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To achieve rational and precise seismic response predictions of large span spatial structures (LSSSs), the inherent non-uniformity and multidimensionality characteristics of earthquake ground motions should be properly taken into consideration. However, due to the limitations of available earthquake stations to record seismic rotational components, the effects of rocking and torsional earthquake components are commonly neglected in the seismic analyses of LSSSs. In this study, a newly developed method to extract the rocking and torsion components at any point along the area of a deployed dense array from the translational earthquake recordings is applied to obtain the rotational seismic inputs for a LSSS. The numerical model of an actual LSSS, the Dalian International Conference Center (DICC), is developed to study the influences of multi-support and multidimensional excitations on the seismic responses of LSSSs. The numerical results reveal that the non-uniformity and multidimensionality of ground motion input can considerably affect the dynamic response of the DICC. The specific degree of influence on the overall and local structural displacements, deformations and forces are comprehensively investigated and discussed.

With the development of unconventional shale gas in the southern Sichuan Basin, seismicity in the region has increased significantly in recent years. Though the existing sparse regional seismic stations can capture most earthquakes with \begin{document}$ {M}_{\mathrm{L}}\ge 2.5 $\end{document} , a great number of smaller earthquakes are often omitted due to limited detection capacity. With the advent of portable seismic nodes, many dense arrays for monitoring seismicity in the unconventional oil and gas fields have been deployed, and the magnitudes of those earthquakes are key to understand the local fault reactivation and seismic potentials. However, the current national standard for determining the local magnitudes was not specifically designed for monitoring stations in close proximity, utilizing a calibration function with a minimal resolution of 5 km in the epicentral distance. That is, the current national standard tends to overestimate the local magnitudes for stations within short epicentral distances, and can result in discrepancies for dense arrays. In this study, we propose a new local magnitude formula which corrects the overestimated magnitudes for shorter distances, yielding accurate event magnitudes for small earthquakes in the Changning−Zhaotong shale gas field in the southern Sichuan Basin, monitored by dense seismic arrays in close proximity. The formula is used to determine the local magnitudes of 7,500 events monitored by a two-phased dense array with several hundred 5 Hz 3C nodes deployed from the end of February 2019 to early May 2019 in the Changning−Zhaotong shale gas field. The magnitude of completeness ( \begin{document}$ {M}_{\mathrm{C}} $\end{document} ) using the dense array is −0.1, compared to \begin{document}$ {M}_{\mathrm{C}} $\end{document} 1.1 by the sparser Chinese Seismic Network (CSN). In addition, using a machine learning detection and picking procedure, we successfully identify and process some 14,000 earthquakes from the continuous waveforms, a ten-fold increase over the catalog recorded by CSN for the same period, and the \begin{document}$ {M}_{\mathrm{C}} $\end{document} is further reduced to −0.3 from −0.1 compared to the catalog obtained via manual processing using the same dense array. The proposed local magnitude formula can be adopted for calculating accurate local magnitudes of future earthquakes using dense arrays in the shale gas fields of the Sichuan Basin. This will help to better characterize the local seismic risks and potentials.

SUMMARYWe compute high-resolution space–time variations of subsurface seismic properties from autocorrelation functions (ACF’s) of noise and local earthquakes, recorded by the Sage Brush Flat dense array deployed around the Clark branch of the San Jacinto fault. The resolved temporal changes are referred to as apparent velocity changes because they reflect both nonlinear response and variations of material properties such as cracking and damage. Apparent velocity changes are estimated at four frequency bands (10–15, 10–20, 15–30 and 20–40 Hz) for two local earthquake data sets. In one analysis, ACF’s from P- and S-wave windows of 31 small events with magnitudes below 3.1 are used to compute the apparent velocity variations with respect to the mean ACF of each phase, and we also use the mean ACF of noise data as reference to estimate the changes. In a further analysis, the temporal evolution of properties is computed using moving time windows in continuous waveform over one-hour long data with noise and earthquake signals. The apparent velocity changes and recovery times are frequency dependent and present a strong spatial variability across the array. The resolved changes are larger and recovery time shorter with data associated with higher frequencies. At frequencies larger than 15 Hz, and using the mean ACF of noise data as a reference, the apparent average velocity changes across the array during the passage of the P and S waves from the small local events are 2.5 per cent and 6 per cent, respectively. The apparent velocity changes decrease by one order of magnitude when the earthquake data are used as a reference. The relatively large changes in response to very low ground motion have important implications on nonlinear processes involving degradation and healing of the subsurface material during common earthquake shaking.

SUMMARY We analyse dominant sources identified in a catalogue of more than 156 000 localizations performed using a 26-d data set recorded by a dense array set on the San Jacinto fault near Anza, in California. Events were localized using an array processing technique called Match Field Processing. As for all array processing techniques, the quality of the event position decrease when the events are outside of the array. We thus separate localizations in and outside the array using simple geometrical conditions. We compare the time distribution of the localization to additional data such as meteorological data, day of human activity as well as existing catalogues to determine the nature of the dominant events located using our method. We find that most of the events located outside of the array could be attributed to a surface structure excited by wind. On the other hand, part of the localizations under the array occur during regional earthquakes and could correspond to diffraction on the fault's heterogeneities. The rest of the localizations inside the array could be generated by the fault itself.

The 3D S-wave velocity of shallow structure, especially the Quaternary sediments at 0-1 km near the surface, is an important issue of concern in urban planning and construction for the requirements...

In this study, we investigate the shallow shear wave velocity structure of the Los Angeles Basin in southern California, using ambient noise correlations between 5 dense arrays and 21 broadband stations from the Southern California Seismic Network (SCSN). We observe clear fundamental mode and first overtone Rayleigh waves in the frequency band 0.25–2.0 Hz, and obtain group velocity maps through tomography. We further derive a 3D shear wave velocity model, covering a large portion of the central LA Basin for the depths shallower than 3 km. We found that the small scale shallow velocity structure heterogeneities are better resolved compared with the SCEC Community velocity models. Our model captures the presence of the Newport‐Inglewood fault by a NW–SE trending high velocity belt. Our model provides more accurate constraints on local ground motion predictions with detailed mapping of structural heterogeneities.

We derive the 3-D S-wave velocity structures of sediments and upper crust in the region off Ibaraki by applying ambient noise tomography to a dense array of short-period ocean bottom seismometers (...

Crevasses are inherent features of glaciers and Ice Sheets. They exert a primary control on glacier dynamics, such as, for example, along shear margins through reducing the overall glacier ice viscosity, or at glacier and Ice Sheet fronts through controlling the onset of serac falls and of ice sheet instabilities (calving, ice shelf disintegration). However, our understanding of crevasse formation and propagation, and in particular the effect of melt water, remains limited due to lacking observations. Here we provide novel observational insights into englacial fracturing, the depth of crevasses and their depth propagation rates using dense seismic array monitoring on an Alpine glacier. We systematically detect and locate englacial seismic events through applying matched-field-processing on a particularly dense seismic array of 98 sensors deployed on the Glacier d’Argentière during 1-month in spring 2018. We observe rupture fronts along crevasses, which propagate from the glacier center to the glacier side at typical velocities of few hundreds of meters per day, i.e. at velocities that are much lower than those of seismic waves but much higher than those of glacier flow. We argue based on a dedicated spatial and temporal analysis that crevasse rupture propagation is set by the migration of water along the crevasse tip. We also observe that crevasses are associated with a wide range of depths, varying from the near surface to the glacier base, which at the present site is located about a hundred meters below the surface. This observation is particularly interesting, since it provides evidences that (i) crevasses are water filled and (ii) crevasses play a role in the supply of water to the bed. These findings are further supported by the observation that surface melt modulates the seismic activity of crevasses including those reaching the bed. Finally, by evaluating coherent structures in the crevasse population, we are able to infer their depth propagation rate, which we find is constant through the ice column, as expected if the surrounding ice stress field is counterbalanced by the water pressure in the crevasse. These observationally-derived findings provide useful grounds to test and improve theories of crevasse dynamics and their control in the overall transfer of water from the surface to the bed.

SUMMARYWe image the internal structure of the San Jacinto fault zone (SJFZ) near Anza, California, with seismic data recorded by two dense arrays (RA and RR) from ∼42 000 local and ∼180 teleseismic events occurring between 2012 and 2017. The RA linear array has short aperture (∼470 m long with 12 strong motion sensors) and recorded for the entire analysed time window, whereas the RR is a large three-component nodal array (97 geophones across a ∼2.4 km × 1.4 km area) that operated for about a month in September–October 2016. The SJFZ at the site contains three near-parallel surface traces F1, F2 and F3 from SW to NE that have accommodated several Mw > 6 earthquakes in the past 15 000 yr. Waveform changes in the fault normal direction indicate structural discontinuities that are consistent with the three fault surface traces. Relative slowness from local events and delay time analysis of teleseismic arrivals in the fault normal direction suggest a slower SW side than the NE with a core damage zone between F1 and F2. This core damage zone causes ∼0.05 s delay at stations RR26–31 in the teleseismic P arrivals compared with the SW-most station, and generates both P- and S-type fault zone trapped waves. Inversion of S trapped waves indicates the core damaged structure is ∼100 m wide, ∼4 km deep with a Q value of ∼20 and 40 per cent S-wave velocity reduction compared with bounding rocks. Fault zone head waves observed at stations SW of F3 indicate a local bimaterial interface that separates the locally faster NE block from the broad damage zone in the SW at shallow depth and merges with a deep interface that separates the regionally faster NE block from rocks to the SW with slower velocities at greater depth. The multiscale structural components observed at the site are related to the geological and earthquake rupture history at the site, and provide important information on the preferred NW propagation of earthquake ruptures on the San Jacinto fault.

In traditional surface wave tomography based on seismic noise, 2D phase or group velocity distribution is obtained by performing pure-path inversion after extracting interstation velocities based on the noise cross-correlation function. In this paper, we show that 2D surface wave phase velocity maps of adequate quality can be obtained directly, without interferometry, by beamforming the ambient noise recorded at array of stations. This method does not require a good azimuthal distribution of the noise sources. The 2D surface wave phase velocity map is obtained by moving the subarrays within a larger dense network of stations. The method is illustrated with seismic noise recorded by over 600 stations of the ChinArray (Phase II). We obtain 2D Rayleigh wave phase velocity maps between 7 and 35 s in Northeastern (NE) Tibetan Plateau and adjacent regions that compare well with results obtained with other methods. The shear wave velocity model is then derived by inverting the phase velocity with depth. The model correlates well with geology and tectonics in NE Tibet. Two clear mid-to-low crustal low-velocity zones are observed at 15- to 35-km depth beneath the Songpan-Ganzi terrane and Northwestern Qilian Orogen, possibly facilitating lower crustal flow in this key region for the tectonic evolution of NE Tibet.

Ultrahigh-resolution 3D (UHR3D) seismic systems employing a dense array of short streamers represent a significant advancement in the field of marine seismic data acquisition and are an appropriate choice for many near-surface survey applications. Data comparisons between conventional 3D, reprocessed 3D, high-resolution 2D, and UHR3D seismic data illustrate the benefits of a UHR3D acquisition strategy. Results from a recent study of seeps imaged in the Gulf of Mexico (GoM) suggest that UHR3D data can serve as a comprehensive data source for some near-surface geophysical characterizations, satisfying many of the needs often addressed by integrating multiple separate types of survey data.

An important factor in mitigation of seismic hazard from induced seismicity is properly established seismic networks suitable for consistent identification of small‐to‐moderate events (magnitudes less than four). Here, we evaluate the performance of the newly established regional broadband seismic network in northeast British Columbia, Canada. The seismic network was designed for monitoring of induced seismicity due to oil and gas operations related to hydraulic fracturing and fluid injection in the region. We use regional and local earthquake catalogs for the period 1985–2015 to analyze magnitude of completeness and epicentral uncertainty. We also perform a theoretical assessment of minimum detectable magnitude across the study region based on analysis of ambient noise and simulated ground motions. From the frequency–magnitude distribution of the reported events in the regional earthquake catalog, the magnitude of completeness has decreased ∼1 magnitude unit from ∼3 in the periods 1985–2013 to ∼2 in the period 2013–2015 as a result of the establishment of new stations. The minimum detectable magnitude in the region is 1.6–2.6 based on the signal‐to‐noise ratio (SNR) of 10 and higher at four or more stations. By comparing the regional and dense array catalogs, we determine that the error in epicentral location in well‐constrained areas by seismic stations is below 3 km in both east–west and north–south directions. However, location uncertainties can be up to 10 km in the east–west direction in areas where the current regional network is sparse. The magnitude detection threshold in the Montney Play, where most of the current oil and gas activities are taking place, can be further reduced by up to 1 magnitude unit with the addition of four new stations.

International audience; The current article presents the results from the analysis of the seismic events recorded from a dense array located on a rock site at Argostoli, Cephalonia Island, Greece. The objective of the study is to explore to what extent the non-direct, diffracted surface waves influence the seismic wavefield at a rock site, to investigate the loss of coherency of ground motions and to compare the results with those from a previously studied similar array located at an adjacent small, shallow sedimentary valley. The array consists of 21 velocimeters encompassing a central station in four concentric circles with diameters 20, 60, 180 and 360 m. The analyzed seismic dataset includes 40 events with magnitudes ranging from 2 to 5 and epicentral distance up to 200 km. MUSIQUE algorithm has been used to analyze the seismic wavefield by extracting the backazimuth and slowness of the dominant incoming waves and identifying the Love and Rayleigh waves. Lagged coherency has been estimated for all the available station pairs in the array and the results from the entire dataset have been averaged at four separation distance intervals, 10-20, 20-30, 30-40, 80-90 m. The results were also compared with those from a similar array located on an adjacent small, shallow sedimentary valley. The analysis suggests that about 20percent energy of the wavefield could be characterized as diffracted Love and Rayleigh waves, primarily arriving from the north-east and north-south directions, respectively. The spatial coherency estimations at the rock site are, generally, observed to be larger than those from the sedimentary array, especially at frequencies below 5 Hz. The directionality of coherency estimates observed from the soil array is absent in case of the rock array data. Comparison with the widely-quoted parametric models reveals that there is little correlation between the decay of coherency observed at the rock site and the models. The significant difference observed between the results of the rock and soil array indicate that the spatial incoherency is largely site dependent and could be potentially associated with the formation of locally generated wavefield

We perform a systematic detection of deep tectonic tremor beneath the southern Central Range in Taiwan using a dense, small‐aperture seismic array. Deployed in February 2011, the array has been recording tectonic tremor nearly continuously for 134 days, including tremor triggered by the 2011 M w 9.0 Tohoku‐Oki earthquake. We use broadband frequency–wavenumber beamforming and moving‐window grid‐search methods to compute array parameters for all continuous data and identify tremor as coherent nonimpulsive seismic signals with deep incidence angles. The obtained array parameters closely match with those of relocated local earthquakes and triggered tremor bursts located by the waveform envelope correlation and clustering (WECC) method, indicating the robustness of our array technique. During the 134‐day study period, we detect tremor for 44 days, with a total duration of 1481 min, three to six times as long as the detection of tremor by the WECC method. We find a relative quiescence in ambient tremor activity for about 20 days following the 2011 Tohoku‐Oki earthquake, suggesting that dynamic stresses from the distant mainshock triggered most tremor close to failure, resulting in a temporary lack of tremor activity. In some cases, we observe rapid tremor migration with a speed at the order of 40–50 km/hr that is similar to the speed of fast tremor migration along dip on narrow streaks in Japan and Cascadia. Our results suggest that dense array techniques are capable of capturing detailed spatiotemporal evolutions of tremor behaviors in southern Taiwan. Online Material: Tables of station coordinates and tremor event parameters, and figures of the moving‐window grid‐search test with local earthquakes, tremor triggered by the 2011 Tohoku‐Oki earthquake, and additional examples of detected ambient tremors.

Over the past two decades, there have been several initiatives to create volunteer‐based seismic networks. The Personal Seismic Network, proposed around 1990, used a short‐period seismograph to record earthquake waveforms using existing phone lines (Cranswick and Banfill, 1990; Cranswick et al. , 1993). NetQuakes (Luetgert et al. , 2010) deploys triaxial Micro‐Electromechanical Systems (MEMS) sensors in private homes, businesses, and public buildings where there is an Internet connection. Other seismic networks using a dense array of low‐cost MEMS sensors are the Community Seismic Network (Clayton et al. , 2012; Kohler et al. , 2013) and the Home Seismometer Network (Horiuchi et al. , 2009). One main advantage of combining low‐cost MEMS sensors and existing Internet connection in public and private buildings over the traditional networks is the reduction in installation and maintenance costs (Koide et al. , 2006). In doing so, it is possible to create a dense seismic network for a fraction of the cost of traditional seismic networks (D’Alessandro and D’Anna, 2013; D’Alessandro, 2014; D’Alessandro et al. , 2014). A rapidly deployable and highly mobile seismic network can collect enormous volumes of data at high spatial density during an aftershock sequence following major earthquakes (Naito et al. , 2013). Although the low‐cost seismic networks described above were primarily designed to detect and characterize earthquakes, the networks have also been used for other purposes such as to monitor building health in Kohler et al. (2013). These types of low‐cost networks may also have other potential applications such as detecting landslides (Azzam et al. , 2011) and locating explosions (Taylor et al. , 2011). The Quake‐Catcher Network (QCN) is another variant of a cyber‐social seismic network, which has been operating since 2008. Cochran, Lawrence, Christensen, and Chung (2009) and Cochran, Lawrence, Christensen, and Jakka (2009) describe the implementation that uses a client software phase‐picking …

For accurate seismic hazard evaluation, both the spatial and frequency-dependent variabilities in the amplitudes of earthquake ground motions are needed. While this information is rarely fully available due to the paucity of relevant seismic data, dense arrays like the 5200-geophone array in Long Beach, California provide the opportunity to study this amplitude variability. Here we show that ambient noise correlation amplitudes from the Long Beach array can be used to directly determine frequency-dependent site amplification factors. We analyze Rayleigh-wavefield amplitude gradients from ambient noise correlations that are processed so that relative amplitudes satisfy the wave equation and are therefore meaningful. Ultimately, we construct maps of site amplification across Long Beach at frequencies of 0.67, 1.0, and 2.0 Hz. These maps correlate well with local structure, notably the Newport-Inglewood Fault and also to known velocity structure. Through this process, we also obtain constraints on average attenuation structure and local scattering.

Old Faithful geyser in Yellowstone National Park has attracted scientific research for almost a century and a half. Temperature and pressure measurements and video recordings in the geyser's conduit led to proposals of many quantitative eruption models. Nevertheless, information on the processes that initiate the geyser's eruption in the subsurface remained limited. Two new studies, specifically Wu et al. (2017) and Ward and Lin (2017) take advantage of recent developments in seismic data-acquisition technology and processing methods to illuminate subsurface structures. Using a dense array of three-component nodal geophones, these studies delineate subsurface structures on a scale larger than previously realized, which exert control on the spectacular eruptions of Old Faithful geyser.

The Canterbury earthquakes of 2010 and 2011 have produced some of the strongest ground motions ever measured in New Zealand. Many of the highest acceleration recordings arose from seismic stations within the city of Christchurch (population ∼377,000). A dense array of strong-motion seismometers was in place prior to the mainshock of 4 September 2010. Subsequent to the mainshock, numerous rapid response accelerometers were installed in the Canterbury Plains, Banks Peninsula, and in the city itself (Gledhill et al. 2011; Cochran et al. 2011). Many of the strongest aftershocks were recorded by this dense amalgamation of permanent and temporary arrays and provide a detailed record of variable ground motion throughout the region during the aftershock sequence. Figure 1. ▴ Vertical and horizontal acceleration vectors and their location relative to the February 22 epicenter (green star). Green horizontal lines on each waveform show the baseline. The most extreme ground motions were recorded during the Mw 6.2 earthquake of 22 February 2011 that struck a few kilometers to the south of Christchurch (Beavan et al. 2011, this issue; Holden et al. 2011, this issue; Bannister et al. 2011, this issue), generating severe damage throughout the city. In fact, damage to the built environment and ground liquefaction was much more widespread in the February event than in the September Mw 7.1 mainshock (Kaiser et al. 2011). This event is one of the best-recorded shallow thrust earthquakes in the near field. Recorded peak ground acceleration (PGA) in February exceeded 2 g near the epicenter and was greater than 0.6 g over much of the central and eastern suburbs (Figure 1). At these near-source stations, vertical accelerations were generally markedly higher than horizontal accelerations. The large accelerations can be reasonably well explained by the combination of the proximity of the February event to Christchurch …

In 1992, a large and dense array of geophones was placed around the geyser vent of Old Faithful, in the Yellowstone National Park, to determine the origin of the seismic hydrothermal noise recorded at the surface of the geyser and to understand its dynamics. Old Faithful Geyser (OFG) is a small-scale hydrothermal system where a two-phase flow mixture erupts every 40 to 100 min in a high continuous vertical jet. Using Matched Field Processing (MFP) techniques on 10-min-long signal, we localize the source of the seismic pulses recorded at the surface of the geyser. Several MFP approaches are compared in this study, the frequency-incoherent and frequency-coherent approach, as well as the linear Bartlett processing and the non-linear Minimum Variance Distorsionless Response (MVDR) processing. The different MFP techniques used give the same source position with better focalization in the case of the MVDR processing. The retrieved source position corresponds to the geyser conduit at a depth of 12 m and the localization is in good agreement with in situ measurements made at Old Faithful in past studies.

The location of the 2006 nuclear explosion in North Korea has been accurately imaged by back-projected regional Pn waves recorded by the Japanese Hi-net array. Based on the determined location, the nuclear explosion site can be identified from geo-referenced FORMOSAT-2 satellite images. The seismically determined epicenter is about 2.5 km northeast of the original estimate of its absolute location. Results indicate that a remote suspect event had been unambiguously detected and accurately located by a dense array within a regional distance. Employing ground truth correction, the satellite images can be referenced for shifting the array-determined epicenter to its absolute position. After correction, this event can be treated as a reference event for accurately locating future nuclear explosions. Our study utilizes public information from a dense seismic network and further demonstrates that commercial observation satellites can accurately monitor compliance with the Comprehensive Nuclear-Test-Ban Treaty, as well as earthquake and tsunami hazards almost in real time.

The Community Seismic Network (CSN) is currently a 500‐element strong‐motion network located in the Los Angeles area of California (see Fig. 1). The sensors in the network are low‐cost microelectromechanical (MEM) accelerometers that are capable of recording on scale up to accelerations of ±2g. The primary product of the network is a set of measurements of ground shaking in the seconds following a major earthquake. An example of this is shown in Figure 2. The shaking information will be contributed to U.S. Geological Survey products such as ShakeMap (Wald et al., 1999) and ShakeCast (Wald et al., 2006), with the goal of providing first responders a proxy for damage that can guide efforts immediately following the event. The basic premise is the strong ground‐motion shaking varies on a subkilometer scale, which will require a dense network to meaningfully measure the shaking. Evidence for this comes from earthquakes recorded by dense oil company surveys in the Los Angeles area (Clayton et al., 2011).

Immediately after a damaging earthquake, emergency managers rapidly seek answers to many important questions: Where is the worst and/or least damage? What equipment and personnel must be mobilized and to what extent? ShakeMap was developed by the U.S. Geological Survey Earthquake Hazards Program to assist in answering these questions by producing near-real-time maps of ground motion and shaking intensity following significant earthquakes (Wald et al. 2004). To produce maps, ShakeMap mimics a dense array of seismometers by using an attenuation relationship to model peak ground motions at virtual or “phantom” stations between existing seismic stations. The modeled data are corrected for site amplification (Borcherdt 1994) and used to produce an instrumental intensity map, a hybrid Modified Mercalli map based on instrumental recording (Wald et al. 1999a). ShakeMap software, implemented predominantly in the Western United States (Northern and Southern California; Salt Lake City, Utah; and Seattle, Washington), requires a degree of regional customization to ensure accuracy of the maps. We report here on the Eastern United States software customization required for implementation of ShakeMap in the New Madrid Seismic Zone (NMSZ) and verification of the resulting maps. The New Madrid region has a relatively high seismic hazard, yet very little data exist for large earthquakes. To estimate peak ground motions for magnitudes greater than 6.0, we qualitatively compared scenario ShakeMaps using different attenuation relationships and intensity regressions to a magnitude 7.4 scenario produced by the USGS Memphis hazard mapping group (Chris Cramer, personal communication 2005; see also http://www.ceri.memphis.edu/usgs/products/regional.html). For smaller events (M < 6) we used the 10 February 2005 magnitude 4.1 earthquake near Blytheville, Arkansas, as an example. The study location is in the Upper Mississippi Embayment of the Central and Eastern United States (CEUS) and is centered on the NMSZ (figure 1). The area covers a four-by-four-degree grid from …

The strong-motion observation system developed and maintained by the Earthquake Research Institute have focused on acquiring data from near earthquake sources and on studying local site effects on strong motion. The observation sites are mostly deployed between the Tokai and southern Kanto region, including the dense array in Ashigara valley.

A detailed analysis of one earthquake recorded by a dense array of seismographs on the surface of an alluvial valley shows two locally-generated waves which propagate down-valley. The faster travelling one is a Rayleigh wave, and the slower one is similar to a Love wave, but has a vertical component thought to arise from the need to meet lateral boundary conditions. These waves can mimic normal modes, and their interaction provides a basis for explaining directional resonances.

The properties of the seimic noise wavefield at Mt. Vesuvius Volcano, Italy, are investigated using a dense array of short-period seismometers deployed on the south-west flank of the volcano and operated on 1997–1998. The array had an aperture of 500 m and average station spacing of 40 m. The main goal of this deployment was to investigate the possible presence of correlated signals, which could be attributed to subsurface magmatic and/or hydrothermal activity (volcanic tremor). The spectral properties of the noise wavefield are investigated using data from the arrays and from the summit stations of the permanent seismic network operated by the Vesuvius Observatory. The seismic noise is characterized by three main spectral peaks at frequencies

We installed a dense array of 41 digital seismographs in San Jose, California, to evaluate in detail the effects of a deep sedimentary basin and shallow sedimentary deposits on earthquake ground motions. This urban array is located near the eastern edge of the Santa Clara Valley and spans the Evergreen sedimentary basin identified by gravity data. Average station spacing is 1 km, with three stations initially spaced 110 m apart. Despite the high-noise urban environment, the stations of the array successfully triggered on and recorded small local earthquakes ( M 2.5–2.8 at 10–25 km distance) and larger regional events such as the M 5.0 Bolinas earthquake (90 km distance), M 4.6–5.6 earthquakes near Mammoth Lakes (270 km distance), M 4.9–5.6 events in western Nevada (420 km distance) and the M 7.1 Hector Mine earthquake (590 km distance). Maps of spectral ratios across the array show that the highest amplitudes in all frequency bands studied (0.125–8 Hz) are generally observed at stations farther from the eastern edge of the Santa Clara Valley. Larger spectral amplitudes are often observed above the western edge of the Evergreen Basin. Snapshots of the recorded wavefield crossing the array for regional events to the east reveal that large, low-frequency (0.125–0.5 Hz) arrivals after the S -wave travel from south to north across the array. A moving-window, cross-correlation analysis finds that these later arrivals are surface waves traveling from the south. The timing and propagation direction of these arrivals indicates that they were likely produced by scattering of incident S waves at the border of the Santa Clara Valley to the south of the array. It is remarkable that the largest low-frequency phases at many of the valley sites for regional events to the east are basin surface waves coming from a direction about 70 degrees different from that of the epicenters. Basin surface waves emanating from the eastern edge of the valley are also identified by the cross-correlation analysis.

Chinese researchers placed a dense array of seismometers around a dangerous-looking seam in the rocks of Sichuan--only to be blindsided by the true killer.

This study addresses the variability of pseudo-velocity response spectra across an array deployed on stiff soil in the San Fernando Valley during the Northridge (Mw 6.7) aftershock sequence. The separation between stations ranged from 0.5 to 5 km, and the aftershock magnitudes ranged from 2.3 to 4.0. We find that 95-percent of observed response spectra are within a factor of 1.9 to 2.6 of the network average. Statistically significant relative amplification factors were found for some of the sites, but the variability of observed response spectra is not significantly reduced by correcting for these effects. This implies that microzonation efforts on less than 5-km distance scales are not warranted at these types of sites. We also found a distance dependence for the response-spectral variability between neighboring sites. 95-percent are within a factor of ∼2.3 at 0.5 km, increasing to 95-percent within a factor of ∼4.2 at 5 km. No frequency dependence in these values could be resolved. Additional work is needed to examine the influence of other factors such as earthquake magnitude.

Summary Micro-earthquake activity is reported for the first 6 months of 2011 in Long Beach, CA using a density urban exploration seismic network. Detected events include at least four times as many man-made events as possible real earthquakes indicated by repeated locations, and distinct patterns in the hour-of-day and day-of-week origin times. Some discrimination appears possible utilized the earthquake spectral behavior in the wavelet transform domain. The back-projection of the seismic data onto a prescribed fault plane of the Newport-Inglewood fault produces interesting slip patterns over a time window spanning a small earthquake.

In a recent paper, Steidl (1993) addresses the estimation of earthquake site response, or sediment amplification, when the motion input to the sediments is estimated from nearby bedrock observations. In particular, Steidl compares spectral-ratio estimates of site response to those based on the cross spectrum. By introducing cross-spectrum estimates as the «statistically optimum method» (p. 295) and by stating in the conclusions that «Cross-spectrum estimates should be considered more reliable [than spectral ratios] when the reference site is a single rock station» (p. 299), Steidl gives the impression that spectral ratios are inferior estimates of site response. However, previously published studies by myself and others (Field et al., 1991, 1992a) have reached different conclusions. This discrepancy is addressed in this letter, and an example is included for illustrative purposes

Topographical and geological amplification that occurred during past earthquakes are quantified using spectral ratios of recorded motions. Several cases are presented from the 1985 Chilean and Mexican earthquakes as well as the 1983 Coalinga (California) and 1987 Supersition Hills (California) earthquake. The strong motions recorded in Mexico City during the 1985 Michoacan earthquake are supplemented by ambient motions recorded within Mexico City to quantify the now well known resonating frequencies of the Mexico City lakebed. Topographical amplification in Canal Beagle (Chile), Coalinga and Superstition Hills (California) are quantified using the ratios derived from the aftershocks following the earthquakes. A special dense array was deployed to record the aftershocks in each case. The implications of both geological and topographical amplification are discussed in light of current code provisions. The observed geological amplifications has already influenced the code provisions. Suggestions are made to the effect that the codes should include further provisions to take the amplification due to topography into account.

International audience; n this paper we expand over the whole of central Greece, the Moho map centered on the Gulf of Corinth from tomographic inversion of PmP traveltime profile data recorded by several tens of temporary stations. Our approach is based on Pn, Moho refracted waves, from a large regional earthquake recorded by both temporary stations and the permanent Hellenic network. The Moho map shows the large Moho depth under the Hellenides belt. It also highlights the shallower Moho domain towards the Aegean Sea south and east of the Corinth Gulf. The domain of shallow Moho is limited along a NE–SW prolongation ahead of the North Anatolian Fault, from the North Aegean Trough to the western tip of the Gulf of Corinth towards the Gulf of Patras. The Pn time-terms provide corrections for the permanent stations that can be used together with the 1D velocity–depth model for a first-order compensation of lateral heterogeneity and contribute to the accurate and fast location of earthquake hypocenters. As a test we relocated the 1995 Aigion earthquake in this way, using only the sparse data of the permanent stations. Hypocentral coordinates then shift close to those derived by a dedicated dense array deployed after the earthquake, implying improvement of the routine location.

[1] High frequency records of deep Mariana earthquakes from a dense array of ocean bottom seismographs deployed in the Mariana arc and back-arc regions are stacked and searched for the phases P660p and S660p to constrain the depth of the 660-km discontinuity near the Mariana slab. Results of the high-resolution study suggest that around 18°N the 660-km discontinuity lies at about 710–730 km (±14 km) depth within or in the vicinity of the slab core. In the region seismicity ceases at ∼620 km depth. This implies that, although tomographic images show the Mariana slab penetrating into the lower mantle, deep seismicity in the region terminates ∼100 km above the base of the transition zone. These findings and similar observations in Tonga argue that factors other than the phase transition at the base of the upper mantle may control the maximum down-dip extent of the deep seismogenic region in the slab.

Utilizing commercial mine blasts and local earthquakes, as well as a dense array of portable seismographs, we have achieved high-resolution crustal refraction profiles across northern Nevada and the central Sierra Nevada Mountains. Using a dense spacing of 411 portable seismographs and 4.5 Hz geophones, the instruments were able to record events ranging from large mine blasts, small local earthquakes (approximately magnitude 2), as well as two larger earthquakes (magnitudes 2.8 and 3.8). Our instruments sensed blast first arrivals out to a distance of approximately 400 km. We have obtained 99% data recovery and clear refractions across the Sierra Nevada and the northern Great Basin regions.

[1] The Japanese dense array of Global Positioning System recorded ionospheric disturbances as changes in Total Electron Content ∼12 minutes after the September 1 2004 eruption of the Asama Volcano, Central Japan. The disturbance had a period of one and a quarter minutes and propagated as fast as ∼1.1 km/s, suggesting its origin as the acoustic wave generated by the explosion. By comparing the disturbance amplitudes with those by a surface mine blast with a known energy, the overall Asama explosion energy is inferred to be about 2 × 1014 J.

In the past decade, Earth scientists have recognized the seismic hazards that crustal faults and sedimentary basins pose to Seattle, Washington (Figure 1). In 1998, the US. Geological Survey and its collaborators initiated a series of urban seismic studies of the upper crust to better map seismogenic structures and sedimentary basins in the Puget Lowland. These studies are called the Seismic Hazard Investigations of Puget Sound (SHIPS). In March 1998, we conducted our first SHIPS study, an investigation of the upper crustal structure of the Puget Lowland, using marine airgun sources and land recorders [Fisher et al., 1999].The study was nicknamed Wet SHIPS. In September 1999, we obtained a seismic refraction line to study the upper crustal structure in the Seattle area in a land-based study nicknamed Dry SHIPS [Brocher et al., 2000] (Figure 1). In March 2000, we recorded the demolition of the Seattle Kingdome sports stadium using a dense array of seismic recorders for a detailed site response study; this study was nicknamed Kingdome SHIPS (Figure 1).

Dense arrays of three-component seismographs have been maintained in the Travale geothermal field during two periods of several weeks. The particular array configuration (20 stations on a 3 × 3 km area) was intended to allow a high horizontal resolution investigation of the field itself, by analysis of steep-incidence waves. Natural and artificial sources at 100 km distances provided phases reflected by deep crustal interfaces, from which differential attenuation factor estimates are obtained. Consequent signatures of the area around and within the productive field are discussed, with respect also to laboratory measurements on porous, fluid-filled rock samples.

Site-response experiments were performed 5 months after the MS = 7.8 central Chile earthquake of 3 March 1985 to identify amplification due to topography and geology.Topographical amplification at Canal Beagle, a subdivision of Viña del Mar, was hypothesized immediately after the main event, when extensive damage was observed on the ridges of Canal Beagle. Using frequency-dependent spectral ratios of aftershock data obtained from a temporarily established dense array, it is shown that there is substantial amplification of motions at the ridges of Canal Beagle. The data set constitutes the first such set depicting topographical amplification at a heavily populated region and correlates well with the damage distribution observed during the main event.Dense arrays established in Viña del Mar also yielded extensive data which are quantified to show that, in the range of frequencies of engineering interest, there was substantial amplification at different sites of different geological formations. To substantiate this, spectral ratios developed from the strong-motion records of the main event are used to show the extensive degree of amplification at an alluvial site as compared to a rock site. Similarly, spectral ratios developed from aftershocks recorded at comparable stations qualitatively confirm that the frequency ranges for which the amplification of motions occur are quite similar to those from strong-motion records. In case of weak motions, the denser arrays established temporarily as described herein can be used to identify the frequency ranges for which amplification occurs, to quantify the degree of frequency-dependent amplification and used in microzonation of closely spaced localities.

Arrival times of seismic waves propagated to a dense array (20 three-component seismometers on a 3 × 3 km area) from natural sources beneath the Travale field, have been analysed for lateral variations which can be related to the extent of the productive reservoir. Significant teleseismic delays between sites only a few hundred meters apart correlate with productive well R9. Spatial variation of both P and S travel arrival times from local earthquakes at the edge of the array and 4 km deep beneath the reservoir consistently delineate its lateral extent to the south and east as documented by drillholes.