Downgoing plate topography stopped rupture in the A.D. 2005 Sumatra earthquake
T. HenstockL.C. McNeillJonathan M. BullBecky CookS. P. S. GulickJames A. AustinHaryadi PermanaY. Djajadihardja
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Earthquakes in subduction zones rupture the plate boundary fault in discrete segments. One factor that may control this segmentation is topography on the downgoing plate, although it is controversial whether this is by weakening or strengthening of the fault. We use multichannel seismic and gravity data to map the top of the downgoing oceanic crust offshore central Sumatra, Indonesia. Our survey spans a complex segment boundary zone between the southern termination of the Mw = 8.7, A.D. 2005 Simeulue-Nias earthquake, and the northern termination of a major 1797 earthquake that was partly filled by an Mw = 7.7 event in 1935. We identify an isolated 3 km basement high at the northern edge of this zone, close to the 2005 slip termination. The high probably originated at the Wharton fossil ridge, and is almost aseismic in both local and global data sets, suggesting that while the region around it may be weakened by fracturing and fluids, the basement high locally strengthens the plate boundary, stopping rupture propagation.Keywords:
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In explaining paleoseismic observations of coastal subsidence during the great AD 1700 Cascadia earthquake, past rupture models have assumed a uniform slip distribution along strike of the Cascadia megathrust. Similar to slip distributions during instrumentally recorded great subduction earthquakes worldwide, we infer heterogeneous slip for the AD 1700 Cascadia earthquake. The assumption of uniform distribution in previous rupture models was due partly to large uncertainties in paleoseismic data that used to constrain the models. In this work, we use more precise estimates of coseismic elevation change in 1700 from detailed tidal microfossil studies. A 3-D elastic dislocation model is developed that allows the slip to vary both along strike and in the dip direction. Despite uncertainties in the updip and downdip extents of coseismic slip, the new, more precise microfossil-based subsidence estimates are best explained by a model with heterogeneous slip in the strike direction, with areas of larger slip separated by areas of smaller slip. For example, the model indicates very little slip near Alsea Bay, Oregon (~ 44.4°N), during the AD 1700 earthquake. This location coincides with a segment boundary previously inferred from gravity anomalies. A probable subducting seamount in this area may be responsible for impeding rupture during great earthquakes. Our results highlight the need for more precise, high-quality estimates of subsidence or uplift during prehistoric earthquakes, especially along the coasts of southern British Columbia, northern Washington, southernmost Oregon, and northern California (north of 47°N and south of 43°N), where slip distributions in our models are poorly constrained.
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We developed velocity models of the crust and sediments offshore south western Greece, between the island of Zakynthos and Messinia. Using these velocity models and depth migrating the seismic data we delineated the main faults and associated them with the tectonic processes of western Greece. This active seismic experiment was essential for defining the limits between the continental domain of western Greece and the oceanic one of the deep Ionian Sea. We successfully linked the onshore with the offshore tectonics and for the first time it was possible to understand how the main dextral fault systems of Cephalonia and Andravida are responsible for the crustal deformation, and its link to the local seismicity. Most of the seismic activity is connected to thrusting, due to crustal shortening or strike-slip faulting that follows the two main dextral wrench faults of Cephalonia and Andravida. It was recognized that the back stop offshore western Peloponnese is floored by thinned continental crust of Preapulia and that the Hellenic Alpine napes do not extend in the back stop domain.
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New types of slow earthquakes have been revealed associated with the subduction of the Philippine Sea plate in southwest Japan based on the development of the dense seismic observation network. At the transition zone, which lies at the deeper extension of the seismogenic zone, deep low-frequency tremors, deep very-low-frequency earthquakes, and short-term slow slip events occur along the slab strike. The short-term slow slip event is the main phenomenon as a stick-slip on the plate interface and other seismic phenomena might be triggered events reflecting the source size. Considering the area of migration and activity of the deep low-frequency tremors, which indicate a transient slip, we can divide the deep slow earthquake belt into segments, each of which has a regular recurrence interval of 6 or 3 months. In each episode, the tremor activity migrates according to the propagation of the transient slip within the segment. Sometimes the migration reaches to the neighboring segment, and a connecting fault rupture occurs. These migrating phenomena are very similar to the rupture process of mega-thrust earthquakes. On the other hand, shallow very-low-frequency earthquakes have been detected in the accretionary prism near the Nankai trough. These events are considered seismic slow slip events at the reverse fault system reflecting the deformation of the accretionary prism. Watching these slow earthquakes is important for the monitoring of the plate motion.
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Abstract Aftershock data recorded on portable seismographs deployed immediately after the MW 6.4 Doubtful Sound earthquake of 1989 May 31 and the MW 6.8 Secretary Island earthquake of 1993 August 10 have been used to constrain the rupture zones of these events. Both earthquakes involved slip at the interface between the subducted Australian plate and overlying Pacific plate. Their rupture zones abut rather than overlap, and the region where they meet lies vertically beneath the surface trace of the East Branch of the Alpine Fault. Slip during the deeper 1989 event was approximately in the plate convergence direction, whereas slip during the shallower 1993 event was approximately down the dip of the subducted plate. This requires slip partitioning in the shallow part of the subduction zone, and suggests that the East Branch of the Alpine Fault is active in this part of Fiordland. The 1989 earthquake produced very few aftershocks, whereas the 1993 earthquake had a rich aftershock sequence. This difference, and the mismatch in slip direction between the two events, can be attributed to changes in the frictional regime at the plate interface with depth. Static stress changes produced by slip in the 1989 earthquake promoted down‐dip thrusting on the rupture zone of the 1993 event. Thus, the 1989 event appears to have triggered the 1993 event, in the sense that it moved it closer to failure.
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This paper reviews the studies on seismic activities and tectonics in southwestern Japan. We start with the stress state of southwestern Japan with reference to movements of the Philippine Sea plate, the Eurasian plate and other concerning plates. Then we describe subcrustal seismic activity associated with the subduction of the Philippine Sea plate and shallow earthquakes occurring in the upper crust in relation to the active faults and mechanical properties of the crust. We also give a brief summary of studies on microearthuakes in the concerned districts.
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The 1995 Amami-Oshima-Kinkai Earthquake occurred near the Nansei-Shoto Trench where the upheaval area of the Philippines Sea plate subducts beneath the Nansei-Shoto islands. The main shock was MJMA 6.6 and its largest aftershock was MJMA 6.5. The aftershock distribution for these two events by Yamada et al. (1996) corresponds to two distinct and nearly vertical fault zones. The focal mechanisms obtained by Kikuchi (1996) are consistent to the aftershock distribution.The authors propose that the seamount found beneath the trench-continental-slope indirectly triggered this earthquake activity. If a subducting oceanic plate is normal oceanic denser than an overriding island arc, the oceanic plate should be faulted near vertically priori to the plate subduction by horizontally tensional force due to plat bending. On the other hand, an oceanic plate with seamounts or an oceanic plateau lighter than a normal oceanic plate, might resist to plate subduction due to its small density and delaying normal faultings might occur in the subducting oceanic plate. The delaying normal faultings between a subducting seamount and a preceding normal portion of the oceanic plate can compensate the subduction process. The compressional convergence margin such as the Nankai Trough, however, may not generate such normal faultings due to the nature of stress field.The low seismicity area existing across the trench axis is also seen both in this aftershock activity and ISC hypocenters. This is the same result as those in other regions. This might imply low earthquake potential for this portion of plate interface due to the existence of low density sediments and water contained in the sediments.
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