The foreshock sequence of the February 4, 1975, Haicheng Earthquake (M = 7.3)

We have examined the locations and radiation patterns of the foreshocks of the February 4, 1975, Haicheng earthquake (M = 7.3). Using arrival times from six local seismic stations, the foreshocks and mainshock were located relative to a master event. The foreshocks occurred in a tight cluster that elongated with time. Before the largest foreshock, the activity was located within a small, approximately equidimensional volume with a diameter of about 2 km. After the largest foreshock, the activity spread northwest and southeast forming a 6‐km‐long, northwest trending zone. First motions and ratios of P to S amplitudes indicate that two different faulting mechanisms occurred during the foreshock sequence. The two radiation patterns can tentatively be correlated with different parts of the zone. The hypocenter of the mainshock was not located on the same fault as that defined by the foreshocks' hypocenters but rather was located 6 km south of and several kilometers shallower than the foreshock cluster. We think this large separation between foreshocks and mainshock in a direction perpendicular both to the plane of rupture of the mainshock and to the trend of the foreshocks might be the result of an en echelon step in the fault that slipped during the mainshock. An analysis of the change in stress due to slip during the foreshocks shows that the increase in shear stress on the mainshock fault caused by the foreshocks is very small and that direct triggering of the mainshock by the foreshocks is unlikely.