The Wharton Basin earthquake occurred about 5° south of the distributed zone of deformation proposed by Gordon et al. You could also do it yourself at any point in time. Abstract Mechanism of the June 4, 2000 southern Sumatra, Indonesia, earthquake (M S =8.0) are estimated from teleseismic body waves recorded by long period seismograph stations of the global seismic network. Three aftershocks lying to the south of subevent 1 (Figure 5a) have similar strike‐slip mechanisms to subevent 1 and so are consistent with our interpretation of a N‐S fault plane within the subducting slab. The 1994 Kurile Islands earthquake (Mw 8.3) has also been interpreted as a tear within the subducting slab, but it did not trigger slip at the plate interface [Tanioka et al., 1995]. By contrast, both the Enggano and Kermadec earthquakes had large numbers of plate interface aftershocks, suggesting that the subduction zone earthquakes are more like continental earthquakes in their ability to trigger aftershocks. Properties of Rocks, Computational Alternatively, those aftershocks to the NE could be related to the second subevent. (1896-1977), Chinese Journal of Geophysics (2000-2018), International [1990], and DeMets et al. [7] The two large earthquakes that occurred on 4 and 18 June 2000 can provide critical constraints on the nature of present‐day deformation in the complex region of the Indian Ocean. Fixing either source as a point source increases the RMS error by about 5% and results in an obvious decrease in visual fit. They recalculated the long period moment tensor and determined that the large non‐double‐couple component in the Harvard CMT solution (35%) is not well resolved. Both the June 2000 earthquakes are consistent with recent models of distributed deformation in the India-Australia composite plate. This agrees well with the preferred fault plane for subevent 1. Seismicity is distributed along and around the Ninety East (90°E) ridge, a prominent feature (Figure 1) that is thought to be an old hot spot trace following a fossil transform fault [e.g., Stein and Okal, 1978]. Earthquake swarms on the Mid‐Atlantic Ridge: Products of magmatism or extensional tectonics? Please check your email for instructions on resetting your password. [16] The maximum depth of the aftershocks decreases to the SE (Figure 5, cross section A), and several aftershocks are at similar depths to the centroid of subevent 1. The occurrence of the Enggano earthquake implies that the stress field within the Indian plate continues to a depth of 50 km in the subducting slab. The 2000 Enggano earthquake took place at the southeastern end of the fault segment that ruptured during the 1833 Sumatra earthquake. [11] He called for a rapid international response to the quake, and said he would keep its victims in his prayers. [34] The occurrence of these earthquakes indicates that fossil fracture zones in the Indian Ocean are being reactivated in the current stress field as strike‐slip faults and that they can rupture in large earthquakes. This first subevent then triggered reverse slip on the subduction interface, rupturing toward the SE. A recent report predicts that Bengkulu is "at risk of inundation over the next few decades from undersea earthquakes predicted along the coast of Sumatra" [9] A series of earthquakes struck Bengkulu during September 2007, killing 13 people. We perform waveform modeling and relocate aftershocks to determine the fault planes of the two earthquakes. There were more than 100 fatalities and up to 2,585 injuries. The orientation of both subevents in our preferred model is consistent with the current stress field in the region. The first (∼65% of the moment) involves strike‐slip motion on a near vertical, approximately N‐S trending fault within the subducting slab, lasting ∼30 s. The rupture propagated toward the north from the hypocenter. It was felt as far away as Singapore where many high‐rise buildings were shaken [ Pan et al., 2001 ]. The six examples include the two cases, 2011 Tohoku-oki and 2007 Bengkulu earthquakes, where interseismic velocities deviate largely from relative plate subduction directions. Composition and Structure, Atmospheric [2001] who only attempt to model the S waves. Physics, Solar [7] "[8] An aftershock measuring 6.2 struck on June 7. [2000] for the 1998 Antarctic earthquake (Mw 8.1) which was the largest intraplate strike‐slip earthquake recorded to date. It is plotted separately in Figure 5b. Seismic behavior of oceanic transform faults, Fault reactivation in the central Indian Ocean and the rheology of oceanic lithosphere, Intraplate shortening in the central Indian Ocean determined from a 2100 km‐long north–south deep seismic reflection profile, Global patterns of radiated seismic energy and apparent stress. No comments yet Add comment. Both subevents show significant directivity. 1999 Jiji earthquake. 4 ). • January 1 - The Year 2000 problem occurs, wrongly and eventually changing the dates of computerized system to wrong dates. Sumatra is often cited as a classic example of a so‐called slip‐partitioned margin [Fitch, 1972]. Both the June 2000 earthquakes are consistent with recent models of distributed deformation in the India–Australia composite plate. The occurrence of the Enggano earthquake implies that the stress field within the Indian plate continues to a depth of 50 km in the subducting slab. We do not use the Harvard CMT as a constraint on the body wave modeling. [1998] find clear evidence for reactivation of N‐S fractures in a left‐lateral sense on seismic profiles through the Wharton Basin. In contrast the subduction thrust subevent had an active aftershock sequence and a lower stress drop, typical of subduction earthquakes. Both the strike‐slip subevents of the Enggano and Wharton Basin earthquakes have few aftershocks. The first motions constrain the mechanism of subevent 1 to within about 2°, but the SH waves are slightly better fit by a mechanism rotated about 10° anticlockwise. The aftershock distribution is also consistent with our preferred rupture geometry of the Enggano earthquake. These values are similar to the stress drop of 16 MPa estimated by Antolik et al. Simple static stress modeling suggests that subevent 1 increased the stress on the plate interface to the SE, in the region of the second subevent. When we constrain the source to be a double couple, however, we cannot fit the waveforms with a single mechanism. The directivity is toward the north and the visual fit to the waveforms decreases significantly (the RMS error increases by 20%) if the solution is fixed as a point source. The Harvard CMT mechanism for the 4 June main shock is joined to its NEIC epicenter (black star) and CMT centroid (white star). The depth of the second subevent can vary by ±10 km with negligible effect on the waveform fits. Indonesia. People were afraid that the computers won’t switch in the proper way from 1999 to 2000 but these concerns weren’t justified. Congratulations on this excellent venture… what a great idea! [3] The 4 June Enggano earthquake occurred within the seismically active Sumatra subduction zone. and you may need to create a new Wiley Online Library account. The poorer constraints result from the relatively small size of this subevent (25% of the moment) and the fact that it occurs entirely within the rupture duration of the larger, first subevent. Use the link below to share a full-text version of this article with your friends and colleagues. Over 730 aftershocks shocked the area afterwards, one just eleven minutes after the mainshock. Both of these earthquakes were largely unexpected, and the aim of our study is to determine their rupture geometry and to investigate what they reveal about the complex seismotectonics of the Indian Ocean and western Indonesia. Newcomb and McCann [1987] cite evidence for fracture zones at 102.5° and 104.5°, based on age offsets and sediment filled troughs. The two earthquakes near its hypocenter in Figure 1 are both aftershocks. In the rupture area of the 1833 earthquake, the upper and lower subducting plates are thought to be fully seismically coupled [Zachariasen et al., 2000], but the coupling appears to decrease near the equator [e.g., Prawirodirdjo et al., 1997]. Therefore, we conclude that the reverse faulting subevent is real and not an artifact of an oversimplified model velocity structure. The westerly of these two is a likely candidate for the fault that ruptured in the 4 June Enggano earthquake. The more recent December 1999 Kodiak Island earthquake was also intrapate with an oblique strike‐slip mechanism, but aftershock relocations imply that it ruptured a steeply dipping fault plane parallel to the trench and was driven by downdip tension in the slab [Ratchkovski and Hansen, 2001]. In September 2001, a M6 reverse faulting earthquake occurred NE of the Wharton Basin earthquake, with a mechanism similar to that of subevent 2 (Figure 7). There … B shows the subducting plate. The onset time and orientation of the second subevent are constrained by the need to fit the P waves to the NW and SW. The mechanisms of earthquakes in the Harvard catalogue, Distribution of seismicity in the Indian Ocean. [32] Both the stress drop and number of aftershocks are of interest as they are first‐order observations as to the differences in earthquake rupture in continental and oceanic lithosphere. The Enggano earthquake was too deep (∼50 km) to be on either the Sumatra or the Mentawai faults. We investigate the uncertainties in the Harvard CMT solution and also find that the best double couple solution (strike, dip, and rake are 168°, 53°, and 11°, respectively) has an RMS error insignificantly higher than that of the solution in the catalogue. It will be interesting to see what motions are observed in the region after the Enggano earthquake. More recently, a three‐plate model has been proposed to explain the region of complex deformation in the central Indian Ocean (Figure 3) [Royer and Gordon, 1997]. We can then include the S waves in the inversion. It will enhance any encyclopedic page you visit with the magic of the WIKI 2 technology. The geometry of the Enggano earthquake and aftershock sequence. [1993] showed that the Kermadec earthquake involved primarily strike‐slip faulting in the subducting slab at a depth of about 45 km. Both the June 2000 earthquakes are consistent with recent models of distributed deformation in the India–Australia composite plate. "The June 2000 Mw 7.9 earthquakes south of Sumatra: Deformation in the India–Australia Plate", "The Sunda megathrust: past, present and future", "Indonesia appeals for help following Sumatran earthquake", "Photo Essay 6/8/2000 - Earthquake in Indonesia", M7.9 Enggano Island-Bengkuku Earthquake, 2000. [23] The Wharton Basin earthquake occurred near to the Investigator Fracture Zone but, although the fault orientation is similar, the locations are too far to the west (∼75 km) for the earthquake to have ruptured the ridge identified as the Investigator Fracture Zone, and it must have occurred on a subparallel feature. We invert only for moment rate functions. We do not see any indication of such reverberations in the part of the waveforms that we model. We fix the first mechanism at strike‐slip solutions consistent with the polarity and amplitude of the first motions (which provide tight constraints of ±2°) (Figure 4) and invert for the mechanism and source parameters of the second subevent. The purely strike‐slip source model of the Wharton Basin earthquake obtained by Robinson et al. The Enggano earthquake had only three strike‐slip aftershocks within 2 orders of magnitude and the Wharton Basin had only one. If you do not receive an email within 10 minutes, your email address may not be registered, However, the area near Enggano Island has been identified as a gap for earthquakes greater than magnitude 8 (ref. [2001]. In the Enggano earthquake the reverse slip subevent was a plate interface thrust comprising at least 35% of the total moment and rupturing to the SE away from the rupture area of the great 1833 earthquake. The relocated epicenters are close to their corresponding NEIC locations, but the NNW‐SSE alignment is a little more tightly defined after relocation. Physics, Comets and They did not observe a tear in the slab, but were unable to resolve a vertical offset in hypocenters of less than 20 km. The distributed deformation is thought to be a consequence of the collision of India with Eurasia impeding the approximately NNE motion of the India–Australia plate in the west and continuing subduction along the Sunda Arc beneath Java and Sumatra. The waveforms (Figure 6) are significantly shorter and less complex than those of the Enggano earthquake. Typically continental earthquakes have one aftershock within an order of magnitude of the main shock, 10 within 2 orders of magnitude and so on (Båth's Law, first cited by Richter [1958]). The hypocenter and strike‐slip subevent are near the southern extent of the great 1833 rupture (Figure 3). The layout and symbols are the same as in, The geometry of the Wharton Basin earthquake and aftershock sequence. A world-wide fear began to spread among people that the world we know will come to an end. The event oc­curred off the coast of south­ern Suma­tra, In­done­sia near Enggano Is­land. Thus it represents another example of tearing of the subducting slab. The Investigator Fracture Zone (IFZ) and Fracture Zone F [, Broadband body wave modeling of the 18 June Wharton Basin earthquake [after, By continuing to browse this site, you agree to its use of cookies as described in our, Journal of Advances New and Popular Countries of the World Quiz. The seismograms are relatively long and complex (Figure 4), even for an earthquake of this size. The largest earthquakes are usually the best recorded, so we start with those aftershocks with M ≥ 5. [2001] failed to model is the phase with the largest amplitude recorded at stations to the NW and starts only about 5 s after the onset. The more recently discovered Mentawai fault in the forearc [Diament et al., 1992] does not appear to be active [Genrich et al., 2000]. [2001] does not fit the P waves to the NW and SW. Robinson et al. We divide the region from 0°S to 7°S into overlapping rectangles, allowing comparison of hypocenter relocations derived for earthquakes included in more than one inversion. They provide sufficient additional constraints to enable us to invert for all the source parameters of both subevents and obtain a stable solution. The Harvard CMT mechanisms of the larger aftershocks exhibit considerable diversity (Figure 2). The aftershocks are all in an area of continued high activity over recent decades. We thus try inverting for two separate mechanisms. The overall rupture area of the earthquake, therefore, could include large regions that slipped aseismically during and following the seismic slip. [2001] also estimated a relatively high stress drop for the Wharton Basin earthquake. Note that the first subevent of the main shock is plotted at the depth determined in the body wave modeling, deeper than the relocated hypocenter. The ISC and NEIC catalogues (since 1970, complete above M ∼ 5) contain no previous earthquakes in the area of Figure 7. Off‐fault aftershocks caused by shear stress increase? The uncertainty in the distribution of slip in the subevents of the Enggano earthquake prevents us from calculating a meaningful model of the static stress change induced by this earthquake in the rupture area of the 1833 earthquake. Comments by D. Kilb, J. Milsom, and the AE J. Freymueller and two anonymous reviewers greatly improved this manuscript. and Chemical Oceanography, Physical We use the GMT software for many of the figures [Wessel and Smith, 1991]. The moment of the second subevent varies from about a third to a half of the total, depending on its orientation. Modeling strike‐slip earthquakes in the Atlantic Ocean with and without a water layer made a negligible difference to the shape of the synthetics, suggesting that such reverberations are small for strike‐slip mechanisms in relatively flat‐lying strata [Abercrombie and Ekström, 2001]. Kanamori and Anderson [1975] observed a difference in stress drop between interplate and intraplate earthquakes and interpreted it as a dependence on strain rate and recurrence time. 14°21'15°00Asc.01°39'. This is significantly lower, despite the wide range of possible rupture areas considered in the calculations to reflect the resolution of the parameters. It is probable that structural heterogeneity is contributing to the smaller‐scale complexity of the waveforms, especially the later arrivals, and so we do not try a more complicated source model that attempts to match every detail. The rupture directivity and hypocenter, centroid and aftershock locations support this interpretation and also imply rupture to the north. Wiens et al. Our relocations are sufficiently accurate to provide the context for the Enggano earthquake sequence that is necessary here. [9], International relief teams arrived in the region within several days. The 1986 Kermadec earthquake appears to have been similar to the Enggano earthquake and that had a stress drop (dominated by the strike‐slip component) of 10–20 MPa [Houston et al., 1993]. In the case of the Enggano earthquake, the low stress drop could be related to the degree of seismic coupling. [12] In order to investigate further the rupture geometry of the sequence we relocate the aftershocks recorded by the NEIC from June to October 2000, using the hypocentroid decomposition method of relative relocation developed by Jordan and Sverdrup [1981] and Bergman and Solomon [1990]. To the east of the 90°E ridge, however, the earthquakes occur on N‐S (or E‐W) striking strike‐slip faults and a few NE‐SW striking reverse faults. [1996] noted that such seismicity is unusual as most subducting fracture zones are aseismic. Thus we do not believe that the rupture direction can be resolved or used to distinguish the fault plane for this second subevent. 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Than initially defined by Gordon et al velocities, and the rupture area of the first subevent ±5! Global study by Boettcher and Jordan [ 2001 ] also estimated a relatively high stress drops and few and! Of about 45 km considerable diversity ( Figure 2 ) fix our inversion parameters to their NEIC epicenters the end! `` sincere sympathy '' for those families stricken by the earthquake, therefore, conclude... Rapid International response to the Harvard CMT as a gap for earthquakes, the... With M ≥ 5 obtained by Robinson et al were impeded by fallen telephone poles, which blocked the.! Causing this reactivation is present to depths of the two subevents is close to the Harvard CMTs are joined their... Constraints to enable the inversion 7.9 Enggano earthquake is in a region of continuously high over... 2 technology who only attempt to model the broadband body wave modeling but poorly resolved, component. 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