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Based on 778 earthquakes (2009—2022) and their observation reports recorded by the Hunan Seismic Network, through the analysis of the cross-correlation of the waveforms, 149 repeated earthquake pairs were identified which were recorded at least 3 stations with cross-correlation coefficients no less than 0.8. After obtaining these data, the location errors and phase picking errors of the Hunan Seismic Network were statistically analyzed. The results show that the horizontal location error is around 3.5 km, and the vertical location error is around 3 km. The Pg and Sg phase picking errors are about 0.4 s and 0.7 s, respectively. There is no significant difference in location error in Hunan region, except for the Changde area, where the error is basically within 4 km.

Copyright of Progress in Earthquake Sciences is the property of China Earthquake Administration, Institute of Geophysics and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Ultra-long-distance and high-precision lightning location technology is an important means to realize low-cost and wide-area lightning detection. This paper carried out research on the high-precision location technology of very-low-frequency (VLF) lightning electromagnetic pulse based on the Asia-Pacific Lightning Location Network (APLLN) deployed in 2018. Two key technologies are proposed in this paper: one is the calculation method of signal arrival time using very-low-frequency lightning electromagnetic pulse waveform, and the other is the compression transmission technology of lightning electromagnetic pulse waveform based on a signal principal component analysis. The results of a comparison and evaluation of the improved APLLN with the ADTD system show that the APLLN has a relative location efficiency of 69.1% and an average location error within the network of 4.5 km. [ABSTRACT FROM AUTHOR]

Abstract The Central and East Java region, which is part of the Sunda Arc, has relatively high seismic rates due to the convergence of two major tectonic plates in the Indonesian region; i.e., the Indo-Australian Plate subducting under the Eurasian Plate. Many devastating earthquakes have occurred in this area as a result of the interaction between these two plates. Two examples are the 1994 Banyuwangi earthquake (Mw 7.6) and the 2006 Yogyakarta earthquake (Mw 6.3). This study aims to determine precise earthquake locations and analyze the pattern of seismic distribution in Central and East Java, Indonesia. We manually re-picked P and S-wave arrival times that were recorded by the Agency for Meteorology, Climatology and Geophysics (BMKG) of the Indonesian earthquake network during the time period January 2009–September 2017. We then determined the earthquake locations using a non-linear method. To improve the accuracy of the earthquake locations, we relocated 1,127 out of 1,529 events, using a double-difference algorithm with waveform cross-correlation data. Overall, the seismicity in the Central and East Java region is predominantly distributed in the south of Java Island; e.g., the Kebumen, Yogyakarta, Pacitan, Malang, and Banyuwangi clusters. These clusters are probably related to the subduction activity in these regions. Meanwhile, there are clusters of earthquakes having shallow depths on the mainland that indicate the activity of inland faults in the region; e.g., the Opak Fault, the Kendeng Thrust, and the Rembang–Madura–Kangean–Sakala (RMKS) Fault Zone. Several other active inland faults have not shown any significant seismicity over the time period mentioned, i.e., the Pasuruan Fault, the Lasem Fault, the Muria Fault, the Semarang Thrust, and the Probolinggo Fault.

In this paper, a template matching and location method, which has been rapidly adopted in microseismic research in recent years, is applied to laboratory acoustic emission (AE) monitoring. First, we used traditional methods to detect P-wave first motions and locate AE hypocenters in three dimensions. In addition, we selected events located with sufficient accuracy (normally corresponding AE events of relatively larger energy, showing clear P-wave first motion and a higher signal-to-noise ratio in most channels) as template events. Then, the template events were used to scan and match other poorly located events in triggered event records or weak events in continuous records. Through cross-correlation of the multi-channel waveforms between the template and the event to be detected, the weak signal was detected and located using a grid-searching algorithm (with the grid centered at the template hypocenter). In order to examine the performance of the approach, we calibrated the proposed method using experimental data of different rocks and different types of experiments. The results show that the proposed method can significantly improve the detection capability and location accuracy, and can be applied to various laboratory and in situ experiments, which use multi-channel AE monitoring with waveforms recorded in either triggering or continuous mode.

Numerous microearthquakes, ML ≤ 3.8, corresponding to background seismicity and swarms were observed from September 3, 2017, to January 1, 2018, mainly in the Tesistán Valley, north of the Guadalajara Metropolitan Zone (GMZ). We located 188 tectonic microearthquakes and identified 11 clusters of similar events from a spatio-temporal analysis and waveform cross-correlations. Our results confirm the presence of continuous seismicity in the GMZ that long went unobserved. Most M L ≥ 2.5 events and some clustered events are located in the northeastern Tesistán, close to the NNE-SSW fault corresponding to the eastern edge of the Zapopan Graben, a structure evidenced by 2015–2016 seismicity. Seismicity recorded during 2020 by a recent local seismic network installed in Zapopan reaffirms that frequent microseismicity is related to active faults that cross the cities of Zapopan and Guadalajara. The microseismicity distribution suggests minor faults with the same orientation and sense of displacement as the main structures bounding the Zapopan Graben, which corresponds to structures known as synthetic faults. This arrangement is common within the Basin and Range tectonic province. The seismicity in the northeast boundary of Jalisco Block is closely related to faults formed by Cenozoic deformation events that might be reactivated due to modern crustal dynamics. Active faults and the possibility of synthetic structures are a hypothesis that necessitates long-term seismic monitoring in order to assess the seismic hazard in the GMZ, which is a crucial factor for urban planning. [ABSTRACT FROM AUTHOR]

The Central and East Java region, which is part of the Sunda Arc, has relatively high seismic rates due to the convergence of two major tectonic plates in the Indonesian region; i.e., the Indo-Australian Plate subducting under the Eurasian Plate. Many devastating earthquakes have occurred in this area as a result of the interaction between these two plates. Two examples are the 1994 Banyuwangi earthquake (Mw 7.6) and the 2006 Yogyakarta earthquake (Mw 6.3). This study aims to determine precise earthquake locations and analyze the pattern of seismic distribution in Central and East Java, Indonesia. We manually re-picked P and S-wave arrival times that were recorded by the Agency for Meteorology, Climatology and Geophysics (BMKG) of the Indonesian earthquake network during the time period January 2009–September 2017. We then determined the earthquake locations using a non-linear method. To improve the accuracy of the earthquake locations, we relocated 1,127 out of 1,529 events, using a double-difference algorithm with waveform cross-correlation data. Overall, the seismicity in the Central and East Java region is predominantly distributed in the south of Java Island; e.g., the Kebumen, Yogyakarta, Pacitan, Malang, and Banyuwangi clusters. These clusters are probably related to the subduction activity in these regions. Meanwhile, there are clusters of earthquakes having shallow depths on the mainland that indicate the activity of inland faults in the region; e.g., the Opak Fault, the Kendeng Thrust, and the Rembang–Madura–Kangean–Sakala (RMKS) Fault Zone. Several other active inland faults have not shown any significant seismicity over the time period mentioned, i.e., the Pasuruan Fault, the Lasem Fault, the Muria Fault, the Semarang Thrust, and the Probolinggo Fault. [ABSTRACT FROM AUTHOR]

The article presents the results of analyzing aftershock sequences initiated by underground nuclear tests conducted in North Korea in 2016 and 2017. Waveform cross-correlation and a multimaster method applied to phase association allowed us to find and analyze 91 aftershocks and group them into clusters associated with the underground explosions on September 9, 2016 (DPRK-5) and September 3, 2017 (DPRK-6). The duration, intensity, and sporadic nature of these two sequences indicate specific mechanisms of seismic energy release, which are probably related to the interaction of the destruction zones of DPRK-5 and DPRK-6 explosions and the collapse of their cavities with the gradual advance of the collapse columns to the free surface. The aftershock activity in 2021 suggested that the collapse process was not yet completed and predicted the occurrence of new aftershocks in the near future, which could end in the collapse column reaching the free surface. These assumptions were confirmed; during the preparation of this work, a strong aftershock with a magnitude of 3.85 was recorded on February 11, 2022, which, according to preliminary estimates, indicates the beginning of the final stage of the advance of the collapse column to the surface, the probability of which is discussed in this study. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Geodesy & Geodynamics (1671-5942) is the property of Editorial Board Journal of Geodesy & Geodynamics and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The seismicity in the Azores plateau can be of volcanic or tectonic origin, the latter usually more destructive, especially in the central group of islands. In this study we aimed to obtain crustal stress indicators from seismic data collected in July 1998, following the July 9th M5.8 earthquake that struck near Faial and Pico islands. The maximum horizontal stress (SHmax) azimuth was obtained from two methods. For the seismogenic volume, the SHmax was obtained from single and composite focal mechanisms, whereas the state of stress beneath the seismic network was derived from shear-wave splitting analysis of local earthquakes. The composite focal mechanisms were obtained from a waveform clustering method applied to identify "identical" earthquakes. The seismic anisotropy analysis could be applied to only six stations, the polarization of the first S arrival being stable, and with a secondary perpendicular pulse arriving 0.6–0.24 s later. The focal mechanisms provided a NW–SE SHmax with azimuth range of 96°–114°, and an associated transition of normal-fault to strike-slip dominant regime consistent with the estimated regional field of 110°–120°, and coherent with movement along three fault systems. Inland the SHmax azimuths present greater variability. In Faial, the stations located north of the Caldeira present a stable azimuth of 151°–153° and are consistent with the effect of a NNW–SSE fault system present in the islands, rotating to 50°–77° in the eastern sector of the Pedro Miguel Graben, probably due to a complex interaction of the WSW–ENE dominant fault system with old eroded volcanic complexes that segmented the island into several blocks. In Pico island, the SHmax direction azimuth ranges from 93° to 135°, in a radial pattern around the island's main stratovolcano, overlapping the regional stress field. [ABSTRACT FROM AUTHOR]

To study the seismogenic structure of the 7 October 2014 Ms 6.6 Jinggu earthquake, we use the cut-and-paste (CAP) method to obtain the focal mechanisms and depths of the mainshock and the two largest aftershocks with Ms > 5. Then, we take advantage of waveform cross-correlations to obtain precise time difference data and apply the double-difference method to relocate the aftershocks between October 7 and December 31. We relocate 2076 earthquakes (approximately 73.6% of the total) with an average horizontal accuracy of approximately 150 m and a depth uncertainty of approximately 700 m. The Ms 6.6 mainshock epicenter originated at 23.378°N, 100.475°E at a depth of 15.38 km. The Jinggu earthquake sequence exhibited a linearly concentrated distribution in the northwest-southeast direction with obvious segmentation features and, thus, it can be divided into four branches spatially and two stages temporally. The aftershocks first appeared around the Ms 6.6 mainshock and then extended on either side of the mainshock along the NW-SE direction (the NW branch). The earthquake rupture direction experienced a slight clockwise deflection at the NW end and rotated clockwise by approximately 90° toward the SW (the NE branch) at the SE end. Then, the rupture propagated a short distance along the SW direction and rotated counterclockwise by approximately 90° into an SSE direction (the NNW branch). There is a small angle between the NW and NNW branches. The depths of the whole sequence with vertical dip angles were shallower toward the NW and deeper with two seismogenic layers toward the SE. According to the distributions of the earthquakes and nearby fractures, it is highly possible that the Jinggu earthquake sequence occurred along a hidden fracture section of the Wuliangshan fault in the NW direction. Moreover, we can confirm that the three large earthquakes all occurred along right-lateral strike-slip faults. [ABSTRACT FROM AUTHOR]

The study of the stress-strain state of a medium in seismically quiet areas is difficult because of the absence of strong events. Under such circumstances, each earthquake, even relatively weak, is of high importance. In this case, all possible information on tectonic stresses and their dynamics, e.g., information on time, location, and magnitude of aftershocks, should be obtained from available seismic data. The earthquake near the town of Mariupol which occurred on August 7, 2016, had a body wave magnitude of 4.5-4.9 from the data of the different seismological centers. We detected 12 aftershocks that occurred within 5 days after the main shock using two seismic arrays (AKASG and BRTR) and one three-component station (KBZ) of the International Monitoring System, as well as two array stations of the Institute of Geosphere Dynamics, Russian Academy of Sciences. For six aftershocks, signals were found at three or more stations. The other aftershocks were detected from the data at two out of three nearest stations. Signal detection and association with aftershocks of the main shock, as well as estimation of magnitude and relative location of the found aftershocks, were carried out using the method of waveform cross-correlation (WFCC). The signals from the main shock that acted as the only master event (ME) for the WFCC method were used as waveform templates. To increase the signal-to-noise ratio and to determine the exact onset time of regular seismic waves from aftershocks, we used waveform templates of different length, from 10 to 180 s depending on the wave type and distance to the station, as well as band filtering in narrow frequency bands. The highest sensitivity of the detector and accuracy of the P-wave onset time estimates were reached when a waveform template included all regular waves from P to Lg. Association of signals with aftershocks was based on back projection of signal arrival times to origin times using the travel time from a master event to the station, which was measured with a very low error, being equal to almost half of the digitization step length. To develop a seismic event hypothesis, the origin times at two or more stations should be spaced within a 2-s interval. [ABSTRACT FROM AUTHOR]

Abstract: We cross-correlated three-component seismic waveforms to find ‘repeating events’ among the aftershocks of the 2008 Wenchuan earthquake centered at the WFSD-1 drilling site during and after the drilling. The aftershocks selected occurred from 2008/10/01 to 2011/09/15, with a magnitude range of M L 1.4–5.5. Using the 1–10Hz bandpass filtered waveforms recorded by the regional and mobile seismic stations, 518 ‘repeating events’ are identified with the criterion that for at least one station the three components of seismograms all have cross-correlation coefficients larger than 0.8. Using 12 ‘repeating multiplets’, defined by the criterion that each pair of events in the ‘multiplet’ must be ‘repeating pairs’, fault slip rates are estimated. Slip rate was averaged at about 5mm/year, being within the range of the deformation rate before the Wenchuan earthquake obtained by ‘repeating earthquakes’, with a weak attenuating trend, and a suspected disturbance by a strong aftershock. [Copyright &y& Elsevier]

Highlights: [•] We have investigated a seismic swarm that occurred in 2007 in Lake Trichonis, Greece. [•] Waveform cross-correlation and relocation was performed using local network data. [•] A complex system of conjugate faults was revealed. [•] Stress-transfer by past major events possibly accelerated the 2007 swarm activity. [•] We gained new insights about important structures in the area of study. [Copyright &y& Elsevier]

Cross-correlation analysis was applied to events in the 2003 Lefkada Island, Greece, sequence in order to identify clusters of seismicity within the extensive aftershock sequence along a fault zone of approximately 100 km length. Data from the small-aperture TRISAR array, covering the first 2 days of aftershock activity, were used. Array-based waveform correlation has a great advantage over single channel correlation analysis in that the validity of waveform matches with relatively low correlation coefficients can be examined by checking the alignment of correlation traces on the different channels. The length of the fault zone leads inevitably to a great diversity in the waveforms, although a small number of clusters of very similar events emerge from the TRISAR data. Events which the correlation analysis had placed within the same cluster were listed in the ISC Bulletin with separations of up to tens of kilometres. This made it necessary to check the validity of the TRISAR clusters by applying the same procedure independently to the three-component stations of the National Seismographic Network of the National Observatory of Athens, located at local to regional distances from the aftershock area. Results suggest that array-based waveform correlation provides a robust tool both for identifying event clusters within large aftershock areas and for identifying situations in which bulletin event location estimates need re-evaluation. [ABSTRACT FROM AUTHOR]

Abstract: The precise relocation of micro-earthquakes using waveform cross-correlation is an efficient tool for defining the spatial characteristics of seismogenic structures in volcanic areas. To be applied and to provide meaningful results, the technique requires, however, the presence of sufficient amounts of similar events. We examined about 870 earthquakes recorded at the Campi Flegrei (CF) caldera by a temporary digital network during the end of the 1982–1984 bradiseismic crisis. The classification of these earthquakes indicates a surprising low amount of similar events, especially according to the clustered aspect of the seismicity. The relocation technique, allows only to identify 3 elongated patterns that may be interpreted as small faults which are dipping inward toward the center of the caldera. To determine up to which point this low similarity rate is unusual, we examined the rates for several data sets coming from various volcanic areas: Hawaii, Mount Pinatubo (Philippines) and Piton de la Fournaise (La Réunion Island). The results show for all areas, percentages of similar events significantly higher than for CF and indicate that the low rate observed there is not just a matter of low detection threshold or network characteristics. At CF, most of the seismicity appears to be diffused and generated along small fractures rather than along well defined fault structures. The low similarity rate is in good agreement with a seismicity generated in a fractured medium by overpressure in the hydrothermal system and low deviatoric local stress. [ABSTRACT FROM AUTHOR]

Abstract: Earthquake precursors are now regularly described but often detected only after a major or moderate seismic event. Presence and influence of fluids in the seismogenic processes are often observed at the time of earthquake studies. Even today, the understanding of the physical processes involved in the source region is a real challenge for seismic hazard assessment. Here, the aftershock sequence of the M L =5.2, 1996 Saint-Paul-de-Fenouillet (Eastern Pyrenees, France) earthquake is first re-examined with P-wave cross-correlations, resulting in extracting three multiplets and in determining new locations. Multiplets and spatio-temporal distribution analysis of the aftershocks allow for quantifying the hydraulic diffusivity D at a maximum value of 5m2/s and the permeability K at 10−15 m2 in the upper Pyrenean crust. Second, a model is established in order to explain the hydrogeochemical transient anomalies, which occurred during the 15day-period preceding the 1996 earthquake. These anomalies consist on a temporal and spatial sequence of gas emissions in the epicentral area and on chloride and lead concentration variations in a bottled mineral water 25km north to the main shock epicenter. The proposed model processed in a standard elastic half-space, consists of creep on a low-angle crustal normal-fault, generating volumetric strain field changes over a distance of 25km from the epicentral area. This model is able to constrain not only the mechanisms and the locations of the geochemical anomalies, but also their timing and probable casual links to the triggering of the impending major event. Also, the active extension proposed here is compatible with seismological observations in the Pyrenees. Thus, the possibility of such creep, which can be considered as a slow-slip event, is discussed in the Pyrenean tectonic and geological context. The model is discussed and compared to previous proposed models on precursor processes of earthquakes, especially concerning the preparation zone concept. Finally, a complete seismic scenario over the period beginning 15days before the quake and ending 5days after is proposed and discussed. [Copyright &y& Elsevier]

In the list of ‘repeating pairs’ or ‘doublets’ of earthquakes in China identified by Schaff and Richards using teleseismic waveform cross-correlation, there were 23 repeating pairs located in Liaoning Province. In this study the waveforms of these events were cross-correlated using records from Liaoning Regional Seismograph Network (LRSN), and the ‘repeating events’ in the sense of regional waveform cross-correlation were obtained. The result was compared with that of Schaff and Richards and was used for the assessment of the seismic phase picking and event location practice of LRSN. The result shows that ‘repeating events’ in the sense of teleseismic waveform cross-correlation and those in the sense of regional waveform cross-correlation have significant difference, although with some overlap. However, the overall assessment of the location accuracy and the phase pick errors of LRSN by using these two sets of ‘repeating events’, respectively, provides similar results, while ‘repeating events’ in the sense of regional waveform cross-correlation seem to be better performing in such an assessment. With the assumption that the separation between the ‘repeaters’ be less than 1 km, the uncertainty in routine earthquake location of LRSN is estimated to be below 5 km, with the average of 2 km. In the observational bulletins of LRSN the time error in phase picking is estimated to be within ±1s for 94% Pg readings and for 88% Sg readings. [ABSTRACT FROM AUTHOR]

A new computer search of the Apollo lunar seismic data set for identification of deep moonquakes using a combination of waveform cross-correlation and single-link cluster analysis has increased the number of positively identified deep moonquakes by more than a factor of five, from the previous 1360 to 7245. At least 88 new deep moonquake source regions (nests) have been discovered, while some source regions previously identified as separate entities are now determined to be identical, reducing the number of previously known source regions from 108 to 77. The newly identified deep moonquakes and their relationships to each other are likely to revise our understanding of their spatial and temporal distributions. [Copyright &y& Elsevier]

We address the uncertainty related to cross-correlation waveform time shift measurements, particularly related to temporal changes in inner core seismic waves. Details of the cross-correlation measurement, including data processing, window length, pick time, sampling rate and the algorithm itself, all affect the calculated time shift. By systematically varying measurement parameters for a set of earthquake doublets we find time shift uncertainties of 0.01 s and 0.02 s for differential times and double differential times respectively. The uncertainty is of a similar magnitude to reported double differential times of PKIKP and PKiKP inner core phases. Accounting for measurement uncertainty results in 80% of values published in a recent study being below the measurement resolution (Yang and Song 2020a). While the temporal variations in inner core phases is not in dispute, properly accounting for uncertainties is needed for robust data interpretation. A re-assessment for the origin of inner core temporal changes is therefore warranted and future studies should account for measurement uncertainty. • Quantify measurement uncertainty of temporal changes of inner core seismic waves. • Uncertainties are similar magnitude to differential times of PKIKP and PKiKP. • Approx. 80% of values in a recent study are below measurement resolution. • Properly accounting for uncertainties is needed for robust data interpretation. [ABSTRACT FROM AUTHOR]

• The 2019 earthquake has occurred at the western part of the 1999 aftershocks zone. • The activated structure in 2019 is likely a blind fault. • The eastern part of the 1999 aftershocks involves more than one activated structures. • The 2019 Mw = 5.1 event is the largest to occur in Attica since the 1999 mainshock. Nearly 20 years after the M w = 6.0 Athens (Greece) earthquake of 7 September 1999, a significant M w = 5.1 event struck on 19 July 2019 in roughly the same epicentral region, south of Mt Parnitha, ∼20 km NW of the Athens metropolitan area. In this work, we present a seismotectonic study of the 2019 sequence and investigate its relation with the 1999 aftershock zone and fault geometry. We also take advantage of this opportunity to re-examine the 1999 sequence, using modern cross-correlation and double-difference relocation tools. Waveforms recorded by stations of temporary local networks that were installed at a common site in both case studies are taken into account to identify strongly correlated earthquakes, in terms of waveform similarity, that may indicate a significant degree of resemblance in their source parameters. Our results indicate that the 2019 sequence partially overlaps the western segment of the 1999 aftershock zone. A spatial gap in the middle of that zone, attributed to a high coseismic slip patch during the 1999 mainshock, is still present in 2019, while an isolated cluster of the eastern segment was also reactivated. The relocated hypocenters of both sequences are distributed on nearly coinciding planes that strike ∼N113°-118 °E and dip 48−55° towards SSW, apparently offset by 1−2 km at seismogenic depths, though this difference is attributed to location biases. Their updip extensions outcrop at Mt Parnitha, 1−3 km north of the mapped Fili fault segments or their westwards extension. The 2019 sequence is considered to have occurred at the same structure as the western group of the 1999 aftershocks, which is likely a blind fault. [ABSTRACT FROM AUTHOR]