Arrival times of compressional (P) and shear (S) waves generated by earthquakes at local and teleseismic distances and recorded by seismographs located in the westem and central Tien Shan are used to determine one-and three-dimensional elastic wave velocity structures of the crust and upper mantle beneath the mountain belt. The best fit one-dimensional structures suggest that the average depth of the Mohorovicic discontinuity in this area is 50 km. The three-dimensional structure of the upper crust reveals thick sediments within each of the major depressions in the region. A 7 km-thick wedge of sediment beneath the Chu Depression is outlined at depth by a south dipping plane of seismic activity, suggesting the presence of an active decollemont. These low velocities extend continuously to the southeast toward Issyk-Kul, suggesting a structural relationship between the two. However, rather than being consumed, it appears that Issyk-Kul is overthrusting the surrounding ranges. The low-velocity sediments in the Fergana basin reach depths of 10 km and are bounded on three sides by amorphous bands of seismicity. Velocities at midcrustal depths generally are lower beneath the central Tien Shan than beneath the western Tien Shan. This pattern becomes more evident in the uppermost mantle, with P velocity contrasts of as much as 10% across a boundary that corresponds roughly to the geographical position of the Talasso-Fergana fault. The low velocities beneath the central Tien Shan exceed 150 km depth but do not appear to be deeper than 300 km depth. There is no evidence for a lithospheric root beneath this part of the range; rather, the low velocities imply the presence of a positive buoyancy force uplifting the mountains. Evidence that this low-velocity region existed before the collision suggests that the Tien Shan may not owe its rejuvenation simply to its location at the northern edge of a strong Tarim basin but rather to an anomalous upper mantle that was easier to deform than the surrounding lithosphere. the Tien Shan can be divided into three fault bounded units [Kravchenko, 1979]. All three units consist largely of sedimentary rocks that formed during the late Proterozoic (in the north) to Cambrian (in the south). These units were accreted onto Eurasia beginning in the early Paleozoic for the northern unit to the late Carboniferous for the southern unit [Burtman, 1975, 1987; Krestnikov, 1962]. The whole area appears to have been stable throughout the Mesozoic, with very little relief in the late Cretaceous. Tectonic activity resumed in the Oligocene, presumably as a consequence of the collision of India with Eurasia and has continued to the present day. The present high level of activity is evidenced in northsouth shortening accommodated along both east-west trending thrust faults and northwest-southeast trending strike-slip faults such as the Talasso-Fergana fault (Figure 1). The region is seismically very active; several earthquakes with M s > 8.0 have occurred in this area since 1900. Analyses of large earthquak...
[1] We report a sequence of crustal quakes that began after the M w = 8.8 thrustsubduction Maule earthquake that affected the Central Chile margin on 27 February 2010. This activity lasted by several months, having the most important events on 11 March 2010 (M w = 6.9 and M w = 7.0) with normal focal mechanisms. Seismicity shows a rupture oriented along a NW-striking and SW-dipping normal fault from the surface down to the interplate contact. Seismicity can be correlated with neotectonics extensional structures similarly oriented in the region, which have coexisted with NNE-SSW reverse faults since the late Pliocene, even though both have older periods of activity since the Paleozoic. This crustal rupture would have been triggered by the high Coulomb stress change produced by the Maule earthquake, enhanced by likely fluid presence along weakened zones of the forearc crust as evidenced by high Vp/Vs ratio. The occurrence of relevant neotectonic activity in coincidence with short-term deformation suggests a relationship with long-term tectonic features of this region, which would have been acting as a barrier during the interseismic period, increasing the strain accumulation and triggering contractional faulting in the crust, as well as producing high slip patches during great subduction ruptures favoring triggering of crustal extensional faulting. Crustal faulting in Pichilemu suggests that this kind of events should be considered in seismic hazard analysis despite the absence of historical crustal seismic activity before the Maule earthquake.
The island of Taiwan is a byproduct of one of the few active collisions between a continent, represented by the Eurasian continental shelf, and an island arc, represented by the northern extension of the Luzon arc on the Philippine Sea plate. To understand better the evolution and current tectonics of this collision, we selected 1260 well‐recorded earthquakes from an initial set of 50,000 located by the Taiwan Telemetered Seismograph Network for the determination of one‐ and three‐dimensional P and S wave velocity structures beneath the island. The results for both structures and earthquake relocations reveal that the island is divided tectonically into three distinct zones. In the east, the Philippine oceanic plate is underthrusting the Eurasian plate east of a north‐south boundary that is well defined by both seismic activity and a region of high velocity. In the south, the Eurasian continental plate is underthrusting the Philippine Sea plate south of an east‐west boundary at about 23°N that is sharply defined by both subcrustal seismicity and a zone of relatively low velocities. The dip of the subducted continent is shallow until it reaches the Luzon island arc 50 km east of the main island. Structure under the main part of the island north of 230N reveals a shallow dipping zone of low velocities in the west above 25 km depth that narrows and steepens below that depth to at least 50 km beneath the central range. The dip of this low‐velocity region is outlined by a narrow zone of seismicity that extends to depths of 100 km. This seismic zone lies in a velocity “saddle” and marks an apparent offset in the central low‐velocity region at 24°N. Evidence for the subduction of the Eurasian continent beneath Taiwan therefore exists everywhere beneath the island, and the low‐velocity regions in the mantle support, but do not require, the subduction of some 6–16 km of lower continental crust to depths of at least 50 km. The distinct change in both the velocity structure and the seismic activity at 23°N and the offset of the low‐velocity regions at 24°N argue for abrupt changes in the nature of subduction across these latitudes. These changes may be caused by an interaction of the Luzon island arc with the subducted continental shelf that mimics a similar interaction evident at the surface. Finally, the velocity structure of the subducting Philippine sea plate suggests that the earthquakes beneath 70 km depth are not occurring within the highest‐velocity regions of the plate, but are probably located near the upper edge of the plate. There is also evidence from earthquake locations and velocity structure that suggests that the subducting plate is segmented, and that subduction is presently occurring as far south as 22°N.
[1] The 27 February 2010 Maule, Chile (Mw=8.8) earthquake is one of the best instrumentally observed subduction zone megathrust events. Here we present locations, magnitudes and cumulative equivalent moment of the first $2 months of aftershocks, recorded on a temporary network deployed within 2 weeks of the occurrence of the mainshock. Using automatically-determined onset times and a back projection approach for event association, we are able to detect over 30,000 events in the time period analyzed. To further increase the location accuracy, we systematically searched for potential S-wave arrivals and events were located in a regional 2D velocity model. Additionally, we calculated regional moment tensors to gain insight into the deformation history of the aftershock sequence. We find that the aftershock seismicity is concentrated between 40 and 140 km distance from the trench over a depth range of 10 to 35 km. Focal mechanisms indicate a predominance of thrust faulting, with occasional normal faulting events. Increased activity is seen in the outer-rise region of the Nazca plate, predominantly in the northern part of the rupture area. Further down-dip, a second band of clustered seismicity, showing mainly thrust motion, is located at depths of 40-45 km. By comparing recent published mainshock source inversions with our aftershock distribution, we discriminate slip models based on the assumption that aftershocks occur in areas of rapid transition between high and low slip, surrounding high-slip regions of the mainshock. Citation: Rietbrock, A.,
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.