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Aftershocks Following the 9 April 2013 Bushehr Earthquake, Iran

Ardalan, Ali; Hajiuni, Alireza; Zare, Mehdi
Fonte: Public Library of Science Publicador: Public Library of Science
Tipo: Artigo de Revista Científica
Publicado em 28/08/2013 Português
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27.43%
On 9 April 2013 at 11:52 UTC (16:22 local time), a Mw 6.2 earthquake occurred at the depth of 20 Km in Dashti district in south-west Iran’s Bushehr province. The macroseismic epicenter was located nearby the city of Shonbeh. During one month after the earthquake, a total of 282 aftershocks hit the epicentral region, mostly at the east and north sides. They ranged from 2.5 to 5.7 on the Richter scale. Seventy aftershocks (24.9%) were M4.0-4.9 and eight (2.8%) were M5.0-5.7. Aftershocks are potentially able to do additional damage. In Bushehr earthquake, a M5.4 aftershock on 10 April in Chahgah village caused at least four injuries and destruction of several buildings that had been already damaged by the main shock. Knowledge about the aftershock induced damages provides opportunities for timely risk communication with the affected people and for long term community education. This will hopefully increase the community awareness and minimize the risk of further loss of lives.

Forecasting large aftershocks within one day after the main shock

Omi, Takahiro; Ogata, Yosihiko; Hirata, Yoshito; Aihara, Kazuyuki
Fonte: Nature Publishing Group Publicador: Nature Publishing Group
Tipo: Artigo de Revista Científica
Publicado em 17/07/2013 Português
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Forecasting the aftershock probability has been performed by the authorities to mitigate hazards in the disaster area after a main shock. However, despite the fact that most of large aftershocks occur within a day from the main shock, the operational forecasting has been very difficult during this time-period due to incomplete recording of early aftershocks. Here we propose a real-time method for efficiently forecasting the occurrence rates of potential aftershocks using systematically incomplete observations that are available in a few hours after the main shocks. We demonstrate the method's utility by retrospective early forecasting of the aftershock activity of the 2011 Tohoku-Oki Earthquake of M9.0 in Japan. Furthermore, we compare the results by the real-time data with the compiled preliminary data to examine robustness of the present method for the aftershocks of a recent inland earthquake in Japan.

Complex networks of earthquakes and aftershocks

Baiesi, Marco; Paczuski, Maya
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Português
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27.64%
We invoke a metric to quantify the correlation between any two earthquakes. This provides a simple and straightforward alternative to using space-time windows to detect aftershock sequences and obviates the need to distinguish main shocks from aftershocks. Directed networks of earthquakes are constructed by placing a link, directed from the past to the future, between pairs of events that are strongly correlated. Each link has a weight giving the relative strength of correlation such that the sum over the incoming links to any node equals unity for aftershocks, or zero if the event had no correlated predecessors. A correlation threshold is set to drastically reduce the size of the data set without losing significant information. Events can be aftershocks of many previous events, and also generate many aftershocks. The probability distribution for the number of incoming and outgoing links are both scale free, and the networks are highly clustered. The Omori law holds for aftershock rates up to a decorrelation time that scales with the magnitude, $m$, of the initiating shock as $t_{\rm cutoff} \sim 10^{\beta m}$ with $\beta \simeq 3/4$. Another scaling law relates distances between earthquakes and their aftershocks to the magnitude of the initiating shock. Our results are inconsistent with the hypothesis of finite aftershock zones. We also find evidence that seismicity is dominantly triggered by small earthquakes. Our approach...

Violation of the scaling relation and non-Markovian nature of earthquake aftershocks

Abe, Sumiyoshi; Suzuki, Norikazu
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 02/07/2008 Português
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The statistical properties of earthquake aftershocks are studied. The scaling relation for the exponents of the Omori law and the power-law calm time distribution (i.e., the interoccurrence time distribution), which is valid if a sequence of aftershocks is a singular Markovian process, is carefully examined. Data analysis shows significant violation of the scaling relation, implying the non-Markovian nature of aftershocks.; Comment: 11 pages, 2 figures, 1 table. Dedicated to Francois Bardou (1968-2006)

Sub-critical and Super-critical Regimes in Epidemic Models of Earthquake Aftershocks

Helmstetter, A.; Sornette, D.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
27.7%
We present an analytical solution and numerical tests of the epidemic-type aftershock (ETAS) model for aftershocks, which describes foreshocks, aftershocks and mainshocks on the same footing. The occurrence rate of aftershocks triggered by a single mainshock decreases with the time from the mainshock according to the modified Omori law K/(t+c)^p with p=1+theta. A mainshock at time t=0 triggers aftershocks according to the local Omori law, that in turn trigger their own aftershocks and so on. The effective branching parameter n, defined as the mean aftershock number triggered per event, controls the transition between a sub-critical regime n<1 to a super-critical regime n>1. In the sub-critical regime, we recover and document the crossover from an Omori exponent 1-theta for t1 and theta>0, we find a novel transition from an Omori decay law with exponent 1-theta fot tt*. The case theta<0 yields an infinite n-value. In this case, we find another characteristic time tau controlling the crossover from an Omori law with exponent 1-theta for t

Scale free networks of earthquakes and aftershocks

Baiesi, Marco; Paczuski, Maya
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 21/09/2003 Português
Relevância na Pesquisa
27.43%
We propose a new metric to quantify the correlation between any two earthquakes. The metric consists of a product involving the time interval and spatial distance between two events, as well as the magnitude of the first one. According to this metric, events typically are strongly correlated to only one or a few preceding ones. Thus a classification of events as foreshocks, main shocks or aftershocks emerges automatically without imposing predefined space-time windows. To construct a network, each earthquake receives an incoming link from its most correlated predecessor. The number of aftershocks for any event, identified by its outgoing links, is found to be scale free with exponent $\gamma = 2.0(1)$. The original Omori law with $p=1$ emerges as a robust feature of seismicity, holding up to years even for aftershock sequences initiated by intermediate magnitude events. The measured fat-tailed distribution of distances between earthquakes and their aftershocks suggests that aftershock collection with fixed space windows is not appropriate.; Comment: 7 pages and 7 figures. Submitted

Aftershocks in Modern Perspectives: Complex Earthquake Network, Aging, and Non-Markovianity

Abe, Sumiyoshi; Suzuki, Norikazu
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 20/02/2012 Português
Relevância na Pesquisa
27.43%
The phenomenon of aftershocks is studied in view of science of complexity. In particular, three different concepts are examined: (i) the complex-network representation of seismicity, (ii) the event-event correlations, and (iii) the effects of long-range memory. Regarding (i), it is shown the clustering coefficient of the complex earthquake network exhibits a peculiar behavior at and after main shocks. Regarding (ii), it is found that aftershocks experience aging, and the associated scaling holds. And regarding (iii), the scaling relation to be satisfied by a class of singular Markovian processes is violated, implying the existence of the long-range memory in processes of aftershocks.; Comment: 28 pages, 6 figures and 1 table. Acta Geophysica, in press

Stress drops and radiated energies of aftershocks of the 1994 Northridge, California, earthquake

Mori, Jim; Abercrombie, Rachel E.; Kanamori, Hiroo
Fonte: American Geophysical Union Publicador: American Geophysical Union
Tipo: Article; PeerReviewed Formato: application/pdf
Publicado em /11/2003 Português
Relevância na Pesquisa
27.43%
We study stress levels and radiated energy to infer the rupture characteristics and scaling relationships of aftershocks and other southern California earthquakes. We use empirical Green functions to obtain source time functions for 47 of the larger (M ≥ 4.0) aftershocks of the 1994 Northridge, California earthquake (M6.7). We estimate static and dynamic stress drops from the source time functions and compare them to well-calibrated estimates of the radiated energy. Our measurements of radiated energy are relatively low compared to the static stress drops, indicating that the static and dynamic stress drops are of similar magnitude. This is confirmed by our direct estimates of the dynamic stress drops. Combining our results for the Northridge aftershocks with data from other southern California earthquakes appears to show an increase in the ratio of radiated energy to moment, with increasing moment. There is no corresponding increase in the static stress drop. This systematic change in earthquake scaling from smaller to larger (M3 to M7) earthquakes suggests differences in rupture properties that may be attributed to differences of dynamic friction or stress levels on the faults.

Fault-Plane Determination of the 18 April 2008 Mount Carmel, Illinois, Earthquake by Detecting and Relocating Aftershocks

Yang, Hongfeng; Zhu, Lupei; Chu, Risheng
Fonte: Seismological Society of America Publicador: Seismological Society of America
Tipo: Article; PeerReviewed Formato: application/pdf; application/zip
Publicado em 01/12/2009 Português
Relevância na Pesquisa
27.55%
We developed a sliding-window cross-correlation (SCC) detection technique and applied the technique to continuous waveforms recorded by the Cooperative New Madrid Seismic Network stations following the 18 April 2008 Illinois earthquake. The technique detected more than 120 aftershocks down to M_L 1.0 in the 2 week time window following the mainshock, which is three times more than the number of aftershocks reported by the seismic network. Most aftershocks happened within 24 hrs of the mainshock. We then relocated all events by the double-difference relocation algorithm. Accurate P- and S-wave differential arrival times between events were obtained by waveform cross correlation. After relocation, we used the L1 norm to fit all located events by a plane to determine the mainshock fault plane. The best-fit plane has a strike of 292°±11° and dips 81°±7° to the northeast. This plane agrees well with the focal mechanism solutions of the mainshock and four largest aftershocks. By combining the aftershock locations and focal mechanism solutions, we conclude that the 18 April earthquake occurred on a nearly vertical left-lateral strike-slip fault orienting in the west-northwest–east-southeast direction. The fault coincides with the proposed left-stepping Divide accommodation zone in the La Salle deformation belt and indicates reactivation of old deformation zone by contemporary stresses in the Midcontinent.

Aftershocks and Triggered Events of the Great 1906 California Earthquake

Meltzner, Aron J.; Wald, David J.
Fonte: Seismological Society of America Publicador: Seismological Society of America
Tipo: Article; PeerReviewed Formato: application/pdf
Publicado em /10/2003 Português
Relevância na Pesquisa
27.7%
The San Andreas fault is the longest fault in California and one of the longest strike-slip faults in the world, yet little is known about the aftershocks following the most recent great event on the San Andreas, the M_W 7.8 San Francisco earthquake on 18 April 1906. We conducted a study to locate and to estimate magnitudes for the largest aftershocks and triggered events of this earthquake. We examined existing catalogs and historical documents for the period April 1906 to December 1907, compiling data on the first 20 months of the aftershock sequence. We grouped felt reports temporally and assigned modified Mercalli intensities for the larger events based on the descriptions judged to be the most reliable. For onshore and near-shore events, a grid-search algorithm (derived from empirical analysis of modern earthquakes) was used to find the epicentral location and magnitude most consistent with the assigned intensities. For one event identified as far offshore, the event's intensity distribution was compared with those of modern events, in order to constrain the event's location and magnitude. The largest aftershock within the study period, an M ∼6.7 event, occurred ∼100 km west of Eureka on 23 April 1906. Although not within our study period...

Magnitude Estimates of Two Large Aftershocks of the 16 December 1811 New Madrid Earthquake

Hough, Susan E.; Martin, Stacey
Fonte: Seismological Society of America Publicador: Seismological Society of America
Tipo: Article; PeerReviewed Formato: application/pdf
Publicado em /12/2002 Português
Relevância na Pesquisa
27.43%
The three principal New Madrid mainshocks of 1811-1812 were followed by extensive aftershock sequences that included numerous felt events. Although no instrumental data are available for either the mainshocks or the aftershocks, available historical accounts do provide information that can be used to estimate magnitudes and locations for the large events. In this article we investigate two of the largest aftershocks: one near dawn following the first mainshock on 16 December 1811, and one near midday on 17 December 1811. We reinterpret original felt reports to obtain a set of 48 and 20 modified Mercalli intensity values of the two aftershocks, respectively. For the dawn aftershock, we infer a M_W of approximately 7.0 based on a comparison of its intensities with those of the smallest New Madrid mainshock. Based on a detailed account that appears to describe near-field ground motions, we further propose a new fault rupture scenario for the dawn aftershock. We suggest that the aftershock had a thrust mechanism and occurred on a southeastern limb of the Reelfoot fault. For the 17 December 1811 aftershock, we infer a M_W of approximately 6.1 ± 0.2. This value is determined using the method of Bakun et al. (2002), which is based on a new calibration of intensity versus distance for earthquakes in central and eastern North America. The location of this event is not well constrained...

Spatial Separation of Large Earthquakes, Aftershocks, and Background Seismicity: Analysis of Interseismic and Coseismic Seismicity Patterns in Southern California

Hauksson, Egill
Fonte: Springer Publicador: Springer
Tipo: Article; PeerReviewed Formato: application/pdf
Publicado em /08/2010 Português
Relevância na Pesquisa
27.43%
We associate waveform-relocated background seismicity and aftershocks with the 3-D shapes of late Quaternary fault zones in southern California. Major earthquakes that can slip more than several meters, aftershocks, and near-fault background seismicity mostly rupture different surfaces within these fault zones. Major earthquakes rupture along the mapped traces of the late Quaternary faults, called the principal slip zones (PSZs). Aftershocks occur either on or in the immediate vicinity of the PSZs, typically within zones that are ±2-km wide. In contrast, the near-fault background seismicity is mostly accommodated on a secondary heterogeneous network of small slip surfaces, and forms spatially decaying distributions extending out to distances of ±10 km from the PSZs. We call the regions where the enhanced rate of background seismicity occurs, the seismic damage zones. One possible explanation for the presence of the seismic damage zones and associated seismicity is that the damage develops as faults accommodate bends and geometrical irregularities in the PSZs. The seismic damage zones mature and reach their finite width early in the history of a fault, during the first few kilometers of cumulative offset. Alternatively, the similarity in width of seismic damage zones suggests that most fault zones are of almost equal strength...

Double-difference Relocation of the Aftershocks of the Tecomán, Colima, Mexico Earthquake of 22 January 2003

Andrews, Vanessa; Stock, Joann; Ramírez Vázquez, Carlos Ariel; Reyes-Dávila, Gabriel
Fonte: Springer Publicador: Springer
Tipo: Article; PeerReviewed Formato: application/pdf
Publicado em /08/2011 Português
Relevância na Pesquisa
27.55%
On 22 January 2003, the M_w = 7.6 Tecomán earthquake struck offshore of the state of Colima, Mexico, near the diffuse triple junction between the Cocos, Rivera, and North American plates. Three-hundred and fifty aftershocks of the Tecomán earthquake with magnitudes between 2.6 and 5.8, each recorded by at least 7 stations, are relocated using the double difference method. Initial locations are determined using P and S readings from the Red Sismológica Telemétrica del Estado de Colima (RESCO) and a 1-D velocity model. Because only eight RESCO stations were operating immediately following the Tecomán earthquake, uncertainties in the initial locations and depths are fairly large, with average uncertainties of 8.0 km in depth and 1.4 km in the north–south and east–west directions. Events occurring between 24 January and 31 January were located using not only RESCO phase readings but also additional P and S readings from 11 temporary stations. Average uncertainties decrease to 0.8 km in depth, 0.3 km in the east–west direction, and 0.7 km in the north–south direction for events occurring while the temporary stations were deployed. While some preliminary studies of the early aftershocks suggested that they were dominated by shallow events above the plate interface...

Waveforms and spectra of preshocks and aftershocks of the 1979 Imperial Valley, California, Earthquake: evidence for fault hetergeneity

Pechmann, James C.; Kanamori, Hiroo
Fonte: American Geophysical Union Publicador: American Geophysical Union
Tipo: Article; PeerReviewed Formato: application/pdf
Publicado em /12/1982 Português
Relevância na Pesquisa
27.7%
We have compared digitally-recorded waveforms of M_L 2.0–2.8 earthquakes that occurred in two small areas along the Imperial fault before and after it broke in the ML 6.6 Imperial Valley earthquake on October 15, 1979. Eight preshocks (1977–1979) from a 4½ by 1½ km area centered 4 km SE of the mainshock epicenter have strikingly similar waveforms over the entire record length (∼30 s), with an average peak cross correlation between seismograms of 0.74. The seismograms are well correlated at frequencies up to at least 4 Hz. This implies similar source mechanisms and hypocenters within ¼ of the 4-Hz wavelengths, i.e., <200–400 m. Five aftershocks from the same area show an average peak cross correlation between seismograms of only 0.23. Any associated changes in mechanism must be small because they are not reflected in the first motion data. Analysis of frequency content of these events using bandpass-filtering techniques showed no systematic temporal changes in spectral shape. Ten preshocks and 24 aftershocks from a 1½ by 2 km source area centered along the fault 16 km NW of the mainshock epicenter were also studied. First motion data suggest that all of the aftershocks and a swarm of six preshocks on December 7–9, 1978...

Geodetic displacements and aftershocks following the 2001 M_w = 8.4 Peru earthquake: Implications for the mechanics of the earthquake cycle along subduction zones

Perfettini, H.; Avouac, J.-P.; Ruegg, J.-C.
Fonte: American Geophysical Union Publicador: American Geophysical Union
Tipo: Article; PeerReviewed Formato: application/pdf
Publicado em 03/09/2005 Português
Relevância na Pesquisa
27.43%
We analyzed aftershocks and postseismic deformation recorded by the continuous GPS station AREQ following the M_w = 8.4, 23 June 2001 Peru earthquake. This station moved by 50 cm trenchward, in a N235°E direction during the coseismic phase, and continued to move in the same direction for an additional 15 cm over the next 2 years. We compare observations with the prediction of a simple one-dimensional (1-D) system of springs, sliders, and dashpot loaded by a constant force, meant to simulate stress transfer during the seismic cycle. The model incorporates a seismogenic fault zone, obeying rate-weakening friction, a zone of deep afterslip, the brittle creep fault zone (BCFZ) obeying rate-strengthening friction, and a zone of viscous flow at depth, the ductile fault zone (DFZ). This simple model captures the main features of the temporal evolution of seismicity and deformation. Our results imply that crustal strain associated with stress accumulation during the interseismic period is probably not stationary over most of the interseismic period. The BCFZ appears to control the early postseismic response (afterslip and aftershocks), although an immediate increase, by a factor of about 1.77, of ductile shear rate is required, placing constraints on the effective viscosity of the DFZ. Following a large subduction earthquake...

Source processes of three aftershocks of the 1983 Goodnow, New York, earthquake: High-resolution images of small, symmetric ruptures

Xie, Jiakang; Liu, Zuyuan; Herrmann, Robert B.; Cranswick, Edward
Fonte: Seismological Society of America Publicador: Seismological Society of America
Tipo: Article; PeerReviewed Formato: application/pdf
Publicado em /06/1991 Português
Relevância na Pesquisa
27.55%
Broadband, large dynamic range GEOS data from four aftershocks (M_L ∼ 2 to 3) of the 1983 Goodnow, New York, earthquake, recorded at hard-rock sites 2 to 7 km away from the epicentral area, are used to study rupture processes of three larger events, with an M_L = 1.6 event as the Green's function event. We analyze the spectra and spectral ratios of ground velocity at frequencies up to 100 Hz, and conclude that (1) there are resolvable P-wave f_(max) (51 and 57 Hz) at two sites; (2) there is abundant information on sources of larger events up to frequencies of 50 to 60 Hz; and (3) there is an unstable, nonlinear instrument resonance at about 90 Hz. We analyze the artifacts of low-pass filters in the deconvolved rupture process, including the limited time resolution and biases in the rise-time measurement. Some extensions of the empirical Green's function (EGF) method are proposed to reduce these artifacts and to precisely estimate the relative locations of events that are close both in time and in space. Applying the EGF method to three aftershocks, we find that these events have ruptures that are simple crack-like, characterized by small fault radii (∼ 70 to 120 m) and static stress drops that vary, depending on the size of the events...

Source parameters of the 23 April 1992 M 6.1 Joshua Tree, California, earthquake and its aftershocks: Empirical Green's function analysis of GEOS and TERRAscope data

Hough, S. E.; Dreger, D. S.
Fonte: Seismological Society of America Publicador: Seismological Society of America
Tipo: Article; PeerReviewed Formato: application/pdf
Publicado em /12/1995 Português
Relevância na Pesquisa
27.55%
Source parameters of the M 6.1 23 April 1992 Joshua Tree mainshock and 86 M 1.8 to 4.9 aftershocks are determined using an empirical Green's function methodology. For the aftershocks, deconvolved P- and S-wave spectra are calculated for 126 pairs of closely spaced events recorded on portable GEOS stations; S-wave spectra from the two horizontal components are averaged. The deconvolved spectra are fit by a ratio of omega-square source models, yielding an optimal (least-squares) corner frequency for both the large and the small event in each pair. We find no resolved difference between the inferred P- and S-wave corner frequencies. Using the standard Brune model for stress drop, we also find no resolved nonconstant scaling of stress drop with moment, although we also conclude that detailed scaling systematics would be difficult to resolve. In particular, a weak increase of stress drop with moment over a limited moment/magnitude cannot be ruled out. For magnitudes smaller than M 3 to 3.5, the inferred stress-drop values will be limited by the maximum observable corner frequency value of 60 Hz. For the mainshock, source-time functions are obtained from mainshock recordings at three TERRAscope stations (PFO, PAS, and GSC) using an M 4.3 foreshock as an empirical Green's function. The results indicate a fairly simple...

Foreshocks, Aftershocks, and Earthquake Probabilities: Accounting for the Landers Earthquake

Jones, Lucile M.
Fonte: Seismological Society of America Publicador: Seismological Society of America
Tipo: Article; PeerReviewed Formato: application/pdf
Publicado em /06/1994 Português
Relevância na Pesquisa
27.55%
The equation to determine the probability that an earthquake occurring near a major fault will be a foreshock to a mainshock on that fault is modified to include the case of aftershocks to a previous earthquake occurring near the fault. The addition of aftershocks to the background seismicity makes its less probable that an earthquake will be a foreshock, because nonforeshocks have become more common. As the aftershocks decay with time, the probability that an earthquake will be a foreshock increases. However, fault interactions between the first mainshock and the major fault can increase the long-term probability of a characteristic earthquake on that fault, which will, in turn, increase the probability that an event is a foreshock, compensating for the decrease caused by the aftershocks.

Foreshocks and Aftershocks of the Great 1857 California Earthquake

Meltzner, Aron J.; Wald, David J.
Fonte: Seismological Society of America Publicador: Seismological Society of America
Tipo: Article; PeerReviewed Formato: application/pdf
Publicado em /08/1999 Português
Relevância na Pesquisa
27.55%
The San Andreas fault is the longest fault in California and one of the longest strike-slip faults anywhere in the world, yet we know little about many aspects of its behavior before, during, and after large earthquakes. We conducted a study to locate and to estimate magnitudes for the largest foreshocks and aftershocks of the 1857 M 7.9 Fort Tejon earthquake on the central and southern segments of the fault. We began by searching archived first-hand accounts from 1857 through 1862, by grouping felt reports temporally, and by assigning modified Mercalli intensities to each site. We then used a modified form of the grid-search algorithm of Bakun and Wentworth, derived from empirical analysis of modern earthquakes, to find the location and magnitude most consistent with the assigned intensities for each of the largest events. The result confirms a conclusion of Sieh that at least two foreshocks (“dawn” and “sunrise”) located on or near the Parkfield segment of the San Andreas fault preceded the mainshock. We estimate their magnitudes to be M ≈ 6.1 and M ≈ 5.6, respectively. The aftershock rate was below average but within one standard deviation of the number of aftershocks expected based on statistics of modern southern California mainshock-aftershock sequences. The aftershocks included two significant events during the first eight days of the sequence...

Tectonic interpretation of aftershock relocations in eastern Papua New Guinea using teleseismic data and the arrival pattern method

Tregoning, Paul; Sambridge, Malcolm; McQueen, Herbert; Toulmin, S; Nicholson, T
Fonte: Blackwell Publishing Ltd Publicador: Blackwell Publishing Ltd
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
27.43%
The arrival pattern (AP) method and teleseismic data have been used to relocate aftershocks following three major (Mw > = 7.8) interplate earthquakes that occurred in eastern Papua New Guinea in 2000 November. The first event was an Mw = 8.0 strike-slip earthquake that ruptured the Weitin fault over a length of ∼100 km. No aftershocks were located in the upper 15 km, suggesting that the accumulated strain was fully released in this event. The two subsequent Mw = 7.8 thrust events occurred on the New Britain trench. Wadati-Benioff zones dipping ∼20° define the locations of the thrust events at 152°E and 153°E on fault planes with strikes of 75° and 65°, respectively. The plot of aftershocks defines a complex seismic zone beneath New Ireland where events have occurred in the overriding Pacific and South Bismarck plates above the deeper zone of subduction of the Solomon Sea Plate. These events cluster in a broad zone to the northeast of the Weitin fault rather than defining a linear feature that might have been expected following a near-vertical major strike-slip event.