Qu J.-H.,China Earthquake Administration |
Li J.,China Earthquake Administration |
Zhang Z.-H.,Earthquake Bureau of Shandong Province |
Zheng J.-C.,Earthquake Bureau of Shandong Province |
Zhang Q.,Earthquake Bureau of Shandong Province
Chinese Journal of Geophysics (Acta Geophysica Sinica) | Year: 2015
There have been many intensive small earthquakes since the 3.8 earthquake of Shandong Rushan occurred on October 1, 2013. Two earthquakes of M4.2 and M4.0 occurred separately on January 7 and April 4, 2014, and there have been never so many earthquakes in a swarm in the history of Rushan. The location results from network show that the distribution direction of Rushan sequence is NW, which is different from the nearest three known faults. So, it is necessary to study the source rupture property, possible seismogenic structure and the relationships with the known faults, which has certain reference significance for understanding the mechanism of Rushan earthquake sequence. This study is based on relocating the Rushan earthquake sequence in 2013-2014 with double-difference algorithm of waveform cross-correlation and analyzing the focal mechanism of stronger earthquake with “Cut and Paste” (CAP) and first motion of P-wave method. The 27 stations with epicentral distance less than 300 km surrounding the epicenter were used to relocate earthquakes. Picking the earthquakes with clear phase records and at least four recording stations, and eliminating the phase records with obvious mistakes and large errors, only considering earthquakes with more than 8 phases, finally, 306 events, 9268 P phases, 9252 S phases are eligible. The CAP method and the P-wave first motion method were used to calculate the focal mechanisms. According to the quality of waveform records and the clarity of first motion of P-wave, 11 broadband stations were selected, including HAY, WED, RCH, LOK, CHD, YTA, BHC, LZH, QID, ANQ, and RZH, which surround the epicenter completely and the epicentral distances are mainly concentrated in 30~250 km. The frequencies of the band-pass filter of Pnl and Snl parts are 0.04~0.2 Hz and 0.02~0.1 Hz respectively. The weight between Pnl and Snl is 2:1.306 earthquakes of Rushan sequence relocated by the method of double-difference got 277 basic earthquake parameters. The relocation results show that the earthquake sequence is distributed along NW direction, the length of long axis is about 35 km, and the length of short axis is about 5 km. The distribution of the Rushan earthquake sequence is dense in a scope of 8 km×3 km, and the focal depths vary from 4 km to 10 km with the predominant distribution in 4~7 km. The focal mechanism results show that the two earthquakes get the best focal mechanism at a depth of 6 km, the parameters of nodal planes are the same, which belongs to strike slip type. At the depth of 6 km, among the 40 phases of M4.2 earthquake recorded by 8 stations 38 have correlation coefficients greater than 0.7, accounting for 95%. The number of correlation coefficients greater than 0.9 is 23, accounting for about 58%. Similarly, at the depth of 6 km, the correlation coefficients of 40 phases of M4.0 earthquake recorded by 8 stations are all greater than 0.7 and the correlation coefficients greater than 0.9 are 31, accounting for about 78%. So, the synthetic seismograms of two earthquakes fit well with the observed seismograms and the inversion results are credible. The focal mechanism shows that the moment magnitude of M4.2 earthquake is Mw4.3, one nodal plane has the strike of 290°, rake of 90°and dip of 23°, and the other nodal plane has the strike of 200°, rake of 67°and dip of 180°. The moment magnitude of M4.0 earthquake is Mw4.2, one nodal plane has the strike of 289°, rake of 90°and dip of 27°, and the other nodal plane has the strike of 199°, rake of 63°and dip of 180°. Using the first motion method of P-waves to calculate the focal mechanism for more than 9 ML3.0 earthquakes, the results show that the 9 earthquake planes are basically the same, with large dip and nearly horizontal slip, which is consistent with the inversion result by CAP method. The hypocentral distance was calculated by the mobile seismic station near the epicenter. The results show that the focal depth should be slightly less than 7 km, which is consistent with the inversion result by CAP method and relocation result. The faults parameters nearest to the epicenter are not consistent with the focal mechanism of larger earthquakes, which shows that they are not the seismic faults. The cross-sections of relocation result show that the hypocentral depth distribution is nearly vertical and relatively the same with the dip of one nodal plane of the focal mechanism. Combining the precise relocation results and larger earthquake focal mechanism, one of the nodal planes of larger earthquakes focal mechanism is consistent with the predominant distribution of earthquake sequence. It is preliminarily concluded that the seismogenic fault of the Rushan earthquake sequence is a nearly vertical strike-slip buried fault in NW direction. ©, 2015, Science Press. All right reserved.