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

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