摘要:
2017年8月8日四川九寨沟发生MS 7.0地震,震中位置(33.20°N,103.83°E),推测发震构造为塔藏断裂南端的分支断裂,为一次走滑型的地震事件,截至目前最大余震为8月9日10时17分发生的MS 4.8地震,已造成25人死亡,约525人受伤.中国地震局8月12日公布的九寨沟地震现场实测最大烈度为9度,但强地面运动观测记录整体偏低,实测最大水平峰值加速度为九寨百河台的-185 cm/s 2,该台站距离震中28.3 km,对应的仪器计算烈度仅为6.2度,明显小于极震区的最大观测烈度.本文首先分析了九寨沟MS 7.0地震震源参数的特征,结果显示主震的地震波辐射能和视应力均偏低,震源破裂过程为应力下调模式;反演的发震断层面滑动集中区最大位移偏小,显著余震较少,且最大余震震级偏低,显示九寨沟序列主震释放了整个序列中绝大部分的地震波能量;破裂沿断层走向相对充分,与实际的余震分布相一致,而断层未出露地表则显示其在应力下调模式下破裂可能受阻;进一步基于震源参数及断层破裂过程,构建有限断层模型模拟九寨沟地震的烈度分布图,计算的最大烈度与观测相一致,但极震区面积相对偏小.综合分析九寨沟地震致灾特征,该地震致灾程度不高的原因可能与地震波辐射能偏少、余震震级偏低、极震区面积较小且人口密度偏小相关.
Abstract:
The Jiuzhaigou, MS 7.0 earthquake of August 8, 2017 in northern Sichuan, China, occurred as the result of shallow strike-slip faulting in the interior of the Eurasia plate, with the epicenter of 33.20°N and 103.83°E. The MS 7.0 earthquake occurred on the eastern boundary of the Bayan Har block, which is several hundred kilometers to the northeast of the convergent India to Eurasia plate boundary. The magnitude of the largest aftershock is MS 4.8, which occurred in 12th Auguset. Twenty-five people were killed, and five hundreds and twenty-five people injured during this earthquake, the damage level is not so severe in consideration about the surface wave magnitude. China Earthquake Administration published the maximum intensity is 9 degrees in the survey intensity map. However, the calculated instrumental intensity based on the ground motion observations is 6.2, which is obvious less than the survey intensity in the meizoseimal area. In this study, we analyzed the dynamic source process according to the characteristics of the source parameters, it can be confirmed that the radiated energy and the apparent stress of the Jiuzhaigou earthquake are small. The results indicate that the frictional undershoot behavior, which is in accord with the less moderate aftershocks. In fact, main shock ruptured adequately along the strike direction and radiated most of the energy of the sequence. And the observed horizontal Peak Ground Acceleration (PGA-H) of the strong ground motion is generally small comparing with the Next Generation Attenuation (NGA) relationship. Furthermore, we constructed one finite fault model to calculate the strong ground motion and simulated the intensity distribution considering the effect of slip distribution on the fault plane. It is found that the less damage level of Jiuzhaigou MS 7.0 earthquake may be caused by the smaller strong ground motion, less proportion of meizoseimal area, and rare population in the near field area.
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