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Song M.-Q.,National Scientific Field Observatory of Continental Rift Dynamics in Taiyuan | Zheng Y.,CAS Wuhan Institute of Geodesy and Geophysics | Ge C.,CAS Wuhan Institute of Geodesy and Geophysics | Ge C.,University of Chinese Academy of Sciences | Li B.,National Scientific Field Observatory of Continental Rift Dynamics in Taiyuan
Chinese Journal of Geophysics (Acta Geophysica Sinica) | Year: 2012

Based on the phase reports of analog seismic stations from 1981-2001, and those of digital seismographs from 2002-2008, we relocated the small to moderate earthquakes with unprecedented precision by absolute and relative relocating methods. Results show that: (1) comparing with precious researches, the percentage of relocated earthquakes with horizontal locating error ≤5 km increases from 65. 8% to 86. 2%, and 7498 earthquakes which were not located previously are located in this work; (2) The general pattern of seismicity has no big changes in horizontal directions. Most earthquakes occurred in the central rift belt, with a few earthquakes located at the two sides of the Fenhe-Weihe Rift While in vertical direction, the relocated earthquakes have an obvious trend that the earthquake depths increase from north to south part of the Fenhe Rift (3) The distribution of relocated earthquakes can outline the profile of the lower seismogenic boundaries of the basins, as well as the boundaries between the extensional basins and the uplifted areas. (4) There is a good correlation between the focal depths and the structures of the basin and range system.


Song M.,National Scientific Field Observatory of Continental Rift Dynamics in Taiyuan | Zheng Y.,CAS Wuhan Institute of Geodesy and Geophysics | Fu R.,CAS Wuhan Institute of Geodesy and Geophysics | Fu R.,University of Chinese Academy of Sciences | And 4 more authors.
Earthquake Science | Year: 2013

Immediately following the MS7.0 Lushan earthquake on April 20, 2013, using high-pass and lowpass filtering on the digital seismic stations in the Shanxi Province, located about 870-1,452 km from the earthquake epicenter, we detected some earthquakes at a time corresponding to the first arrival of surface waves in high-pass filtering waveform. The earthquakes were especially noticed at stations in Youyu (YUY), Shanzizao (SZZ), Shanghuangzhuang (SHZ), and Zhenchuan (ZCH), which are located in a volcanic region in the Shanxi Province,but they were not listed in the Shanxi seismic observation report. These earthquakes occurred 4-50 min after the passage of the maximum amplitude Rayleigh wave, and the periods of the surface waves were mainly between 15 and 20 s following. The Coulomb stresses caused by the Rayleigh waves that acted on the four stations was about 0.001 MPa, which is a little lower than the threshold value of dynamic triggering, therefore, we may conclude that the Datong volcanic region is more sensitive to the Coulomb stress change. To verify, if the similar phenomena are widespread, we used the same filtering to observe contrastively continuous waveform data before, and 5 h after, the MS7.0 Lushan earthquake and MS9.0 Tohoku earthquake in 2011. The results show that the similar phenomena occur before the earthquakes, but the seismicity rates after the earthquakes are remarkably increased. Since these weak earthquakes are quite small, it is hard to get clear phase arrival time from three or more stations to locate them. In addition, the travel time differences between P waves and S waves (S-P) are all less than 4 s, that means the events should occur in 34 km around the stations in the volcanic region. The stress of initial dynamic triggering of the MS9.0 Tohoku earthquake was about 0.09 MPa, which is much higher than the threshold value of dynamic triggering stress. The earthquakes after the MS9.0 Tohoku earthquake are related to dynamic triggering stress, but the events before the earthquake cannot be linked to seismic events, but may be related to the background seismicity or from other kinds of local sources, such as anthropogenic sources (i.e., explosions). Using two teleseismic filtering, the small background earthquakes in the Datong volcanic region occur frequently, thus we postulate that previous catalog does not apply bandpass filter to pick out the weak earthquakes, and some of the observed weak events were not triggered by changes in the dynamic stress field. © 2013 The Seismological Society of China, Institute of Geophysics, China Earthquake Administration and Springer-Verlag Berlin Heidelberg.

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