Source Parameters of the 2000 Yao'an Earthquakes

On Jan.15, 2000, an earthquake sequence with the main shock MS6.4 occurred in the Yao'an region of Yunnan province. We study the physical process of this sequence based upon the data of aftershocks recorded by the near field small aperture observational network. By using the seismic scaling law, we have calculaded the parameters of the mainshock. They are: seismic moment M0 = 1:58×1018N.m; moment magnitude MW=6.0; average dislocation =0.63m; fault length L=16.6km; and fault width W=5.6km. The the accurate locating for the aftershock sequence and strike of aftershocks' energy distribution have verified that the fault rupture strike of the mainshock with MS6.4 is about N50°W. They also indicate that the Maweiqing fault in seismic region is the seismogenic structure of the mainshock, and the fault property is of right‐lateral strike‐slip type. The focal parameters of these earthquakes are estimated by utilizing the recorded data of transverse wave and the method of spectrum analysis. The seismic moment M0 is between 1010 and 1016N·m; the rupture radius of seismic focus varies from 80m to 500m; and seismic stress drop is in the range of 0.01~9.5MPa. The higher stress drop (Δσ > 1:0MPa) arranges in a linear manner along the main fault; and the high stress drops (Δσ > 2:0MPa) are related with earthquakes of ML ≥ 3.0. Both energy release of aftershocks and the earthquakes with high stress drop occur mostly at depths from 6km to 11km, which means the stress in the earth's crust is mainly concentrated in this depth range.