Determination Of Epicenters Research Articles

Båth's law and the self‐similarity of earthquakes

We revisit the issue of the so‐called Båth's law concerning the difference D1 between the magnitude of the main shock and the second largest shock in the same sequence. A mathematical formulation of the problem is developed with the only assumption being that all the events belong to the same self‐similar set of earthquakes following the Gutenberg–Richter magnitude distribution. This model shows a substantial dependence of D1 on the magnitude thresholds chosen for the main shocks and the aftershocks and in this way partly explains the large D1 values reported in the past. Analysis of the New Zealand and Preliminary Determination of Epicenters (PDE) catalogs of shallow earthquakes demonstrates a rough agreement between the average D1 values predicted by the theoretical model and those observed. Limiting our attention to the average D1 values, Båth's law does not seem to strongly contradict the Gutenberg–Richter law. Nevertheless, a detailed analysis of the D1 distribution shows that the Gutenberg–Richter hypothesis with a constant b‐value does not fully explain the experimental observations. The theoretical distribution has a larger proportion of low D1 values and a smaller proportion of high D1 values than the experimental observations. Thus, Båth's law and the Gutenberg–Richter law cannot be completely reconciled, although based on this analysis the mismatch is not as great as has sometimes been supposed.

An Assessment of Seismic Body-wave Magnitudes Published by the Prototype International Data Centre

A significant initiative in the reporting of global seismicity began in January 1995 with the publication of daily “Reviewed Event Bulletins” (REB's), a few days in arrears. This work was and still is done in the context of international efforts aimed at verification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). When it was finalized in 1996 the CTBT specified that the seismic monitoring network of the International Monitoring System (IMS) would consist of 50 primary stations contributing data continuously to an International Data Centre (IDC) in Vienna, plus 120 auxiliary stations that record continuously but contribute data to IDC only on request (Richards, 2002). Until IDC began daily operations on 21 February 2000, the daily REB was published by the Prototype International Data Centre (PIDC) and a predecessor organization. We shall refer to all the REB's from 1 January 1995 to 20 February 2000 as having been published by PIDC. This paper focuses on the PIDC teleseismic body-wave magnitudes ( mb ). They are important as a modern effort to measure short-period magnitudes based on what is expected to become a fixed network of global stations, all operating in a standardized way. Although seismic magnitudes are intended to be a characteristic of the source, it is well known that in practice they are influenced by numerous additional factors such as the particular set of stations reporting individual magnitudes, the instrument responses, and the details of how each station magnitude is assigned. Global bulletins of seismicity, including mb values, have long been reported, months in arrears, by the Preliminary Determination of Epicenters (PDE) and Earthquake Data Report (EDR) of the U.S. Geological Survey's National Earthquake Information Center (USGS NEIC). They have also been reported, about two years in arrears, by the International Seismological Centre (ISC), based in England. To obtain its …

Seismotectonics of the Explorer region

The Explorer region offshore western Canada is a tectonically complex area surrounded by the Pacific, North America, and Juan de Fuca plates. Existing tectonic models for the region differ fundamentally. Proposed plate configurations range from multiple independent plate fragments to an Explorer plate now fused to North America along the continental margin and cut by Pacific–North America transform faults in the west. We present new seismological data constraining the region's current tectonics. We use three‐component regional waveforms to determine the source parameters of 84 earthquakes with magnitude greater than 4. Combined with 34 Harvard centroid moment tensor solutions, they represent the region's largest earthquake source parameter data set obtained by robust waveform modeling techniques. In addition, we perform joint epicenter determination to relocate larger earthquakes recorded since 1918. The source parameters and improved locations provide a consistent tectonic picture. Earthquake slip vector azimuths along the Pacific plate boundary change smoothly and are significantly less northerly oriented than the Pacific‐North America plate motion direction, requiring an independent Explorer plate. The present‐day Pacific‐Explorer boundary is formed by transform faults subparallel to the Revere‐Dellwood‐Wilson fault. Plate motion vectors indicate that the Winona block is part of the Explorer plate. Current Explorer motion is more northerly than indicated by magnetic anomalies prior to 2 Ma, implying a recent change, possibly coinciding with a northwestward ridge jump near Explorer plate's northern end transferring the Winona block from the Pacific to the Explorer plate. In response to these plate motion changes the region north of the western Sovanco fracture zone was assimilated into the Pacific plate. The region around the eastern Sovanco fracture zone, characterized by broadly distributed seismicity, is composed of well‐defined sets of conjugate faults bounding rotating crustal blocks. Earthquake fault strikes agree with the dominant northwest‐southeast fault sets; however, the conjugate sets must be also active to fully accommodate present‐day Explorer plate motion. The SW portion of the strike‐slip Nootka fault zone, the Explorer‐Juan de Fuca plate boundary, is well defined by focused seismicity; however, its full extent under Nootka Island remains unresolved. The Explorer–North America boundary shows sporadic low‐magnitude seismicity. Our Explorer–North America rotation pole predicts convergence varying from negligible at the boundary's northwest end to ∼2 cm/yr at the SE end. This convergence can be accommodated either by subduction or by crustal thickening extending to the North American continent. We favor subduction based on low deformation rates observed by onshore GPS sites. The present Explorer plate system configuration is a result of stepwise reorientation of the Explorer ridge system, each step successively reducing the subduction rate relative to North America.

Depths of Eastern Canadian Earthquakes from Regional Data

While it is a relatively well understood problem to determine hypocentral depths from teleseismic data or from dense local networks, many eastern Canadian earthquakes are too small to be recorded teleseismically and occur in regions where the network density is adequate for reliable epicentral determination but insufficient for hypocentral determination. Using stacked, filtered (2–6 Hz bandpass) regional data (recorded at 250 to 500 km), we have developed a method for rapid depth estimation of small and moderate earthquakes. We pick the largest amplitude arrival in the Pn wavetrain, which we call P max, and measure its arrival time relative to the onset of Pn. Because P max appears to be related to the same phase, sPn , we can use it reliably to determine depths. The method was calibrated using earthquakes in the Charlevoix, Quebec, seismic zone, where the network density is sufficient for depth determination as part of the location procedure. The standard error is 2.3 km, assuming that the Charlevoix network depths are correct. In only one case (out of twelve) did the network and Pn-Pmax depths differ by more than 5 km, implying that the assumption that Pmax is usually related to the same phase is valid. The Pn-Pmax method was then applied to earthquakes in the western Quebec and Lower St. Lawrence seismic zones. Results show that earthquakes in western Quebec can occur over a wide range of crustal depths but, on average, are shallower than the 18-km depth currently used as a default. Earthquakes in the lower St. Lawrence appear to be deeper than the default, but this conclusion is based on only two earthquakes.

Synthesis of magnitude and focal mechanism computations for the M ≥ 4.5 earthquakes in France for the period 1995–2000

This paper proposes a synthesis of thestudies made in terms of source parametersevaluation for the last earthquakes oflocal magnitude greater than 4.5 whichoccurred in or nearby France during thelast five years. Focal mechanisms andseismic moments have been computed for thethree most important events, largely feltby the population: St Paul de Fenouillet(February 18th 1996, ML 5.6),Annecy-Epagny (July 15th 1996, ML5.2) and St-Beat (October 4th1999, ML 4.8). These focal mechanismshave been obtained either by regionalmoment tensor inversion or from firstmotion polarities and are compared withcomplementary studies made on theseearthquakes. In addition, for the otherearthquakes of local magnitude greater than4.5 which occurred nearby French borderssince the beginning of the recording ofbroadband data by the ReNaSS(Reseau National de SurveillanceSismique, French Seismological Survey) inmid–1995, several magnitude calculationsconcerning the following earthquakes arepresented: Pamplona (25/2/1996, ML4.7), Aoste (31/03/1996, ML 4.6),Imperia (24/2/1997, ML 4.5),Barcelonette (31/10/1997, ML 4.8),Pamplona (27/10/1998, ML 4.9), andBonifacio (26/4/2000, ML 4.5). Localmagnitudes are usually higher thanthe Mb magnitudes reported by the PDE(Preliminary Determination of Epicenters),while the extension of the Msz scale toregional magnitudes and the Mw magnitudesderived from seismic moments give smallervalues. The relative importance of thevarious earthquakes in terms of surfacewave magnitude or seismic moment does notalways agree with that implied by localmagnitudes.

How to Locate Epicenters

Earthquake location is one of the most basic and important activities in seismology. Hypocentral location is a complex topic that involves technical difficulties such as the reliability of absolute time at seismographs, observational difficulties such as the picking of arrival times, and theoretical difficulties such as error estimates in nonlinear inverse theory. Even so, earthquake location is such a fundamental topic that we want students to do exercises in epicenter location. While there are many resources available for the ( S-P ) method of epicentral determination, this column discusses a new addition to the EduQuakes online lab manual which shows students how to use just P waves to locate an epicenter. The classic earthquake location method found in nearly all introductory seismology books is the ( S-P ) method. The simplest case is for local earthquake location where the epicenter-to-station distances are less than two hundred kilometers, hypocentral depth is less than 15 kin, and we already know the crustal P …

GSETT-3: Evaluation of the Detection and Location Capabilities of an Experimental Global Seismic Monitoring System

This article presents the results of the seismological evaluation of the first 18 months (January 1995 to June 1996) of the Group of Scientific Experts third Technical Test (GSETT-3). Performance measures such as completeness, estimates of detection thresholds, and equivalent circle areas as an estimate of location accuracies are obtained by comparing the REB (Reviewed Event Bulletin) of GSETT-3 to global and regional reference bulletins, and also by intrinsic methods. Incremental detection thresholds are stated on the PDE (Preliminary Determination of Epicenter) m b scale: magnitude conversion formulae are derived to convert REB m b and other reference bulletin magnitudes to this scale. At the 90% confidence level, an incremental detection threshold of about m b (PDE) 3.5 or better is attained in northwest Europe and parts of North America, Central Asia, and Australia. Apart from these areas, the threshold is generally around 4.0 in the Northern Hemisphere. In the Southern Hemisphere it is 4.2 or better in most of South America and northern and equatorial Africa, and between 4.2 and 4.6 in other regions with the highest values being attained in some remote parts of the southern oceans. Still at the 90% confidence level, equivalent circle areas for the m b (PDE) 4–4.5 magnitude range come close to 1,000 km 2 in northern Europe. The global average is about 8,000 km 2 . At the 50% level (the median of all detected events), most northern hemisphere regions have equivalent circle areas at or below 1,000 km 2 , and even in the southern hemisphere 5,000 km 2 are not exceeded. Observed performance is used to calibrate network simulation codes, which allow to interpolate system capabilities over aseismic regions and to extrapolate for the capabilities of the future International Monitoring System (IMS) seismic network. It is concluded that at the 90% confidence level, except in remote ocean areas, a global incremental detection threshold of m b (PDE) 4 and a location accuracy of the order of 1,000 km 2 are attainable with the IMS seismic network. However this will require a full calibration of the network and an optimized usage of the auxiliary station network. New computational methods as well as calibration will help to improve the depth determination.

Explosion site recognition; neural net discriminator using single three-component stations

In monitoring of local seismicity, the occurrence of many chemical explosions poses sever practical problems of two kinds: (i) such recordings add significantly to the analyst workload and (ii) in extreme cases, pollute the seismicity data base to the extent of rendering it useless for serious scientific studies. In some countries, the local seismicity is equivalent to felt earthquakes but the problem remains since both earthquakes and explosions are and will be recorded by local stations. These events will therefore enter the network data center processing system, and thus, be subjected to further analysis. Source classification schemes are not always well suited for this kind of needed analysis at local distance ranges (not easily transportable). Besides, epicenter determinations may be less accurate in cases of few station reportings. The common denominator for failures is the modest usages of the information potential contained in seismic recordings being represented by a few time/amplitude parameters for the Pn- and Lg-phases. On the other hand, seismic waveform similarities for closely spaced earthquakes and explosions in particular are well established observationally. In this study period, we explore the possibility of using single station three-component (3C) covariance matrix traces from a priori known explosion sites (learning) for automatically recognizing subsequent explosions from the same site. To ensure adequate sampling, we used the nine different complex covariance time domain elements in combination with a suit of 12 bandpass filters equivalent to 108 observation pieces for a single event recording. We used a neural net scheme for teaching the computer to recognize new explosion recordings from a specific site through scanning of hundreds of detector segmented waveform files. No epicenter information was used in this analysis. The output was a single number between Zero and Four (log-scale) with an acceptance threshold (repeated explosion) of 1.2–analyst friendly usage. Actual tests were performed on two explosion sites of western Norway for which ground truth information was available. The 100% correct decisions were obtained between `site explosions' and hundreds of non-site events. A brute force test on the relative merits of the 12 frequency bands used gave that the 2–4 Hz and 8–12 Hz spectral parts were most informative. Experiment tied to two quarry sites for which ground truth information was lacking gave less sharp separation of site and non-site events. Since covariance elements comprise P- and S-waveform jointly, good spatial sampling is ensured even for single site 3C stations; beyond 10 km from the epicenter the scores were in the non-site category. The above approach to event discrimination is very flexible as we may combine several 3C stations with even z-component stations and besides, no analyst action in any way will be needed.

An approach to automatic location of regional events

The conventional network location algorithms for automatic processing are based on P and S first arrivals. These procedures have the following shortcomings: (1) it is often difficult to identify the proper phase: (Pg or Pn, Sg or Sn); and (2) first arrivals are often masked by noise. Both factors may cause significant location errors. An alternative is to look for maximums of seismic energy time–distance distribution, which are less sensitive to these factors. We measure the time-of-maximum of P and S waves envelopes vs. distance for each available station of the Israel Seismic Network (ISN), thus providing a travel time curve (TTC). The record envelopes are obtained using 1D χ 2 optimal detector and 3–6 Hz `short-time-average' time-curves, having enhanced sensitivity for seismic signal arrivals. The corresponding P and S time-of-maximum vs. distance functions are approximated linearly by a least-square method for a set of local earthquakes and quarry blasts. Travel time inversion for location of small events comprises three main steps: (1) cleaning of the records from noise bursts and computation of the envelopes, (2) triggering and identification of the P and S phases and computation of their energy maximums, and (3) maximization of a sum of station residuals as a function of epicenter coordinates. The maximum of the functional is looked for on a 60×60 km grid, step of 2 km, covering different parts of Israel and Jordan. The algorithm is not sensitive to the source depth, thus providing epicenter determination only, but shows to be convenient and robust. As a result of the preliminary study for a set of relatively weak 74 local earthquakes and 58 quarry blasts, M L∼1.5–2.5, we have obtained the accuracy of epicenter estimation ±6 km for 80–90% of both types of events, which is satisfactory for automatic location. The accuracy is measured relative to the ISN bulletin locations for earthquakes and the ground truth information for quarries, respectively.

Spatial aftershock distribution: Effect of normal stress

We study the spatial clustering of shallow aftershock hypocenters with respect to focal mechanisms of mainshocks. We use the Harvard centroid moment tensor (CMT) global catalog, the Preliminary Determination of Epicenters (PDE) earthquake list, the California Institute of Technology/U.S. Geological Survey catalog of earthquakes in southern California, and a catalog of focal mechanisms for all earthquakes since 1850 in southern California with magnitude larger than 6. We need to account for possible systematic bias in hypocenter distribution due to the geometry of seismogenic zones, especially that of subduction zones. We also select only strike‐slip earthquakes from the catalogs to investigate aftershock clustering in circumstances more favorable for direct observation. We compare the spatial distribution of hypocenters before each strong earthquake with the distribution during the first 250 days after the earthquake and for the time interval extending beyond 250 days. If the friction coefficient in the Coulomb criterion is positive one expects that after a strong earthquake, aftershocks and other earthquakes would concentrate in the direction of the P axis (dilatational quadrant) rather than in the direction of the T axis (compression quadrant). Such correlations have been pointed out previously for selected earthquakes sequences, but is such correlation a general feature of earthquake occurrence? We study spatial earthquake distributions before and after each event for several choices of focal sphere partition, cutoff magnitude, focal mechanisms of large events, time periods, distance from a mainshock, etc. Although some earthquake distributions agree with a nonzero friction coefficient, others produce the opposite pattern, suggesting that the concentration of events along the P and T axes is due to random effects. This result implies that the friction coefficient in the Coulomb law is close to zero.

Robust and reliable epicenter determinations: Envelope processing of local network data

Abstract Local event recordings are complex in the sense that relevant P and S phases vary in an unpredictable manner even between closely spaced stations; thus, manual analysis of such records is still commonplace. Our approach to solving this long-standing problem of observational seismology is to bandpass filter (3 to 6 Hz) to ensure good signal-to-noise ratio SNR and then form envelopes to ensure simple signals across a seismograph network. The physical basis is that Pg and Lg are crustal wave-guide phases reflecting P- and S-energy propagation. Extensive tests on envelope analysis of local records from different areas found that arrival times of the maxima of Pg and Lg envelopes increase very consistently with distance even in different tectonic regimes, typical velocities being 6.1 and 3.5 km/sec, respectively. These arrival-time parameters are easy to extract in a semi-automatic manner and are highly suitable for local epicenter determinations. Extensive tests on locating mining explosions were conducted, and on average, the “envelope” location errors relative to “true” locations were similar to those in bulletins that are based on conventional phase pickings. Occasionally, the Pg/Pn envelope may be very weak but can be replaced by the easily pickable (non-envelope) Pn phase. Additional advantages with envelope locations are transportability (not overly sensitive to details of crustal structure), and that envelope amplitudes can be directly converted to ground motion and magnitudes. For modern stations, envelopes can be formed in situ with low sampling rates of 1 to 2 Hz, thus greatly reducing transmission costs.

Source Time Functions

INTRODUCTION Seismologists have always striven to determine earthquake parameters as soon as possible after an earthquake occurs. Now there are several research groups that routinely “broadcast” their rapid determinations of earthquake location, focal mechanism, size, and depth. We report on our efforts to produce and “broadcast” source time functions of earthquakes. The steady march of technical progress in seismology has provided first for the rapid determination of epicenter and magnitude (e.g., from the National Earthquake Information Center [NEIC] and various regional networks). Technical requirements for this effort are that several stations have their output telemetered to some location where the arrivals can be picked and that at least one of these stations be well-calibrated so that magnitude can be estimated. This technical level and service has been available for several decades. In some cases, this procedure has been completely automated with earthquake alarms issued with no human intervention. It is...

Constraints on Macquarie Ridge tectonics provided by Harvard focal mechanisms and teleseismic earthquake locations

In this study we evaluate teleseismically determined focal mechanisms and epicenters for earthquakes along the Macquarie Ridge Complex (MRC) from 45°S to 61°S and 155°E to 168°E, a region characterized by some previous investigators as undergoing subduction initiation. From 65 centroid moment tensors reported by Harvard, we develop statistical guidelines for choosing 26 which represent better determined, more reliable focal mechanisms for tectonic analysis. Although thrust mechanisms occur in the north, near Fiordland, elsewhere along the MRC the better determined mechanisms virtually all indicate that present‐day motion along most of the MRC is strike‐slip. This is consistent with sidescan sonar and multichannel reflection data collected between 50°S and 57°S on 1994 and 1996 cruises; the active plate boundary zone appears to be quite narrow (<5 km wide), and no active compressional features can be observed on the seafloor. If we determine a rotation pole for plate boundary motion using only slip vectors from better determined Harvard mechanisms along the MRC, the best fitting “instantaneous” pole is at 57.4°S, 179.4°E, about 2.5° north of the NUVEL‐I Australian‐Pacific pole, which averages motion over the last 3.0 my. If the MRC pole was formerly farther south than at present, this could explain the existence of relict features associated with crustal shortening, such as bathymetric highs and troughs; yet, the absence of active features such as thrust faults, etc., suggests ongoing compression or subduction initiation. We also carefully read arrival times for P phases for 53 earthquakes at 16 teleseismic stations, selected to represent a range of azimuths surrounding the earthquakes; we relocated these earthquakes using standard joint epicentral determination (JED) methods. While most of the better quality relocations lie on or very close to the Australia‐Pacific boundary as determined on the 1994 cruise, a few epicenters occur well away from the boundary, apparently on Cenozoic fracture zones. Thus, on the Macquarie Ridge Complex and other major strike‐slip boundaries (e.g., in California), it appears that the very largest earthquakes occur on the principal plate boundary fault but that other earthquakes, some quite large, may occur away from the boundary along zones of preexisting weakness.

"Near" Realtime Seismology

How often have we heard the term “realtime” applied to some sort of seismic analysis or process? “Realtime epicenter determination,” “realtime moment tensor solution,” and “realtime earthquake notification” are terms being routinely used to describe current seismic processing. Of course, this is usually a misapplication of the term “realtime.” According to Webster, realtime means “the actual time during which something takes place” and according to the Oxford English Dictionary, it means “the actual time during which a process or event occurs, especially, one analyzed by a computer.” In the strictest sense, for earthquake locations and mechanisms “realtime” would mean during the actual faulting process. A more liberal interpretation might be analysis during the arrival of seismic waves. Currently seismologists seem to use the term “realtime” to mean “automatic” or “pretty soon after.” A 1991 report of the National Research Council entitled “Realtime Earthquake Monitoring” included discussions of early warning and...

Do seismologists agree upon epicentre determination from macroseismic data? A survey of ESC Working Group ' Macroseismology'

In contrast to the case of instrumental data, the procedures for epicentral parameter determination (coordinates and I0) from macroseismic data are not very well established. Although there are some "rules", upon which most seismologists agree (centre of the isoseismal of largest degree, and so on), the practical application of, such rules displays many problems. Therefore, it is commonly seismologists' practice to find their own pro cedures and solutions; this is particularly evident in the more complicated cases, Such as offshore epicentres or, as in many cases of historical earthquakes, poor sets of data. One of the major consequences is that parametric catalogues are not homogeneous with respect to macroseismic parameters; moreover, merging catalogues compiled according to different criteria can introduce high noise in any catalogue built in such a way. In order to survey the current practice of epicentre determination from macroseismic data in Europe, a set of cases was distributed to the participants of the first meeting of the ESC WG "Macroseismology". A comparison of the 15 sets of results provided by 16 authors, who gave their own solutions and the explanation., of the adopted procedures is given, showing that in some cases the ideas and results are rather distant.

Inaccuracies in seismicity and magnitude data used by Varotsos and Coworkers

The claims of successful earthquake predictions by VAN are based on magnitudes said to be Ms (surface wave). However, they are larger by 1.0 unit, on the average, than those published in the Preliminary Determination of Epicenters (PDE), the authoritative source for Ms values. This discrepancy arises because VAN calculate Ms from ML (local magnitude) using the incorrect transformation Ms(VAN)=ML+0.5. Least squares regressions, based on the PDE data for Greece, yield Ms=l.50ML−2.62 and Ms=1.82mb−4.16 instead. An important part of many author's evaluations of VAN's claims is an assessment of whether their performance is better than that expected by random chance. If the random chance probability is defined using standard (PDE) seismicity data and magnitudes, whereas VAN's claims are based on a non‐standard scale, the significance of VAN's successes will be overestimated strongly. Additional discrepancies are introduced because Ms(VAN) is based on preliminary ML values), or sometimes on newspaper reports, rather than those published in the final bulletin of the Seismological Institute of the National Observatory Athens (SI‐NOA). The correct final magnitude should be used in all studies claiming success and evaluating these claims years after the earthquakes occurred. Inaccuracies in presenting an earthquake catalog also exist in Varotsos et al. [1981b], who claimed a one‐to‐one correlation of seismic electric signals (SES) with the list of earthquakes with mostly ML>2.9 published by the SI‐NOA. In fact, VAN had used instead a list called “Ten Days Preliminary Seismogram Readings at Athens” which is not a bulletin and contains no epicenters. As a consequence 37% of the earthquakes reported in the SI‐NOA Monthly Bulletin (including the largest one) were not contained in the list of VAN, and 40% of the earthquakes VAN claimed had occurred, were not in the SI‐NOA Bulletin. In addition, VAN added another 25% of events to the list. These latter events were identified as added, but there is no evidence that they were not explosions or other cultural disturbances. It is concluded that some of VAN's claims of correlations between SESs and earthquakes, and some of the evaluations of these claims, are based on incorrect earthquake lists.

P-wave backazimuth anomalies observed by a small-aperture seismic array at Pinyon Flat, southern California: Implications for structure and source location

AbstractSeismograms of earthquakes and explosions recorded at local, regional, and teleseismic distances by a small-aperture, dense seismic array located on Pinyon Flat, in southern California, reveal large (±15°) backazimuth anomalies. We investigate the causes and implications of these anomalies by first comparing the effectiveness of estimating backazimuth with an array using three different techniques: the broadband frequency-wavenumber (BBFK) technique, the polarization technique, and the beamforming technique. While each technique provided nearly the same direction as a most likely estimate, the beamforming estimate was associated with the smallest uncertainties. Backazimuth anomalies were then calculated for the entire data set by comparing the results from beamforming with backazimuths derived from earthquake locations reported by the Anza and Caltech seismic networks and the Preliminary Determination of Epicenters (PDE) Bulletin. These backazimuth anomalies have a simple sinelike dependence on azimuth, with the largest anomalies observed from the southeast and northwest directions. Such a trend may be explained as the effect of one or more interfaces dipping to the northeast beneath the array. A best-fit model of a single interface has a dip and strike of 20° and 315°, respectively, and a velocity contrast of 0.82 km/sec. Application of corrections computed from this simple model to ray directions significantly improves locations at all distances and directions, suggesting that this is an upper crustal feature. We confirm that knowledge of local structure can be very important for earthquake location by an array but also show that corrections computed from simple models may not only be adequate but superior to those determined by raytracing through smoothed laterally varying models.

Array analysis of core-phase caustic signals from underground nuclear explosions: Discrimination of closely spaced explosions

Abstract The GRF (Gräfenberg) seismic array in Germany is located at epicentral distances immediately before the PKP caustic B generated by underground nuclear explosions on the French atolls Mururoa and Fangataufa in the South Pacific. I use the large-amplitude PKP waveforms to improve the locations of underground explosions near these French atolls. The core phases are investigated using theoretical seismograms calculated with an extended reflectivity method. Near the caustic range, the waveforms are strongly distance dependent due to a rapid decrease in PKPdiff amplitudes before the caustic B. On the basis of their different slownesses, beam-forming and velocity-spectral (vespagram) analyses are used to identify PKPdiff, PKIKP, and PKiKP, which arrive within a few seconds of each other. Amplitudes and travel times of the weak secondary arrivals PKIKP and PKiKP are measured from the corresponding beams using a matched-filter technique. Theoretical seismograms show a change in the amplitude ratio PKIKP/PKPdiff by a factor of 2 for explosions separated by 30 km. This distance-dependent amplitude ratio is tested as a discriminant for closely spaced explosions using 31 Mururoa and five Fangataufa events. On average, the observed amplitude ratios agree with the theoretically expected amplitude-ratio behavior with distance. A combined analysis of travel time and amplitude data indicates that the investigated Mururoa explosions were detonated beneath a relatively large area of the atoll (ca. 80%), whereas on Fangataufa, explosion epicenters were concentrated within a small portion of the island. This finding contrasts the ISC/PDE epicenter determinations, which, due to insufficient station coverage, locate the majority of the explosions outside of the rim of the atolls.

Bolivian earthquake ranks as largest deep quake on record

Early on June 9, an exceptionally large earthquake occurred 630 km beneath a remote area in the Amazon rain forest of northern Bolivia. (USGS National Earthquake Information Center (NEIQ Quick Epicenter Determination at 13.86°S and longitude 67.54°W).Minutes later, strong ground motion was felt over much of central South America. Fortunately, no loss of life was reported in initial local news accounts, and damage was relatively minor considering that it was the largest deep event in the seismological record for which accurate estimates are available.

Catalog of locations of nuclear explosions at Balapan, Kazakhstan, 1965 to 1985

Abstract Geographic locations for 81 explosions at the Balapan test site in Kazakhstan are derived with a precision of about 200 m from time-sequence LANDSAT MSS and SPOT satellite images combined with teleseismic epicenter estimates. The LANDSAT images were registered to ground control points from a SPOT image to provide high relative accuracy for feature comparison. Initial seismic location estimates for the explosions were derived using joint epicenter determination. For most events, there is little or no ambiguity in the association between seismic events and image features, owing to the frequency of the LANDSAT images and the spatial separation between explosions.