关键词: 漾濞6.4级地震, 前震时空丛集, 震前快速扩展, 单向传播, 震前b值下降

Abstract:

In 2018, a short-period seismic network was set up in Eryuan area of Yunnan Province to carry out continuous field observation of the sub-instability process of the earthquake. The relevant data of the Yangbi M_S6.4 earthquake sequence are mainly from the waveforms recorded by this network, combined with some other stations from Yunnan regional seismic network. The Yangbi earthquake sequence shows that the events in this area began to occur intensively on May 18. A total of 2 000 earthquakes with M>0.1 were recorded from May 18 to 23, including 770 foreshocks.

Seismicity analysis shows that two clusters of foreshocks occurred successively in the adjacent area of the main earthquake in the northwest segment of the rupture strip within 3 days, then in the subsequent impending period(within 1 hour before the main shock)about 60 events spread symmetrically from the center of the fracture zone to the ends. The spatial distribution of foreshocks in different periods shows the spatial migration of local fractures and accelerated expansion prior to the main shock. The spreading speed is about 5km/d from foreshock clustering process to 96km/d in impending earthquake period. The epicenter of the main shock is located at the edge of the cluster foreshocks and the northwest end of the final rupture zone. Subsequent aftershocks extend southeastward to the whole fracture zone in about half an hour, and the final fracture zone is more than 20 kilometers long, showing unilateral propagation of the rupture. Since 2018, b-value in the Yangbi area has been stable(0.9~1.1)for the past three years. After March this year, the b-value abnormally decreased to 0.6 before the main shock, reflecting that there was a significant process of continuous increase of local stress before the Yangbi earthquake.

The identification of short-term precursors and somehow definite information is one of the focus problems in earthquake prediction research. On the basis of the experimental results, Ma Jin proposed the theory of seismic meta-instability stage based on the characteristics of the load stress after the peak value from rock experiments and the corresponding change of related physical field, and considered that the degree of fault activity synergy was a sign to determine the stress state of the fault. When the fault activity changes from the expansion and increase of the stress releasing points in the early stage of meta-instability to the connection between the released segments at the late stage of meta-instability, that is, the quasi dynamic instability stage, the stress release on the fault will accelerate, and the acceleration mechanism is the strong interactions between the fault segments. In the context that the macroscopic stress state cannot be known directly, the original intention of the “meta-instability” test area is to try to capture the characteristic signal of the meta-instability stage described by the experimental phenomenon through the deformation and seismicity of the actual faults during the earthquake preparation process. It is clear that in this stage, the fault will continue to expand in the pre-slip zone theoretically, and it will enter into the quasi dynamic fracture expansion before the impending earthquake. This theory is obviously embodied in the foreshocks of this earthquake, forming the phenomenon of rapid migration of small earthquakes as mentioned above. From the current understanding of the meta-instability, it can be seen that the seismogenic fault is in the state of overall stress release at this stage, rather than the continuous increase of stress. Therefore, the decrease of b value before the earthquake shows that local faults have been activated and entered the final stage of nucleation process. The quasi dynamic spreading phenomenon before this kind of moderate-strong mainshock displayed by small earthquake activity can be identified as the precursor of a kind of earthquakes.

Key words: the M_S6.4 Yangbi earthquake, foreshock clustering, fast expansion, unilateral propagation, decreasing of b-value