Abstract:

A significant seismic swarm occurred in Yigong, Bomi, Tibet, in July and August of 2020. 25 earthquakes with M_L≥4.0 occurred during about 30 days and the magnitude of the maximum earthquake reached M_L4.9(hereinafter referred to as the Bomi swarm). The proportion of large to small earthquakes in Bomi swarm is unbalanced, the number of earthquakes with larger magnitude is somewhat higher, and the proportionality coefficient, b value, of Gutenberg-Richter relationship is about 0.3, obviously smaller than the average b value of 1.0 of the whole seismic sequence. The seismicity of Bomi swarm has two dense stages, one is from July 19 to August 1 and another is from August 8 to 18, few earthquakes occurred between these two stages. For spatial distribution of earthquakes, the main areas of earthquake distribution in these two stages are almost overlapped. However, comparing with the previous stage, the southern boundary of the dense distribution of earthquakes in the latter stage has an extending trend to SE direction. The focal mechanism and the centroid depths of 20 earthquakes with M_L≥4.0 have been calculated by CAP method. Results show that the centroid depths are shallow, most of them are distributed in the range of 3~4km. Viewing from the focal mechanism, taken July 27, 28 as the time boundary, the focal mechanisms before that time are mainly thrust with strike-slip component, the strike directions of nodal planes are inconsistent. After that time, the focal mechanism shows a good consistency with near EW-trending tensile rupture.

The retroactive statistical results on historical earthquake catalogue have shown that earthquakes in Bomi region mostly occurred during July and August, indicating the obvious seasonal characteristics, and earthquakes mainly concentrated in a very small area(about 15km×20km)in space. The magnitude of maximum earthquake in each year is generally stable in the range of M_L4.5~5.0, the annual average seismic energy release is roughly equivalent to one earthquake with M_L4.9. It should be pointed out that swarms or significant earthquakes do not occur every year. During a total of 51 years from 1970 to 2020, significant swarms or earthquakes with M_L≥4.0 occurred only in 18 years, accounting for about 35% of total time period.

The correlation between seasonal meteorological factors and the seismicity in Bomi region is studied in this paper and the results show that there is a close but very complex relationship between them. Generally, the seismicity in Bomi region is closely related to the rainfall intensity and precipitation process in the first half of the year. The swarms mainly occurred during the periods with the peak precipitation, and generally followed the end of the first significant precipitation process in the year. The contrastive analysis shows that the strength of the seismicity is qualitatively proportional to the starting time of precipitation above designated scale, the days of precipitation above designated scale during the first half year, as well as the increasing rate of precipitation from April to June. Specificly, the earlier the starting time of precipitation above designated scale, the more the number of days with precipitation above designated scale in the first half of the year, the longer the time interval from the starting of the precipitation above designated scale to the seismicity, the higher the increasing rate of the monthly average precipitation from April to June, and the more the expected rainfall in June, the higher the seismicity level of this year will be.

Bomi swarm is located to the north of Jiali fault zone and obviously off the Jiali fault zone. The seismicity in Bomi region is not the result of the fault activity of the Jiali fault zone, nor is related to the aftershock activity of Milin M6.9 earthquake in 2017, which occurred about 44km south of Jiali fault zone, since there is no obvious tectonic correlation among of them. Viewing from the geographical terrain, the seismicity in Bomi region mainly concentrated in the middle part of the NE-trending Lequ Zangbo River and its branches on both sides. Due to the lower terrain, it becomes an area for fast convergence of water from surrounding regions in the summer, which provides the basic conditions for fluid-triggered earthquakes in July and August every year. The lithology in the earthquake densely distributed area is mainly quartz sandstone and siltstone with relatively higher permeability, which is convenient for fluid penetration and leads to the pore pressure increasing in shallow crustal medium, thus, is liable to trigger seismicity. The local area with dense earthquake distribution in Bomi region is truncated and confined by several faults. The faults may act as a “water-retaining wall”, which has a certain confining effect on water infiltration and diffusion. On the other side, the faults, especially for normal faults, have better fluid conductivity, which is convenient for fluid infiltrating rapidly. Under the action of both the gravity and load pressure of the surface water, the fluid infiltrates rapidly along the fracture zone and the sandstone-like rock medium with good permeability, resulting in the rapid increase of the pore pressure in the underground cracks, faults and porous medium, therefore leading to the decrease of the strength for faults or cracks, and consequently triggering the seismicity. Considering the contribution of accumulated precipitation, groundwater level change, as well as warming and snowmelt to surface water level uplift in the first half of the year, the temporal variation of pore pressure at different depths are simulated by the numerical methods under the simplified conditions. The simulation results support the mechanism explanation on seismicity in Bomi region proposed in the paper.

Key words: Bomi earthquake swarm, seasonal precipitation, precipitation scale and precipitation process, lithology of medium, fault structure, earthquake triggering, variation of pore pressure

摘要：

2020年7—8月西藏波密易贡乡发生M_L4.9显著震群, 1个月内共发生25次M_L≥4.0地震, 引起了广泛关注。1970年以来的资料显示, 波密震群区域的地震活动时间基本集中于每年的7、 8月, 空间上主要分布于15km×20km的区域范围内, 最大震级基本稳定在M_L4.5~5.0之间, 年均释放的地震能量大体相当于1次M_L4.9地震。波密震群区域的地震活动与上半年降水规模及降水过程关系密切, 震群活动发生于降水量的峰值时段, 一般起始于当年首次较显著降水过程结束之后。地震活动水平与规模以上降水开始时间的早晚、 上半年规模以上降水的天数及4—6月降水量增加速率等反映降水规模和降水过程的因素呈正相关。波密震群区域的地震活动主要集中分布于近NE向勒曲藏布中段及两侧分支流域, 该区夏季成为周边水体快速汇聚的区域, 这为每年7、 8月流体触发地震活动提供了基本条件; 地震密集分布区域地表出露的岩石以石英砂岩、 粉砂岩为主, 相对而言具有更高的渗透率, 便于流体下渗引起地下介质孔隙压力增加; 地震密集区域被多条断层所围限, 断层一方面对地表聚集水体下渗扩散的区域具有一定围限作用, 另一方面又具有较好的流体导通性, 便于流体快速下渗, 其中张性断层更是如此。在流体自重及地表水体载荷压力的共同作用下, 流体沿断裂破碎带及具有较好渗透性的砂岩类岩石介质快速下渗, 导致地下断层、 裂隙及孔隙介质中的孔隙压力快速增大, 断层、 裂隙强度随之降低, 进而导致地震活动。不同深度处的孔隙压力随时间变化的模拟计算结果支持上述关于波密震群区域地震活动的机理解释。

关键词: 波密震群, 降水规模和降水过程, 介质岩性和断层构造, 地震触发, 孔隙压力变化