MOMENT DEFICITS ON THE MAJOR FAULTS AND EARTHQUAKE HAZARD ASSESSMENT IN THE EASTERN HIMALAYAN SYNTAXIS

doi:10.3969/j.issn.0253-4967.2020.01.003

Abstract

Abstract:

The Eastern Himalayan Syntaxis(EHS)is a critical region for studying the tectonic evolution of Tibetan plateau, which was affected by the intense seismic activities. We use the theory of moment balance, GPS velocities and historical earthquake records to analyze the moment deficits in the EHS, assess the future seismicity and further to predict the recurrence interval of the 1950 Chayu M_S8.6 earthquake.
We first collected multiple sets of GPS velocity fields and combined them to reduce the systematic bias. Then a micro-blocks model, constrained by GPS velocities, was built by TDEFNODE software to simultaneously invert the fault elastic strain parameters and rigid motion parameters based on the grid research and simulated annealing methods. The long-term slip rates on the faults were further estimated by the differential motions between the neighboring blocks. The results show that the nearly NS dextral strike-slip faults, Naga Fault and Sagaing Fault, slip with the average rates of ~10.6 and ~16.6mm/a, which are consistent with the lateral extrusion in the Tibetan plateau. However, the Main Frontal Thrust shows a distinguished sinistral strike-slip feature(6~10mm/a), possibly caused by the NNE pushing from the Indian plate to the Eurasian plate. On the other hand, because the EHS is located in frontal area of the collision between Indian and Eurasian plate, most faults show thrusting feature. The most obvious one is the Mishimi Fault, slipping with the rate of 23.3mm/a, implying that the convergence rate of the Indo-European plates is largely absorbed by this fault. The moment accumulation rate in the EHS is higher than the average rate in the Tibetan plateau and the total moment accumulation is(1.15±0.03)×10²² N·m in the last 200a. About 59.7% and 21.6% of the moment accumulation rate concentrate on the Main Frontal Thrust and Mishimi Fault.
Second, we selected the earthquake records occurring on the upper crust since 1800AD to analyze the moment release in the EHS based on the data from the International Seismological Centre, United States Geological Survey, and catalogue of historical strong earthquakes in China and some other previous studies. In addition, the Global Centroid Moment Tensor Project and linear regression method were adopted to estimate the relationship between body wave magnitude(m_b), surface wave magnitude(M_S), local magnitude(M_L)and the moment(M₀). Then we further estimated the total fault moment release in the EHS, (5.50±2.54)×10²¹N·m, which is significantly lower than the total moment accumulation. About 79.2% of the moment release occurs on the Mishimi Fault, this is because the 1950 M_S8.6 Chayu earthquake is assumed to have ruptured on this fault.
Finally, the present-day moment deficits on the faults in the EHS were calculated by the differences between the moment accumulation and release, which represent the possibility to produce earthquakes on the upper crust faults in the future. The largest moment deficit was found on the Main Frontal Thrust near Bhutan, which is able to rupture with M_W8.1+. Similarly, earthquakes with M_W7.5+ and M_W7.3+ have the potentials to occur on the Naga Fault and the Jiali Fault near Tongmai. However, the future earthquake scales may be less than M_W7.1 on the remaining faults. Moderate minor earthquakes are the main activity in the area near the Yarlung Zangbo Suture zone and the southern Sagaing Fault. Although the Chayu M_S8.6 earthquake occurred near the Mishimi Fault and the eastern MFT, the earthquake risk on those two faults cannot be ignored. Meanwhile, no matter which fault produced the Chayu earthquake, its recurrence will likely be 660a to 1 030a.

Key words: GPS, fault slip rates, moments deficit, earthquake recurrence period, Eastern Himalayan Syntaxis

摘要：

喜马拉雅东构造结是研究青藏高原构造演化的关键地区, 地震活动频繁。文中基于地震矩平衡理论, 利用GPS资料与历史地震目录分析东构造结地区的地震矩亏损, 继而评估该区未来的地震活动。结果表明: 研究区总体的地震矩累积率高于青藏高原的平均水平, 近200a内的累积总量达(1.15±0.03)×10²²N·m, 明显高于地震矩的释放总量(5.50±2.54)×10²¹N·m。而地震矩亏损量最高的主前缘断裂不丹段具备发生M_W8.1以上地震的可能, 那加山断裂及嘉黎断裂通麦段则不排除未来发生震级大于M_W7.5与M_W7.3地震的可能, 其余断层发生强震(M_W7.1以上)的概率相对较低。而对于米什米断层与主前缘断层东段, 虽然察隅M_S8.6地震发生于此, 但这2条断层未来的地震危险性仍不容忽视, 且无论察隅地震发生于哪条断层, 其复发周期均为660~1 030a。

关键词: GPS, 滑动速率, 地震矩亏损, 地震复发周期, 东构造结

CLC Number:

P315.2

TIAN Zhen, YANG Zhi-qiang, WANG Shi-di. MOMENT DEFICITS ON THE MAJOR FAULTS AND EARTHQUAKE HAZARD ASSESSMENT IN THE EASTERN HIMALAYAN SYNTAXIS[J]. SEISMOLOGY AND GEOLOGY, 2020, 42(1): 33-49.

田镇, 杨志强, 王师迪. 喜马拉雅东构造结主要断裂的地震矩亏损与危险性评估[J]. 地震地质, 2020, 42(1): 33-49.

Figures/Tables 8

Fig. 1 a Tectonic setting and seismicity of the Eastern Himalayan Syntaxis(M≥4); b The number and the moment(M0)time series of the earthquakes(M>6).

Fig. 2 GPS velocities in the study area(relative to the stable Eurasia, error ellipses represent the 90% confidence level).

Fig. 3 Combined GPS velocities(relative to the stable Eurasia)and strike-slip rates on the major faults(a);Residuals of the micro-block model and the dip-slip rates(error ellipses represent the 90%confidence level)(b);Histogram of residuals(c).

Fig. 4 The relation between residuals and locking depths.

Fig. 5 Moment accumulation rate in the Eastern Himalayan Syntaxis(unit: N·m/a).

Fig. 6 Centroid moment tensor from GCMT in the study area(a), the linear relation between scalar moment and the body-wave(b), and the surface-wave magnitude(c).

Fig. 7 Moment accumulation(a)and release(b)since 1800AD in the study area(unit:N·m).

Fig. 8 Estimated regional moment deficit since 1800AD(unit: N·m).

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