Earthquake Data Report Research Articles

Correlations Between the Mainshock and the Largest Aftershock for Taiwan Earthquakes

From the mainshocks of 85 Taiwan earthquakes (Ms > 5) and their aftershocks reported in the Earthquake Data Report by NEIC PDE catalogue, several correlations between mainshocks and their aftershocks are studied. Results show that the magnitude difference (ΔMs) between the mainshock and the largest aftershock slightly increases with the mainshock magnitude (Ms) and varies from 0.1 to 2.2, with two values of 0.3 and 0.9 having the largest numbers of events. The mean value of ΔMs is 0.83 ± 0.46. The differences of occurrence time and epicenter between the mainshock and the largest aftershock do not clearly correlate to Ms as well as ΔMs. For the whole data set, 36.5 and 77.7% of the largest aftershocks are located near their individual mainshocks within a distance of 10 and 40 km, respectively. The difference in occurrence time somewhat increases as the difference of epicenters increases.

Regional and teleseismic double‐difference earthquake relocation using waveform cross‐correlation and global bulletin data

We have developed a double‐difference algorithm to relocate earthquakes recorded at global seismic networks, using differential arrival times for first and later arriving regional and global phases to invert for the vectors connecting the hypocenters. Differential times are formed from global seismic bulletins and are accurately measured on similar seismograms by time domain waveform cross correlation. We evaluate the performance of this spherical, multiphase double‐difference algorithm using three‐dimensional regional‐scale synthetic data and two sets of earthquake data in different tectonic settings. The first includes 3783 intermediate depth earthquakes that occurred between 1964 and 2000 in the subducting Nazca plate beneath northern Chile, where the relocated seismicity confirms a narrowly spaced double seismic zone previously imaged with temporary local seismic data. Residual statistics and comparison with accurately known locations indicate mean relative location errors at the 90% confidence level of 2.4 km laterally and 1.8 km vertically. Later events typically constrained by cross‐correlation data have errors of 1.6 km laterally and 1.4 km vertically. The second data set includes 75 crustal earthquakes in the 1999 Izmit and Düzce, Turkey, aftershock sequences, where the double‐difference solutions image orientation and dip of individual fault segments that are consistent with focal mechanisms and near‐surface information. Fault complexity likely causes a low level of waveform similarity in this aftershock sequence and thus generates fewer correlated events compared to the Chile earthquakes. Differences between the double‐difference locations and corresponding locations in global seismicity catalogs (Earthquake Data Report, EDR; International Seismological Centre, ISC; Engdahl‐Hilst‐Buland, EHB) are typically greater than 10 km. We evaluate the potential of cross‐correlation and double‐difference methods to improve hypocenter locations on a global scale.

Using Small, Temporary Seismic Networks for Investigating Tectonic Deformation: Brittle Deformation and Evidence for Strike-Slip Faulting in Bhutan

SUMMARY We processed data from a small, five-station temporary seismic network deployed from January 2002 until March 2003 within the Kingdom of Bhutan. We detected, associated, and located approximately 2,100 teleseismic, regional, and local events; approximately 900 were not in the United States Geological Survey (USGS) Earthquake Data Report catalog. We supple mented our data for these 900 events with data from the Global Seismographic Network (GSN) stations in the region. After relocation of these events, we focused on approximately 175 events that occurred near or within the borders of Bhutan. We reviewed each solution, manually timing the P- and S-waves for each event, and inverted for event locations and an average 1-D velocity model for the region. Our model was tested with other models appropriate for the region. We found a high amount of microseismicity throughout southern Bhutan and almost no seismicity under northern Bhutan and southern Tibet. Our results showed that analysis of data from small in-country seismic networks resulted in new scientific findings. In this case, we found the crust under southern Bhutan brittlely deforming, and there was evidence for strike-slip faulting, supporting previous results for the region.

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 …

Improving Regional Seismic Event Location in China

— In an effort to improve our ability to locate seismic events in China using only regional data, we have developed empirical propagation path corrections and applied such corrections using traditional location routines. Thus far, we have concentrated on corrections to observed P arrival times for crustal events using travel-time observations available from the USGS Earthquake Data Reports, the International Seismic Centre Bulletin, the preliminary International Data Center Reviewed Event Bulletin, and our own travel-time picks from regional data. Location ground truth for events used in this study ranges from 25 km for well-located teleseimic events, down to 2 km for nuclear explosions located using satellite imagery. We also use eight events for which depth is constrained using several waveform methods. We relocate events using the EvLoc algorithm from a region encompassing much of China (latitude 20°–55°N; longitude 65°–115°E). We observe that travel-time residuals exhibit a distance-dependent bias using IASPEI91 as our base model. To remedy this bias, we have developed a new 1-D model for China, which removes a significant portion of the distance bias. For individual stations having sufficient P-wave residual data, we produce a map of the regional travel-time residuals from all well-located teleseismic events. Residuals are used only if they are smaller than 10 s in absolute value and if the seismic event is located with accuracy better than 25 km. From the residual data, correction surfaces are constructed using modified Bayesian kriging. Modified Bayesian kriging offers us the advantage of providing well-behaved interpolants and their errors, but requires that we have adequate error estimates associated with the travel-time residuals from which they are constructed. For our P-wave residual error estimate, we use the sum of measurement and modeling errors, where measurement error is based on signal-to-noise ratios when available, and on the published catalog estimate otherwise. Our modeling error originates from the variance of travel-time residuals for our 1-D China model. We calculate propagation path correction surfaces for 74 stations in and around China, including six stations from the International Monitoring System. The statistical significance of each correction surface is evaluated using a cross-validation technique. We show relocation results for nuclear tests from the Balapan and Lop Nor test sites, and for earthquakes located using interferometric synthetic aperture radar. These examples show that the use of propagation path correction surfaces in regional relocations eliminates distance bias in the residual curves and significantly improves the accuracy and precision of seismic event locations.

Detection Capability of Selected Seismic Stations in Central Europe

The 50% and 90% P-wave detection thresholds of 3 Austrian, 2 Czech, 5 German, 1 Hungarian and 3 Slovak seismic stations have been determined for four intervals of epicentral distances by a direct method using the maximum-likelihood technique. The USGS-NEIC Earthquake Data Reports covering the period from January 1990 to November 1994 were used as the reference system. The seismic stations Kasperske Hory (KHC, Czech Republic) and Collmberg (CLL, Germany) are the stations with the best detection capabilities for all investigated intervals of epicentral distances.

Aspects of Large Taiwan Earthquakes and Their Aftershocks

From the data of 31 large Taiwan earthquakes and their aftershocks published by Hsu (1971, 1980, and 1985) and reported in the Earthquake Data Report (EDR)by USGS, several aspects of main shocks and their aftershocks are studied. Results show that the magnitude difference (ΔM) between main-shock and the largest aftershock increases with mainshock magnitude (M). The linear relations of ΔM vs. M are different for two data sets. The difference of original times of mainshock and the largest aftershock more or less decreases with magnitude of mainshock. The departure of the M(subscript s)-m(subscript b) scaling of the mainshock from that of aftershocks is bigger for larger main-shocks than for smaller ones. The mainshock magnitude cannot be estimated from the log N-M relation from aftershocks and is actually larger than the maximum magnitude evaluated from the relation.

P-wave traveltime and crust-upper mantle velocity 1976, structure in the Indian region

P-arrival times and residuals from shallow focus earthquakes of Indian origin, as reported in the Earthquake Data Report of the United States Department of the Interior, Geological Survey (EDR-USGS), at seismological station in India have been collected and analysed. Based on the past studies a thicker two layered crust and higher P-wave velocities in each of the two layers compared to a Jeffrey-Bullen model (J-B model), which is in conformity with the traveltimity data, has been chosen for the Indian plains. A comparatively higher P-wave velocity than in the J-B model in the upper mantle down to a depth of 185 km has been interpreted. J- B model has been accepted beyond this depth.
 Hypocentre location of Cachar earthquake on 30December 1984 has been obtained using this model. This has been found to be in good agreement With both those determinations by USGS and Dubeet al. (1986).

Considerations for the body-wave magnitude determination in the recent earthquake data report of the United States Geological Survey

Considerations for the body-wave magnitude determination in the recent earthquake data report of the United States Geological Survey

Magnitudes of large shallow earthquakes from 1904 to 1980

In order to obtain a uniform magnitude catalogue, surface-wave magnitudes M s and broad-band body-wave magnitudes m B have been determined for large shallow earthquakes from 1904 to 1980. In making the catalogue homogeneous, the author consistently adheres to the original definitions of M s and m B given by Gutenberg (1945) and Gutenberg and Richter (1956). The determinations of M s and m B are all based on the amplitude and period data listed in Gutenberg and Richter's unpublished notes, bulletins from stations worldwide, and other basic information. m B is measured on broad-band instruments in periods of ∼8 s. Consistency of the magnitude determinations from these different sources is carefully checked in detail. More than 900 shallow shocks of magnitude 7 and over are catalogued. The meaning of the magnitude scales in various catalogues is examined in terms of M s and m B. Most of the magnitudes listed by Gutenberg and Richter (1954) in their “Seismicity of the Earth” are basically M s for large shocks shallower than 40 km, but are basically m B for large shocks at depths of 40–60 km. The surface-wave magnitudes given by “Earthquake Data Reports” are higher than M s by 0.2 unit whether the combined horizontal amplitude or the vertical amplitude is used. m B and the currently used 1 s body-wave magnitude are measured at different periods and should not be directly compared.

A Seismic Refraction Line Along the Axis of The Southern Piedmont and Crustal Thicknesses in the Southeastern United States

Arrival times of Pn and Sn phases from southeastern United States earthquakes listed in the Earthquake Data Reports from July 1970 to June 1977 show a scatter of 2 seconds (one standard deviation). In contrast, the Pg phase arrivals show a scatter of only 0.5 seconds. The difference in scatter was interpreted as indicating variation in the depth to the Moho. In order to evaluate Moho depths, a detailed refraction line and crustal model were developed along the axis of the Southern Piedmont Province from Central Georgia across South Carolina. Accurately located recent earthquakes and explosions occurring and recorded along the axis of the Southern Piedmont Province were used to define the travel time curves. The resulting travel times for the Southern Piedmont are described by; P = x/(5.5 km/sec) Pg = 0.4 + x/(6.05 km/sec) and Pn = 7.6 + x/(8.2 km/sec) with a scatter of ±0.5 seconds. The Sg and Sn velocities were 3.5 and 4.5 km/sec respectively. The implied crustal structure consists of a 2.6 km thick surface layer over a 30.4 km thick crust. � No evidence for an intermediatelayer in the crust was observed in the traveltimes but a 6.7 km/sec layer less than 6 km thick could exist at the base of the crust. Crustal thicknesses in the southeastern United States were computed from Pn-Pg times by comparison to the Southern Piedmont crustal model. In the Coastal Plain Moho depths of 40 km shallow toward the southeast to 29 km near the coast. Moho depths of 36 to 54 km were measured for the mountains of north Georgia, eastern Tennessee and western North Carolina.

Magnitudes of great shallow earthquakes from 1953 to 1977

The surface-wavemagnitudes M s are determined for 30 great shallow earthquakes that occurred during the period from 1953 to 1977. The determination is based on the amplitude and period data from all available station bulletins, and the same procedure as that employed in Gutenberg and Richter's “Seismicity of the Earth” is used. During this period, the Chilean earthquake of 1960 has the largest M s , 8.5. The surface-wave magnitudes listed in “Earthquake Data Reports” are found to be higher than M s on the average. By using the same method as that used by Gutenberg, the broad-band body-wave magnitudes m B are determined for great shallow shocks for the period from 1953 to 1974. m B is based on the amplitudes of P, PP and S waves which are measured on broadband instruments at periods of about 4–20 s. The 1-s body-wave magnitudes listed in “Bulletin of International Seismological Center” and “Earthquake Data Reports” are found to be much smaller than m B on the average. Through the examination of Gutenberg and Richter's original worksheets, the relation between m B and M s is revised to m B = 0.65 M s+ 2.5 which well satisfies the mg and M s data for M s between 5.2 and

Evidence for ω-cube scaling from amplitudes and periods of the Rat Island sequence (1965)

Abstract An attempt has been made to deduce seismic scaling information from the amplitudes A and periods T of P waves from 868 earthquakes of the Rat Island sequence of 1965, with m b values ranging from ∼4 to 5. Consistently reliable data were reported in the Earthquake Data Reports from five VELA array stations in the U.S., falling in an azimuthal sector of 60° to 80° from the source region with distances ranging from 44° to 68°. The most striking feature of the data is the strong linear trend of log ( A/T ) with T at each station over a wide period range of 0.5 to >1.5 sec. Using a subset of 245 events reported jointly at all five VELA stations we calculated a least squares magnitude versus period relation given by m b = (3.24 ± 0.08) + (1.32 ± 0.09) T . In order to interpret this trend in terms of source models, synthetic records, including earth attenuation were calculated for ω-square models of Sharpe and Brune, and an ω-cube model suggested by Savage. Under the assumption of geometric similarity, whereby the zero-frequency source level is proportional to source volume, the measured trend of log ( A/T ) on the synthetic records indicates that an ω-cube rather than either ω-square model is more appropriate for the Rat Island data. Extrapolation of the synthetic source spectra to D.C. for m b 4.7 yields a moment of 6.65 × 10 22 dyne cm and for the sequence a stress drop of ∼25 bars, in reasonable agreement with previous results obtained from long-period P -wave spectra. These results indicate that crude seismic scaling information for moderate-sized earthquakes can be extracted from good bulletin data without using spectral analysis. Large amounts of data are required, however, to establish a reliable trend for interpretation.

On the magnitude calibration function for core phases with special reference to the PKP1-PKP2 caustic.

An empirical method is developed for determining magnitude calibration functions independently of the standardized approach after Gutenberg and Richter. The method is applied to the worldwide data in the Earthquake Data Report (EDR) Of NOAA and the Bulletins of the ISC. Magnitude calibration functions are derived through the method for events in the three seismic regions (a) Alaska-Aleutian Arc, (b) Andean South America, and (c) Japan-Kurile-Kamchatka. Characteristics of the calibration functions thus obtained are summarized for the three epicentral ranges of i) the near (0°≤Δ<20°), ii) the teleseismic (20°≤Δ<110°) and iii) the PKP (Δ°≥110°) distance ranges.In the PKP distance range, variations of log (A/T) are interpreted in relation to the travel time of compressional core phases. It is shown that seismic signals at the PKP1-PKP2 caustic (142°-152°) from small events of magnitude about 4 can possibly be detected by observations under normal routine circumstances with a magnification of about 5×104 and the signals at the caustic range are highly suitable for worldwide magnitude determination.