The detection and location of low magnitude earthquakes in northern Norway using multi-channel waveform correlation at regional distances

A fortuitous sequence of closely spaced earthquakes in the Rana region of northern Norway, during 2005, has provided an ideal natural laboratory for investigating event detectability using waveform correlation over networks and arrays at regional distances. A small number of events between magnitude 2.0 and 3.5 were recorded with a high SNR by the Fennoscandian IMS seismic arrays at distances over 600 km and three of these events, including the largest on 24 June, displayed remarkable waveform similarity even at relatively high frequencies. In an effort to detect occurrences of smaller earthquakes in the immediate geographical vicinity of the 24 June event, a multi-channel correlation detector for the NORSAR array was run for the whole calender year 2005 using the signal from the master event as a template. A total of 32 detections were made and all but 2 of these coincided with independent correlation detections using the other Nordic IMS array stations; very few correspond to signals detectable using traditional energy detectors. Permanent and temporary stations of the Norwegian National Seismic Network (NNSN) at far closer epicentral distances have confirmed that all but one of the correlation detections at NORSAR in fact correspond to real events. The closest stations at distances of approximately 10 km can confirm that the smallest of these events have magnitudes down to 0.5 which represents a detection threshold reduction of over 1.5 for the large-aperture NORSAR array and over 1.0 for the almost equidistant regional ARCES array. The incompleteness of the local network recordings precludes a comprehensive double-difference location for the full set of events. However, stable double-difference relative locations can be obtained for eight of the events using only the Lg phase recorded at the array stations. All events appear to be separated by less than 0.5 km. Clear peaks were observed in the NORSAR correlation coefficient traces during the coda of some of the larger events; the local stations confirm that these are in fact aftershocks exhibiting very similar waveforms to the main events. Many of the more marginal correlation detections are not made when the calculations are repeated using shorter signal segments, fewer sensors or more distant stations. We demonstrate in addition how these almost repeating seismic sources have been exploited to detect and measure timing anomalies at individual sites within the arrays and network.     

Determination of Mislocation Vectors to Evaluate bias at GSETT-3 primary stations

Using the database provided by the Reviewed Event Bulletins (REBs) for the first 2.5 years of the Group of Scientific Experts Technical Test-3 (GSETT-3) experiment, we compiled mislocation vectors for both arrays and selected three-component stations of the primary network from the published slowness and azimuth information gained through f-k- and polarization analysis. Imposing constraints such as a minimum signal-to-noise ratio (SNR) and number of defining phases, we aim at eliminating location bias as the hypocentral parameters are taken from the REBs. Results from 14 arrays with apertures from about 1 km to more than 20 km are presented as well as from 18 three-component stations, which indicate that the mislocation vectors in many cases can improve location accuracy considerably. If these mislocation vectors are compiled to provide coverage of a sufficient portion of the slowness domain these empirical corrections can easily be applied prior to location processing. In the context of the Comprehensive Nuclear Test-Ban Treaty (CTBT), these mislocation patterns could be essential for providing accurate event location of suspicious low-magnitude events, as these location parameters will be used to pinpoint the area where to conduct an on-site inspection.     

Source Specific Station Corrections for Regional Phases at Fennoscandian Stations

—?The IASPEI91 global travel-time curves are used as the default for event location at the Prototype International Data Center (PIDC). In order to improve event location, a 1-D Baltic travel-time model was implemented at the PIDC in 1997 for locating events using regional phases from Fennoscandian stations. Where a single model is insufficient for characterizing the regional geology, path-dependent corrections, or Source Specific Station Corrections (SSSCs), are more appropriate for event locations. We have developed SSSCs for regional phases at the Fennoscandian stations by interpolating travel times through different 1-D models. SSSCs for stations NRIS and SPITS are also derived, given the fact that paths from both stations to high latitude events are within the Fennoscandia regionalization as Baltic.¶Validation testing of the SSSCs demonstrates that using SSSCs in event location is superior to not using SSSCs, a nd, in most cases, to using the 1-D model directly when locating events. For a ground-truth data set which includes events in the Baltic Shield with location accuracy better than 2?km, the average improvement in location due to SSSCs is 9?km, and the median coverage ellipse is reduced by 2710?km² (from 3830 to 1120?km²). These results are similar to those obtained using the 1-D Baltic model. For a CEB (Calibration Event Bulletin) data set which includes events along the North Atlantic oceanic ridge and in central/southern Europe, using SSSCs the ridge events move closer to the ridge axis, and the European events move closer to CEB locations than 1-D Baltic locations. For a constrained JHD (Joint Hypocenter Determination) data set of events in the Novaya Zemlya region, when using SSSCs or the 1-D Baltic model, relative to the JHD locations mislocations are less or similar to those without SSSCs. All coverage ellipses are smaller but sti ll contain the JHD solutions.¶Our SSSCs are strongly dependent on the 1-D regional models and regionalization. Future development in 1-D velocity models and travel-time curves should improve such SSSCs, event locations, and uncertainties. It is hoped that the implementation and demonstration of SSSCs in the PIDC software will encourage these further developments. These SSSCs were implemented at the PIDC for Reviewed Event Bulletin (REB) location in April 1999.     

Location Calibration Data for CTBT Monitoring at the Prototype International Data Center

—?Ground-truth information is essential for location calibration of the International Monitoring System network being developed under the Comprehensive Nuclear-Test-Ban Treaty. The objective of the calibration effort is to improve the accuracy of seismic event locations and to reduce the size of the error ellipse, both in automatic and in human analyst-reviewed bulletins, in order to meet the On-Site Inspection requirement for the size of the inspection area. Several databases were compiled and are maintained at the Prototype International Data Center (PIDC) to support calibration efforts. The Nuclear Explosion Database contains most readily accessible information about all nuclear explosions worldwide. The events in the Calibration Event Bulletin (CEB) carefully selected well located events from the PIDC Reviewed Event Bulletin and relocated using additional arrivals from regional networks requested from various National Data Centers. The Ground-Truth Database contains carefully selected events with known or well estimated location accuracies from the Nuclear Explosion Database, CEB, as well as from bulletins of U.S. National Earthquake Information Center and International Seismic Centre. It also contains data on chemical explosions and quarry blasts when confirmed by local or national authorities. Ground-truth events are subdivided into various ground-truth categories according to their location accuracy. The databases have been used in various calibration studies to derive and test corrections to improve event locations. Several location calibration techniques are briefly described. The validation test for any proposed operational change requires that the results meet the location calibration metrics developed and implemented at the PIDC.     

Use of GSETT-3 gamma data in the Slowness-Azimuth Calibration of IMS primary arrays at regional distances

For the purpose of verifying compliance with the CTBT seismic monitoring is one of the four techniques used by the IDC. In order to improve the accuracy of the automatic and the reviewed bulletin epicenter locations the IDC uses SASC for the IMS seismic stations. SASC determination is a straightforward calculation done by comparing for selected events the azimuth and slowness from the waveform processing using array techniques to the theoretical values based on the event locations and the velocity model.The main problem, however, is to build a set of reference events, whose locations are accurate enough and not based on information from the stations to be calibrated. A reference event list assumed to meet this requirement is the Gamma bulletin, which was collected since 1993 and was compiled in the framework of the GSETT-3. In this work calculation of SASC for regional to teleseismic distances (up to 30 degrees) was performed for 11 IMS primary arrays. The calculation was done using Pg, Pn, P, Sg, Sn, and S phases based on the detection list obtained from the pIDC and the Gamma bulletin for 6 years (1993–1999). The number of Gamma events varies from several hundreds for some arrays (BRAR) to several tens of thousands for others (i.e. ARCES, ILAR). Due to the fact that the Gamma bulletin is purely voluntary, the coverage is non-uniform both in time and in space and the location accuracy is non-uniform. This drawback can be overcome by encouraging signatory states to submit quality Gamma bulletin data to the IDC. The work presented here can be used as a routine procedure for improving IMS array performance, especially at regional distances.     

Detection and Analysis of Near-Surface Explosions on the Kola Peninsula

Seismic and infrasonic observations of signals from a sequence of near-surface explosions at a site on the Kola Peninsula have been analyzed. NORSAR’s automatic network processing of these events shows a significant scatter in the location estimates and, to improve the automatic classification of the events, we have performed full waveform cross-correlation on the data set. Although the signals from the different events share many characteristics, the waveforms do not exhibit a ripple-for-ripple correspondence and cross-correlation does not result in the classic delta-function indicative of repeating signals. Using recordings from the ARCES seismic array (250 km W of the events), we find that a correlation detector on a single channel or three-component station would not be able to detect subsequent events from this source without an unacceptable false alarm rate. However, performing the correlation on each channel of the full ARCES array, and stacking the resulting traces, generates a correlation detection statistic with a suppressed background level which is exceeded by many times its standard deviation on only very few occasions. Performing f-k analysis on the individual correlation coefficient traces, and rejecting detections indicating a non-zero slowness vector, results in a detection list with essentially no false alarms. Applying the algorithm to 8 years of continuous ARCES data identified over 350 events which we confidently assign to this sequence. The large event population provides additional confidence in relative travel-time estimates and this, together with the occurrence of many events between 2002 and 2004 when a temporary network was deployed in the region, reduces the variability in location estimates. The best seismic location estimate, incorporating phase information for many hundreds of events, is consistent with backazimuth measurements for infrasound arrivals at several stations at regional distances. At Lycksele, 800 km SW of the events, as well as at ARCES, infrasound is detected for most of the events in the summer and for few in the winter. At Apatity, some 230 km S of the estimated source location, infrasound is detected for most events. As a first step to providing a Ground Truth database for this useful source of infrasound, we provide the times of explosions for over 50 events spanning 1 year.     

Operational Processing of Hydroacoustics at the Prototype International Data Center

— The Prototype International Data Center (PIDC) has designed and implemented a system to process data from the International Monitoring System's hydroacoustic network. The automatic system detects and measures various signal characteristics that are then used to classify the signal into one of three categories. The detected signals are combined with the seismic and infrasonic detections to automatically form event hypotheses. The automatic results are reviewed by human analysts to form the Reviewed Event Bulletin (REB). Continuous processing of hydroacoustic data has been in place since May 1997 and during that time a large database of hydroacoustic signals has been accumulated. For a two-year period, the REB contains 13,582 T phases that are associated to 8,437 events. This is roughly 25% of REB events after taking station downtime into account. Predicted travel times used in locations are based on the arrival time of the peak a mplitude mode calculated from a normal mode propagation model. Global sound velocity and bathymetry databases are used to obtain reliable 2-D, seasonally dependent, travel-time tables for each hydroacoustic station in the PIDC. A limited number of ground-truth observations indicate that the predicted travel times are good to within 5 seconds for paths extending to over 7,000?km – corresponding to a relative error of less than 0.1%. The ground truth indicates that the random errors in measuring arrival times for impulsive signals are between 1 and 6 seconds. This paper describes and evaluates the automatic hydroacoustic processing compared to the analyst reviewed results. In addition, special studies help characterize the overall performance of the hydroacoustic network.     

Contribution of Local Seismic Networks To The Regional Velocity Model of The Bohemian Massif

The routine location of regional seismic events using data from the Czech National Seismological Network (CNSN) is based on Pn, Pg, Sn, Sg phases. A simple velocity model derived from Kárník's (1953) interpretation of an earthquake in Northern Hungary in 1951 has hitherto been used. At present, numerous local seismic networks record and locate local events, which are occasionally recorded at regional distances as well. Due to the relatively small dimensions of local networks, hypocenters (and origin times) determined by a local network might be considered as nearly exact from the point of view of regional-scale CNSN. The comparison of common locations performed by CNSN and by a local network enables us to estimate the accuracy of CNSN locations, as well as to optimize a simple velocity model. The joint interpretation of the CNSN bulletin and the catalogues of four local seismic networks WEBNET, OSTRAVA, KLADNO and LUBIN produced a new ID velocity model. The most frequent epicentral error in this model is less than 5 km, and most foci lie up to 15 km from the true position. The performed analysis indicates bimodal distribution of Sn residuals.     

Results of locating the IASPEI GT(0-5) reference events using the standard ISC procedures

The International Seismological Centre (ISC) is charged with production of the definitive global bulletin of seismic events, based on the most comprehensive set of parametric data collected from all over the world. Almost every event in the bulletin retains the original hypocentral solutions reported to the ISC by contributing agencies. In addition, where possible, the ISC computes its own solution, which is intended to be the most accurate where the data from several networks are used. It is because of the requirement for consistency of the bulletin over the years that the procedures used at the centre to compute hypocentres have remained rather conservative despite considerable advances made in the field of earthquake location.The ISC has developed and put into operation a new data management system. As a result, it is now possible to review and subsequently introduce more up-to-date methods of locating seismic events into the operations. The ISC Governing Council called for a workshop dedicated to location procedures, which was held during the 2005 IASPEI General Assembly in Santiago, Chile.To compare the accuracy of different location algorithms, a list of 156 reference events (IWREF) was selected prior to the workshop. The list includes geographically well distributed earthquakes and explosions with positions known with an accuracy of up to 5 km. It covers the period of 1954-2001 and includes all station readings and hypocentral solutions of different agencies available for these events in the ISC bulletin. Although the original ISC solutions are included, these may be different from the solution obtainable now due to changes in the ISC procedures over the years. This paper presents the results of relocation of these events using standard ISC location procedures as of 2005. These new ISC locations and analysis of their shifts with respect to reference locations present a benchmark for further improvement.     

Locating Small Aftershocks Using a Small-Aperture Temporary Seismic Array

In this paper, we developed a specialized method to locate small aftershocks using a small-aperture temporary seismic array. The array location technique uses the first P arrival times to determine the horizontal slowness vector of the incoming P wave, then combines it with S–P times to determine the event location. In order to reduce the influence of lateral velocity variation on the location determinations, we generated slowness corrections using events well-located by the permanent broadband network as calibration events, then we applied the corrections to the estimated slownesses. Applications of slowness corrections significantly improved event locations. This method can be a useful tool to locate events recorded by temporary fault-zone arrays in the near field but unlocated by the regional permanent seismic network. As a test, we first applied this method to 64 well-located aftershocks of the 1992 Landers, California, earthquake, recorded by both the Caltech/USGS Southern California Seismic Network and a small-aperture, temporary seismic array. The average horizontal and vertical separations between our locations and the well-determined catalogue locations are 1.35 and 1.75 km, respectively. We then applied this method to 132 unlocated aftershocks recorded only by the temporary seismic array. The locations show a clear tendency to follow the surface traces of the mainshock rupture.     

Accurate Location of Synthetic Acoustic Emissions and Location Sensitivity to Relocation Methods, Velocity Perturbations, and Seismic Anisotropy

Acoustic emission (AE) monitoring is a non-invasive method of monitoring fracturing both in situ, and in experimental rock deformation studies. Until recently, the major impediment for imaging brittle failure within a rock mass is the accuracy at which the hypocenters may be located. However, recent advances in the location of regional scale earthquakes have successfully reduced hypocentral uncertainties by an order of magnitude. The least-squares Geiger, master event relocation, and double difference methods have been considered in a series of synthetic experiments which investigate their ability to resolve AE hypocentral locations. The effect of AE hypocenter location accuracy due to seismic velocity perturbations, uncertainty in the first arrival pick, array geometry and the inversion of a seismically anisotropic structure with an isotropic velocity model were tested. Hypocenters determined using the Geiger procedure for a homogeneous, isotropic sample with a known velocity model gave a RMS error for the hypocenter locations of 2.6 mm; in contrast the double difference method is capable of reducing the location error of these hypocenters by an order of magnitude. We test uncertainties in velocity model of up to ±10% and show that the double difference method can attain the same RMS error as using the standard Geiger procedure with a known velocity model. The double difference method is also capable of precise locations even in a 40% anisotropic velocity structure using an isotropic model for location and attains a RMS mislocation error of 2.6 mm that is comparable to a RMS mislocation error produced with an isotropic known velocity model using the Geiger approach. We test the effect of sensor geometry on location accuracy and find that, even when sensors are missing, the double difference method is capable of a 1.43 mm total RMS mislocation compared to 4.58 mm for the Geiger method. The accuracy of automatic picking algorithms used for AE studies is ±0.5 μs (1 time sample when the sampling rate is 0.2 μs). We investigate how AE locations are effected by the accuracy of first arrival picking by randomly delaying the actual first arrival by up to 5 time samples. We find that even when noise levels are set to 5 time samples the double difference method successfully relocates the synthetic AE.     

Local seismic network for monitoring of a potential nuclear power plant area

This study presents a plan for seismic monitoring of a region around a potential nuclear power plant. Seismic monitoring is needed to evaluate seismic risk. The International Atomic Energy Agency has set guidelines on seismic hazard evaluation and monitoring of such areas. According to these guidelines, we have made a plan for a local network of seismic stations to collect data for seismic source characterization and seismotectonic interpretations, as well as to monitor seismic activity and natural hazards. The detection and location capability of the network were simulated using different station configurations by computing spatial azimuthal coverages and detection threshold magnitudes. Background noise conditions around Pyhäjoki were analyzed by comparing data from different stations. The annual number of microearthquakes that should be detected with a dense local network centered around Pyhäjoki was estimated. The network should be dense enough to fulfill the requirements of azimuthal coverage better than 180° and automatic event location capability down to ML?～?0 within a distance of 25 km from the site. A network of 10 stations should be enough to reach these goals. With this setup, the detection threshold magnitudes are estimated to be ML?=??0.1 and ML?=?0.1 within a radius of 25 and 50 km from Pyhäjoki, respectively. The annual number of earthquakes detected by the network is estimated to be 2 (ML?≥?～ ?0.1) within 25 km radius and 5 (ML?≥?～?0.1 to ～0.1) within 50 km radius. The location accuracy within 25 km radius is estimated to be 1–2 and 4 km for horizontal coordinates and depth, respectively. Thus, the network is dense enough to map out capable faults with horizontal accuracy of 1–2 km within 25 km radius of the site. The estimation is based on the location accuracies of five existing networks in northern Europe. Local factors, such as seismic noise sources, geology and infrastructure might limit the station configuration and detection and location capability of the network.     

Precise Relative Location of 25-ton Chemical Explosions at Balapan Using IMS Stations

—?We test how well low-magnitude (m _bLg 1.8 to 2.6), 25-ton chemical explosions at Balapan, Kazakhstan, can be located using IMS stations and standard earth models, relying on precisely determined relative arrival times of nearly similar, regional and teleseismic waveforms. Three 1997 Balapan explosions were recorded by a number of currently reporting and surrogate IMS stations. Three regional stations and two teleseismic arrays yielded consistent waveforms appropriate for relative picking. Master-event locations based on the AK135 model and ground-truth information from the first, shallowest and best-recorded explosion, fell under 1 km from known locations, for depths constrained to that of the master event. The resulting 90% confidence ellipses covered 12–13?km² and contained the true locations; however, results for depth constrained to true depth were slightly less satisf actory. From predictions based on ground truth, we found a P _g -coda phase at Makanchi, Kazakhstan to be misidentified and poorly modeled. After accounting for this, 90% ellipses shrank to 2–3?km² and true-depth mislocation vectors became more consistent with confidence-ellipse orientations. These results suggest that a high level of precision could be provided by a tripartite array of calibration shots in cases where models are poorly known. We hope that the successful relocation of these small Balapan shots will support the role of calibration explosions in verification monitoring and special event studies, including on-site inspection.     

The Finnish seismic array

This paper deals with the Finnish seismic array, which in its present state consists of two tripartite arrays, HESA and JYSA. The first of these arrays, situated near Helsinki, has been in continuous operation since 1964 and uses analog recording by frequency-modulated telemetry. The JYSA array is sited in central Finland near the town of Jyväskylä. This array consists of three substations equipped with vertical-component SP seismometers. The signals are transmitted by digital FM telemetry in the UHF band and recorded at Helsinki on magnetic tape. Events are detected visually from monitoring drums. The digital data of the events detected are handled by a microprocessor with a graphic display and fed to a Burroughs 6700 analysing computer.Comparison with the NORSAR array indicates that at frequencies below 2 Hz the absolute noise level at JYSA is about 6 dB lower, but above 2 Hz the noise level for single instruments at the two arrays is about the same. As regards detection capability, the incremental threshold is about the same for JYSA as for Hagfors and 0.5 magnitude units higher than for NORSAR, but 0.5 magnitude units lower for JYSA than for the WWSSN station NUR in southern Finland.     

Analysis of the IMS Location Accuracy in Northern Eurasia and North America Using Regional and Global Pn Travel-time Tables

—?Joint Research Program of Seismic Calibration of the International Monitoring System (IMS) in Northern Eurasia and North America has been signed by the Nuclear Treaty Programs Office (NTPO), Department of Defense USA, and the Special Monitoring Service (SMS) of the Ministry of Defense, Russian Federation (RF). Under the Program historical data from nuclear and large chemical explosions of known location and shot time, together with appropriate geological and geophysical data, has been used to derive regional Pn/P travel-time tables for seismic event location within the lower 48 States of the USA and the European part of the RF. These travel-time tables are up to 5?seconds faster in shields than the IASPEI91 tables, and up to 5?seconds slower in the Western USA. Relocation experiments using the regional Pn travel-time curves and surrogate networks for the IMS network generally improved locations for regional seismic events. The distance between true and estimated location (mislocation) was decreased from an average of 18.8?km for the IASPEI91 tables to 10.1?km for the regional Pn travel-time tables. However, the regional travel-time table approach has limitations caused by travel-time variations inside major tectonic provinces and paths crossing several tectonic provinces with substantially different crustal and upper mantle velocity structure.¶The RF members of the Calibration Working Group (WG): Colonel Vyacheslav Gordon (chairman); Dr. Prof. Marat Mamsurov, and Dr. Nikolai Vasiliev. The US members of the WG: Dr. Anton Dainty (chairman), Dr. Douglas Baumgardt, Mr. John Murphy, Dr. Robert North, and Dr. Vladislav Ryaboy.     

利用人工爆破资料研究福建一维P波速度结构和地震定位

通过人工爆破资料研究地球结构的独特优点是震源时间和位置精确知道.2010—2012年间福建省进行了一系列的爆破实验.本文利用手工拾取来自省地震台网记录的爆破地震初至Pg、Pn以及续至Pg波到时数据,采用联合反演方法构建了新的一维P波速度模型,即福建爆破模型(FJEM).与华南模型相比,FJEM模型对走时的拟合程度提高了45%,有明显改善.利用不同爆破地震数据组合得到稳定类似的福建地区一维速度模型,显示福建地区存在较简单的一维速度结构.对爆破地震的重定位显示传统使用的华南模型在福建地区具有较小的水平定位误差(平均0.52±0.45km),但存在较大深度误差(平均4.7±1.2km).FJEM模型表现出与华南模型相似的水平定位能力,但是震源深度误差更小(1.3±1.1km).对基于FJEM模型的合成天然地震目录的重定位,华南模型显示出相似的定位结果:(1)台站方位覆盖较好的福建中部地区的水平定位误差小;(2)台站方位覆盖差的福建海岸及海峡区域水平定位误差大;(3)震源深度误差则跟台站数目及方位分布没有明显的关系,而是与发震时间误差有互易关系.从中可以看出,地震水平定位误差基本上受台站方位覆盖影响,而受参考速度模型影响不大;而在深度方面,本文改进的FJEM模型不仅更加接近真实的速度结构(拟合走时更好)而且也减小了深度误差.因此建议在福建及其邻近区域的日常定位中用FJEM模型替代华南模型.     

Location of Moderate-Sized Earthquakes Recorded by the NARS�CBaja Array in the Gulf of California Region Between 2002 and 2006

We relocated the hypocentral coordinates of small to moderate-sized earthquakes reported by the National Earthquake Information Center (NEIC) between April 2002 and August 2006 in the Gulf of California region and recorded by the broadband stations of the network of autonomously recording seismographs (NARS?CBaja array). The NARS?CBaja array consists of 19 stations installed in the Baja California peninsula, Sonora and Sinaloa, Mexico. The events reported by the preliminary determinations of epicenters (PDE) catalog within the period of interest have moment magnitudes (M _w) ranging between 1.1 and 6.7. We estimated the hypocentral location of these events using P and S wave arrivals recorded by the regional broadband stations of the NARS?CBaja and the RESBAN (Red Sismológica de Banda Ancha) arrays and using a standard location procedure with the HYPOCENTER code (Lienert and Havskov in Seism Res Lett 66:26?C36, 1995) as a preliminary step. To refine the location of the initial hypocenters, we used the shrinking box source-specific station term method of Lin and Shearer (J Geophys Res 110, B04304, 2005). We found that most of the seismicity is distributed in the NW?CSE direction along the axis of the Gulf of California, following a linear trend that, from north to south, steps southward near the main basins (Wagner, Delfin, Guaymas, Carmen, Farallon, Pescadero and Alarcon) and spreading centers. We compared the epicentral locations reported in the PDE with the locations obtained using regional arrival times, and we found that earthquakes with magnitudes in the range 3.2?C5.0?mb differ on the average by as much as 43?km. For the M _w magnitude range between 5 and 6.7 the discrepancy is less, differing on the average by about 25?km. We found that the relocated epicenters correlate well with the main bathymetric features of the Gulf.     

Updating default depths in the ISC bulletin

The International Seismological Centre (ISC) publishes the definitive global bulletin of earthquake locations. In the ISC bulletin, we aim to obtain a free depth, but often this is not possible. Subsequently, the first option is to obtain a depth derived from depth phases. If depth phases are not available, we then use the reported depth from a reputable local agency. Finally, as a last resort, we set a default depth.In the past, common depths of 10, 33, or multiples of 50 km have been assigned. Assigning a more meaningful default depth, specific to a seismic region will increase the consistency of earthquake locations within the ISC bulletin and allow the ISC to publish better positions and magnitude estimates. It will also improve the association of reported secondary arrivals to corresponding seismic events.We aim to produce a global set of default depths, based on a typical depth for each area, from well-constrained events in the ISC bulletin or where depth could be constrained using a consistent set of depth phase arrivals provided by a number of different reporters.In certain areas, we must resort to using other assumptions. For these cases, we use a global crustal model (Crust2.0) to set default depths to half the thickness of the crust.     

Crustal structure and accurate hypocenter determination along the Longmenshan fault zone

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