Phase Arrival Times Research Articles

A Seismic Structure Study in the Kaoping Area, Southwestern Taiwan

The difference between S ‐wave and S ‐to‐ P ‐wave conversion ( S P phase) arrival times is enhanced with rectilinear motion detector filtering to describe alluvial‐sediment thickness in the Kaohsiung–Pingtung (Kaoping) plains area. A more complete understanding of the underground structures of the Kaoping area is provided in this paper and explains why the surrounding regions in Taiwan experience more earthquakes than the Kaoping area. Data are based on seismic activity recorded by the portable array for numerical data acquisition (PANDA) for the period from 1995 to 1997. The difference between S ‐wave and S P ‐phase arrival times shows that the sedimentary layer is thicker along the west and southwest coasts. P ‐wave travel‐time residuals, high‐frequency attenuation parameters Kappa, and quality factor Q P , Q S , and coda waves confirm this result. We also determined the orientation of the Chaochou fault using the first motion of P ‐wave arrivals. To the east of the Chaochou fault, stress trends southeast–northwest, while to the west, it trends northeast–southwest. The change in stress trends east and west of Chaochou fault suggests the presence of a highly fluid accretionary wedge in the Kaoping area. The Chaochou fault forms a seismically active tectonic boundary with the uplift of the hanging wall leading to westward tilting of the basement of the Kaoping plains. We demonstrate that these features are the reason there are relatively few earthquakes in the Kaoping area. The presence of a highly fluid accretionary wedge is indicated by a thick alluvial layer in the west and southwest Kaoping coasts; the Peikung High acts as the indenter that may allow seismic energy to escape and reduce the number of earthquakes in the region. Online Material: Figures illustrating calculations of Kappa, Q c , P ‐ and S ‐wave spectra, Q P , and Q S from ground‐motion data.

AIMBAT: A Python/Matplotlib Tool for Measuring Teleseismic Arrival Times

Seismic travel‐time tomography, a common method for studying the Earth’s three‐dimensional seismic velocity variations, relies on robust measurement of seismic phase arrival times. Absolute arrival times are estimated by visually picking the emergence of the seismic phase of interest on seismograms. Subtracting theoretical arrival times (calculated using a known source location and origin time) from these picked absolute times yields absolute delay times. A widely used method whereby careful picking of absolute arrival times is not required is multi‐channel cross‐correlation (MCCC), developed by VanDecar and Crosson (1990). In MCCC, phase arrival times relative to an unknown average time are obtained by finding the maximum of the cross‐correlation function between each possible pair of seismograms. The preprocessing time of marking the phase to be cross‐correlated in MCCC has increased with the tremendous growth in seismic data volume over the past decade. In this paper, we introduce an efficient and robust computer tool, Automated and Interactive Measurement of Body‐wave Arrival Times (AIMBAT), to measure teleseismic body‐wave arrival times for phases for which arrival time predictions exist. The tool is based on MCCC, but an iterative cross‐correlation and stack (ICCS) algorithm replaces the initial phase marking part of the MCCC procedure, which significantly reduces the need for early user labor. The tool is written in Python (http://www.python.org) and utilizes its open‐source packages Numpy (http://numpy.scipy.org) and Scipy (http://scipy.org) for numerical array computation and Matplotlib (Hunter, 2007) for two‐dimensional plotting and graphical user interface (GUI) applications. We also transcribed the Fortran version of MCCC from VanDecar and Crosson (1990) into Python, which was validated by running both versions on the same data. Python is an efficient, high‐level, object‐oriented, and platform‐independent (Linux, Mac, and Windows) …

A new automatic S-onset detection technique: Application in local earthquake data

Algorithms that deal with the automatic [Formula: see text]-onset time identification problem are a topic of ongoing research. Modern dense seismic networks used for earthquake location, seismic tomography investigations, source studies, early warning, etc., demand accurate automatic S-wave picking. Most of the techniques that have been proposed up to now are mainly based on the polarization features of the seismic waves. We propose a new time domain method for the automatic determination of the [Formula: see text]-phase arrival onsets, and present its implementation on local earthquake data. Eigenvalue analysis takes place over small time intervals, and the maximum eigenvalue which is obtained on each step is retained for further processing. In this way, a time series of maximum eigenvalues is formed, which serves as a characteristic function. We obtain a first [Formula: see text]-phase arrival time estimation by applying the kurtosis criterion on the derived characteristic function. Furthermore, a multiwindow approach combined with an energy-based weighting scheme is also applied, to reduce the algorithm’s dependence on the moving window’s length and provide a weighted [Formula: see text]-phase onset. Automatic picks were compared against manual reference picks, resulting in mean residual time of 0.051 s. Moreover, the proposed technique was subjected to a noise robustness test and sustained a good performance. The mean residual time remained lower than 0.1 s, for noise levels between [Formula: see text] up to 8 dB. The proposed method is easy to implement, because it is almost parameter free and demands low computational resources.

Bias Problems in Existing Teleseismic Travel Time Databases: Ignore or Repair?

An increasing number of earth scientists use teleseismic data for investigations of local, regional, and global geoscientific problems. The open access to global data makes this possible also for scientists outside the formal and informal communities of seismologists who produce and quality‐control these global data sets. In order to investigate the crust and upper mantle structure in the region of southern Scandinavia, Europe, we acquired waveform data from temporary stations contributed by several European instrument pools. The mobile station arrays were deployed in different time intervals (Fig. 1). One of the investigation methods was absolute travel time tomography. Here, we discuss some specific problems and their remediation in connection with tomographic inversion of teleseismic travel times when data from both permanent and moving temporary arrays are combined. Figure 1. Study area of southern Scandinavia with permanent and temporary seismological stations analyzed for tomographic velocity modeling (Medhus et al. , 2012). The non‐permanent arrays, marked with closed‐symbol markers, were deployed in different projects for 1–2 years covering about 10 years all together. The bold dashed outline shows the first case area of data for Figures 2 and 4. Stations studied in the second case are indicated by name boxes. Teleseismic travel times, defined as the difference between phase arrival time at the seismometer and the hypocenter time, are prone to errors of several kinds. These errors contaminate velocity modelling, such as tomographic inversion. Uncorrelated errors tend to average out, and therefore it has been very popular to take in massive data sets like the International Seismological Centre (ISC) (2001), Bulletin data (e.g., Widiyantoro et al. , 1997, Bijward et al. , 1998, Weidle et al. , 2005). However, bias errors do not average out. We discuss two sources of bias error in teleseismic …

Surface wave eikonal tomography in heterogeneous media using exploration data

SUMMARY Surface wave tomography often involves the construction of phase (or group) velocity maps through linearized inversion of measured phase (group) arrival times. Such inversions require a priori information about the medium (that is, a reference model) to calculate source–receiver paths as well as regularization. Surface wave eikonal tomography bypasses these limitations and has the advantage of being simple to implement and use, with virtually no input parameters. It relies on the accurate measurement of phase arrival time, which can be challenging for dispersive waves and complex waveforms. We present a measurement method based on the evaluation of phase arrival time differences at nearby receivers. We show, using an exploration data set, that the produced Rayleigh wave velocity maps are in agreement with results from traditional tomography. The latter reveals less detail, however, because of the regularization needed to accommodate for the heterogeneity of the study area and noise in the data. Eikonal tomography requires averaging over results from multiple sources to produce a proper image; for the scattering environment considered we estimate that a source spacing of 200 m is sufficient. Finally, we show that combining seismic interferometry and eikonal tomography is effective when the source coverage is inadequate.

S-Wave Identification by Polarization Filtering and Waveform Coherence Analyses

Abstract High‐resolution imaging with microseismic events requires the use of large and consistent data sets of seismic phase arrival times. In particular the S phase is important to derive physical parameters of the subsurface. Typically this phase is identified on one of the horizontal seismogram components by a change of signal amplitude and frequency as compared to the previous P phase. However, reliable S ‐phase identification can be difficult for local events because of a signal overlap with the P coda, the presence of converted phases, and possible S ‐wave splitting due to anisotropy. In this study we propose a new data processing technique aiming at uniquely identifying the S ‐phase arrival using all available records from a seismic network. The technique combines polarization analysis of single three‐component recordings of an event with analysis of lateral waveform coherence across the network. This makes it possible to construct seismic sections in which the first arrival is the S phase. This graphical representation can support an operator in both the analysis of single events and in semiautomatic analyses of large datasets. In addition, an automated stacking velocity analysis provides S ‐wave velocities from these sections. We demonstrate the applicability of this technique using synthetic seismograms, and we evaluate the efficacy on a dataset of three‐component velocimeter records from local earthquakes of the Campania–Lucania Apennines (southern Italy) recorded by the Irpinia Seismic Network (ISNet).

BENCHMARK TESTS FOR STRONG GROUND MOTION PREDICTION METHODS: CASE FOR NUMERICAL METHODS (PART 2)

We performed a benchmark test for strong motion simulation methods using numerical methods (finite difference method and finite element method). We considered a four-layered model, a symmetric basin model and a slant basement basin model. All the results calculated by six teams from different institutions generally show good agreement to each other. We found minor differences of phase arrival time between institutions and minute differences between FDM and FEM for some models.

Empirically Based Ground Truth Criteria for Seismic Events Recorded at Local Distances on Regional Networks with Application to Southern Africa

Wepresentanewapproachtoobtainingempiricallybased(EB)criteriafor estimating the epicentral location accuracy (i.e., ground truth, GT) of seismic events recorded at local distances on a regional network. The approach has been developed usinga jackknife resampling method appliedto carefully pickedPgphasearrivaltimes for GT reference events from several South African gold mines. The events were well recorded locally by Southern African Seismic Experiment (SASE) stations within the Archean Kaapvaal craton, an area of relatively simple crustal structure. The region- specific criteria obtained specify an EBGT395% level of epicentral accuracy if events are recorded on eight or more stations at distances less than the Pg=Pn crossover (215 km) when the stations have a primary azimuthal gap <202 degrees. In addition, when nine or more stations are used for event location and one of them is within 79 km of the event, we find that a focal depth accuracy of 4 km at the 95% confidence level can be obtained and that an accuracy of 6 km can be obtained if eight stations are used. This result illustrates that GT criteria commonly applied to global event catalogs can be relaxed if an accurate velocity model and carefully picked phase-arrival times are used for event locations. Consequently, it is likely that additional events can be added to GT compilations by developing EBGT criteria for other regional networks and using them to identify candidate GT events. For example, the EBGT criteria developed in this study, when applied to the SASE seismicity catalog, yields 10 new GT events.

Automated determination ofP-phase arrival times at regional and local distances using higher order statistics

P>We present an algorithm for automatic P-phase arrival time determination for local and regional seismic events based on higher order statistics (HOS). Using skewness or kurtosis a characteristic function is determined to which a new iterative picking algorithm is applied. For P-phase identification we apply the Akaike Information Criterion to the characteristic function, while for a precise determination of the P-phase arrival time a pragmatic picking algorithm is applied to a recalculated characteristic function. In addition, an automatic quality estimate is obtained, based on the slope and the signal-to-noise ratio, both calculated from the characteristic function. To get rid of erroneous picks, a Jackknife procedure and an envelope function analysis is used. The algorithm is applied to a large data set with very heterogeneous qualities of P-onsets acquired by a temporary, regional seismic network of the EGELADOS-project in the southern Aegean. The reliability and robustness of the proposed algorithm is tested by comparing more than 3000 manually derived P readings, serving as reference picks, with the corresponding automatically estimated P-wave arrival times. We find an average deviation from the reference picks of 0.26 +/- 0.64 s when using kurtosis and 0.38 +/- 0.75 s when using skewness. If automatically as excellent classified picks are considered only, the average difference from the reference picks is 0.07 +/- 0.31 s and 0.07 +/- 0.41 s, respectively. However, substantially more P-arrival times are determined when using kurtosis, indicating that the characteristic function derived from kurtosis estimation is to be preferred. Since the characteristic function is calculated recursively, the algorithm is very fast and hence suited for earthquake early warning purposes. Furthermore, a comparative study with automatically derived P-readings using Allen's and Baer & Kradolfer's picking algorithms applied to the same data set demonstrates better quantitative and qualitative performance of the HOS approach. This study shows, that precise automatic P-onset determination is feasible, even when using data sets with very heterogeneous signal-to-noise ratio.

Validation of Teleseismic Inversion of the 2004 Mw 6.3 Les Saintes, Lesser Antilles, Earthquake by 3D Finite-Difference Forward Modeling

We study the 21 November 2004 Mw 6.3 Les Saintes earthquake that occurred south of the island of Guadeloupe at a shallow depth, damaging some buildings on the island. The objective of this work is to assess the potential of a teleseismic source study to reproduce local ground motion. The velocity model of this area is not currently well known, and the near-field seismograms are affected by paths and site effects. We first analyze this earthquake as a point source and then as an extended fault using teleseismic broadband waveform inversion approach from the P and SH waveforms at stations well distributed in azimuth. We obtain two stable and reliable solutions of the spatiotemporal history of the earthquake rupture on each nodal plane. The models show a rupture duration of 6 sec and a maximum slip of about 1 m but have different locations of the main asperity. We then carry out the forward modeling of the seismic wave propagation at the regional scale, coupled with the obtained kinematic source models taking into account the topography, the bathymetry, and the sea layer. We investigate if one of the fault models could produce synthetics that match the local observed ground motions. The simulation results provide a better understanding of the uncertainty of source, path, and site effects even if our knowledge of the underground geological structure is limited to the 1D stratified model. It is inferred that the 1D model does not seem to be appropriate in the northern direction (middle of Guadeloupe Island) due to the unknown basin structure, while it is relatively sufficient in the eastern and the northwestern directions from the source. By careful comparison of the synthetic and observed displacement seismograms in terms of main phase forms and arrival times at low frequency, especially at the closest station GBGA, the fault model with northeast faulting is estimated to be the more relevant to the measured local ground motions.

Near-Real-Time Double-Difference Event Location Using Long-Term Seismic Archives, with Application to Northern California

We present a real-time procedure that uses cross-correlation and double- difference methods to rapidly relocate new seismic events with high precision relative to past events with accurately known locations. Waveforms of new events are auto- matically cross correlated with those archived for nearby past events to measure accu- rate differential phase arrival times. These data, together with delay times computed from arrival time picks, are subsequently inverted for the vector connecting the new event to its neighboring events using the double-difference algorithm. The new seis- mic monitoring technique is applied to earthquakes recorded in northern California, using near-real-time data feeds from the Northern California Seismic Network (NCSN) and the Northern California Earthquake Data Center, and a locally stored copy of the NCSN seismic archive. New events are automatically relocated in near-real time (tens of seconds) relative to a high-resolution double-difference earthquake catalog for northern California. Back testing using past events across northern California indi- cates that the real-time solutions are on average within 0.08 km laterally and 0.24 km vertically of the double-difference catalog locations. We show that the precision with which new events are located using this technique will improve with time, helped by the continued increase in density of recorded earthquakes and growth of the digital seismic archives. Real-time double-difference location allows for monitoring spatio- temporal changes in seismogenic properties of active faults with unprecedented res- olution and therefore has considerable social and economic impact in the immediate evaluation and mitigation of seismic hazards.

Seismic hybrid swarm precursory to a major lava dome collapse: 9–12 July 2003, Soufriere Hills Volcano, Montserrat

A swarm of ≈ 9500 hybrid earthquakes preceded the 12–13 July 2003 dome collapse at Soufriere Hills Volcano, Montserrat. Most events had nearly identical waveforms and cross-correlation was applied to measure inter-event periods as well as phase arrival times to determine accurate relative location. Hypocenter depths were shallow (< 3 km), and relative locations were confined to a radius of < 150 m. This small source volume is consistent with the observed waveform similarity. Changes in inter-event periods and energy release, measured from the seismic records, showed that the volcano evolved through several energetic states, possibly linked to cyclic magma movement. Shorter inter-event periods were linked to higher energy release rates and possibly reflect increased pressurization during periods of low extrusion rates.

Upper Mantle Heterogeneity beneath North America from Travel Time Tomography with Global and USArray Transportable Array Data

The large volume of broadband waveforms that are being acquired by USArray (http://www.iris.edu/USArray/), the seismology component of the national Earth science program EarthScope (http://www.earthscope.org/), offer unique opportunities for seismic imaging. Constraining structures on a range of length scales and understanding their physical and chemical causes is a prerequisite for understanding the relationship between near surface and deeper mantle processes. One can expect that, eventually, full wave tomography ( e.g. , De Hoop and van der Hilst 2005; De Hoop et al. 2006; Tromp et al. 2005; Zhao and Jordan 2006) with broadband USArray waveforms will produce superior insight into the structure of the mantle beneath North America, but linearized tomographic inversion of phase arrival time data readily yields exciting results in regions where data from dense seismograph networks is available. We will use travel times from the transportable component of USArray, hereinafter referred to as USArrayTA, and from other sources, such as the EHB data base (Engdahl et al. 1998), to constrain 3-D mantle heterogeneity beneath North America. Tomographic images based on USArrayTA data will help us understand first-order geological structure of and processes in the mantle beneath North America. Examples include, but are not restricted to: 1) the transition from the stable continental lithosphere at the center of the North American continent to the tectonically active domains farther west, 2) the Cascadian subduction system, 3) the Yellowstone hotspot, and 4) the relationship between current and past episodes of subduction and upper mantle upwellings and processes deeper in the mantle. Before the advent of USArrayTA, insight into mantle structure beneath the western United States was obtained either from pieced-together regional P -wave studies ( e.g. , figure 1A, after Dueker et al. 2001) or from global travel time or surface wave tomography (figures 1B, C, and D, …

Three‐dimensional S velocity of the mantle in the Africa‐Eurasia plate boundary region from phase arrival times and regional waveforms

A new model of the three‐dimensional shear velocity structure of the Africa‐Eurasia plate boundary is presented. The new model is derived by jointly inverting different types of seismic data. The two main sources of information are regional waveforms and teleseismic S wave arrival times. We show that it is possible to find a model that fit the different data types nearly as well as when inverting solely one type of data. The main improvement in resolving power is achieved between depths of 300 and 700 km, though the improvements are not limited to this depth range. Our model reflects the complicated evolution of this plate boundary area. The transition zone is dominated by high‐velocity anomalies which we infer to represent a mix of lithosphere that subducted relatively recently or is not sufficiently cold and dense to traverse the 660‐km discontinuity. The only low‐velocity zone in the transition zone is beneath the Ionian Sea. The high‐velocity Hellenic slab is continuous throughout the upper mantle and into the lower mantle to about 1200 km, most likely representing subducted Neo‐Tethys lithosphere. The uppermost mantle is dominated by low velocities, consistent with the high level of tectonic activity. Low‐velocity regions are relatively strong beneath the Mid‐Atlantic Ridge, Turkey, and the Dead Sea region. The region's current lithosphere is relatively thin, except beneath the Adriatic and Ionian seas and the easternmost Atlantic Ocean.

Systematic Relocation of Early Instrumental Seismicity: Earthquakes in the International Seismological Summary for 1960 1963

Abstract We have relocated 2556 events that were reported in the bulletins of the International Seismological Summary (ISS) during the period 1960–1963 using the phase arrival time data listed in the bulletins. Parts of the data were already available in digital form, and the rest we obtained by scanning the printed bulletins and applying an optical character recognition procedure. Earthquakes were then relocated with a teleseismic location method that uses an improved 1D global travel-time Earth model, depth phases, and corrections for ellipticity, bounce-point bathymetry/topography, and station elevation. The aspherical velocity structure below stations was partially taken into account through the use of teleseismic station (patch) corrections. The quality of the newly obtained locations and the improvements in data fit are similar to relocations of earthquakes in the early bulletins of the International Seismological Centre (ISC). The most significant improvement of the new locations is in focal depth, and a significant number of blatant mislocations (such as earthquakes deeper than 100-km below midocean ridges) have also been corrected. The relocations presented here extend by 10% the time period for which homogeneous and reliable hypocenters are available on a global basis. This is the first of a series of studies addressed towards relocating, using the same methodology, all the instrumentally recorded earthquakes listed in the bulletins of the ISS (1918–1963).

Temporal and spatial properties of some deep moonquake clusters

Using the event search method of Bulow et al. (2005), we have found 503 new deep moonquakes among the eight largest (in terms of total number) nearside source regions, increasing the number of identified events for each cluster an average of 36% over the existing catalog. These new events provide an improved deep event catalog, with which we explore some temporal and spatial aspects of deep moonquakes. First, we examine the spectra of moonquake occurrence times at each deep source region, and observe known tidal periodicities, notably those at ∼27 days and 206 days. Application of spectral methods for the analyses of point processes (discrete events) allows us to resolve closely spaced tidal periods not previously seen in moonquake data. Second, we pick seismic phase arrival times from optimized stacks of events from each source region. We use these picks, along with published velocity models, to relocate the nine source regions. Source regions A1 and A18 are the best located, with 95% confidence bounds of less than ±5° in latitude and longitude, and consistent with estimates from different studies. The locations of source regions A8 and A9 are poorly constrained, with uncertainties in latitude of up to ±28° resulting from the absence of clear phase arrivals at station 15. Large trade‐offs exist between relocation estimates and choice of velocity model, and the lack of reliable seismic phase arrivals severely affects location error.

Vibration source probe system

A vibration source probe system for detecting the surface waves of vibration coming over the ground or the floor of a building and specifying and displaying a vibration source at a factory or on a road. The position of a vibration source is estimated from a phase difference or arrival time difference between vibrations of surface waves detected by the vibration sensors which are separated from each other by a predetermined distance, an image near the estimated position of the vibration source is picked up by a camera, and the estimated position of the vibration source is displayed on the image displayed on the display screen of a personal computer.

Using beamforming for the global network location

The aim of this study is the design and trial of a novel sparse network beamforming (NB) technique to improve earthquake location. For this purpose bulletin phase arrival time data were processed via the use of complex exponents form and then used in an optimization via grid-search to search for the maximum semblance in hypocenter space. The use of the robust semblance statistic, provides reliable location results. The NB location results for a set of test events were compared to standard iterative ISC location procedure “iscloc” and its prototype the Jeffreys maximum likelihood estimator. For this purpose a data-base of 139 reference ground-truth events was extracted from the catalog of the International Seismological Centre (ISC) (97 GT5 events, assumed earthquakes, and 42 GT0 nuclear tests). The tuned NB procedure has shown excellent location results for events with a “good” ISC location and demonstrated the large epicenter deviations have been decreased in “bad” cases. Further developments of the algorithm would include allowances for 3D earth structure and a priori site-specific information.

Application of an improved algorithm to high precision relocation of ISC test events

New location features for possible implementation by the International Seismological Centre in its standard location procedures are tested using a set of 156 well-located and geographically well-distributed earthquakes and explosions. The tests are performed using the Engdahl et al. ([Engdahl, E.R., Van der Hilst, R.D., Buland, R.P., 1998. Global teleseismic earthquake relocation with improved travel times and procedures for depth determination. Bull. Seism. Soc. Am. 88, 3295–3314]; EHB) location algorithm with the 1-D reference Earth model ak135 [Kennett, B.L.N., Engdahl, E.R., Buland, R., 1995. Constraints on seismic velocities in the Earth from travel times, Geophys. J. Int. 122, 108–124]. Weighting by phase variance as a function of distance improves location accuracy by 7%. Use of later phase arrival times does not result in a significant improvement in location or depth for events with observing stations well distributed in azimuth. However, with application of an improved phase identification technique, depth phases provide significantly better estimates of focal depth.

A Digital Hypocenter Catalog for the International Seismological Summary

Research Article| September 01, 2005 A Digital Hypocenter Catalog for the International Seismological Summary Antonio Villaseñor; Antonio Villaseñor Institute of Earth Sciences “Jaume Almera” CSIC Lluís Solé i Sabarís s/n 08028 Barcelona, Spain antonio@ija.csic.es (A.V.) Search for other works by this author on: GSW Google Scholar E. Robert Engdahl E. Robert Engdahl University of Colorado Department of Physics Campus Box 390 UCB Boulder, CO 80309-0390, USA engdahl@colorado.edu (E.R.E.) Search for other works by this author on: GSW Google Scholar Seismological Research Letters (2005) 76 (5): 554–559. https://doi.org/10.1785/gssrl.76.5.554 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Antonio Villaseñor, E. Robert Engdahl; A Digital Hypocenter Catalog for the International Seismological Summary. Seismological Research Letters 2005;; 76 (5): 554–559. doi: https://doi.org/10.1785/gssrl.76.5.554 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 A reliable catalog of earthquake locations and magnitudes spanning long periods of time is a fundamental requirement for a wide variety of seismological studies, including seismotectonics, Earth structure, and seismic hazard. On a global scale, the most comprehensive source of earthquake locations, phase data, and magnitudes is the Bulletin of the International Seismological Centre (ISC). Locations, magnitudes, the data used to compute them (i.e., phase arrival times and amplitude/period readings), and other data are collected and processed at ISC and made readily accessible in digital form (Willemann and Storchak, 2001). ISC, which published its first bulletin... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.