2015年Mw 7.8尼泊爾地震破裂特徵研究

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Title:	2015年Mw 7.8尼泊爾地震破裂特徵研究 Rupture Features of the 2015 Mw 7.8 Nepal Earthquake
Authors:	李昀宸
Contributors:	地學研究所地質組
Keywords:	2015年尼泊爾地震破裂方向性震源歷時破裂速度時間域解迴旋震源輻射能量靜應力降
Date:	2018
Issue Date:	2018-05-03 10:25:21 (UTC+8)
Abstract:	2015年4月25日Mw 7.8尼泊爾地震發生在歐亞板塊與印澳板塊的碰撞帶上。地震發生後，已有許多研究以P波反投影及有限斷層逆推來探討其震源運動學上的破裂特徵。本研究以表面波破裂方向性及P波多重破裂分析再一次探討尼泊爾地震的破裂特徵，除決定其斷層參數外，也提供動力學破裂過程的訊息。首先，以週期100秒雷利波走時得到的隨測站方位變化的視震源歷時，並與雷利波頻譜節點週期聯合逆推得到尼泊爾地震的斷層參數，其最佳破裂方位為北偏東127，與餘震的分布一致，總震源歷時70.2秒、上揚時間11.5秒、破裂時間58.7秒，所估算的破裂長度為149公里，其破裂速度約2.5 km/sec，約是S波在地殼內波速的0.74倍，符合先前研究所提破裂速度的範圍。以遠場P波在時間域進行解迴旋得到的震源時間函數分析尼泊爾地震的多重破裂，結果顯示尼泊爾地震由7個子破裂所組成，總震源歷時約59.5秒，略低於表面波破裂方向性分析的結果，但與一些P波反投影的研究相若。總地震矩為7.25×1020 (Mw 7.84)，在地震發生後45秒內釋放了約90%的能量，震源時間函數分別在地震發生後的22秒與33秒處有峰值出現，其中，最大能量釋放約在33秒處，少許能量持續釋放至約59.5秒。總震源輻射能量為1.02×1016 ，整體的ES/M0為1.41×10-05，略低於一般大地震的比值，此反應尼泊爾地有偏低的靜應力降，表示破裂時在斷層面上的摩擦緩慢下降。最大的應力降出現在約38至48秒處，約為23 bars，較其他破裂相對上有較快速的摩擦下降，平均靜應力降為20 bars，與板塊交界地震的靜應力降相當。尼泊爾地震的動應力降13.7 bars，配合上揚時間反映出斷層的破裂寬度為93.5 km，斷層破裂面積約13744 km2。 On April 25, 2015, the Mw 7.8 Nepal earthquake occurred at the collision zone between the Indian and Eurasian plates. Previous studies using P-wave back-projection and finite fault modeling revealed the kinematic features about the earthquake. In this study, we used Rayleigh-wave phase velocity to perform the rupture directivity analysis and adopted P-wave to analyze multiple sources of the Nepal earthquake. Here, we not only determined the fault parameters but also provided information about the dynamic features for the 2015 Nepal earthquake. First, we used the 100-s Rayleigh-wave phase-velocity to obtain the azimuth-dependent source duration, and then merged with the spectral-node periods of Rayleigh waves to complete a joint inversion to derive the fault parameters of Nepal earthquake. Results showed an optimal rupture azimuth of 127, consistent with the aftershock distributions, and a total source duration of 70.2 s. The rupture time and rises time were 58.7 s and 11.5 s, respectively. The earthquake had a rupture length of ~147 km and an average rupture velocity of 2.5 km/s, about 0.74 times of the crustal S-wave velocity. Such results were comparable with previous studies. By using the time-domain deconvolution of teleseismic P-wave to retrieve the relative source time function, the Nepal earthquake was composed of 7 sub-events. The total duration was 59.5 s, less than that from surface-wave rupture directivity analysis but similar to those from the P-wave back-projection. The seismic moment (M0) was estimated to be 7.251020 Nm (corresponding to Mw 7.84). The energy release reached about 90 % of total seismic moment for the first 45 s. From the source time function, two energy peaks appeared at 22 s and 33 s after the initiation of the earthquake. The total radiated seismic energy (ES) was 1.02×1016 Nm, leading to the ES/M0 ratio of 1.41×10-5, less than ordinary large earthquakes. This implied that the Nepal earthquake had a lower static stress drop and the friction on the fault plane slowly dropped off during the rupture on average. Maximum static stress drop, ~23 bars, occurred between 38 and 48 s. This indicated that the friction on the fault plane quickly dropped off relative to the rest of ruptures. The average static stress drop was approximately 20 bars for the Nepal earthquake, corresponding to that for interplate earthquakes. We calculated the dynamic stress drop of the Nepal earthquake to be 13.7 bars from the derived rise time. Then, the rupture width and rupture area were estimated at 93.5 km 13744 km2, respectively, for the 2015 Nepal earthquake.
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