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Abstract:
Although the physical mechanism of earthquake nucleation processes is under debate and it is still challenging to distinguish foreshocks during an ongoing seismic sequence, foreshocks may still provide unique and valuable information for earthquake nucleation process. Investigating the temporal and spatial evolution of foreshock sequences with high resolution and monitoring b-values in real time may shed light on these key issues. Here we investigate a series of earthquake sequences in western Yunnan, China, where a number of moderate to strong earthquakes were well recorded by the newly established seismic network. To find missing earthquakes and build more comprehensive earthquake catalogs, we first carry out earthquake detection using the matched-filter detector. We use events in the standard catalog of China Earthquake Networks Center as templates to scan through continuous waveforms 3-6 months before and after the mainshock. We then estimate the b-value and its temporal changes based on the newly developed catalogs. An obvious reduction in b-values before the major earthquake was observed in both the 2016 Ms5.1 Yunlong and 2021 Ms6.4 Yangbi sequences. However, we didn’t observe such a feature before the 2017 Ms5.1 Yangbi mainshock, showing spatial variations in the region. After the relocation of the detected earthquakes and deriving their source parameters including focal mechanisms and stress drops, we found that the foreshocks in western Yunnan exhibit mostly a cascading rupture manner, rather than indicating slow-slip driving nucleation of mainshocks.
