The Mw 7.7 Tocopilla Earthquake of 14 November 2007 at the Southern Edge of the Northern Chile Seismic Gap: Rupture in the Deep Part of the Coupled Plate Interface
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The slip distribution of the Mw 7.7 Tocopilla earthquake was obtained from the joint inversion of teleseismic and strong-motion data. Rupture occurred as underthrusting at the base of the seismically coupled plate interface, mainly between 35 and 50 km depth. From the hypocenter, located below the coast 25 km south of the town of Tocopilla, the rupture propagated 50 km northward and 100 km southward. Overall, the slip distribution was dominated by two slip patches, one near the hypo- center and the other 70 km to the south where slip reached its maximum value (3 m). An additional branch of moderate slip propagated at shallower depth toward the west near the northern tip of the Mejillones peninsula. Rupture velocity remained close to 2:8 km=sec, with a total rupture duration of 45 sec. The first 2 weeks of aftershocks located with a local seismic network display a strong correlation with the slip distri- bution. The 2007 rupture ended below the Mejillones peninsula, where the 1995 An- tofagasta rupture also ended (Ruegg et al., 1996; Delouis et al., 1997; Pritchard et al., 2006). This corroborates the role of barrier played by this structure. The downdip end of the seismically coupled zone at 50 km depth, evidenced by previous studies for the 1995 event, is also confirmed. The 2007 Tocopilla earthquake contributed only mod- erately to the rupturing of the great northern Chile seismic gap, which still has the capacity for generating a much larger underthrusting event.Keywords:
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Abstract The 2007 Noto Hanto Earthquake occurred on March 25, 2007, in the Noto Peninsula, central Japan. A half day after the main shock, we started installing temporary seismic stations in order to determine the precise locations of its aftershocks. Ten universities and two research institutes deployed 88 temporary seismic stations in and around the source area. The observation lasted for about 2 months. We relocated 1318 aftershocks with arrival time corrections at each station. The relocated hypocenters show relatively small errors—less than 0.2 km in the horizontal direction and less than 0.4 km depth. Most of the relocated hypocenters are about 2.0 km shallower than those determined by JMA. The distribution of the aftershocks forms a southeast-dipping plane. The main shock is located at the bottom part of their distribution. The precise aftershock distribution extends into a shallower area than the original, and it coincides with sea floor-ward extension of the active faults previously known from a sonic reflection survey. Heterogeneous distribution of the aftershocks on the fault plane shows low seismicity just above the main shock hypocenter, and middle-size aftershocks are distributed on the periphery of the main shock. A precursory event ( M 4.4) that occurred 0.6 s before the main rupture is located close to the M 2.2 foreshock that occurred 12 min before it.
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Abstract The 2004 Mid Niigata Prefecture Earthquake (Mj = 6.8) occurred on 23 October 2004 in the northeastern part of the Niigata-Kobe Tectonic Zone where large contraction rates were observed. The mainshock was followed by an anomalously intense aftershock activity that included nine Mj ≥5.5 aftershocks. We deployed three temporary online seismic stations in the aftershock area from 27 October, combined data from the temporary stations with those from permanent stations located around the aftershock area, and determined the hypocenters of the mainshock and aftershocks with a joint hypocenter determination (JHD) technique. The resulting aftershock distribution showed that major events such as the mainshock, the largest aftershock (Mj = 6.5), the aftershock on 27 October (Mj = 6.1), etc. occurred on different fault planes that were located nearly parallel or perpendicular to each other. This might be due to heterogeneous structure in the source region. The strain energy was considered to have been enough accumulated on the individual fault planes. These features are probably a cause of the anomalous intensity of the aftershock activity.
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On May 29, 2017, an earthquake (Mw 6.6) occurred in the Poso District, central Sulawesi. This earthquake caused damage to houses and infrastructures and several people were seriously injured. About one minute after the mainshock, aftershock sequences occurred around the fault area. Up to June 4, 2017, the BMKG network had recorded 293 aftershock events. We have successfully relocated 238 out of the 293 aftershocks using hypocenter double-difference method. The results indicate improvement in hypocenter location, where the initial earthquakes focal depths fixed at a depth of 10 km have been updated. Our results show that there are two event clusters in a northwest-southeast direction. The first cluster is indicated as aftershocks and the second one is probably the triggered seismicity by the mainshock. A validation through the histogram of travel-time residuals depicts statistically fairly good relocation results, in which the residuals are mostly close to zero.
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In order to clarify the origin of aftershocks, we precisely analyze the hypocenters and focal mechanisms of the aftershocks following the 2000 Western Tottori Earthquake, which occurred in the western part of Japan, using data from dense seismic observations. We investigate whether aftershocks occur on the mainshock fault plane on which coseismic slip occurred or they represent the rupture of fractures surrounding the mainshock fault plane. Based on the hypocenter distribution of the aftershocks, the subsurface fault structure of the mainshock is estimated using principal component analysis. As a result, we can obtain the detail fault structure composed of 8 best-fit planes. We demonstrate that the aftershocks around the mainshock fault are distributed within zones of 1.0–1.5 km in thicknesses, and their focal mechanisms are significantly diverse. This result suggests that most of the aftershocks represent the rupture of fractures surrounding the mainshock fault rather than the rerupture of the mainshock fault. The aftershocks have a much wider zone compared with the exhumed fault zone in field observations, suggesting that many aftershocks occur outside the fault damage zone. We find that most aftershocks except in and around the large-slip region are well explained by coseismic stress changes. These results suggest that the thickness of the aftershock distribution may be controlled by the stress changes caused by the heterogeneous slip distribution during the mainshock. The aftershock is also distributed within a much wider zone than the hypocenter distribution observed in swarm activity in the geothermal region, which is thought to be caused by the migration of hydrothermal fluid. This result implies a difference in generation processes: Stress changes due to the mainshock contribute primarily to the occurrence of aftershocks, whereas earthquake swarms in the geothermal region are caused by fluid migration within the localized zone.
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Abstract The M 7.0 Lushan earthquake and its aftershocks in 9 days were relocated by the double difference algorithm and a layered velocity model, using the seismic phases recorded at stations within 150 km including the fixed stations of the Sichuan earthquake network center, temporary stations, and reservoir stations. The events analyzed include the main shock and its 3324 aftershocks. The results show that the origin time of the main shock is 8:02:46.8 on 20 April 2013, hypocenter location 30.278°N, 102.989°E and the focal depth 16.67 km. The main rupture is about 40 km long, 20 km wide with an apparent area 800 km 2 . It strikes in southwest and dips about 40°. Most aftershocks occurred at depths 10∼22 km, concentrating on the hanging wall of the Dayi‐Mingshan fault, forming a southwest‐trending belt on the surface. Based on these and other data, this paper makes a preliminary analysis of the seismogenic structure of the Lushan event.
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Yogyakarta earthquake, Mw 6.3, 27 May 2006 had killed 5,571 victims and destroyed more than 1 million buildings. This incident became the most destructive earthquake disaster over the last 11 years in Indonesia. Earthquake mitigation plan in the area has been carried out by understands the location of the fault. The location of the fault is still unclear among geoscientists until now. In this case, analysis of the aftershocks using oct-tree importance sampling method was applied to support the location of the fault that responsible for the 2006 Yogyakarta earthquake. Oct-tree importance sampling is a method that is recursively subdividing the solution domain into exactly eight children for estimating properties of a particular distribution. The final result of the subdividing process is a cell that has a maximum Probability Density Function (PDF) and identified as the location of the hypocenter. Input data consists of the arrival time of the P wave and S wave of the aftershocks catalog from 3-7 June 2006 and the coordinate of the 12 seismometers, and 1D velocity model of the study area. Based on the hypocenter distribution of the aftershocks data with the proposed method show a clearer trend of the fault compared with the aftershocks distribution calculated with the Hypo71 program. The fault trend has a strike orientation of N 42° E with a dip angle of 80° parallel with the fault scarp along the Opak River at the distance of about 15 km to the east. This fault trend is similar with the fault orientation obtained using the Double Difference Algorithm.
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Temporal features of the aftershock activity following some large shallow earthquakes of M≥7 in Japan have been studied quantitatively. The earthquakes concerned were accompanied by large aftershocks which triggered their own aftershock activity. The purpose of the present study is to seek any anomalous change in aftershock activity of the main shock before the occurrence of such large aftershocks. Aftershock activity shows an appreciable decrease from the level expected from the modified Omori formula before the occurrence of a large aftershock. The aftershock activity then recovers to the normal level or even increases beyond the normal level shortly before the occurrence of the large aftershock. The recovered activity generally occurs near the hypocenter of the forthcoming large aftershock. Such a feature has been recognized ha fourteen cases out of eighteen for which sufficient data are available. We have the possibility of predicting the occurrence of a large aftershock which might be as large and disastrous as the main shock, if we keep watch on the change of the aftershock activity immediately following the main shock. Moreover, a rough prediction of the place can be made by checking the hypocenter location of aftershocks occurring in the recovered stage.
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The focal process of the 1964 Niigata Earthquake was reinvestigated on the basis of hypocentral distribution of its aftershocks. This study indicates that the aftershocks are distributed on a fault plane dipping westward.Although it has been clear that the fault strike of the mainshock was in N20°E direction, the dip of the fault was not still clear due to a poor resolution of hypocenter of aftershocks. To resolve the difficulty, we reexamined seismological data obtained by the Japan Meteorological Agency (JMA).Reexamination of seismograms of nearby stations enabled us to supplement more than 1200 new P and S arrivals of aftershocks. We also dentified a number of P and S arrivals from the data which were previously reported as unidentified phases. The Joint Hypocenter Determination method was used to get a more reliable aftershocks distribution. The number of located aftershocks much increased, as about 380 aftershocks are well located by this study.Aftershocks on the vertical cross section which is normal to the fault strike shows that aftershocks are on a westward dipping plane. The dip of the plane is estimated as 50 degrees which is consistent with the focal mechanisms reported by several studies. Although the dip angle depends on the velocity model used in hypocenter location, westward dipping of aftershocks is valid, independent of several different velocity models. Therefore we estimate that the subduction of the Japan Sea under the north-east Honshu does not occur in the southern part of the eastern margin of the Japan Sea.The aftershock activity is found to be low around the hypocenter of mainshock which is located near the bottom of aftershock region, suggesting a large strain release around the nucleation point of mainshock. Relative position of forerunning seismic activity which preceded the mainshock by two years seems to be within the shallow part of the aftershock region east of Awashima-island which is located in the western middle of the focal region. The epicentral distribution of aftershocks indicates that aftershock occurrence is scarce around Awashima-island. A similar relation was reported in the case of the 1983 Nihonkai-chubu earthquake between its aftershocks and Kyurokujima-island, which is situated east of the middle of the aftershock region. Few aftershocks occurred in the area around Kyurokujima-island. In spite of the difference in relative location, that is, Awashima is situated west of the aftershock region while Kyurokujima is in the east, this suggests possibilities that crust around the islands cannot sustain enough strain to generate aftershocks or it behave as an earthquake barrier.
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ABSTRACT In this article, we created a well-resolved aftershock catalog for the 2015 Gorkha earthquake in Nepal by processing 11 months of continuous data using an automatic onset and hypocenter determination procedure. Aftershocks were detected by the NAMASTE temporary seismic network that is densely distributed covering the rupture area and became fully operational about 50 days after the mainshock. The catalog was refined using a joint hypocenter determination technique and an optimal 1D velocity model with station correction factors determined simultaneously. We found around 15,000 aftershocks with the magnitude of completeness of ML 2. Our catalog shows that there are two large aftershock clusters along the north side of the Gorkha–Pokhara anticlinorium and smaller shallow aftershock clusters in the south. The patterns of aftershock distribution in the northern and southern clusters reflect the complex geometry of the Main Himalayan thrust. The aftershocks are located both on the slip surface and through the entire hanging wall. The 1D velocity structure obtained from this study is almost constant at a P-wave velocity (VP) of 6.0 km/s for a depth of 0–20 km, similar to VP of the shallow continental crust.
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