Because they are recognized as one of the most reliable premonitory events of the subsequent earthquake, foreshocks are often very useful to pinpoint the time and location of the forthcoming earthquake. However, the most challenging questions, including why foreshocks precede some earthquakes but not others and what key factors control their occurrence, are not yet satisfactorily answered. In the present study, a group of strike-slip earthquakes (M-s >= 6.0) in China and the western United States are classified into three types according to the configuration of the seismologic ruptures: type 1, type 2, and type 3. They refer to earthquakes of ruptures containing an extensional rock bridge, a contractional rock bridge, and both extensional and contractional rock bridges, respectively. Foreshocks are more likely to occur in type 1 or type 3 earthquakes, whereas they are less likely in type 2 earthquakes. These phenomena may indicate that, in strike-slip earthquakes, extensional interlock is a heterogeneity facilitating the occurrence of foreshocks. Being inconsistent with some published results, the incidence of foreshock seems to be independent of the depth of mainshock (shallower than 30 km) as far as this study is concerned, suggesting that the increase of normal stress may exert dissimilar influences on the foreshock occurrence for different earthquake focal mechanisms or locations. In the study data set, no definite connection is observed between the occurrence of foreshock and the magnitude of the forthcoming mainshock.
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