Finite-Source Modeling of the South Napa Earthquake

On August 24 2014 an Mw 6.0 earthquake struck south-southwest of the city of Napa, California. As part of the Berkeley Seismological Laboratory (BSL) Alarm Response a seismic moment tensor solution and preliminary finite-source model were estimated. The preliminary finite-source model used high quality three-component strong motion recordings, instrument corrected and integrated to displacement, from 8 stations of the BSL BK network for stations located between 30 to 200 km. The BSL focal mechanism (strike=155, dip=82, rake=-172), and a constant rise time and rupture velocity were assumed. The GIL7 plane-layered velocity model was used to compute Green’s functions using a frequency wave-number integration approach. The preliminary model from these stations indicates the rupture was unilateral to the NNW, and up dip with a average slip of 42 cm and peak slip of 102 cm. The total scalar moment was found to be 1.15*10²⁵ dyne cm giving a Mw 6.0.The strong directivity from the rupture likely leads to the observed elevated local strong ground motions and the extensive damage to buildings in Napa and surrounding residential areas. In this study we will reevaluate the seismic moment tensor of the mainshock and larger aftershocks, and incorporate local strong motion waveforms, GPS, and InSAR deformation data to better constrain the finite-source model. While the hypocenter and focal parameters used in the preliminary model are consistent with the mapped surface trace of the west Napa fault, the mapped surface slip lies approximately 2 km to the west. Furthermore there is a pronounced change in strike of the mapped surface offsets at the northern end. We will investigate the location of the fault model and the fit to the joint data set as well as examine the possibility of multi-segmented fault models to account for these apparently inconsistent observations.