Comparing Ambient-Noise-Based Seismic Velocity Variations with Dynamic and Static Strain Changes Associated with Major Earthquake Rupture at Parkfield

Significant stress perturbations across a fault zone can change seismic wave velocity in the adjacent area, with implications for rock properties and fault mechanics. Ambient noise seismic interferometry is increasingly used to retrieve information about wave propagation between seismic stations, constrain seismic velocity changes, and potentially monitor the subsurface strain field. Comparing seismic velocity changes with strain changes in space and time helps us understand the damage and healing processes of fault rocks. Here we study the seismic velocity and strain changes in the shallow crust associated with the 2004 Mw 6.0 Parkfield earthquake in California. We process continuous seismic recordings from 13 stations of the High Resolution Seismic Network (HRSN) from 2001 to 2007. Then we use an open-source software NoisePy (Jiang and Denolle, 2020) to cut band-pass filtered data into 1-hour chunks and perform cross-correlation, stack hourly cross-correlation functions into 30-day averages, and retrieve reference Green's functions based on a criteria of cross-correlation coefficients. We also test the influence and quantify errors of different stacking methods. To estimate the history of relative velocity changes, dv/v, we use a moving-window cross spectrum method with a moving window of 30 days with a 1-day step. To resolve potential spatial variations in velocity changes, we select reference Green's functions from station pairs having low background dv/v fluctuations, and then derive S-wave velocity changes over different depth ranges from six frequency bands. We aim to quantify the relation between dv/v time series and dynamic strain changes from strong-motion seismograms, and coseismic and postseismic static strain inferred from geodetic measurements. These efforts may help us improve strategies to obtain robust dv/v time series and distinguish the causes of seismic velocity changes of the subsurface.