, etc., can be obtained from the observed ratio of M w / M e . The value of this ratio is not unique but depends on the nature of faulting and the tectonic environment. For self‐similar earthquakes α =3, M w =(4/3) M e . For most earthquakes in western North America, α varies from 2 to 3, leading to a range of values for the ratio M w / M e , whereas for most earthquakes in eastern North America ( α ≈2 and M w ≈(8/9) M e ). This type of scaling for small‐to‐intermediate earthquakes may be due to earthquake fault zones being shaped by the geometry of sloping rock layers within the crust.

Among other methods, passive imaging technique is widely applied to obtain surface‐wave velocities. This technique implies that the average cross correlations between diffuse wavefields recorded at two observers is proportional to the imaginary part of the Green’s function. For this purpose, most applications rely on both seismic ambient noise and the coda of earthquakes. Instead, we use a generalized diffuse field (GDF), defined as the waves produced by a multiplicity of distant seismic sources. These wavefields undergo multiple scatterings along their way and at the local surface geology. In this communication, we use GDF to extract the locally generated surface waves in a 2D alluvial valley model for both inplane and antiplane cases from the retrieved Green’s function. For the inplane case, an equipartitioned cocktail of plane P , SV , and Rayleigh waves is used, whereas for the antiplane case, the incidence is a set of plane SH waves. In addition to isotropic illumination, we explore the partial illumination from one side of the valley. In both cases, we obtain dispersion curves for the Rayleigh and Love waves’ group velocities from the retrieved Green’s functions and found good agreement with the exact result for the fundamental modes of both Love and Rayleigh waves in an infinite horizontal layer. This theoretical validation is a proof of concept within an ongoing project whose goal is to improve the characterization of Mexico City subsoil throughout tomography maps of surface‐wave velocities using a collection of historical strong earthquakes recorded by the Mexico City Accelerometric Network.

The aftershocks of the 24 August 2014 M w 6.0 South Napa earthquake generated prominent fault‐zone trapped waves (FZTWs) that were recorded on two 1.9‐km‐long seismic arrays deployed across the northern projection (array 1, A1) and the southern part (A2) of the surface rupture of the West Napa fault zone (WNFZ). We also observed FZTWs on an array (A3) deployed across the intersection of the Franklin and Southampton faults, which appear to be the southward continuations of the WNFZ. A1, A2, and A3 consisted of 20, 20, and 10 L28 (4.5 Hz) three‐component seismographs. We analyzed waveforms of FZTWs from 55 aftershocks in both time and frequency to characterize the fault damage zone associated with this M w 6.0 earthquake. Post‐ S coda durations of FZTWs increase with epicentral distances and focal depths from the recording arrays, suggesting a low‐velocity waveguide along the WNFZ to depths in excess of 5–7 km. Locations of the aftershocks showing FZTWs, combined with 3D finite‐difference simulations, suggest the subsurface rupture zone having an S ‐wave speed reduction of ∼40%–50% between A1 and A2, coincident with the ∼14‐km‐long mapped surface rupture zone and at least an ∼500‐m‐wide deformation zone. The low‐velocity waveguide along the WNFZ extends further southward to at least A3, but with a more moderate‐velocity reduction of 30%–35% at ray depth. This last FZTW observation suggests continuity between the WNFZ and Franklin fault. The waveguide effect may have localized and amplified ground shaking along the WNFZ and the faults farther to the south (see a companion paper by Catchings et al. , 2016).

Coda waves are an important instrument to study the elastic properties of the lithosphere. The attenuation of coda waves in western Mexico is analyzed using seismograms recorded during the mapping of the Rivera subduction zone experiment. The coda‐wave attenuation is measured by means of Q within the single‐backscattering model and the single‐isotropic scattering model schemes. The frequency dependence of Q is analyzed using the power fit formulation Q ( f )= Q f α . The resulting Q , Q , and α are then used to study the seismic attenuation properties in western Mexico. We compare our results with the crustal structure by means of the surface geology, seismic activity, crustal thickness, and P ‐wave velocity patterns in the study area. There is a poor correlation of the attenuation parameters with such crustal features. However, an analysis of the variations of the P ‐wave velocity shows that the seismic attenuation parameters correlate to some extent with seismic velocity variations previously noted in the study area, where low attenuation and weak frequency dependence correlate with low‐velocity variation.

Reliable moment magnitude estimates for seismic events in the Middle East region can be difficult to obtain due to the uneven distribution of stations, the complex tectonic structure, and regions of high attenuation. In this study, we take advantage of the many new broadband seismic stations that have become available through improved national networks and numerous temporary deployments. We make coda envelope‐amplitude measurements for 2247 events recorded by 68 stations over 13 narrow frequency bands ranging between 0.03 and 8 Hz. The absolute scaling of these spectra was calculated based on independent waveform modeling solutions of the moment magnitudes for a subset of these events to avoid circularity. Using our 1D path calibrations, we determined coda‐based magnitudes for a majority of the events. We obtain fairly good agreement with waveform‐modeled seismic moments for the larger events ( M w >4.5) at low frequencies ( 0.7 Hz) because of unaccounted 2D path effects, as well as mixing of both Sn coda and Lg coda, which have different attenuation behavior. This scatter leads to increased variance in the magnitudes estimated for smaller events in which low‐frequency amplitudes are below the noise levels and the higher frequencies are the only signals available. We quantify the expected variance in coda envelope amplitudes as a function of frequency using interstation scatter as our metric. The net results of this study provide thousands of new 1D coda magnitude estimates for events in the broad region, as well as the necessary initial starting model for use in a new related 2D coda study (Pasyanos et al. , 2016). Online Material: Table of site terms and moment magnitudes.

Using seismograms recorded at 66 Canadian seismic stations, coda Q was estimated from earthquakes in southwestern British Columbia and northern Washington State, employing the single backscattering approximation. A total of 580 earthquakes with magnitudes ranging from 1.2 to 6.4, depths from 0 to 67 km, and epicentral distances of 5–110 km were selected to obtain 3022 high signal‐to‐noise ratio traces for analysis. An average of all the data yields a relationship for coda Q of Q C =72 f 0.91. There is little variation of this coda Q relationship when using either crustal or in‐slab sources, which represent uniform sampling of the crust and upper mantle. Crustal earthquakes result in a relationship of Q C =73 f 0.89, and for in‐slab events Q C can be expressed as Q C =69 f 0.94. In general, Q ( Q C at 1 Hz) increases from the west coast of Vancouver Island to the east‐southeast within the Coast belt. Stations on west‐central Vancouver Island closest to the landward projection of the Nootka fault zone, and the location of the only two known large crustal earthquakes (1918 M ∼7 and 1946 M ∼7.3) on Vancouver Island, have the lowest Q values in our study area, suggesting a contrast in Q between the north and south of the island. Online Material: Figure showing principal tectonic units and station locations, and tables of average Q and alpha values with estimated uncertainties.

Coda amplitudes have proven to be a stable feature of seismograms, allowing one to reliably measure magnitudes for moderate‐to‐large ( M ≥3) earthquakes over broad regions. Because smaller ( M

A 1D normal moveout (NMO)‐corrected and stacked pseudoprofiling method was applied to analyze the characteristic features shown on primary P ‐ and S ‐wave coda and on Sp waveforms from local microearthquakes in an attempt to image prominent reflectors and to resolve shallow crustal velocity structure (∼5 km) in the upper Mississippi embayment. Acoustic well log data were used to constrain the P ‐wave velocity in the upper 5 km. Events at close distances and with clear P and S arrivals were selected to ensure reliable NMO correction for reflections and transmissions. The observed reflections and transmissions are important controlling factors on modeling waveforms. We analyzed local earthquake data recorded at all broadband and one short‐period station of the Cooperative New Madrid Seismic Network. Despite polarity differences among P , S , and Sp waveforms, consistent reflectors in the sedimentary section can be imaged across the three wave types. Correlation with a basement‐penetrating well indicates that reflectors at the base of the Upper Cretaceous–Holocene Mississippi Embayment Supergroup, the base of the Cambrian–Ordovician Knox Group, and the high‐velocity lower Upper Cambrian Bonneterre Formation are shown in pseudoprofiles among stations in the upper Mississippi embayment. Our study finds that a one‐layer homogeneous velocity model of sediments in the ranges of 1.95–2.42 km/s for V P and 0.60–0.73 km/s for V S overlying a half‐space of Paleozoic rocks with velocities in the ranges of 6.0–6.2 km/s for V P and 3.26–3.6 km/s for V S can represent shallow crustal structure in the upper Mississippi embayment. Differential times of P – PpPhp and S – SsShs appear linearly proportional to sediment thicknesses, which best fits a one‐layer sediment structure with average V P 2.042±0.041 km/s and V S 0.709±0.051 km/s, in the least‐squares sense predicted by the wave propagation effects. Online Material: Figures illustrating seismograms, process procedure, imaged reflectors, and resolved velocity structure for six broadband stations and one short‐period station.

In the present study, the scattering and intrinsic attenuation are separated using the S ‐wave attenuation ( Q β ) and coda‐wave attenuation ( Q c ) employing the Wennerberg (1993) method, and the frequency‐dependent Q s and Q i relations have been developed for the region. The Q i and Q s show the frequency‐dependent character in the frequency range 1.5–24 Hz. The average scattering and intrinsic relationships are obtained for the region as Q s =(31±1) f 1.04±0.02, Q s =(48±1) f 1.05±0.02, and Q s =(61±1) f 1.05±0.02 and as Q i =(68±1) f 0.95±0.06, Q i =(134±1) f 1.01±0.05, and Q i =(167±1) f 0.96±0.03 for lapse time windows of 30, 40, and 50 s, respectively. The quality factor for the P wave ( Q α ) and the S wave ( Q β ) are estimated using the extended coda‐normalization method of Yoshimoto et al. (1993). The frequency dependence Q α and Q β relationships are obtained as Q α =(25±1) f (1.24±0.04) for the P wave and Q β =(58±1) f (1.16±0.04) for the S wave. The quality factor for the coda wave ( Q c ) is estimated using the single backscattering model of Aki and Chouet (1975). The comparison of Q β and Q c with Q i and Q s shows that both Q i and Q s are lower than the Q β , as well as Q c , at 30 s lapse time. As the lapse time increases, both Q i and Q s increase in such a manner that Q c will increase because it contains the effects of both. This agrees with the theoretical as well as the laboratory measurements. Also Q c is higher than Q β ; this supports the model given by Zeng et al. (1991), which predicts that the combination of Q i and Q s should be such that Q c is more than Q β .

Using the single backscattering method, coda quality factor functions through coda window lengths of 20, 30, 40, 50, and 60 s have been estimated for the New Madrid seismic zone (NMSZ). Furthermore, geometrical spreading functions for distances less than 60 km have been determined in this region at different center frequencies exploiting the coda normalization method. A total of 284 triaxial seismograms with good signal‐to‐noise ratios (SNR>5) from broadband stations located in the NMSZ were used. The database consisted of records from 57 local earthquakes with moment magnitudes of 2.6–4.1, and hypocentral distances less than 200 km. Q ‐factor values were evaluated at five frequency bands with central frequencies of 1.5, 3, 6, 12, and 24 Hz. Vertical components were utilized to estimate vertical coda Q ‐factor values. Horizontal coda Q ‐factor values were determined using the average amount of the Q ‐factor values estimated from two orthogonal horizontal components. The coda Q ‐factor increases with increasing of the coda window length implying that with increasing the depth, the coda Q ‐factor increases. The intermediate values of the Q ‐factor and intermediate values of the frequency dependency indicate that the Earth’s crust and upper mantle beneath the entire NMSZ is tectonically a moderate region with a moderate to relatively high degree of heterogeneities. The geometrical spreading factors of S ‐wave amplitudes are frequency dependent and determined to be −0.761, −0.991, −1.271, −1.182, and −1.066 for center frequencies of 1.5, 3, 6, 12, and 24 Hz, respectively, at hypocentral distances of 10–60 km. The geometrical spreading factors for lower frequencies are not recommended to be used due to the greater impact of the radiation pattern and directivity effect on low frequencies, as well as the greater sensitivity of band‐pass‐filtered seismograms of small earthquakes to the noise in low frequencies.

Published results of coda Q show a large variation in values. These variations are often claimed to be related to different tectonics, whereas they might just be related to using different assumptions in the processing, leading to different input parameters for the analysis. In this study, the effect of using different processing parameters is investigated and significant differences, particularly at low frequencies, are observed. We find a new set of optimal parameters, which we recommend using in future studies. Using a short lapse time of 30 s and optimal parameters, data from both similar and very different tectonic regions are used to calculate coda Q using the same program and the same parameters. The regions considered are eastern Anatolia, the Azores, Jan Mayen, northwestern and central Argentina, the Shanxi rift system in China, and southwestern Norway. We obtain the following relations: eastern Anatolia ( Q =88 f 0.66 ), Azores ( Q =86 f 0.70 ), Jan Mayen ( Q =90 f 0.72 ), northwestern and central Argentina ( Q =89 f 0.94 ), Shanxi rift system ( Q =99 f 0.89 ), and southwestern Norway ( Q =124 f 0.91 ). The results show that coda Q is very similar for regions of similar tectonics and significantly different for regions with varying tectonics. Using alternative, more common parameters gives different Q , but the regional differences remain, so which parameters to use to get correct coda Q values is still uncertain. However, coda Q can clearly distinguish different tectonic areas provided identical processing parameters are used, even if they are not optimal.

Operation WIGWAM was a test of a 30 kt nuclear depth charge conducted in deep water 500 miles southwest of San Diego on 14 May 1955. Its primary purpose was to determine the effectiveness of that device as an antisubmarine weapon. The acoustic pulse from the test, initially an intense shockwave, radiated throughout the North and South Pacific Oceans. Acoustic reflections from topographic features were recorded for several hours after the explosion by SOund Fixing And Ranging (SOFAR) hydrophones at Point Sur, California, and Kaneohe, Hawaii. Sheehy and Halley (1957) identified peaks of the recorded coda with reflections from specific topographic features at great distances (e.g., the Hawaiian Islands, French Polynesia, or Fiji). With modern data for seafloor topography and ocean sound speed, these coda were computed with surprising accuracy using simple geodesic rays reflected from islands and seamounts. The intensity variations of the coda are mostly determined by simple ray geometry, together with modest attenuation. Coda peaks are often obtained from rays arriving simultaneously from multiple, but disparate, topographic features. Online Material: Figures of computed and measured coda and associated geodesic paths.

The diffusive approximation predicts that the ratio of S ‐to‐ P energy in earthquake coda depends only on the ratio of S and P wavespeeds. However, for seismic applications, the near‐surface geologic conditions can introduce complications to the original models of coda diffusion. Specifically, the free‐surface and near‐surface site effects can conspire to significantly affect the ratio of body‐wave energy when observed at the Earth’s surface. To address these effects, previous workers developed physics‐based models of coda diffusion to account for near‐surface effects such as body‐to‐surface‐wave mode conversions and Earth stratification. These models predict that the value of the S ‐to‐ P energy in coda will be strongly affected by near‐surface effects; however, there have been very few controlled experiments to verify this. In this article, we describe the results of an experiment that was designed to observe the effects of the near surface on the S ‐to‐ P strain energy ratio of the coda. Two arrays were deployed simultaneously at two different sites, each of which contained significantly different near‐surface geology. We observe that the energy ratio of the coda is significantly affected by the near‐surface geology and is highly frequency dependent.

The quality factor of coda ( Q c) waves has been estimated by using single backscattering and single isotropic scattering models. The earthquakes used were recorded by three permanent and one temporary network located in the central and eastern Alborz, Iran. The database was composed of 746 local earthquakes with local magnitude from 1.1 to 5.7. The estimated Q c has been found to be similar for lapse times greater than twice the S ‐wave travel time (2 t S ) for both methods. The estimated Q c for central frequencies

Cross‐correlating seismic random signals, such as coda waves or ambient noise, at two sites can extract seismic wavefields as if a source is at one site and a receiver is at the other if certain conditions are met (e.g., Aki, 1957; Campillo and Paul, 2003). Given the recent development of dense seismic network, the last decade has witnessed the rapid emergence of imaging Earth’s local (Nagaoka et al. , 2012), regional (Shapiro et al. , 2005; Nishida et al. , 2008; Lin et al. , 2011), and global (Nishida et al. , 2009) seismic structures and their temporal changes (Brenguier, Campillo, et al. , 2008; Brenguier, Shapiro, et al. , 2008; Nagaoka et al. , 2010; Brenguier et al. , 2014) from these random signals. With this trend of research, we fortuitously found peculiar pulses in the cross correlations of the Japanese high‐sensitivity seismograph network (Hi‐net; Obara et al. , 2005) seismic records (Fig. 1). Here, we used one‐month records for January 2012 at 142 Hi‐net stations in southwest Japan. Applying a band‐pass filter of 10–20 s to correlograms reveals sharp pulses at lag times of every 60 s (Fig. 1a). Unfiltered correlograms nicely demonstrate a propagation of Rayleigh waves, but a closer look at locations where no wavetrain is extracted reveals sharp pulses at lag times of every second (Fig. 1b). Figure 1. Correlograms of ambient seismic noise aligned along the interstation distance. (a) Correlograms with a band‐pass filter between 10 and 20 s. (b) Unfiltered correlograms of a portion where no extracted wave propagation is expected. (c,d) Same as (a) and (b) but the quantization errors are subtracted from the raw data before cross correlation. (e,f) Same as (a) and (b) but the difference of ZR and RZ correlations is taken without subtraction of the quantization error. The amplitude scales of (c,e) and (d,f) are the same as those of (a) and (b), …

Accurate magnitude determinations are important issues when producing seismicity catalogs aimed at assessing seismic hazard, especially in moderate seismicity regions. When only a few seismic stations are available, the stability of coda‐wave amplitudes presents great advantages as compared to S / Lg ‐wave amplitudes for estimating magnitude. We develop a method for processing coda waves to determine the moment magnitude M w that is valid both for recent digital short‐period records and old analog short‐period seismograms. Because spectral analyses of paper records are most often not possible, we fit the coda envelopes of the raw time signal. Our empirical representation of the coda envelope is tested with synthetics, and its parameters are determined using 109 earthquakes digitally recorded at 12 stations of the Laboratoire de Detection et de Geophysique (LDG) French velocimetric network between 1997 and 2013. The coda parameters are adapted to the four broad geological zones that can be defined in France. Within each zone, this model matches the observed coda‐wave envelope for M w between 3.4 and 5.3. Synthetic coda signals computed from the regional quality factors Q c ( f ) within the [0.3–7 Hz] frequency band of the instrument show that the frequency and regional variation of Q c are enough to explain the coda shape of the raw signal. This empirical coda model is only valid for events with corner frequency larger than the 1 Hz cutoff frequency of the LDG instrument. When applying this raw signal envelope model to actual data, the site effect at each station must be accounted for. From this empirical coda model, we obtain stable corrected coda amplitudes that can be directly converted into M w by linear regression.

An earthquake dataset consisting of 327 records from 67 earthquakes is used to study the frequency‐dependent amplitude decay of seismic coda waves in the Kumaon Himalayas, India. Coda amplitude decays with time t are measured by empirical t − α dependences, with exponent α being variable regionally with frequencies and lapse times. The results show that α varies from 1 to 3, with an average of about 1.93. The lapse‐time dependence of α is noticeable but weak, and no significant azimuthal dependence of α is obtained. Significant regional variations of α are found, ranging from α ≈2.1 for the Lesser Himalaya to α ≈1.76 for the Greater Himalaya. These variations correlate with the geology of the region and crustal structure. The values of α are mostly frequency independent, indicating low intrinsic attenuation within the crust. When interpreted according to the conventional coda‐ Q ( Q c ) model, these values of α lead to Q c ≈78 f 1.06 in the Kumaon Himalaya, Q c ≈90 f 1.09 in the Kumaon Lesser Himalaya, and Q c ≈90 f 0.92 in the Kumaon Greater Himalaya. A frequency‐dependent α is found within a localized area, giving an estimate of near‐surface Q ≈200. Three general conclusions of this study may be significant for coda studies in other areas. First, the values of α are variable regionally, whereas its average level is remarkably constant and correlates with rock types and upper‐crustal structure. Second, the traditionally assumed value of α =1 is much lower than the actual spreading rates, which shows that the coda cannot be viewed as body waves scattered within a uniform crust. Using the value of α =1 causes a systematic underestimation of coda amplitude decays in the data. Third, the dependence of α on frequency is relatively weak and occurs in localized areas. Combined with an underestimated α , this weak frequency dependence may cause biases in the estimation of secondary parameters, such as coda Q .

We have obtained tomographic maps of Lg coda Q and its frequency dependence at 1 Hz for all of North America assuming that all of our coda Q data can be described by a power law. Our map of Q 0 displays highest values (700–1000) throughout a large region that coincides with the Canadian Shield and exhibits sharp changes in Q 0 across much of the boundary separating the high values of the shield from lower values outside it. A second high Q region appears as a small oval of high Q 0 values (650–700) overlying much of the Ozark uplift in the central United States. A band of low values (200–350) covers most of Central America and Mexico and extends northward from California into western Canada before broadening in Alaska. A north–south‐trending band of high η values, extending from northern Canada to the gulf coast, separates the North American Cordillera from stable portions of central North America. We compared our Q 0 and η determinations with those found in five earlier studies in two regions of the United States. In the present study, in each region these values agree with values found in the earlier studies when earthquake–station distances are greater than 300 km but differ when distances are less than that. The similarity of our Q 0 measurements to previous direct Lg and Lg coda measurements and the inverse correlation we find of Q 0 with upper mantle temperatures suggest that we may predominantly be measuring intrinsic Q 0 . We propose a mechanism for explaining why scattering dominates over intrinsic attenuation for Lg and Lg coda propagation at short distances but can be secondary to intrinsic attenuation at long distances. It can be nicely described using a ray characterization for Lg and understanding that at short distances many near‐vertical traveling rays are multiply reflected in the crust and much energy is lost to the mantle. At greater distances (say 300 km), virtually all of the reflected waves exceed the critical angle and so cannot easily be lost to the mantle.

Source depth estimation is a key process in earthquake location, hazard assessment, and nuclear monitoring. For instance, hypocentral depth is an important parameter affecting the spatial attenuation of ground motions resulting from an earthquake and an important indicator of natural versus man‐made events. Resolving transition zone phase complexity is important in sparsely monitored areas, for improved location and magnitude estimation of a seismic event at regional‐to‐teleseismic distances. In response to this need, we demonstrate a new approach to first‐order depth estimation. Depth phases are notoriously difficult to identify, specifically for shallower events. The usual way of determining the depth of focus of an earthquake is to measure the difference Δ t P − pP between the arrival time of the direct P wave and the arrival time of the surface reflection pP . Once Δ t P − pP is calculated, the depth of the focus can then be estimated closely, assuming that the velocity structure between the surface and the event focus is satisfactorily known. Although this method sounds simple, one major challenge remains: depth‐phase identification. For this reason, many monitoring agencies pick few depth phases from earthquakes shallower than 70 km. For example, depth‐phase observations were reported for only about 5% of the analyst‐reviewed events listed in the revised event bulletins of the Prototype International Data Centre during the Group of Scientific Experts Technical Test No 3 (GSETT‐3) experiment (Murphy and Barker, 2006). Routine depth‐phase identification is complicated by issues including (1) focal mechanism (affecting the relative P and pP or sP amplitude), (2) differential attenuation effects, (3) variations of surface topography and of subsurface structure at the depth‐phase bounce points, (4) earthquake epicentral distance (e.g., complex coda of P phases bottoming at the transition zone of the upper and lower mantle makes pP identification difficult), (5) rupture duration for large earthquakes ( M …

The teleseismic virtual‐source reflection (TVR) profiling method uses free‐surface reverberations of the teleseismic P ‐wave ( PPdp phase) data to produce an image of the subsurface. Upper‐crustal reflections can be retrieved by deconvolving the average source wavelet for all stations in an array from the teleseismic P ‐wave coda recorded at each station of the array. We present a case study for which we construct the upper‐crustal TVR images in the Three Gorges (TG) region, China, based on a dataset of four teleseismic events recorded by a linear regional array of geophones. The features in the resultant TVR images, such as the geometry of the Zigui and Sichuan basins, correlate well with the surface geologic observations at the TG region. We identified reflectors in the Zigui and Sichuan basins that indicated their maximum depths at 2.6 and 7 km, respectively. The basin structure is consistent with the compressional stress regime that is evident by two reverse faults in the region: the Gaoqiao fault, corresponding to a basement ridge in the TVR image that separates the two basins, and the Xiannvshan fault. Online Material: Figures and discussion on choosing source length.

The usage of seismic ambient noise has recently proved its efficiency in different contexts, from imaging to monitoring. The impulse response (or Green’s function [GF]) between two sensors can be reconstructed from the correlation of seismic noise recorded (Campillo and Paul, 2003). This method has provided excellent results in imaging the Earth’s interior, from global to regional or local scales. More recently, the method was extended to study time‐dependent variations in those GF. A change in the delay times might originate from a change in the medium velocity or from a dramatic change in the position of the source or of one/many scatterers. Several studies using seismic ambient noise have shown that small perturbations within a volcanic edifice can be detected as changes in seismic‐wave properties (Sens‐Schonfelder and Wegler, 2006; Brenguier, Shapiro, et al. , 2008; Duputel et al. , 2009; Mordret et al. , 2010; Brenguier et al. , 2011; Anggono et al. , 2012). Contrary to the use of active sources or earthquake coda waves, the technique takes advantage of the continuous sampling of the medium using around‐the‐clock records from seismic stations. The method has proven its ability to evidence temporal physical changes in fault zones (Wegler and Sens‐Schonfelder, 2007; Brenguier, Campillo, et al. , 2008), the lunar environment (Sens‐Schonfelder and Wegler, 2011), or to detect instrumental errors (Stehly et al. , 2007; Sens‐Schonfelder, 2008). Some codes have already been presented to compute cross correlations of seismic noise, for example, within Seismic Analysis Code (SAC) (Goldstein et al. , 2003) or within Computer Programs in Seismology (CPS) 3.3 (Herrmann, 2002). To our knowledge, no integrated solution has been published to go …

It has been recently demonstrated that averaging the autocorrelations of fields produced by various almost‐vertical incoming elastic body plane waves upon a layered system approximately leads to the imaginary part of the corresponding 1D Green’s functions for deep sources located underneath the receiver (Kawase et al. , 2011). Thus, the ensemble of these waves from deep earthquakes recorded in a station located in the epicentral zone is interpreted as a diffuse field. In this short note, we extend the study to consider earthquakes recorded in a station located at epicentral distances of up to hundreds of kilometers. We consider the horizontal‐to‐vertical spectral ratio (HVSR) of the averaged P , S , and coda waves and full earthquake records at the Cibeles station (Mexico City Accelerometric Network) and compare these with the results obtained with the corresponding HVSR for the 1D (Kawase et al. , 2011) and the 3D (Sanchez‐Sesma, Rodriguez, et al. , 2011) diffuse fields models. Using the signals of 90 earthquakes recorded at Cibeles, we find that the experimental results have distinctive features compatible with the 3D signature of a diffuse field. We interpret this result as a consequence of the multiple paths that seismic waves undergo from the subducting slab to the Mexico City valley and to the multiple scattering in a complex tectonic environment. Our study strongly suggests that we can use strong‐motion records from earthquakes and apply similar techniques to the ones used to analyze the ambient seismic field. Online Material: Earthquake catalog.

Seismic coda waves scattered by small‐scale heterogeneities contain information on stress changes of the medium, because of changes in the physical state of materials. Based on the ultrasonic measurements under different stresses for a cylindrical sandstone sample, we investigate the influence of stress changes on ultrasonic S ‐coda attenuation and aim to characterize its stress/frequency‐dependent pattern. Considering the complexity of ultrasonic coda waveforms measured from finite‐size rock samples in laboratory experiments, the Monte Carlo simulation is employed to synthesize ultrasonic envelopes, which act by incorporating the effect of multiple scatterings and boundary reflections on coda waves. The optimal simulation parameters, estimated by minimizing the residual between the observed and synthesized envelopes, indicate that the rock sample under study presents moderate heterogeneities. The relationship between attenuation and stress is similar for direct and coda S waves and remains stable in the range of high‐effective stresses around 30–60 MPa, with less stress sensitivity. Enhanced attenuation for both types of waves occurs at lower‐effective stresses, but with coda attenuation much faster and stronger, presenting a quite different nonlinear behavior with respect to stress. Coda attenuation increases drastically at extremely low‐effective stresses below 15 MPa because of the increase in rock compliance, showing much greater sensitivity to high‐pore pressure than intrinsic attenuation. This study improves our understanding of the mechanism of ultrasonic coda attenuation and its scaling dependence on stress and frequency.

Coda quality factor ( Q c) estimates for the Faryb region in the southeast Zagros have been obtained by using aftershock data of the Tiab earthquake of 28 February 2006, recorded within an epicentral distance of 100 km. More than 394 earthquakes were recorded by a local temporary network consisting of 9 short‐period stations installed for one month after 28 February 2006. Q c was estimated using the single‐backscattering model in frequency bands of 0.5–24 Hz. In this research, the spatial and temporal variation of Q c in the Sanandaj–Sirjan zone is studied. To explore the lateral variation, Q c is calculated for nine stations and in different parts of the covered area. The lowest values are derived in KALI station in the southern part and the higher value in BAGA station in the northern part, but generally there is an absence of significant lateral variation in coda Q c in the Faryab region. To investigate the attenuation variation with depth, Q c value was calculated for eight lapse times. The average frequency relation for this region is Q c=(28±1.1) f (1.14±0.01) at 5 s lapse time window length and Q c=(105±1.3) f (0.98±0.047) at 30 s lapse time window length. Q c increases with depth; however, the rate of increase of Q c is not uniform with depth in the study area. Beneath the Faryab region, the rate of increase of Q c is greater at depths less than 49 km (compared to larger depths), indicating a low attenuation structure at less than ∼49 km depth, which is the Moho depth in this region.

Reliable use of regional seismic phases for discrimination and magnitude estimation requires accurate corrections. Procedures that simultaneously invert for source, attenuation ( Q ), spreading, and site parameters have trade‐offs that result in large errors for source and distance corrections. This motivates our efforts to improve corrections by constraining trade‐offs. Using an empirical Green’s function approach, relative spectra of regional phases are computed for nearby, similar earthquake pairs of different moments, to cancel path, site, and focal mechanism effects, giving reliable estimates of source corner frequencies and relative moments. Many such pairs are available for this analysis throughout Eurasia. A large dataset of three‐component regional seismograms from Incorporated Research Institutions for Seismology (IRIS) is assembled and processed for events listed in the preliminary determination of epicenters from 1989 to 2009. A relative Brune (1970) source model is fit to network‐median relative spectra for over 46,000 pairs, corresponding to about 9400 unique events. Pseudorelative spectra are also computed from coda envelopes in 16 frequency bands. Coda is less sensitive to focal mechanism, event separation, and station coverage (Mayeda et al. , 2007) but more prone to data quality issues. Results are presented with good corroboration of the moments and corner frequencies from coda and direct phases. Detailed case studies are shown to indicate the level of agreement, interstation variability, comparisons to published results based on local networks, and causes of various discrepancies between coda and direct phases. The spectra subsequently are corrected for source terms to estimate more reliable Q , geometric spreading rates, and site effects. Examples are compared to amplitude tomography results. Our estimated spreading rates are consistent with published studies, except for long distance Pn and mantle P . As important, the spreading analysis also provides a very consistent set of absolute scalar moments. Further details and comparisons to independent Q and frequency‐dependent site terms are presented in a companion paper (Fisk and Phillips, 2013).

Coda waves arise from scattering to form the later arriving components of a seismogram. Coda‐wave interferometry (CWI) is an emerging tool for constraining earthquake source properties from the interference pattern of coda waves between nearby events. A new earthquake location algorithm is derived which relies on coda‐wave‐based probabilistic estimates of earthquake separation. The algorithm can be used with coda waves alone or in tandem with arrival‐time data. Synthetic examples (2D and 3D) and real earthquakes on the Calaveras fault, California, are used to demonstrate the potential of coda waves for locating poorly recorded earthquakes. It is demonstrated that CWI: (a) outperforms traditional earthquake location techniques when the number of stations is small; (b) is self‐consistent across a broad range of station situations; and (c) can be used with a single station to locate earthquakes.

Fisk and Phillips (2013) motivated the need to constrain trade‐offs among correction parameters for regional phase amplitudes to improve seismic discrimination and magnitude (yield) estimation. Using an empirical Green’s function approach to cancel path and site effects, relative spectra of direct regional phases and coda were fit for many thousands of nearby, similar earthquake pairs of different moments, to estimate reliable source corner frequencies and relative moments. Detailed comparisons demonstrated the benefit of using independent measurements of coda and direct phases to provide a large set of corroborated source terms for earthquakes throughout Eurasia. The spectra were subsequently corrected for source terms to estimate more reliable Q , geometric spreading rates, and site effects. Regression analysis was used to estimate geometric spreading rates and to establish a very consistent set of absolute moments. Here, frequency‐dependent Q and site terms are examined for , Sn , , and Pn spectra. Comparisons to independent Q estimates from amplitude tomography exhibit good agreement for many paths. Large discrepancies are shown for higher frequencies, in low Q regions, and/or at the edges of the tomography grid, which significantly impact P / S discrimination. Our frequency‐dependent site terms are compared with independent estimates from coda tomography, showing good agreement (even for and Pn , at many stations) but also some substantial differences.

Co‐ and postseismic slip of the great earthquakes can give rise to temporal changes in the medium either due to strong ground motion damaging near‐surface sediment layer or stress perturbations modulating crack density and/or fluid movement at depth. Such time‐varying crustal properties can result in fractional change in seismic velocity that can be probed from cross‐correlating waveforms and measuring their time lags within a repeating earthquake sequence. This study analyzes lag times of high‐frequency (0.5–2.0 Hz) P ‐ and S ‐coda waves as well as long‐period (0.03–0.1 Hz) surface waves from repeating aftershock sequences of the great 2004 Sumatra–Andaman and 2005 Nias–Simeulue earthquakes. The observed lag times reveal several major characteristics: (1) lag‐time series τ ( t ) of S coda for the 2004 sequences fluctuate around zero and are sometimes negative as a function of lapse time, whereas τ ( t ) of S coda for the 2005 sequences exhibit a monotonic increase as a function of lapse time; (2) average velocity reduction of S coda (− δV S ) is about two times larger than that of P coda (− δV P ); average velocity reduction of Rayleigh waves (− δV LR ) is 3–4 times larger than that of Love waves (− δV LQ ); and (3) δV S and δV LR display temporal velocity recovery with calendar time, especially for the 2005 sequences. The form of temporal velocity recovery of δV S of the 2005 sequences is similar to available displacement time series of the nearby geodetic station. Whereas we discuss potential artifacts, such as source separation, temporal changes of the noise field, and instrument response, observations collectively point to nonuniform temporal velocity reduction in the crust modulated by co‐ or/and postseismic slip of the 2004/2005 great earthquakes. Online Material: Table of source parameters and figure showing tests on lag‐time measurements of long‐period Rayleigh waves.

The behavior of earthquake source scaling has been the topic of significant debate in the earthquake source community over the past two decades, and validating recent results has been difficult. In this study, we focus on high quality records from the 21 February 2008 M w 5.9 Wells, Nevada, earthquake, and its aftershocks, which provide an unprecedented opportunity to take an in‐depth look at the source scaling and ground motions. For this earthquake sequence, conflicting scaling relations were reported in two previous studies (Baltay et al. , 2010; Mayeda and Malagnini, 2010). In addition, recent comparisons of the ground‐motion prediction equations (GMPEs) with this data set have shown significant overestimation of ground motions for the aftershocks, while being in rough agreement with the mainshock ground motions (Petersen et al. , 2011). In order to evaluate the reported scaling relationships and better understand the observed discrepancy in the GMPE’s, we chose to revisit the Wells, Nevada, earthquake sequence. We investigate the source parameters of the earthquakes using the S ‐wave and coda spectral ratio methods (Mayeda et al. , 2007) and find that the stress drops of the aftershocks are 2–5 times lower than that of the mainshock. We compute pseudospectral acceleration (PSA) ratios using direct S waves from broadband records and compare with theoretical source ratios assuming the self‐similar and non‐self‐similar source scaling assumptions as well as ratios derived from state‐of‐the‐art GMPE estimates. We find that we can only simultaneously match source ratios between the mainshock and selected aftershocks if we use non‐self‐similar scaling. Accounting for the significant differences in the stress drop between small and large earthquakes will help to enhance the prediction capability of ground motions for this region. By validating the source scaling with PSA ratios, these results can be used as constraints in stress parameterization used in the GMPE’s.

Online Material: Time sequences of the fk analysis for two of the largest events recorded by the Long Beach array. Seismic‐scattering theories describe high‐frequency coda waves as a combination of waves from random scatterers superimposed on direct waves from the source. The direct waves are expected to be spatially coherent whereas the scattered waves, arriving with random phase, will be spatially incoherent. Our objective is to use data from an extreme high‐resolution seismic experiment in Long Beach, California, to determine the transition from coherent to incoherent coda. The network, deployed by Nodal Seismic, comprises ∼5400 vertical component instruments, spaced every ∼100 m over an area of ∼5×7 km2. It was deployed for a period of six months with the primarily target to image the geological structure of the area for oil exploration. During the deployment, several thousand earthquakes and microearthquakes were recorded. We examine coda waves from the two largest events that occurred in the vicinity of the array. We compute frequency–wavenumber diagrams to determine the sources of coda and their evolution in time. Entropy analysis of the propagation of seismic waves through the array indicates the transition between the coherent direct body waves and the onset of incoherent coda waves. Our analysis illustrates that after the arrival of the body waves, the seismic coda is initially dominated by a dispersing wave train composed of spatially coherent body waves, forward scattered from 1D crustal layering. This is then followed by omni‐directional, spatially incoherent coda waves that can be described as scattered waves from 3D random sources. Pioneering work by Aki (1969) first recognized that coda is the result of scattering due to the random distribution of heterogeneities in the crust. In later work, Aki and Chouet (1975) developed a theoretical model of single scattering, using the Born approximation, that fits …

The coda quality factor of short‐period S waves ( Q c ) excited by local earthquakes in the Pyrenees has been measured as a function of the length of the coda window ( L W ) for different choices of the onset time of the coda ( t W ). In the 2–16 Hz frequency band, we observe a transient regime characterized by an increase of Q c with L W , followed by a stabilization around a plateau the value of which depends on the central frequency of the signal. Using Monte Carlo simulations of wave transport in a variety of random media (≈1200 models), we demonstrate that the lapse‐time dependence of Q c in the Pyrenees may be modeled by multiple anisotropic scattering of seismic waves, without invoking any depth dependence of the attenuation properties in the crust. In our model, anisotropic scattering is quantified by the ratio between the transport mean free path and the mean path ( l */ l ). At 6 Hz, the data require an anisotropy factor l */ l ≥5, a transport mean free path l *≈400 km, and an intrinsic quality factor Q i ≈800. From the frequency‐dependent plateau of Q c at large lapse time, we infer an intrinsic quality factor of the form Q i ≈400 f 0.4 in the Pyrenees. We also show how the rapid increase of the lapse‐time dependence of Q c with frequency may be exploited to put constraints on the power spectrum of heterogeneities in the crust.

We address a classic problem in observational seismology, namely the precise estimation of the focal depth of small to moderate earthquakes. The data available are three‐component records including Pn onsets and their coda. We believe that the Pn coda contains P ‐type phases that originate in the source area and lag behind due to longer travel paths. Phases such as pP and sP are generally not included in seismic bulletins, and even cepstrum and array f ‐ k techniques have not been successful in extracting such arrivals. We use recent developments in polarization analysis (Roberts and Christoffersson, 1990; Fedorenko et al. , 2008) to extract secondary P phases in local and regional records from five earthquakes. Phase characteristics are typically slowness and lag time relative to the preceding Pn phase. However, information is incomplete for confidently deciding whether an arriving wavelet should be classed as pP , sP , or something else. This validation problem is resolved by picking several coda arrivals that can uniquely be fitted to travel‐time curves for suites of Pn ‐coda waves. Focal‐depth estimates, even for Ground Truth 05 events, are seldom more accurate than to 5 km, and therefore we compared our results with those from moment tensor analysis. Differences in depth estimates were less than 5 km and for H Pn arrival is generally sPn . The Dt ( Pn − pPn ) lag time for shallow events is 2.0 s or less and hence difficult to detect in the coda. Most techniques for focal‐depth estimation are not user friendly. In contrast, the polarization schemes tested are easy to use and depth estimates are more accurate than those derived from ground truth studies based exclusively on P ‐wave arrivals.

Receiver‐function analysis is an effective tool for investigating crustal seismological structure. Here, we present the extraction of the Moho‐reflected PpPp that emerges in teleseismic P coda via a deconvolution process. Using nonlinear waveform analysis (an approach using simulated annealing technique) we estimate the source wavelet of a teleseismic P wave from records of the vertical component observed at an array of seismometers. PpPp recorded on the vertical component can be extracted by deconvolving individual vertical components by the resulting source wavelet. By employing this technique in a case study in southwestern Japan, seismic images from PpPp , as well as from Ps and PpPs , successfully image the continental Moho, the oceanic Moho, and the top surface of the Philippine Sea slab. In addition, we found that the amplitude of PpPp is useful in precisely determining crustal properties, such as vertically averaged V P / V S and the crustal thickness, by grid‐search techniques. It is also important to take into account the variations of the conversion/reflection coefficients for decreasing errors of the parameters in the grid‐search technique. Moreover, we demonstrate that improved seismic images of horizontal discontinuities can be obtained by using a stacking technique.

The spatial and temporal variations of coda attenuation (![Graphic][1] ) were studied in the source region of the 2011 Off the Pacific Coast of Tohoku, Japan ( M w 9) earthquake. The ![Graphic][2] values were determined from the amplitude decay rate of the S ‐wave coda in narrower overlapping frequency bands in the range f =1.0–24 Hz, based on a single isotropic scattering model for more than 400 earthquakes ( M JMA 3∼6.5) in the region recorded in a period from January 2005 to August 2011, including pre‐ and postseismic period. Our estimates of the spatially averaged ![Graphic][3] value, ![Graphic][4] in f =1.0–24 Hz, is almost stable with small variations ( increases by about 10%–16% after the 2011 Tohoku event in f =1.25–3.5 Hz in some stations of northern Japan, which is confirmed by a statistical t ‐test at 99.9% confidence level. The change in ![Graphic][6] is spatially limited to the rupture zone, while other paths remain nearly unaffected, suggesting local changes of scattering properties in the vicinity of the Tohoku‐Oki source volume. This change may be attributed to an increase in the density of open microcracks in the mainshock source volume (such as due to increase in stress induced by the 2011 event) and probably the fluid content in fractures in the rocks. A model of heterogeneity due to coseismically opened cracks (dominant scale length of a =0.6–1.8 km in f =1.25–3.5 Hz) enhanced by increased stress change possibly controls the increased ![Graphic][7] after the 2011 event in the source rupture region. [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif [3]: /embed/inline-graphic-3.gif [4]: /embed/inline-graphic-4.gif [5]: /embed/inline-graphic-5.gif [6]: /embed/inline-graphic-6.gif [7]: /embed/inline-graphic-7.gif

A tsunami coda lasting more than 5 days was clearly recorded across the Pacific Ocean following the 2011 Tohoku‐Oki earthquake. We found that the coda energy per unit area (elapsed time of 60–96 hr from the earthquake origin time) does not show a systematic variation with respect to the travel distance at the stations located at sea depths deeper than 2000 m, whereas the maximum amplitude near the source is significantly larger than those of the far‐field stations. This feature is similar to that of a seismic coda in the sense that the coda energy is distributed uniformly in space. The average tsunami coda decays with a decay time of ∼24 hr. When we carefully examined the tsunami waveform near Japan, a large‐amplitude wave was identified at an elapsed time of ∼48 hr, which corresponds to the round‐trip travel time for waves reflected from Chile. To investigate the excitation mechanisms of the tsunami coda, we conducted a tsunami simulation using a global model. Although linear long‐wave tsunami equations were found to overestimate the observed coda amplitude, the equations including an intrinsic energy loss could successfully simulate the observed coda decay if a bottom friction coefficient of 10 −5 was assumed. This indicates that not only wave scattering but also intrinsic energy loss plays an important role in the tsunami coda excitation and decay process.

We observe enhanced high‐frequency radiation of P waves from aftershocks of the 4 April 2010 ( M w 7.2), El Mayor–Cucapah, Baja California, earthquake that may reflect isotropic radiation generated by rock damage in the source regions. To eliminate path, radiation pattern, and site effects we use spectral ratios of four pairs of collocated events with similar size and focal mechanism that occurred within short time intervals (less than 24 hrs) and were recorded at the same stations. The P / P and the S / S spectral ratios calculated for the selected pairs of events show that events with similar size may have differences of high‐frequency radiation up to a factor of 4 at 10 Hz and up to a factor of 10 at 4 Hz for P and S waves, respectively. To evaluate the differences between P ‐ and S ‐wave energy radiated at high frequencies, we calculate the ( P / P )/( S / S ) ratio of the ratios at high frequencies ( f >1.5 Hz) in a band for which the signal‐to‐noise ratio is adequate. Since the pairs of used events have approximately the same size, the ratio of ratios is expected to be near unity. We observed high ( P / P )/( S / S ) spectral ratios at high frequencies ( f >6 Hz), up to a factor of 9, which may reflect isotropic radiation associated with rock damage. To find additional possible signatures of rock damage, we evaluate changes of coda Q using the same data. The results show small change of Q for a doublet located on the Laguna Salada fault. However, for the doublets located south of the Sierras El Mayor–Cucapah, where there was no surface rupture before the sequence, changes of Q at 5 Hz by a factor of about 3 indicate significant temporal variations of rock properties along the source–receiver path.

The difference between S ‐wave and S ‐to‐ P ‐wave conversion ( S P phase) arrival times is enhanced with rectilinear motion detector filtering to describe alluvial‐sediment thickness in the Kaohsiung–Pingtung (Kaoping) plains area. A more complete understanding of the underground structures of the Kaoping area is provided in this paper and explains why the surrounding regions in Taiwan experience more earthquakes than the Kaoping area. Data are based on seismic activity recorded by the portable array for numerical data acquisition (PANDA) for the period from 1995 to 1997. The difference between S ‐wave and S P ‐phase arrival times shows that the sedimentary layer is thicker along the west and southwest coasts. P ‐wave travel‐time residuals, high‐frequency attenuation parameters Kappa, and quality factor Q P , Q S , and coda waves confirm this result. We also determined the orientation of the Chaochou fault using the first motion of P ‐wave arrivals. To the east of the Chaochou fault, stress trends southeast–northwest, while to the west, it trends northeast–southwest. The change in stress trends east and west of Chaochou fault suggests the presence of a highly fluid accretionary wedge in the Kaoping area. The Chaochou fault forms a seismically active tectonic boundary with the uplift of the hanging wall leading to westward tilting of the basement of the Kaoping plains. We demonstrate that these features are the reason there are relatively few earthquakes in the Kaoping area. The presence of a highly fluid accretionary wedge is indicated by a thick alluvial layer in the west and southwest Kaoping coasts; the Peikung High acts as the indenter that may allow seismic energy to escape and reduce the number of earthquakes in the region. Online Material: Figures illustrating calculations of Kappa, Q c , P ‐ and S ‐wave spectra, Q P , and Q S from ground‐motion data.

The Iwate‐Miyagi Nairiku earthquake (14 June 2008, M 7.2, depth 8 km) caused severe damage in the southern part of Iwate and the northern part of Miyagi Prefectures in Japan. The KiK‐net Ichinoseki‐Nishi vertical array site (IWTH25), operated by the National Research Institute for Earth Science and Disaster Prevention (NIED), is situated near the epicenter. The observed peak ground accelerations at the surface in the up–down, east–west, and north–south directions were, respectively, 3866 cm/s 2 , 1432 cm/s 2 , and 1143 cm/s 2 . The extreme ground motions recorded at this site provide valuable information about the wave propagation and soil degradation at a large strain level. We examined the nonlinear soil behavior at the site due to the earthquake on the basis of normalized input‐output minimization (NIOM) analysis. First, we analyzed 279 records observed before the Iwate‐Miyagi Nairiku earthquake and estimated the vertical propagation time (initial value) between the bottom of the well and the ground surface by taking into account incidence angles. Then we analyzed the records of the Iwate‐Miyagi Nairiku earthquake and 179 events that occurred after it (from 14 June 2008 to 19 October 2009). The results revealed that (1) the S ‐wave propagation time increased from 0.265 s to about 0.35 s because of the nonlinear behavior of the soil; (2) this propagation time change corresponded to a 60% reduction in the shear moduli of the upper soil layers (surface to 64‐m depth) and a strain level of 1×10 −3 ; (3) a gradual decrease in the propagation time with decreasing seismic intensity was observed soon after the principal motions; however, (4) the propagation times in the coda part and for the events after the Iwate‐Miyagi Nairiku earthquake were still larger than the initial value, indicating that the shear rigidities of soils remained less than their values before the earthquake for more than a year.

Source processes commonly posed to explain instances of remote dynamic triggering of tectonic (nonvolcanic) tremor by surface waves include frictional failure and various modes of fluid activation. The relative potential for Love‐ and Rayleigh‐wave dynamic stresses to trigger tectonic tremor through failure on critically stressed thrust and vertical strike‐slip faults under the Coulomb–Griffith failure criteria as a function of incidence angle are anticorrelated over the 15‐ to 30‐km‐depth range that hosts tectonic tremor. Love‐wave potential is high for strike‐parallel incidence on low‐angle reverse faults and null for strike‐normal incidence; the opposite holds for Rayleigh waves. Love‐wave potential is high for both strike‐parallel and strike‐normal incidence on vertical, strike‐slip faults and minimal for ∼45° incidence angles. The opposite holds for Rayleigh waves. This pattern is consistent with documented instances of tremor triggered by Love waves incident on the Cascadia megathrust and the San Andreas fault (SAF) in central California resulting from shear failure on weak faults (apparent friction is μ * ≤0.2). Documented instances of tremor triggered by surface waves with strike‐parallel incidence along the Nankai megathrust beneath Shikoku, Japan, however, are associated primarily with Rayleigh waves. This is consistent with the tremor bursts resulting from mixed‐mode failure (crack opening and shear failure) facilitated by near‐lithostatic ambient pore pressure, low differential stress, with a moderate friction coefficient ( μ ∼0.6) on the Nankai subduction interface. Rayleigh‐wave dilatational stress is relatively weak at tectonic tremor source depths and seems unlikely to contribute significantly to the triggering process, except perhaps for an indirect role on the SAF in sustaining tremor into the Rayleigh‐wave coda that was initially triggered by Love waves.

We introduce a new method to use narrowband regional amplitude envelopes for event analysis. Building on the success of the coda‐wave method, we construct synthetic template envelopes that attempt to fit the entire waveform, including multiple direct phases and their coda, across a broad frequency band. The method makes use of our understanding of earthquake and explosion source models, regional wave propagation, and the relationship between direct amplitudes and their respective codas. We demonstrate the power of the method by examining earthquake and nuclear explosions in the Korean Peninsula at regional distance stations MDJ (Mudangjing, China) and TJN (Taejon, South Korea). In order to implement the method, however, we need to account for propagation through the use of an attenuation model for the region, which we have developed, in addition to an empirical correction to provide for unaccounted effects in the direct‐to‐coda transfer functions. Under the assumption that our explosion and attenuation models and the empirically obtained P ‐to‐ P ‐coda and S ‐to‐ S ‐coda transfer functions are correct, we determine that the 2006 test by the Democratic People’s Republic of Korea (DPRK) is consistent with a yield between 200 and 800 tons and a depth between 20 and 300 m, with our best fit at 500 tons at a depth of 100 m. Similarly, the 2009 DPRK test is consistent with a yield range of 1–5 kt and a depth range of 70–600 m, with our best fit at 2 kt at a depth of 200 m.