International Seismological Centre Bulletin Research Articles

A new moment tensor catalogue of light to moderate earthquakes in Palu-Koro and Matano faults, Sulawesi, based on the regional network in Indonesia: Fault analysis and insight

Starting from 2009, substantial improvements in the regional seismic network operating in Sulawesi, as part of the Meteorological, Climatological, and Geophysical Agency (BMKG) station deployment, allowed us to determine earthquake mechanism for 140 shallow (5–60 km) light to moderate size (3.8 ≤ Mw ≤ 6.1) earthquakes between 2009 and 2018 along the Palu-Koro and Matano faults, through moment tensor inversion. Ten (10) out of the 140 solutions obtained in this study were found in the International Seismological Centre Bulletin and used as validation. The double-couple mechanism of the best solutions (variance reduction ≥0.4 and condition number ≤ 7.5) is further used to analyze the Palu-Koro and Matano segments. We observed that the Donggala segment of the Palu-Koro fault shows variable earthquake mechanism types. Whereas the two subsequent segments, the Palu and Saluki segments, show predominate oblique strike-slip with normal and reverse mechanisms. The Matano fault is characterized by strike-slip mechanisms, with normal mechanisms also observed. The normal mechanisms depict the pull-apart system in the Matano segment, manifested as Matano Lake. The results of this work improve our understanding of regional tectonics.

Homogenization of magnitudes of the ISC Bulletin

SUMMARY We implemented an automatic procedure to download the hypocentral data of the online Bulletin of the International Seismological Centre (ISC) in order to produce in near real-time a homogeneous catalogue of the Global and EuroMediterranean instrumental seismicity to be used for forecasting experiments and other statistical analyses. For the interval covered by the reviewed ISC Bulletin, we adopt the ISC locations and convert the surface wave magnitude (Ms) and short-period body-wave magnitude (mb) as computed by the ISC to moment magnitude (Mw), using empirical relations. We merge the so obtained proxies with real Mw provided by global and EuroMediterranean moment tensor catalogues. For the most recent time interval (about 2 yr) for which the reviewed ISC Bulletin is not available, we do the same but using the preferred (prime) location provided by the ISC Bulletin and converting to Mw the Ms and mb provided by some authoritative agencies. For computing magnitude conversion equations, we use curvilinear relations defined in a previous work and the chi-square regression method that accounts for the uncertainties of both x and y variables.

Complementing regional moment magnitudes to GCMT: a perspective from the rebuilt International Seismological Centre Bulletin

Abstract. Seismologists and geoscientists often need earthquake catalogues for various types of research. This input usually contains basic earthquake parameters such as location (longitude, latitude, depth, and origin time), as well as magnitude information. For the latter, the moment magnitude Mw has become the most sought after magnitude scale in the seismological community to characterize the size of an earthquake. In this contribution we provide an informative account of the Mw content for the newly rebuilt Bulletin of the International Seismological Centre (ISC, http://www.isc.ac.uk, last access: May 2021), which is regarded as the most comprehensive record of the Earth's seismicity. From this data, we extracted a list of hypocentres with Mw from a multitude of agencies reporting data to the ISC. We first summarize the main temporal and spatial features of the Mw provided by global (i.e. providing results for moderate to great earthquakes worldwide) and regional agencies (i.e. also providing results for small earthquakes in a specific area). Following this, we discuss their comparisons, by considering not only Mw but also the surface wave magnitude MS and short-period body wave magnitude mb. By using the Global Centroid Moment Tensor solutions as an authoritative global agency, we identify regional agencies that best complement it and show examples of frequency–magnitude distributions in different areas obtained both from the Global Centroid Moment Tensor alone and complemented by Mw from regional agencies. The work done by the regional agencies in terms of Mw is fundamental to improve our understanding of the seismicity of an area, and we call for the implementation of procedures to compute Mw in a systematic way in areas currently not well covered in this respect, such as vast parts of continental Asia and Africa. In addition, more studies are needed to clarify the causes of the apparent overestimation of global Mw estimations compared to regional Mw. Such difference is also observed in the comparisons of Mw with MS and mb. The results presented here are obtained from the dataset (Di Giacomo and Harris, 2020, https://doi.org/10.31905/J2W2M64S) stored at the ISC Dataset Repository (http://www.isc.ac.uk/dataset_repository/, last access: May 2021).

A 24-Yr-Long Seismic Bulletin for the European Arctic

AbstractTo understand the seismic activity in the European Arctic, the seismic bulletins for the region should be as complete as possible. We present the collection results of a new seismic event bulletin of the European Arctic (70° to 90° N, −15° to 75° E) for the 24-yr-long period 1990–2013. The contribution describes in detail the merging of the different sources taken in account for the compilation, the homogenization of the data, and the relocation of the seismic events. With respect to the International Seismological Centre bulletin, the new bulletin contains 5957 new seismic events and 58,242 new seismic onset readings from stations mostly located at regional distances. The gains are distributed over the entire study region, with the most significant contributions across the Svalbard archipelago, along the Knipovich and northern Mohns ridges, as well as northern Fennoscandia.

Indications of seismic station phase reversals detected from parametric data in the ISC bulletin

The goodness of fit of first motion manually picked polarities from the reviewed Bulletin of the International Seismological Centre (ISC) is examined versus available earthquake mechanism solutions in order to study the behavior of seismic stations, and/or reporting agencies, over time. The probability of the reporting polarity data fitting (and not fitting) source mechanisms is quantified by calculating the probability distribution of several Bernoulli trials over a randomly perturbed set of hypocenters and velocity models for each earthquake mechanism–station polarity combination. The overall agreement of first motion polarities against source mechanisms is found to be good with a few cases of seismic stations showing indications of systematic phase reversals over certain time periods. Specifically, results were obtained for 50% of the registered stations at the ISC, and from these stations 70% show reliable operation during the operational time period under investigation, with only 3% showing the opposite, and 7% showing evidence of systematic changes in the quality of the reported first motion polarities. The rest showed great variability over short periods of time, which does not allow one to draw any conclusions. Comparing waveform data with the associated reported polarities revealed a mixture of cases of either questionable picking or true station phase reversals. The ISC is planning to make available on its website the detailed results for each station in order to make data users and seismic network operators aware of cases that need attention.

Seismic Attenuation and Velocity Measurements of the Uppermost Mantle Beneath the Central and Eastern United States and Implications for the Temperature of the North American Lithosphere

AbstractWe constructed a detailed map of the uppermost mantle seismic structure beneath the Central and Eastern United States (CEUS) using the International Seismological Centre Bulletin and seismic waveforms from USArray. Identical Pn and Sn travel time data sets were inverted to tomographically image the uppermost mantle P wave (Pn) velocity, S wave (Sn) velocity, and the velocity ratio (VPn/VSn). Furthermore, we modified the two‐station method in order to limit the contributions of site response and estimate the effective attenuation of Sn phase (Q−1Sn).Lower Q values generally correspond with lower velocities in terms of both Pn and Sn wave speeds (e.g., New England and the Mississippi Embayment), but some regions, including southern Georgia, eastern South Carolina, and the New Madrid Seismic Zone, show high velocity and low QSn values. This can be explained by scattering attenuation of the Sn phase. To estimate the uppermost mantle temperature, a constrained grid‐search algorithm was conducted using the observed VSn, VPn, and QSn with the calculated velocities of specific compositional models. The uppermost mantle temperature result shows ~300–500 °C beneath the northern midcontinent and 1100 °C beneath New England. Although our temperature results appear to be well resolved, we found that VPn, VSn, and QSn are not enough to constrain the detailed uppermost mantle composition model. Our results highlight significant temperature heterogeneity in the uppermost mantle across the CEUS and is consistent with there not being any melt within the uppermost mantle beneath the CEUS.

Enhanced performance of ISC focal mechanism computations as a result of automatic first-motion polarity picking optimization

The International Seismological Centre (ISC) routinely calculates and makes available automatic earthquake focal mechanisms by combining reported parametric data (first-motion polarities) available in the reviewed ISC Bulletin and auto-picked first-motion polarities obtained from waveform data using a broadband automatic picker. In order to further enhance the robustness of the auto-picked polarities, we set up an optimization strategy which is carried out using the neighbourhood algorithm on a 24-processor mini computer cluster. The aim is to minimize an objective misfit function which takes into account the data uncertainties and compares the first P-wave arrival times and polarities of a large dataset of nearly 18,000 manual picks and the associated auto-picked waveform phase arrivals. The optimization yielded an overall increase of matching auto-picks from 15 to 30% in comparison with the default setup of the automatic picker. We then applied the optimized automatic picker to a set of earthquakes from the reviewed ISC Bulletin where we could not obtain well-constrained mechanism solutions using its default setup. As a result of using the optimized picker, we obtained well-trusted mechanism solutions for 28% of these cases by increasing the number of first motion auto-picked polarities, and hence minimizing the station azimuthal gap in some cases, and/or correcting some of the erroneous auto-picked polarities where possible.

The ISC Bulletin as a comprehensive source of earthquake source mechanisms

Abstract. In this article we summarize the availability of earthquake source mechanisms in the Bulletin of the International Seismological Centre (ISC). The bulletin in its current status contains ∼81 000 seismic events with only one associated mechanism solution and ∼25 000 events with at least two associated source mechanisms. The main sources of earthquake mechanisms in the ISC Bulletin are reported solutions provided by data contributors and ISC-computed focal mechanisms based on first motion polarities. Given the importance of using pre-determined fault plane solutions in different types of studies, here we briefly discuss the methodologies adopted by major data providers to the ISC and investigate the intra-event variability of the source mechanisms. We conclude that the overall agreement among different earthquake mechanisms for the same event as reported by different sources can show a similarity coefficient as high as 80 %, based on the rotation angles of their best-fitting double couple solutions, for the majority of the cases. The earthquake source mechanisms discussed in this work are freely available within the ISC Bulletin websearch at http://doi.org/10.31905/D808B830.

A scheme to set preferred magnitudes in the ISC Bulletin

One of the main purposes of the International Seismological Centre (ISC) is to collect, integrate and reprocess seismic bulletins provided by agencies around the world in order to produce the ISC Bulletin. This is regarded as the most comprehensive bulletin of the Earth’s seismicity, and its production is based on a unique cooperation in the seismological community that allows the ISC to complement the work of seismological agencies operating at global and/or local-regional scale. In addition, by using the seismic wave measurements provided by reporting agencies, the ISC computes, where possible, its own event locations and magnitudes such as short-period body wave m b and surface wave M S . Therefore, the ISC Bulletin contains the results of the reporting agencies as well as the ISC own solutions. Among the most used seismic event parameters listed in seismological bulletins, the event magnitude is of particular importance for characterizing a seismic event. The selection of a magnitude value (or multiple ones) for various research purposes or practical applications is not always a straightforward task for users of the ISC Bulletin and related products since a multitude of magnitude types is currently computed by seismological agencies (sometimes using different standards for the same magnitude type). Here, we describe a scheme that we intend to implement in routine ISC operations to mark the preferred magnitudes in order to help ISC users in the selection of events with magnitudes of their interest.

Regression Relations for Conversion of Various Magnitude Types and Catalogs for the Earthquakes of Turkey and Vicinity

Online Material: Table of regression parameters. Homogeneous earthquake catalogs are needed for various seismological studies, such as hazard analysis and seismicity. In most of the research in question, it is preferred to use moment magnitude M w. Turkey is a seismically active region due to the complex tectonic structure. This high seismicity requires studies on earthquakes that in turn must rely on a uniform catalog, that is, the catalog must be based on a single‐magnitude type. Several national and international organizations record and evaluate the earthquake data to calculate and catalog the magnitude values in various types. This study was carried out to introduce the regression relations between these magnitude scales to provide uniform catalogs for Turkey. A number of studies have been implemented to estimate the relationship between various magnitude scales for global and regional datasets. Although the moment magnitude is the most reliable magnitude scale (Kagan, 2002b, 2003; Castellaro et al. , 2006), it is the most rare. In the regional earthquake catalog of Turkey, compiled by Bogazici University Kandilli Observatory and Earthquake Research Institute–National Earthquake Monitoring Centre (B.U.KOERI–NEMC), there are 2110 body‐wave magnitudes, 1958 local magnitudes, 1511 duration magnitudes, 405 moment magnitudes, and 254 surface‐wave magnitude values published (all magnitudes equal or greater than 4.0) for 4074 events within the period from 1900 to 2014. Similarly, there is a shortage of moment magnitude data in the catalogs of the International Seismological Centre (ISC). A search of the ISC bulletin for events in Turkey with magnitude equal or greater than 4.0 yielded 4246 body‐wave magnitude and 636 surface‐wave magnitude values. There are only 151 moment magnitude reported in the Global Instrumental Earthquake Catalogue released by the ISC and Global Earthquake Model (GEM) effort. With the same search parameters, there are 219 earthquakes with moment magnitude values …

Long-term accumulation and improvements in seismic event data for the polar regions by the International Seismological Centre

The International Seismological Centre (ISC) is a non-governmental non-profit making organization funded by 62 research and operational institutions around the world and charged with the production of the ISC Bulletin – the definitive summary of the global seismicity based on reports from over 130 agencies worldwide, including those active in Polar regions. Jointly with the National Earthquake Information Center (NEIC) of the United States Geological Survey (USGS), the ISC runs the International Seismic Station Registry. The ISC is also charged with maintaining the International Association of Seismology and Physics of the Earth Interior (IASPEI) Reference event List. The new ISC product, the ISC Event Bibliography allows users to obtain references to scientific articles describing specific seismic events, natural and anthropogenic. In this paper we demonstrate how these products and services are applicable to seismic events both in Arctic and Antarctic regions. We also give a summary of the ISC data in polar regions and provide credit to Institutions that report these data to the ISC.

A New ISC Service: The Bibliography of Seismic Events

Research Article| March 01, 2014 A New ISC Service: The Bibliography of Seismic Events D. Di Giacomo; D. Di Giacomo International Seismological Centre, Pipers Lane, Thatcham, Berkshire RG19 4NS, UK Search for other works by this author on: GSW Google Scholar D. A. Storchak; D. A. Storchak International Seismological Centre, Pipers Lane, Thatcham, Berkshire RG19 4NS, UK Search for other works by this author on: GSW Google Scholar N. Safronova; N. Safronova International Seismological Centre, Pipers Lane, Thatcham, Berkshire RG19 4NS, UK Search for other works by this author on: GSW Google Scholar P. Ozgo; P. Ozgo International Seismological Centre, Pipers Lane, Thatcham, Berkshire RG19 4NS, UK Search for other works by this author on: GSW Google Scholar J. Harris; J. Harris International Seismological Centre, Pipers Lane, Thatcham, Berkshire RG19 4NS, UK Search for other works by this author on: GSW Google Scholar R. Verney; R. Verney International Seismological Centre, Pipers Lane, Thatcham, Berkshire RG19 4NS, UK Search for other works by this author on: GSW Google Scholar I. Bondár I. Bondár International Seismological Centre, Pipers Lane, Thatcham, Berkshire RG19 4NS, UK Search for other works by this author on: GSW Google Scholar Author and Article Information D. Di Giacomo International Seismological Centre, Pipers Lane, Thatcham, Berkshire RG19 4NS, UK D. A. Storchak International Seismological Centre, Pipers Lane, Thatcham, Berkshire RG19 4NS, UK N. Safronova International Seismological Centre, Pipers Lane, Thatcham, Berkshire RG19 4NS, UK P. Ozgo International Seismological Centre, Pipers Lane, Thatcham, Berkshire RG19 4NS, UK J. Harris International Seismological Centre, Pipers Lane, Thatcham, Berkshire RG19 4NS, UK R. Verney International Seismological Centre, Pipers Lane, Thatcham, Berkshire RG19 4NS, UK I. Bondár International Seismological Centre, Pipers Lane, Thatcham, Berkshire RG19 4NS, UK Publisher: Seismological Society of America First Online: 14 Jul 2017 Online ISSN: 1938-2057 Print ISSN: 0895-0695 © 2014 by the Seismological Society of America Seismological Research Letters (2014) 85 (2): 354–360. https://doi.org/10.1785/0220130143 Article history First Online: 14 Jul 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation D. Di Giacomo, D. A. Storchak, N. Safronova, P. Ozgo, J. Harris, R. Verney, I. Bondár; A New ISC Service: The Bibliography of Seismic Events. Seismological Research Letters 2014;; 85 (2): 354–360. doi: https://doi.org/10.1785/0220130143 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySeismological Research Letters Search Advanced Search The International Seismological Centre (ISC) is a not‐for‐profit organization with the primary mission of producing the definitive summary of the seismicity of the Earth (ISC Bulletin; ISC, 1964–2013). This is achieved thanks to a unique international cooperation in the seismological community that allows the ISC to collect, integrate, and finally process seismological bulletins (location parameters, station data, moment tensor solutions, felt reports, etc.) from approximately 130 agencies worldwide. Seismic (earthquakes or man‐made) events parameters and station data are publicly available via the ISC website (www.isc.ac.uk, last accessed January 2014). In 2012 we started to set up a... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Improved location procedures at the International Seismological Centre

SUMMARY The International Seismological Centre (ISC) is a non-governmental, non-profit organization with the primary mission of producing the definitive account of the Earth's seismicity. The ISC Bulletin covers some 50 yr (1960–2011) of seismicity. The recent years have seen a dramatic increase both in the number of reported events and especially in the number of reported phases, owing to the ever-increasing number of stations worldwide. Similar ray paths will produce correlated traveltime prediction errors due to unmodelled heterogeneities in the Earth, resulting in underestimated location uncertainties, and for unfavourable network geometries, location bias. Hence, the denser and more unbalanced the global seismic station coverage becomes, the less defensible is the assumption (that is the observations are independent), which is made by most location algorithms. To address this challenge we have developed a new location algorithm for the ISC that accounts for correlated error structure, and uses all IASPEI standard phases with a valid ak135 traveltime prediction to obtain more accurate event locations. In this paper we describe the new ISC locator, and present validation tests by relocating the ground truth events in the IASPEI Reference Event List, as well as by relocating the entire ISC Bulletin. We show that the new ISC location algorithm provides small, but consistent location improvements, considerable improvements in depth determination and significantly more accurate formal uncertainty estimates. We demonstrate that the new algorithm, through the use of later phases and testing for depth resolution, considerably clusters event locations more tightly, thus providing an improved view of the seismicity of the Earth.

The effects of data quality in local earthquake tomography: Application to the Alpine region

Despite the increase in quality and number of seismic stations in many parts of the world, accurate timing of individual arrival times remains crucial for many tomographic applications. To achieve a data set of high quality, arrival times need to be picked with high accuracy, including a proper assessment of the uncertainty of timing and phase identification, and a high level of consistency. We have investigated the effects of data quantity and quality on the solution quality in local earthquake tomography. We have compared tomographic results obtained with synthetic and real data of two very different data sets. The first data set consisted of a large set of arrival times of low precision and unknown accuracy taken from the International Seismological Centre (ISC) Bulletin for the greater Alpine region. The second high-quality data set for the same region was seven times smaller and was obtained by automated quality-weighted repicking. During a first series of inversions, synthetic data resembling the two data sets were inverted with the same amount of Gaussian distributed noise added. Subsequently, during a second series of inversions, the noise level was increased successively for ISC data to study the effect of larger Gaussian distributed error on the solution quality. Finally, the real data for both data sets were inverted. These investigations showed that, for Gaussian distributed error, a smaller data set of high quality could achieve a similar or better solution quality than a data set seven times larger but about four times lower in quality. Our results further suggest that the quality of the ISC Bulletin is degraded significantly by inconsistencies, strongly limiting the use of this large data set for local earthquake tomography studies.

Comparison of hypocentre parameters of earthquakes in the Aegean region

The Aegean Sea is one of the more seismically active areas in the Euro-Mediterranean region. The seismic activity in the Aegean Sea is monitored by a number of local agencies that contribute their data to the International Seismological Centre (ISC). Consequently, the ISC Bulletin may serve as a reliable reference for assessing the capabilities of local agencies to monitor moderate and low magnitude earthquakes. We have compared bulletins of the Kandilli Observatory and Earthquake Research Institute (KOERI) and the ISC, for the period 1976–2003 that comprises the most complete data sets for both KOERI and ISC. The selected study area is the East Aegean Sea and West Turkey, bounded by latitude 35–41°N and by longitude 24–29°E. The total number of events known to occur in this area, during 1976–2003 is about 41,638. Seventy-two percent of those earthquakes were located by ISC and 75% were located by KOERI. As expected, epicentre location discrepancy between ISC and KOERI solutions are larger as we move away from the KOERI seismic network. Out of the 22,066 earthquakes located by both ISC and KOERI, only 4% show a difference of 50 km or more. About 140 earthquakes show a discrepancy of more than 100 km. Focal Depth determinations differ mainly in the subduction zone along the Hellenic arc. Less than 2% of the events differ in their focal depth by more than 25 km. Yet, the location solutions of about 30 events differ by more than 100 km. Almost a quarter of the events listed in the ISC Bulletin are missed by KOERI, most of them occurring off the coast of Turkey, in the East Aegean. Based on the frequency-magnitude distributions, the KOERI Bulletin is complete for earthquakes with duration magnitudes Md > 2.7 (both located and assigned magnitudes) where as the threshold magnitude for events with location and magnitude determinations by ISC is mb > 4.0. KOERI magnitudes seem to be poorly correlated with ISC magnitudes suggesting relatively high uncertainty in the magnitude determinations. It is apparent that KOERI, like other agencies in the region, will greatly benefit from a much improved regional cooperation.

The Euro-Mediterranean Bulletin: A Comprehensive Seismological Bulletin at Regional Scale

The Euro-Mediterranean Seismological Center (EMSC) is in charge of collecting parametric data to operate a real-time information and alert system for potentially damaging earthquakes and to publish a comprehensive seismological bulletin for the Euro-Mediterranean region. The region is a seismically active area associated with a complex tectonic structure (Dercourt et al. 1986) but monitored by numerous local or national seismological networks. To obtain a coherent image of the seismicity, it is important to gather and merge data from all countries. The Euro-Mediterranean bulletin computed at the EMSC therefore provides a coherent source of information for the scientific community, for seismic hazard studies, or for tomographic and tectonophysic studies. At the global scale, the three main seismological bulletins are produced by the International Seismological Centre (ISC), the International Data Center (IDC), and the National Earthquake Information Center of the United States (NEIC). The ISC produces a worldwide and exhaustive bulletin with a two-year delay and is considered the most comprehensive database (Willemann et al. 2001). The ISC is a nongovernmental and nonprofit organization mostly funded by academic institutions; it works closely with the International Association of Seismology and Physics of the Earth's Interior (IASPEI). Until 2001, the ISC bulletin location had been solely based on P arrivals. After 2001, all phases have been included in the location. The magnitude completeness varies among regions, but the ISC aims to report all events as well as review and relocate those recorded by more than one network. In addition to known earthquake location determination, the ISC searches for events unidentified by the local networks by associating groups of phases from different agencies. The other global bulletin available is provided by the NEIC, which provides three bulletin publications: the Quick Epicenter Determinations (QED), produced daily; and the Preliminary Determination of Epicenters (PDE), produced both weekly …

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.

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.

Global Mantle Tomography: Progress Status in the Past 10 Years

▪ Abstract We review the present status of global mantle tomography and discuss two main classes of models that have been developed in the past 10 years: P velocity models based on large datasets of travel times from the International Seismological Centre bulletins, often referred to as “high resolution” models, and S velocity models based on a combination of surface wave and hand picked body wave travel times, or waveforms, referred to as “long wavelength” models. We discuss their respective strengths and weaknesses, as well as progress in the resolution of other physical parameters, such as anisotropy, anelasticity, density, and bulk sound velocity using tomographic approaches. We present the view that future improvements in global seismic tomography require the utilization of the rich information contained in complete broadband seismic waveforms. This is presently within our reach owing to theoretical progress as well as the increase in computational power in recent years.

Global Test of Seismic Event Locations Using Three-Dimensional Earth Models

To prepare to update location procedures at the International Seismological Centre (ISC), we conduct a global test of location accuracy using two 3D Earth models and three 1D models (Jeffreys-Bullens [J-B], Preliminary Reference Earth Model [PREM], ak135) with initial phase arrival times from different distance ranges. The 3D models include one spherical harmonic model and one irregular grid model, representing two important alternative model parameterizations. Our tests include a worldwide distribution of reference events from a previous study by Smith and Ekstrom (1996) and the ground-truth database of the prototype International Data Centre. For each test we use phase arrival times associated with the events in the ISC Bulletin and recompute hypocenters with the same algorithm used in the ISC operations. With teleseismic arrival times alone (Δ ≥ 25°), the overall accuracy of our recomputed hypocenters is similar using either of the 3D models. The teleseismic hypocenters from 3D models are, on an average, closer to the reference locations than the hypocenters computed using any of the 1D models; but the improvement in results using ak135 is small. When we use the irregular grid model with teleseismic, regional, and local initial arrival times together and fix to reference depths, the accuracy of the epicenters is improved over the results with teleseismic times alone. With a 3D model we are able to invert for depth for a larger fraction of earthquakes, and our newly computed depths are more tightly clustered in a subduction zone than depths from the ISC Bulletin, but not as tightly as depths computed by Engdahl et al. (1998) who used depth phases as well as direct phases. In a test of all the locatable events from 1 month of the ISC Bulletin the irregular grid model improved residuals and formal location uncertainties without introducing large systematic changes in the hypocenters.