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[en] 1988 Spitak Earthquake is obtained. It is shown that besides crust aftershocks, mantle aftershocks are also observed. Most of the Spitak Earthquake aftershocks are situated in the depth of 0-90 km, certain cases are observed with the depth of 150 km. The coordinates of the hypocenter of the main shock are northern latitude φ=40.867°, and eastern longitude λ=44.199°, estimated depth is H= 5.0 km
[en] The majority of original seismograms recorded at the very beginning of instrumental seismology (the early 1900s) did not survive till present. However, a number of books, bulletins, and catalogs were published including the seismogram reproductions of some, particularly interesting earthquakes. In case these reproductions contain the time and amplitude scales, they can be successfully analyzed the same way as the original records. Information about the Atushi (Kashgar) earthquake, which occurred on August 22, 1902, is very limited. We could not find any original seismograms for this earthquake, but 12 seismograms from 6 seismic stations were printed as example records in different books. These data in combination with macroseismic observations and different bulletins information published for this earthquake were used to determine the source parameters of the earthquake. The earthquake epicenter was relocated at 39.87° N and 76.42° E with the hypocenter depth of about 18 km. We could further determine magnitudes mB = 7.7 ± 0.3, MS = 7.8 ± 0.4, MW = 7.7 ± 0.3 and the focal mechanism of the earthquake with strike/dip/rake − 260°± 20/30°± 10/90°± 10. This study confirms that the earthquake likely had a smaller magnitude than previously reported (M8.3). The focal mechanism indicates dominant thrust faulting, which is in a good agreement with presumably responsible Tuotegongbaizi-Aerpaleike northward dipping thrust fault kinematic, described in previous studies.
[en] Earthquakes with small magnitude and shallow depths are one form of manifestation of existence a fault line in Yogyakarta. Accurate hypocenter determination is needed to identify tectonic and fault conditions based on earthquake hypocenter distribution recorded by seismograf. Hypocenter was determined by using two methods Geiger’s Adaptive Damping for early hypocenter and Modified Joint Hypocenter Determination (MJHD) for relocate hypocenter, we used 2 weeks aftershock data after main quake occurred on June 10-14 2006 with total of 625 events earthquake obtained from the results of picking waves. The difference of hyposenter result on the longitude and latitude is about 2 -25 km and depth change around 0.3-5 km with major distribution to the east from the fault. The final result of this relocation shows that the area is more easily releases energy is in the eastern section of the fault consisting of sedimentary rocks. (paper)
[en] Many geophysical methods at various scales have been applied to understand the internal structure beneath Merapi volcano and its magmatic process. As part of the DOMERAPI project, a seismic experiment was conducted from October 2013 to mid April 2015 in order to determine the deep magma source beneath the volcano through seismic travel-time tomography. The earthquake events were identified and picked manually and carefully to determine the hypocenters. They were then relocated to get precise hypocenter locations before running the seismic tomographic imaging. The data from the BMKG network from the same period of time as mentioned above were also incorporated to minimize azimuthal gap, because the majority of events occurred outside the DOMERAPI network. The checkerboard resolution test result depicts that the area around the network can be well resolved. Compared to previous studies, our result shows a higher resolution at shallow depths, i.e., less than 35 km and a low velocity material imaged to ascend diagonally from the deeper area. (paper)
[en] The 6,4 Mw earthquake occurred in Yogyakarta on 27th May 2007 has been renowned as a manifestation from Opak fault activity. Double-Difference (HypoDD) method was implemented to relocate hypocenters location, due to its poor accuracy caused by un-modeled velocity structure. The relocation was carried out on aftershocks data from 15th June to 19th June 2006. This method relocated 296 out of 303 aftershock events. Seismicity map after relocation showed concentrated epicenters by the east side of Opak fault. Distribution of the hypocenters which previously ranged from 0 to 40 km significantly shifted to the range of 10-20 km. RMS values obtained after relocation reduced close to 0 which indicated an outstanding improvement of hypocenters location in Opak fault zone. (paper)
[en] The Hokkaido Eastern Iburi Earthquake (M = 6.7) occurred on Sep. 6, 2018 in the southern part of Central Hokkaido, Japan. Since Paleogene, this region has experienced the dextral oblique transpression between the Eurasia and North American (Okhotsk) Plates and the subsequent collision between the Northeast Japan Arc and the Kuril Arc due to the oblique subduction of the Pacific Plate. This earthquake occurred beneath the foreland fold-and-thrust belt of the Hidaka Collision zone developed by the collision process, and is characterized by its deep focal depth (~ 37 km) and complicated rupture process. The reanalyses of controlled source seismic data collected in the 1998–2000 Hokkaido Transect Project revealed the detailed structure beneath the fold-and-thrust belt, and its relationship with the aftershock activity of this earthquake. Our reflection processing using the CRS/MDRS stacking method imaged for the first time the lower crust and uppermost mantle structures of the Northeast Japan Arc underthrust beneath a thick (~ 5–10 km) sedimentary package of the fold-and-thrust belt. Based on the analysis of the refraction/wide-angle reflection data, the total thickness of this Northeast Japan Arc crust is only 16–22 km. The Moho is at depths of 26–28 km in the source region of the Hokkaido Eastern Iburi Earthquake. Our hypocenter determination using a 3D structure model shows that most of the aftershocks are distributed in a depth range of 7–45 km with steep geometry facing to the east. The seismic activity is quite low within the thick sediments of the fold–thrust belt, from which we find no indication on the relationship of this event with the shallow (< 10–15 km) and rather flat active faults developed in the fold-and-thrust belt. On the other hand, a number of aftershocks are distributed below the Moho. This high activity may be caused by the cold crust delaminated from the Kuril Arc side by the arc–arc collision, which prevents the thermal circulation and cools the forearc uppermost mantle to generate an environment more favorable for brittle fracture. .
[en] Before the ML 6.6 Meinong earthquake in 2016, intermediate-term quiescence (Qi), foreshocks, and short-term quiescence (Qs) were extracted from a comprehensive earthquake catalog. In practice, these behaviors are thought to be the seismic indicators of an earthquake precursor, and their spatiotemporal characteristics may be associated with location, magnitude, and occurrence time of the following main shock. Hence, detailed examinations were carried out to derive the spatiotemporal characteristics of these meaningful seismic behaviors. First, the spatial range of the Qi that occurred for ~ 96 days was revealed in and around the Meinong earthquake. Second, a series of foreshocks was present for ~ 1 day, clustered at the southeastern end of the Meinong earthquake. Third, Qs was present for ~ 3 days and was pronounced after the foreshocks. Although these behaviors were recorded difficultly because the Qi was characterized by microseismicity at the lower cut-off magnitude, between ML 1.2 and 1.6, and most of the foreshocks were comprised of earthquakes with a magnitude lower than 1.8, they carried meaningful precursory indicators preceding the Meinong earthquake. These indicators provide the information of (1) the hypocenter, which was indicated by the area including the Qi, foreshocks, and Qs; (2) the magnitude, which could be associated to the spatial range of the Qi; (3) the asperity locations, which might be related to the areas of extraordinary low seismicity; and (4) a short-term warning leading of ~ 3 days, which could have been announced based on the occurrence of the Qs. Particularly, Qi also appeared before strong inland earthquakes so that Qi might be an anticipative phenomenon before a strong earthquake in Taiwan.
[en] The Hokkaido Eastern Iburi earthquake (MJMA = 6.7) occurred on September 6, 2018, in the Hokkaido corner region where the Kurile and northeastern Japan island arcs meet. We relocated aftershocks of this intraplate earthquake immediately after the main shock by using data from a permanent local seismic network and found that aftershock depths were concentrated from 20 to 40 km, which is extraordinarily deep compared with other shallow intraplate earthquakes in the inland area of Honshu and Kyushu, Japan. Further, we found that the aftershock area consists of three segments. The first segment is located in the northern part of the aftershock area, the second segment lies in the southern part, and the third segment forms a stepover between the other two segments. The hypocenter of the main shock, from which the rupture initiated, is located on the stepover segment. The centroid moment tensor solution for the main shock indicates a reverse faulting, whereas the focal mechanism solution determined by using the first-motion polarity of the P wave indicates strike-slip faulting. To explain this discrepancy qualitatively, we present a model in which the rupture started as a small strike-slip fault in the stepover segment of the aftershock area, followed by two large reverse faulting ruptures in the northern and southern segments. .
[en] On November 15, 2014, an Mw4.3 earthquake occurred 2 km west of Mihoub village, 60 km SE of Algiers. In this study, we retrieve the relative source-time functions of the mainshock and largest aftershock (Mw3.9) for rupture analysis using the empirical Green’s function method. The two events are nearly colocated with a smaller aftershock (Mw3.5), which is treated as the empirical Green’s function. Moreover, these three events have similar focal mechanisms, suggesting that deconvolution is well posed in this case. The three events were recorded by nine stations of the Algerian permanent network. We use mainly P-wave data. The focal mechanism solution shows dominant reverse faulting with a strong strike-slip component. The two nodal planes align almost E-W, dipping to the south, and NNE-SSW, dipping to the NW, respectively; the fault and auxiliary planes cannot be resolved from hypocenter locations alone because too few aftershocks were recorded by the permanent network. The results show unilateral rupture propagation to the ENE and complex rupture with multiple episodes for the mainshock. The largest aftershock shows similar behavior with slightly less pronounced directivity at some sites. The rupture directivity for the mainshock is estimated at about N66° E, and the rupture velocity is Vr = 0.66β. The E-W nodal plane of the best-fit focal mechanism is the preferred fault plane because it best agrees with the directivity direction and is consistent with the E-W faulting that dominates in the region.
[en] In the past 2 years, we have been conducting routine hypocenter relocation in Indonesia. We compiled and conducted hypocenter relocation of ∼50.000 events from a magnitude of 1.4 to 8.5 recorded by 451 stations around and outside Indonesia region. We used local, regional, and teleseismic arrival time data from Indonesian Agency of Meteorology, Climatology, and Geophysics (BMKG) catalog from April 2009 to March 2018. We performed teleseismic double-difference relocation inversion using our previous study of 3D seismic velocity model beneath the Indonesian region with grid size 1∘ by 1∘ for inside Indonesia region and 1D global seismic velocity model from AK135 for outside Indonesia region. This method improved limitation from BMKG earthquake data catalog in which events were recorded from scattered seismic station array and insufficient azimuthal gap around Indonesia. Our results show that travel-time RMS residuals were greatly reduced compared to those of the BMKG catalog and the hypocenter location shows significant improvement, refining to the geological structure, especially trench and major faults. These hypocenter relocation results can be helpful for another seismic study in Indonesia region that required a precise hypocenter location e.g. body wave tomography and probability seismic hazard analysis. (paper)