May 22, 2019,
noon - 1 p.m.
We study temporal changes of seismic velocities associated with the June 2016 Mw 5.2 Borrego Springs earthquake in the San Jacinto fault zone, using 9 component Green’s function estimates reconstructed from daily cross correlations of ambient noise. The analyzed data are recorded by stations in two dense linear arrays crossing the fault surface trace ~3 km northwest (DW) and southeast (JF) of the event epicenter. The two arrays have 9-12 stations each with instrument spacing of 25-100 m. Relative velocity changes (δv/v) are estimated from arrival time changes in the daily correlation coda waveforms compared to a reference stack. The obtained array-average δv/v time series exhibit seasonal variations, linear trends and changes associated with the Borrego Springs event. The earthquake signal is characterized by rapid co-seismic velocity drop followed by a gradual recovery. This is observed consistently using both time- and frequency-domain δv/v analysis methods on data of different components in various frequency bands at both arrays. After removal of the seasonal and linear components, the velocity drops estimated with a daily resolution from the vertical-vertical correlation data are around 6%, 0.3%, and 0.2% at the DW array, and 1%, 0.3%, and 0.2% at the JF array, for 0.1-0.4 Hz, 0.5-2 Hz, and 1-4 Hz, respectively. Normalizing the co-seismic changes by corresponding amplitudes of seasonal variations helps suppressing site-specific responses. The larger changes at lower frequencies imply that the variations are not limited to the subsurface material. A decreasing co-seismic velocity reduction with coda wave lapse time indicates larger co-seismic structural perturbations within the fault zone compared to the surrounding host rock. The observed larger changes at the DW array compared to the changes at the JF array may reflect larger damage produced by the northwestward rupture directivity of the Borrego Springs earthquake.