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In this introductory article we give a general description of the ISC-GEM Global Instrumental Earthquake Catalogue (1900–2009). We also provide the background for four further articles that describe the effort in collecting and digitizing parametric earthquake bulletin data as well as the methodologies developed to compute homogeneous earthquake parameters. The result of the two and a half year project is a catalogue of approximately 20,000 large earthquakes covering 110 years with hypocentres and uncertainties computed using the same technique and velocity model. We show that the overall homogeneity of the main earthquake parameters in the catalogue was achieved despite changes in instrumentation and routine measurement practices concurrent with developments in instrumental seismology from almost a ground level to its current state. For each earthquake, MW magnitude values and uncertainties were computed either based on available estimates of seismic moment or using new empirical relationships between MW, MS and mb. Further important results of this project include the electronic availability of a considerable volume of seismic wave arrival time and amplitude measurements from early instrumental printed station bulletins. These newly recovered amplitude measurements provided a basis for computation of many previously unavailable MS magnitudes with uncertainties. In this article we describe why such a catalogue is required for a comprehensive assessment of global and regional seismic hazard. We also describe other potential uses of the catalogue in many other fields of Earth Sciences. We discuss the catalogue availability and lay out the plans of further development.

We present the final results of a two-year project sponsored by the Global Earthquake Model (GEM) Foundation. The ISC-GEM global catalogue consists of some 19 thousand instrumentally recorded, moderate to large earthquakes, spanning 110 years of seismicity. We relocated all events in the catalogue using a two-tier approach. The EHB location methodology (Engdahl et al., 1998) was applied first to obtain improved hypocentres with special focus on the depth determination. The locations were further refined in the next step by fixing the depths to those from the EHB analysis and applying the new International Seismological Centre (ISC) location algorithm (Bondár and Storchak, 2011) that reduces location bias by accounting for correlated travel-time prediction error structure. To facilitate the relocation effort, some one million seismic P and S wave arrival-time data were added to the ISC database for the period between 1904 and 1970, either from original station bulletins in the ISC archive or by digitizing the scanned images of the International Seismological Summary (ISS) bulletin (Villaseñor and Engdahl, 2005, 2007). Although no substantial amount of new phase data were acquired for the modern period (1964–2009), the number of phases used in the location has still increased by three millions, owing to fact that both the EHB and ISC locators use most well-recorded ak135 (Kennett et al., 1995) phases in the location. We show that the relocation effort yielded substantially improved locations, especially in the first half of the 20th century; we demonstrate significant improvements in focal depth estimates in subduction zones and other seismically active regions; and we show that the ISC-GEM catalogue provides an improved view of 110 years of global seismicity of the Earth. The ISC-GEM Global Instrumental Earthquake Catalogue represents the final product of one of the ten global components in the GEM program, and is available to researchers at the ISC (www.isc.ac.uk) w

The International Seismological Centre Global Earthquake Model (ISC–GEM) Global Instrumental Earthquake Catalogue (1900–2009) is the result of a special effort to substantially extend and improve currently existing global catalogs to serve the requirements of specific user groups who assess and model seismic hazard and risk. The data from the ISC–GEM Catalogue would be used worldwide yet will prove absolutely essential in those regions where a high seismicity level strongly correlates with a high population density (Fig. 1 ). The Catalogue is also expected to have a multidisciplinary use in a wide range of studies such as global seismicity assessment, tectonics, as well as the rapid determination of seismic hazard. Because of the large volume of digitized seismic traveltime and amplitude data generated during this project, we expect the Catalogue to be useful in studies of the inner structure of the Earth as well as in nuclear monitoring research. This global catalog was also designed to serve as a reference to be used for calibration purposes by those compiling regional seismicity catalogs that contain events of much smaller magnitudes. This will guarantee that the catalogs prepared by other teams for different regions will contain comparable earthquake locations and magnitude parameters, especially in border areas. The work on the Catalogue was funded by the GEM Foundation as part of the five Global Hazard Components and is a result of a 27-month-long project. This project was led by the ISC and performed by the team of international experts in accordance with the requirements of the Scientific Board of GEM and following recommendations of the International Association of Seismology and Physics of the Earth’s Interior (IASPEI ) observers. Further, eight IT, administrative, and technical staff at the ISC worked on this project. Details can be found in the Acknowledgments.

We present a new tomographic model of the mantle in the area of the 2010 M8.8 Maule earthquake and surrounding regions. Increased ray coverage provided by the aftershock data allows us to image the detailed subducting slab structure in the mantle, from the region of flat slab subduction north of the Maule rupture to the area of overlapping rupture between the 1960 M9.5 and the 2010 M8.8 events to the south. We have combined teleseismic primary and depth phase arrivals with available local arrivals to better constrain the teleseismic earthquake locations in the region, which we use to conduct nested regional–global tomography. The new model reveals the detailed structure of the flat slab and its transition to a more moderately dipping slab in the Maule region. South of the Maule region, a steeply dipping relic slab is imaged from ∼200 to 1000 km depth that is distinct from the moderately dipping slab above it and from the more northerly slab at similar depths. We interpret the images as revealing both horizontal and vertical tearing of the slab at ∼38°S to explain the imaged pattern of slab anomalies in the southern portion of the model. In contrast, the transition from a horizontal to moderately subducting slab in the northern portion of the model is imaged as a continuous slab bend. We speculate that the tearing was most likely facilitated by a fracture zone in the downgoing plate or alternatively by a continental scale terrane boundary in the overriding plate.

We are investigating the crustal and upper mantle structure of the region in central Asia bounded approximately by longitude 41-67 deg E and latitude 20-44 deg N by using in-country datasets of seismic phase arrival times, supplemented by ground truth datasets developed from our previous research efforts. We present the results of Pn tomography using a dataset consisting entirely of GT5 events in the region. From 27 calibrated earthquake clusters we extract 832 events that qualify as ground truth (GT)5 at the 90% confidence level. The raypaths of Pn readings from these events provide good coverage of most of the region except for the Makran region in the southeast. To determine an appropriate reference model for the region we compared empirical travel times calculated from the GT5 dataset to those predicted by several models: the ak135 global model (Kennett et al., 1995), the CUB2.0 model (Ritzwoller et al., 2003), and the so-called "Unified" model a 3-D velocity model of the crust and upper mantle in Eurasia, constructed from models developed by researchers at Lawrence Livermore National Laboratory (LLNL) and Los Alamos National Laboratory (LANL) (Pasyanos et al, 2004; Begnaud et al., 2004; Steck et al., 2004; Flanagan et al., 2007). Although the CUB2.0 and Unified models do vary laterally, in the study region they vary quite slowly and capture little of the variability of observed travel times. All three models predict Pn travel times that are systematically too fast. These baseline errors range from 2-4 seconds. For this initial round of tomography we stretched the ak135 model to a crustal thickness of 45 km for the reference model., Presented at 2009 Monitoring Research Review - Ground-Based Nuclear Explosion Monitoring Technologies held in Tucson, AZ on 21-23 Sep 2009. Published in the Proceedings of 2009 Monitoring Research Review - Ground-Based Nuclear Explosion Monitoring Technologies, v1 p22-31, 2009. Sponsored in part by DOE. The original document contains color images. All DTIC reproductions will be in black and white.

We present the final results of our three-year research project to produce a high-confidence global GT5 data set. During the course of this project we have developed, tested, and validated the hybrid HDC-RCA (Hypocentroidal Decomposition and Reciprocal Cluster Analysis) methodology to produce new GT5 or better event locations from event clusters. The HDC algorithm uses regional and teleseismic data to estimate precise relative event locations with respect to the cluster centroid. The RCA algorithm uses local data to precisely locate the cluster centroid. We have demonstrated that the HDC-RCA multiple event location methodology is able to produce high-confidence GT5 (epicenter and depth) or better event locations using only a few local stations, without reliance on independent GT information. A posteriori assessment procedures and a priori applicability criteria have been developed and tested to assure the quality and high-confidence of the resulting GT5 events. We have developed a novel, adaptive approach to waveform cross-correlation for improved differential arrival time measurements. The method finds the optimal time-bandwidth product to perform waveform cross-correlation, thus maximizing the similarity between waveforms for a wide range of seismic phases. Correlations are accepted or rejected based on their significance level derived from the estimated time-bandwidth product. We have further developed an error model to estimate the a priori uncertainties in differential time measurements in order to facilitate their inclusion with bulletin arrival time picks in the HDC algorithm. We demonstrated differential times contribute to significant improvements in resolving the relative event locations in the HDC analysis and validated the cross-correlation differential time measurement model., The original document contains color images.

High-resolution cluster analysis (multiple-event relocation) of earthquakes and other seismic sources is developed as a tool for assembling catalogs of reference events, especially those whose locations can be determined with an accuracy of 5 km or better [Ground Truth (GT) 5]. We use the Hypocentroidal Decomposition (HDC) method of Jordan and Sverdrup (1981), which is well suited to the rigorous statistical analysis required for this task. Candidate reference events typically arise from local seismic networks and from temporary deployments for aftershock studies that can yield very high-resolution hypocenters that, nevertheless, must be validated. We utilize arrival time data (as reported to the International Seismological Centre and to the U.S. Geological Survey's National Earthquake Information Center) at regional and teleseismic distances in the cluster analysis to validate candidate reference events, and in some cases, to generate new reference events. HDC analyses have now been performed on a number of earthquake and explosion sequences in Eurasia and Africa, resulting in reference events with locations known to GT5 accuracy. In this paper we review and evaluate our analyses of these clusters to date, and address problem areas. In particular, we find that some candidate reference events cannot be validated because either the reported local network solutions are in error, or the coverage of reported arrival times used in the HDC analysis is not sufficient to constrain the locations. Some discrepancies may arise when local networks locate small precursors or low-energy early stages of rupture in larger earthquakes, while teleseismic stations record only the main pulse of energy release. We have found several cases in which there appear to be systematic biases in the time base used for local network solutions. In another case, we obtained "reference event" locations from two different sources for the same cluster., Presented at the Seismic Research Review: Worldwide Monitoring of Nuclear Explosions (23rd) held in Jackson Hole, WY on 2-5 October 2001. Published in the Proceedings of the Seismic Research Review: Worldwide Monitoring of Nuclear Explosions (23rd), p205-214, 2001.

High-resolution cluster analysis is applied to explosions and earthquake sequences in Asia and North Africa for which one or more of the associated events is known to an accuracy of 5 km or better (GT5). In particular, we analyze phase data reported to the ISC and NEIC for the 1997 Ghaen-Birjand earthquake sequence in eastern Iran and the 1997 Jiashi swarm in western China. In both cases, reference event data is available from short-term portable seismograph deployments following the initiation of seismic activity in those regions. For the cluster analyses we used 92 events of the Iran sequence and 123 events of the Jiashi swarm, ranging in magnitude from 3.4 to 7.1. The analyses produce new locations (relative to the centroid) and 90% confidence ellipses that sharpen the spatial relationship of the earthquakes to known faults and modes of deformation in these regions. Shifting the centroid to match reference event locations produces source-station path corrections (to ak135) for regional seismic phases recorded by IMS and surrogate stations in Asia. The standard error of these path corrections is typically only a few tenths of seconds. Hence, these data can be used to validate 3-D models and to support the calculation of travel-time curves and source-specific station corrections for the Asian and north African regions., Proceedings of the Annual DoD/DOE Seismic Research Symposium (22nd): Planning for Verification of and Compliance with the Comprehensive Nuclear-Test-Ban Treaty (CTBT) held in New Orleans, Louisiana on 13-15 September 2000. U.S. Government or Federal Rights. The original document contains color images.

As an intermediate step to producing a integrated P and S model of the crust and upper mantle beneath most of Eurasia to predict regional station specific travel time correction surfaces, we present maps of isotropic Pn and Sn velocities across Eurasia. These maps are constructed using a groomed version of the ISC/NEIC data that are described, in part, by Engdahl et al. (1998). The locations and origin times of the events are replaced with local ground truth locations whenever possible (e.g., Sultanov et al., 1999). We use the tomographic method of Barmin et al. (2000) to estimate Pn and Sn maps on a 1.5 x 1.5 degree grid, augmented to include station and event static corrections and an epicentral distance or range correction. The starting model is CRUST5.1 (Mooney et al., 1998). We call the Pn and Sn maps together with these corrections the CU Pn=Sn model. Data are used in the inversion if the residual relative to the prediction from the spherical model AK135 (Kennett et al., 1995) is less than 7.5 s for P and 15 s for S, if the event depth is within the crust or less than 50 km deep, if the azimuthal gap to all reporting stations for the event is less than 180 degrees, and if the nominal error ellipse is less than 1000 km2 in area. We define the phases Pn and Sn as arriving between epicentral distances of 3 and 15 degrees. The resulting data set consists of about 1,600,000 Pn and 400,000 Sn travel times inhomogeneously distributed across Eurasia., Proceedings of the 22nd Annual DoD/DOE Seismic Research Symposium: Planning for Verification of and Compliance with the Comprehensive Nuclear-Test-Ban Treaty (CTBT) held in New Orleans, Louisiana on September 13-15, 2000. U.S. Government or Federal Rights. The original document contains color images.

Southern California straddles the boundary between the North American and the Pacific plates. The relative motion between these two plates has been determined from paleomagnetic lineations in the Gulf of California, from global solutions to known slip rates along plate boundaries, from geology, and from geodesy (Minster and Jordan, 1978; Minster and Jordan, 1978; DeMets and others, 1987) to be primarily horizontal at a rate of about 48 mm/yr (DeMets and others, 1987). This results in one of the highest levels of seismicity in the conterminous United States (e.g., Evernden, 1970). In southern California, the deformation is spread over a large area, encompassing numerous normal, strike-slip, and reverse faults. A majority of the plate motion appears to be accommodated by the San Andreas fault, with the rest distributed among the dozen or so other major faults (Weldon and Humphreys, 1986). This is in contrast to the plate boundary in northern California, where the plate motion is more concentrated near the San Andreas fault than it is in southern California (e.g., Hill and others, this volume). The diffuse deformational pattern leads to the high level of seismic activity and to a complicated tectonic structure.