The effect of focal mechanisms of Jiashi strong earthquake swarm on the characteristics of long-period ground motions

In 1997 a strong earthquake swarm which contained 7 earthquakes with magnitude greater than 6 occurred in Jiashi, Xinjiang, China. Another earthquake with magnitude 6.9 occurred near Jiashi in 1996. In 2003, several strong earthquakes occurred in Jiashi again. During these earthquakes, digital broad-band records were obtained in Wushi seismic station, which is one of the Geoscope stations of France. The digital broad-band records are suitable for studying long-period ground motions. The repeated strong earthquakes in same place give us a very good chance to explore the effects of earthquake source on the characteristics of long-period ground motions, because it eliminates the effects of propagation path. Fortunately the focal mechanisms of these earthquakes contain strike-slip, normal and reverse faults. By analyzed the digital broad-band records of Jiashi strong earthquake swarm, we can conclude that: (1) The magnitude of earthquake has strong effects on the long-period ground motions. Larger earthquake has more long-period ground motion contents; W Compared with the strike-slip earthquakes, the oblique-faulting earthquakes have more long-period contents in vertical component; (3) The long-period horizontal acceleration response spectra of strike-slip earthquakes are higher than that of the normal-faulting earthquakes, but it is difficult to figure out whether it is due to the focal mechanism or the lower stress-drop of normal-faulting earthquakes.