THE KINEMATIC CHARACTERISTICS OF THE MS6.5 LUDIAN YUNNAN EARTHQUAKE IN 2014

doi:10.3969/j.issn.0253-4967.2014.04.018

Abstract

Abstract:

We employ multistep inversions in the frequency or time domain to infer the kinematic characteristics of the M_S6.5 Ludian, Yunnan earthquake in 2014, mainly using regional broadband waves recorded by the China Digital Seismic Network. In this paper, we firstly invert the focal mechanism solution and the centroid depth of the Ludian earthquake, and then determine the best-fitting finite-fault model and the dominant rupture direction. According to the above results, we further analyze and discuss the kinematic characteristics of the Ludian earthquake, and explore preliminarily the reason for the serious disaster caused by this event.
We take into account some factors which could have effects on the inversion results, e.g. the use of different waves and simplified 1-D velocity models. Several test results indicate that the misfit between observed waves and synthetics is better, if we use the full waves and the 1-D velocity model (Model M2) in this study area. According to the point-source model (focal mechanism solution), this event occurs on a true rupture plane (strike=342°, dip=83°, and rake=-34°), which shows a left-lateral strike-slip faulting with a minor normal oblique component. The centroid in the horizontal direction is located at nearly 5.4km southeast of the epicenter(27.109°N/103.354°E), and the best-fitting centroid depth is around 4.4km. The total scalar moment, M₀, is retrieved with an average value of 2.1×10¹⁸N·m (or moment magnitude M_W6.1).
The rupture history indicates the event can be considered to have an asymmetric bilateral rupture source with a radius of 10km. The total rupture area is about 227.6 km² with an average slip of nearly 0.16m. Most of the energy releases within about 6s. From 0s to 2s, the energetic rupture starts at the nucleation center, then propagates bilaterally along the fault plane. After 2s, the rupture mainly extends south-east, showing an obvious rupture directivity. Finally, the rupture ends at nearly 6s.
In order to investigate rupture directivity of the Ludian earthquake, we retrieve the apparent source duration at different stations, using the method developed by Cesca et al.(2011). Rupture directivity of the Ludian earthquake is detected on the basis of a frequency domain inversion of the apparent duration at each station and the further interpretation of its azimuthal variation. The result indicates that it is obvious that the apparent source durations at majority of stations located southeast of the epicenter of the Ludian earthquake are relatively shorter, and the shortest one is round 2s. However, the apparent source durations of majority of stations distributed northwest of the epicenter are much longer, and the longest one is up to 9s. The mean value of apparent source durations of all stations is about 4.96s. Based on the principle of the Doppler effect, this result provides a clear indication for the rupture propagating towards southeast, and thus can be used to discriminate the true fault plane (NW-SE).
In the end, we speculate that one of the most important reasons why the Ludian earthquake caused the devastating damage is that most of the energy is instantly released within relatively short duration in the shallow layer of the upper crust.

Key words: the Ludian Yunnan earthquake, eikonal source model, focal mechanism solution, rupture history

摘要：

使用中国数字地震台网记录的区域宽频带波形, 通过频率域和时间域多步反演, 研究了2014年云南鲁甸M_S 6.5地震震源运动学特征.基于点源的震源机制解揭示:地震发震断层面参数分别为走向342°/倾角83°/滑动角-34°, 表现为一次左旋走滑型为主兼有少量正断倾滑分量的事件.质心在水平方向位于震中(103.354°E, 27.109°N)东南约5.4km, 最佳波形拟合的质心深度约4.4km, 平均总标量地震矩M₀为2.1×10¹⁸N·m, 矩震级M_W约6.1.破裂过程图像显示:此地震为一次不对称双侧破裂事件, 破裂半径约10km, 整个破裂面积为227.6km², 平均滑动量约0.16m.破裂在6s内释放了大多数能量, 震后0～2s, 破裂以孕震点为中心向NW和SE两侧同时扩展, 2s后, 破裂表现出明显的方向性, 主要向SE(沿走向342°相反方向)扩展, 故导致SE向多数台站视破裂持续时间总体偏小, 最小值为2s.约6s后破裂基本趋于停止.推断鲁甸地震破裂在上地壳浅层集中释放了大多数能量是导致灾害严重的主要原因之一.

关键词: 云南鲁甸地震, Eikonal震源模型, 震源机制解, 破裂过程

CLC Number:

P315.2

ZHAO Xu, LIU Jie, FENG Wei. THE KINEMATIC CHARACTERISTICS OF THE M_S6.5 LUDIAN YUNNAN EARTHQUAKE IN 2014[J]. SEISMOLOGY AND GEOLOGY, 2014, 36(4): 1157-1172.

赵旭, 刘杰, 冯蔚. 2014年云南鲁甸M_S6.5地震震源运动学特征[J]. 地震地质, 2014, 36(4): 1157-1172.

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