International Seismological Centre Data Research Articles

Seismic Characteristics and Seismic Hazard Assessment: Source Region of the 2015 Nepal Earthquake Mw 7.8 in Central Himalaya

Seismicity and seismic characteristics like b value, fractal dimension, energy release and recurrence period are assessed for the region (80°E–89°E and 26°N–31°N), which includes the 2015 large Nepal earthquake Mw 7.8 in central Himalaya. The International Seismological Centre (ISC) catalogue of earthquakes with mb ≥ 3.8 for the period August 1964 – January 2013 are used. The b values are estimated using the maximum likelihood and least square methods. The fractal dimension is estimated using the correlation integral method. The events are also used for estimating radiated energy in the region. The probability of occurrence of moderate earthquakes (mb ≥ 5.0 using ISC data and Mw ≥ 6.0 using USGS data for the period 1964–2015) during a specified interval of time is estimated on the basis of three probabilistic models, namely, Weibull, gamma and lognormal. The model parameters are estimated by the maximum likelihood estimates. It is found that there is a 99% probability for occurrence of at least one earthquake mb ≥ 5.0 in this region in a time window of 2–3 years from the year 2013, and in 20–30 years for an earthquake Mw ≥ 6.0 from the year 2015. The b value maps identified variable stressed zones, and the fractal dimension maps fractal characteristics of the active fault zones. The energy release map identified the zones of higher and lower energy release. The percent probability maps, prepared using the Poisson distribution, identify the zones of higher- and lower-probability earthquakes and thus indicate the areas of future probable earthquakes.

ISC-EHB: reconstruction of a robust earthquake data set

The Engdahl-van der Hilst-Buland (EHB) Bulletin of hypocentres and associated traveltime residuals was originally developed with procedures described by (Engdahl et al. 1998) and ended in 2008. It is a widely used seismological data set, which is now expanded and reconstructed, partly by exploiting updated procedures at the International Seismological Centre (ISC), to produce the ISC-EHB. The reconstruction begins in the modern period (2000–2013) to which new and more rigorous procedures for event selection, data preparation, processing and relocation are applied. The selection criteria minimize the location bias produced by unmodelled 3-D Earth structure, resulting in events that are relatively well-located in any given region. Depths of the selected events are significantly improved by a more comprehensive review of near station and secondary phase traveltime residuals based on ISC data, especially for the depth phases pP, pwP and sP, as well as by a rigorous review of the event depths in subduction zone cross-sections. The resulting cross-sections and associated maps are shown to provide details of seismicity in subduction zones in much greater detail than previously achievable. The new ISC-EHB data set will be especially useful for global seismicity studies and high-frequency regional and global tomographic inversions.

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.

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.

A new absolute arrival time data set for Europe

SUMMARY The main aim of this study is to create a data set of accurate absolute arrival times for stations in Europe which do not report to the International Seismological Centre (ISC). Waveforms were obtained from data centres and temporary experiments and a semi-automatic picking method was applied to determine absolute arrival times for P and S phases. 85 000 arrival times were picked whose distribution of residuals shows generally low standard deviations on the order of 0.5‐0.7 s. Furthermore, mean teleseismic station residuals reflect the properties of the underlying crust and uppermost mantle. Comparison to ISC data for matching event-stationphase combinations also confirms the good quality of the new absolute arrival time picks. Most importantly, this data set complements the ISC data as it fills regional data coverage gaps in Europe.

A new earthquake catalogue for Bulgaria and the conterminous Balkan high hazard region

Abstract. A new homogenous earthquake catalogue covering Bulgaria and the surrounding Balkan area has been created with intention of performing a consistent seismic hazard assessment across the region. In keeping with modern requirements of cataloguing seismicity, this catalogue has been made homogenous as far as possible with regards to magnitude, which has been provided on any of four different reported scales for each event; mb, Ms, Mw and ML. A key historical catalogue for the region has been used to represent the early instrumental period of earthquake recording (1900 to 1963), whilst data have been obtained from the International Seismological Centre (ISC), National Earthquake Information Center (NEIC) and National Observatory of Athens (NOA) to cover the instrumental period of earthquake recording (1964 to 2004). ISC data have also been used to develop a new mb→Ms magnitude conversion equation for the catalogued region. Application of this new magnitude conversion relation, in combination with other selected magnitude scale correlations, ensures reported magnitudes can be systematically rendered onto homogenized Ms and Mw scales for all earthquakes. This catalogue contains 3681 events with homogenized magnitudes ≥4.0 Mw, for the time interval 1900 to 2004 (inclusive), located in the region bounded by 39°–45° N, 19°–29° E, at focal depths of 0.0 km to 401.0 km and in a magnitude range 4.0≤Mw≤7.2. Selected large magnitude (M≥6.0 Ms) earthquakes have had their reported magnitudes reassessed – and adjusted if necessary – in light of work by other authors. Applied statistical approaches aimed at determining the lower threshold to magnitude completeness suggest this catalogue is complete down to a homogenized surface-wave magnitude of 4.6 Ms.

Seismological Center heads in a new direction

The International Seismological Centre (ISC), funded mostly by contributions from 55 national institutes in 47 countries, has produced the most comprehensive global seismicity bulletin for more than 30 years. Seismologists use ISC data to investigate the Earth's internal structure, analyze seismic hazards, and evaluate tectonic models. They depend on a consistently high level of qualityin the bulletin from year to year but, due to the growing volume of data, change may be coming to the ISC.Thanks to an increasing number of regional and national networks around the world/the ISC bulletin has grown to cover more than 60,000 events annually. But even as the bulletin tripled in size over the last 20 years, budgetary restrictions have prevented growth of the ISC staff or significant development of its computer resources. With publication of the bulletin beginning to experience delays, the strain is starting to show.

Comparison of P and S station corrections and their relationship to upper mantle structure

We use International Seismological Centre data between the years 1964 and 1994 to derive a data set of P and S station corrections. By restricting the raw P and S data to those which share the same source to receiver paths, we minimize any bias to the station corrections from differential P and S data distributions. We find that both static and azimuth dependent terms are highly correlated. In addition, we perform synthetic tests to determine the sensitivity of our data to mantle structure and show that static terms are sensitive to structure, at least throughout the upper mantle. In determining the regression slope of the static terms, we allow for regional offsets and find a best‐fit global slope of S to P correction a=2.85±0.19. This is lower than that obtained in previous station correction studies but is in agreement with results in the upper mantle using differential travel times and with values derived for the upper parts of the lower mantle. Regionalizing the data by tectonics does not give statistically distinct ratios of S to P station correction. The result for shield areas, however, is significantly smaller than previously believed, probably because of our ability to derive station corrections from similar data sets. Variations in the second azimuthal terms are similar in both fast direction and relative magnitude. In another synthetic experiment we find that their general trends can be predicted well by isotropic models of heterogeneous velocity structure. From this result and also because they do not correlate with SKS splitting data, we conclude that station correction second azimuthal terms should not be interpreted in terms of anisotropy.

Regionalized temperature variations in the upper 400 km of the Earth's mantle

Tectonically regionalized variations in the temperature of the upper 400 km of the Earth's mantle are estimated from analysis of global seismic travel-time data cataloged by the International Seismological Centre (ISC). Seismic parameter profiles are determined from estimates of P and S velocities obtained by tau inversion. Summary phase diagrams for the olivine and pyroxene-garnet subsystems are constructed in conjunction with a thermodynamic potential formulation that allows self-consistent determination of density, bulk modulus and adiabats throughout the pressure and temperature regimes of the mantle. Perturbations in estimated seismic parameters are expressed in terms of variations in temperature using the model temperature derivatives of the bulk modulus and density at a given temperature and pressure. Confidence bounds on the velocity estimates are used to place corresponding bounds on the constructed seismic parameters. A simple differential relationship is solved iteratively to obtain a temperature variation for a given variation in seismic parameter. This approach allows the estimation of a range of seismically determined temperature variations by employing a given compositional model. Results indicate that whereas the P and S velocity variations in the upper mantle are consistent with the tectonic regionalization, variations in V p V s ratios are irregular. This leads to unstable estimates of the seismic parameters and thus estimates of mean temperature anomalies, typically within 600°C of the weighted mean, that are inconsistent with the regionalized seismic data. A comparison of two compositional models is used to show the trade-off with estimated temperature variations. A refined regionalization and analysis of a larger ISC data set are suggested to stabilize the S velocity inversion, reduce statistical uncertainties on the seismic parameters, and thus improve constraints on estimated temperature variations.

Constraints on inner core anisotropy from PKP(DF) travel times

PKP(DF) travel times reported by the International Seismological Centre (ISC) are analyzed to study the depth dependence of inner core anisotropy. Plots of PKP(DF) travel time residuals versus range for different subsets of the data clearly show large anomalies at ranges beyond about 149° for those paths with inner core ray angles within 30° of the rotation axis. On average, the times are about 3 to 5 s earlier for these paths than for rays at other angles, consistent with recent waveform measurements of anomalous PKP(DF) times by Creager (1992) and Song and Helmberger (1993), and indicative of inner core anisotropy in which the P velocity is about 3% faster for N‐S paths than E‐W paths. The size of these apparent anomalies in the ISC data are much larger than those inferred from several previous analyses of ISC PKP(DF) data which indicated 0.5 to 2 s anomalies at these ranges. The earlier studies applied binning and averaging schemes to the data using windows that were overly tight, thus excluding some of the most anomalous data points and biasing the results toward smaller anomalies. To avoid this problem, I apply a method for computing summary ray residuals that finds the maxima in smoothed histograms of the residuals and does not rely on a fixed interval to window the data points. Standard errors on each summary ray residual are estimated using a bootstrap technique that randomly resamples the data. Plots of the summary ray residuals for several range intervals reveal a small number of independent paths nearly parallel to the rotation axis for which the arrivals are 3 to 5 s early. Although the data are sparse for paths within 30° of the rotation axis, anisotropy does not appear to diminish with depth within the inner core and may cause a much larger anomaly near 180° than the 2‐s value obtained in several prior studies of ISC data. The uniform anisotropy models of Creager (1992) and Song and Helmberger (1993) fit the bulk of the data reasonably well. However, the observed residuals at ranges between 132° and 140° are smaller than those predicted by most anisotropy models, suggesting that the anisotropy may diminish within the outermost 50 km of the inner core.

Robust inversion of IASP91 travel time residuals for mantle P and S velocity structure, earthquake mislocations, and station corrections

Using both P and S arrival time information, 41,108 events in the International Seismological Centre (ISC) catalog for the years 1964 to 1987 are relocated relative to the IASP91 velocity model. The mean absolute horizontal relocation is 7.7 km and the mean absolute depth relocation is 9.1 km, The mean absolute origin time shift is 1.2 s. The relocation procedure increased the P residual standard deviation slightly from 2.3 s to 2.5 s while decreasing the S residual standard deviation from 6.8 s to 6.1 s. When plotted as bottoming point averages, the resulting IASP91 P and S arrival time residuals show coherent patterns as a function of geographic location. An iterative lp residual norm minimization algorithm is used to estimate the set of P and S velocity variations as well as the earthquake relocation and seismographic station parameters which best explain the travel time residuals. The procedure is robust in that extremely large travel time residuals, which are common in the ISC data, do not unduly influence the velocity estimates. Both the P and S models of lateral heterogeneity contain prominent circum‐Pacific low velocities, 1% to 2% perturbations, underlying the back arc basins between 35 and 200 km depth. This ring of negative deviations continues into the depth interval 200–400 km. The continental cratons are underlain by high‐velocity anomalies with maximum amplitudes of 2%. Iceland and the Azores are underlain by low‐velocity mantle material that extends down to at least 400 km. The Benioff zones are only intermittently imaged as 1–2% high‐velocity regions in the uppermost 400 km. They are best resolved in the P velocity variations. Both the P and S velocity models contain a circum‐Pacific ring, beneath the Benioff zones, of 1–2% positive velocity deviations in the depth range 660–870 km. Coherent high‐velocity features are seen below South America, Borneo, Tonga‐Fiji, the Marianas, and the northern Kuriles. The anomalies beneath South America and Borneo extend into the 870–1070 km depth range. Below depths of 1270 km for P variations and 1070 km for S variations the amplitude of the heterogeneity has decreased significantly. It is only in the lowermost mantle, 2670 km to the core‐mantle boundary, that the level of P heterogeneity rises significantly above the estimated noise level. In this depth range a partial ring around the Pacific basin is observed, although this.

Travel-time tomography of the European-Mediterranean mantle down to 1400 km

The 3-D P-wave velocity structure of the mantle below Europe, the Mediterranean region and a part of Asia Minor is investigated. This study is a considerable extension of an earlier tomographic experiment that was limited to imaging upper-mantle structure only. Here, the Earth's volume under study encompasses the mantle to a depth of 1400 km, and we increase the number of International Seismological Centre (ISC) data for inversion by a factor of four by taking more years of observation, and by including data from teleseismic events. The most important departure from the earlier study is that we do not use the Jeffreys-Bullen model as a reference model, but an improved radially symmetric velocity model, the PM2 model, which is appropriate for the European-Mediterranean mantle. Our inversion procedure consists of two steps. First, the radial model PM2 is determined from the ISC delay times by a nonlinear trial-and-error inversion of the data. As opposed to the Jeffreys-Bullen model, the new reference model has a high-velocity lithosphere, a low-velocity zone, and seismic discontinuities at depths of 400 and 670 km. Next, the ISC data are corrected for effects related to the change in reference model and inverted for 3-D heterogeneity relative to the PM2 model. We follow this two-step approach to attain a better linearizable tomographic problem in which ray paths computed in the PM2 model provide a better approximation of the actual ray paths than those computed from the Jeffreys-Bullen model. Hence, the two-step scheme leads to a more credible application of Fermat's Principle in linearizing the tomographic equations. Inversion results for the 3-D heterogeneity are computed for both the uncorrected ISC data and for the PM2 data. The data fit obtained in the two-step approach is slightly better than in the inversion of ISC data (using the Jeffreys-Bullen reference model). A comparison of the tomographic results demonstrates that the PM2 data inversion is to be preferred. To assess the spatial resolution an analysis is given of hit count patterns (sampling of the mantle by ray paths) and results of sensitivity tests with 3-D synthetic velocity models. The spatial resolution obtained varies with position in the mantle and is studied both in map view and in cross-section. In the well-sampled regions of the mantle the spatial resolution for larger-scale structure can (qualitatively) be denoted as reasonable to good, and at least sufficient to allow interpretation of larger-scale anomalies. A comparison is made of the results of this study with independent models of S-velocity heterogeneity obtained in a number of investigations, and with a prediction of the seismic velocity structure of the mantle computed from tectonic reconstructions of the Mediterranean region. In the context of this comparison, interpretations of large-scale positive anomalies found in the Mediterranean upper mantle in terms of subducted lithosphere are given. Specifically addressed are subduction below southern Spain, below the Western Mediterranean and Italy, and below the Aegean. In the last region a slab anomaly is mapped down to depths of 800 km.

Lower bound estimate of average earthquake mislocation from variance of travel-time residuals

We introduce a novel method by which one can estimate absolutely (i.e. independent of forward modelling assumptions, e.g. velocity model) a lower bound on the average contribution of incorrect hypocentral parameters (total mislocation, sum of contribution due to spatial mislocation and origin-time error) to the variance of travel-time residuals. The method involves estimating the variance of fans of rays with a common station, and also the variance of fans of rays with a common event. One estimates the variances for fans of different extent by evaluating the variance in grid cells of different size. The lower bound estimate of the contribution of total mislocation is derived from the difference between the two variances in the limit of small grid size. Applying this method to P-wave travel-time residuals from the Bulletins of the International Seismological Centre (ISC), we demonstrate that the mislocation signal is appreciable, of order 35% of the total signal. We also find that the ISC data have very little intrinsic, spatially uncorrelated noise. The observation that the variance decreases for all grid-sizes, including very small grid sizes, is suggestive of structural signal at all scale lengths. The method has also been investigated in simple synthetic models, in both one and three dimensions. The synthetic studies illustrate how the method can lead to underestimates of total mislocation when the total mislocation is spatially coherent at the smallest separations. They also reinforce the fact that relocation masks structural signal.

PKP(BC) versus PKP(DF) differential travel times and aspherical structure in the Earth's inner core

PKP(BC) versus PKP(DF) differential travel times show coherent patterns of residuals indicating aspherical structure within the Earth's inner core. Using a waveform correlation method, we measure 318 differential PKP(BC) versus PKP(DF) travel times from 5 years of Global Digital Seismograph Network (GDSN) short‐period data at ranges between 145° and 155°. In addition, we obtain 19,470 probable BC versus DF travel times at these ranges by searching through 23 years of the International Seismological Centre (ISC) catalog. Plots of residuals versus turning point location and residuals versus ray direction show clear patterns which indicate that the differential travel times are highly correlated along similar ray paths. This pattern is robust with respect to range and source depth. These anomalies are almost certainly due to aspherical inner‐core structure since PKP(BC) and PKP(DF) have nearly coincident ray paths in the mantle and outer core. The GDSN and ISC residual patterns are in general agreement, but the ISC residuals underpredict the GDSN residuals by about a factor of 2. The ISC data have better spatial coverage, particularly for N‐S ray paths within the inner core. These N‐S ray paths show positive residuals of about 0.5 s, indicating faster inner‐core P velocities in this direction, a result consistent with previous studies of absolute PKP(DF) travel times but derived from an independent data set. Due to the sparse ray coverage of our data, either heterogeneity or anisotropy within the inner core with about 1% velocity variations could explain these residual patterns. However, inversions of the ISC data for heterogeneity require many more free parameters to achieve the same variance reduction as a simple two‐parameter anisotropy model.

Single-Link Cluster Analysis As A Method to Evaluate Spatial and Temporal Properties of Earthquake Catalogues

SUMMARY Single-link cluster analysis is a straightforward method to quantitatively measure the degree of clustering or isolation of groups of elements in a set, such as a catalogue of earthquakes. to apply single-link cluster analysis to a set of N earthquakes, individual earthquakes are first linked to their nearest neighbours to form event sub-groups. the process is then repeated and each sub-group linked to its nearest neighbour, recursively, until N-1 links are found to join all the earthquakes. This paper shows how knowledge of these links can be used to divide a group of earthquakes into any number of spatial clusters. With various modifications this analysis can identify earthquake nests, isolated events, aftershock sequences, and zones of seismic quiescence. to illustrate this method we apply it to; (i) a global data set of 2178 earthquakes having mb of 5.8 or greater reported by the International Seismological Centre (ISC) between 1964 and February 1986, and (ii) sets of earthquakes having mb of 4.9 and greater as reported by the ISC, occurring in Central America and in the Aleutians. to facilitate comparison with real data, this study also investigates the distribution of link lengths for synthetic events placed randomly on 1-D (circular line), 2-D (circular area) and 3-D (spherical volume) geometries. If we remove links between nuclear explosions from the ISC data, the resulting link distribution is quite similar to that for synthetic events placed along a 1-D fault. For the shortest links, the similarity improves markedly if the synthetic fault has a finite width, or if relative location errors cause events to be located away from the fault trace, with a standard deviation of about 15 km.