A PRELIMINARY DISCUSSION ON THE FOCAL MECHANISM AND SEISMIC TECTONIC FEATURE OF THE 2013 LAIZHOU M4.6 EARTHQUAKE SEQUENCE

doi:10.3969/j.issn.0253-4967.2015.02.004

Abstract

Abstract:

An M_S4.6 earthquake occurred at noon on Nov. 23, 2013 at Laizhou, Shandong Province, China. This earthquake is the largest event since the Sept. 20, 1995 Cangshan M_S5.2 earthquake in Shandong area, and shook the whole Shandong Peninsula. The local area has low seismic activities, only one M_L3 earthquake sequence was recorded from 1970 to 2012. But since 2012, small shocks break out every now and then, up to the recent M_S4.6 sequence.We investigate the faulting process of the 2012—2014 Laizhou M4.6 earthquake sequence by combining relocated hypocenters and focal mechanisms. CAP method and additional bootstrap technique are employed to stably invert the moment tensor solution and to estimate its uncertainties. The average faulting parameters are: A. strike=239.6°, dip=75.0°, rake=174.4°; B. strike=331.1°, dip=84.6°, rake=15.0°, and error range of P, T axes is about 20°。We use HASH method to solve the focal mechanism solutions for 12 small events(M_L≥3.0)in the sequence, and adopt double difference method(HypoDD)to analyze precisely the aftershock distribution.Relocation images show that, except 3 small shocks away from the swarm, the concentrated area of Laizhou sequence presents a NE-oriented major axis, and the sources distribution indicates a NW dipping fault, with a dip angle about 70°, which is in accord with the solutions for small events retrieved by HASH method.Finally, a discussion on the structural features of seismic tectonic and faulting process is made by using of all the results and relative geological data, and several opinions are concluded as follows:(1) There was an ordered rupture process at the earlier stage. At the very beginning(Jan 1, 2012 M_L 3.2), rupture spread towards northeast. After the M_S4.6 mainshock, rupture of the aftershocks became disordered, and sources distribution became more stochastic.(2) Small events before the mainshock scattered around the main rupture area; the occurrence of M_S4.6 event filled up the gap.(3) Strike-slipping is the dominant faulting type in the earlier stage of the sequence. Two foreshocks right before the mainshock display some thrust component. This maybe implicates the strengthening of regional stress relative to the mainshock. The focal mechanism variation of small aftershocks indicates stress field's adjustment at deep source area after the mainshock.(4) Slipping vectors of the fault are in accord with accurate location results, which reveals the dynamics of faulting process.(5) The seismotectonic characters of Laizhou earthquake sequence revealed by this paper are consistent with other regional geology data. Focal mechanisms conform to the orientation of regional maximum horizontal principal compressive stress. This implies that Laizhou earthquake sequence occurred under the regional stress field, and has relationship with the relative motion between tectonic blocks.

Key words: focal mechanism, waveform inversion, amplitude ratios, HypoDD, fault features

摘要：

2013年11月23日13时44分山东莱州发生M4.6地震, 造成较大影响, 并且在主震前后发生了持续较长时间的小震序列活动。文中使用CAP方法研究了莱州M4.6主震的震源机制, 使用自助抽样(Bootstrap)方法进行稳定反演, 给出了误差估计; 使用HASH方法基于振幅比和初动符号求解了序列中12次M_L≥3.0的小震震源机制; 使用双差方法对莱州地震序列进行重新定位。最后结合有关地质资料对此次莱州震群的发震构造以及序列发展过程中断层的破裂进行了探讨。

关键词: 震源机制, 波形反演, 振幅比, 双差定位, 断层性质

CLC Number:

P315.3⁺3

ZHENG Jian-chang, LI Dong-mei, WANG Peng, ZHAO Jin-hua, XU Chang-peng. A PRELIMINARY DISCUSSION ON THE FOCAL MECHANISM AND SEISMIC TECTONIC FEATURE OF THE 2013 LAIZHOU M4.6 EARTHQUAKE SEQUENCE[J]. SEISMOLOGY AND GEOLOGY, 2015, 37(2): 384-399.

郑建常, 李冬梅, 王鹏, 赵金花, 徐长朋. 2013年莱州M4.6地震序列震源机制与发震构造初探[J]. 地震地质, 2015, 37(2): 384-399.

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