Tectonic implication of the March 5th, 2005, doublet earthquake in Ilan, Taiwan

Focal mechanism of the March 5th, 2005 earthquake doublet was determined as strike-slip 30 faulting from Harvard and BATS moment tensor inversion. However, based on first motion 31 polarities, the first shock has a normal focal mechanism (Wu et al., 2008a). The discrepancy 32 causes a debate in the focal mechanism solution, since different focal mechanisms have 33 different tectonic implications. On the basis of dislocation determination from Global Position 34 System (GPS) measurements, we find this event includes both tensile and strike-slip 35 components. Thus, this finding illustrates the reason for the differences in the determined 36 focal mechanisms using two different types of seismic data and analyzing methods. Results of 37 field mapping and microstructure examination indicates that the ductile deformation around 38 the study area was characterized by the evolution from transpression to transtension with 39 predominant strike-slip component, but present-day active structures may be dominated by 40 normal faulting. Thus, the result of active tensile slip determined from dislocation modeling 41 strongly suggests that the backarc extension of the Okinawa trough have influenced the stress 42 state in this region, and switched the major component of transtension from strike-slip 43 faulting to normal faulting. 44 .