Estimating stress drops using inter-station phase coherence: comparison with the Ridgecrest stress drop validation study

Stress release is a fundamental physical parameter of earthquakes. Yet the direct measurement of stress drop at depth is practically difficult; different estimations of stress drop often give different values, likely because of biases and uncertainties of the various methods and dataset. To better understand these biases and uncertainties, it is important to estimate stress drop with a wide range of approaches, which have different potential errors.  So here we use and attempt to validate a different approach to determining stress drop, focusing on variability in the apparent source time functions observed at a range of stations.

 

Specifically, we estimate stress drops of earthquakes in the 2019 Ridgecrest, CA sequence.  Over the past few years, researchers have used a variety of approaches to estimate stress drops of these earthquakes.  We compare their estimates with stress drops obtained from our inter-station coherence approach.  With these comparisons, we aim to assess which models of earthquake rupture are plausible.

 

The coherence method works by examining the differences in the apparent source time functions (ASTFs) observed at a range of stations.  We note that signals coming from different locations within the rupture area have different arrival time at the observing stations. The arrival time differences are proportional to the largest distance between generated seismic waves, which is proportional to the rupture diameter D.  Thus longer-period inter-station differences can arise when the rupture diameter is larger, and we identifythe periods where inter-station differences exist in order to identify the rupture diameter. 

 

The 2019 Ridgecrest earthquake sequence occurred between 4 July 2019 to 17 July 2019. We aim to focus on 55 well-located earthquakes in the range magnitude of 2.01 to 4.52 at depths between 2 km to 10 km.  For each earthquake signal, we perform signal pre-processing to the vertical component of seismogram by remove the effect of trend from the trace. We taper the signal to minimize the effect of discontinuities at the beginning and the end of time series and remove the instrument response of the seismogram. Then we obtain high-quality arrival times and cross-correlate signals from nearly co-located earthquakes to partly remove the path effect.  After the path effect removal, we can examine differences in the source time functions among stations and infer the earthquake diameters.

 

Initial results for M3.81 and M3.99 earthquakes imply stress drops of 2 MPa and 19 MPa, respectively.  These estimates are scattered around estimates obtained by other researchers, and we are currently working to obtain more stress drops to compare and to examine synthetic ruptures and understand why the estimates differ.