Precise determination of focal parameters for July 20, 1995 M L=4.1 earthquake sequence in the Huailai basin

Abstract

On July 20, 1995 an M L=4.1 earthquake occurred in Huailai basin, northwest of Beijing, with epicenter coordinates 40.326°N, 115.448°E and focal depth 5.5 km. Following the main shock, microseismicity sharply increased in the basin. This earthquake sequence was recorded by Sino-European Cooperative Huailai Digital Seismograph Network (HDSN) and the hypocenters were precisely located. Using 8 aftershocks as empirical Green’s function (EGF) we calculated the source time function (STF) of the M L=4.1 event respectively, and by superposing these results, we obtained the average time function which has better signal to noise ratio. The final result shows that the M L=4.1 earthquake consisted of two events, one of which was stronger, the other was weaker, STFs from each station demonstrate an obvious “seismic Doppler effect”. Through a trial-and-error method, the following results were drawn: for the 1st event, the rupture length is 0.44 km, rupture velocity is 4.0 km/s, while for the 2nd event those parameters are 0.25 km, 3.0 km/s, the two events ruptured in the direction of 140° and 90° respectively (measured from the strike of the fault, anticlockwise positive), the two events were separated by 0.57 km in space, and 0.09 s in time. Using single EGF, we also retrieved the source time function of other 13 events with M L≥2.1 in the Huailai earthquake sequence. These results were reached: each of the 13 events consisted of single event, with the duration of the source time functions ranging between 0.05–0.16 s. Using far-field ground displacement measurement method, the scalar seismic moment, stress drop and rupture radii of 25 events with 0.95≤M L≤4.1 were obtained. The trend that scalar seismic moment and stress drop increase with magnitude was obvious. In the magnitude range 0.9≤M L≤2.4, the obtained radii show no correlativity with magnitude, we tend to believe, in such small magnitude range, precise measurement for radii of such small events is beyond the data used in this study.