Deciphering the transient dip-slip behavior on the southern segment of the Hayward Fault, California

Authors: Schmidt, D. A., Bürgmann

We explore the effects of regional dynamics and fault structure on creep rates along the southern portion of the Hayward Fault. We evaluate the depth of creep by performing a joint inversion of GPS displacements, radar interferometric data, and surface creep data for fault geometry and slip rates. Observations from radar interferometry would suggest a rate of 17 mm/yr if all of the range change is accommodated by strike-slip motion (Burgmann et. al., 1997). The geologic slip rate along the fault is reported as ~9mm/yr of strike-slip motion with an insignificant component (<< 1 mm/yr) of dip-slip motion. Field observations suggest that horizontal creep does not exceed 9 mm/yr. Burgmann et al. suggest that an appreciable transient component of dip-slip motion would explain the high rate observed from InSAR data because the satellite range change is most sensitive to vertical displacements. Since InSAR data alone are incapable of resolving the three dimensional displacement field, we incorporate GPS data to provide additional constraints on the vertical component. The high creep rate suggests that surface creep extends to great depth, which would agree with the lack of microseismicity directly below the surface trace. However this segment is assumed to represent the southern tip of the fault which may restrict slippage through the seismogenic zone. Alternatively, the surface trace may join with the observed microseismicity that diverges to the east suggesting a complex fault structure. We attempt to explain the observed behavior using GPS, InSAR, and surface creep measurements in conjunction with numerical modeling of the mechanics of this complex system.