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Duration and Dynamic Stress Drop During the Initial Rupture “Breakaway” Stage of Ridgecrest Earthquakes

Description: 

The initial slope of P wave velocity seismograms often starts with a relatively low rate for a short time before growing quickly with an approximately ramp-like shape (Iio, 1992; Ellsworth and Beroza, 1995, Meier et al., 2016). The latter stage was named the “breakaway” stage by some authors. We adopt the self-similar dynamic circular growth model (Kostrov, 1964; Boatwright, 1980) and use P wave records at hypocentral distances less than 50 km to constrain the duration and dynamic “breakaway” stress drop for Mw>3.9 Ridgecrest earthquakes. The “breakaway” phases that we examined are generally well recorded by multiple local seismic stations, allowing us to assess the measurement uncertainties. All of the selected earthquakes have reliable moment tensor solutions (Wang and Zhan, 2020), which are used to estimate the P-wave radiation pattern. We calculate the P-wave response using a 1D Southern Sierra model and a station-dependent near-surface velocity structure. The latter is constrained using the P wave polarization information (Ni et al., 2014; Park and Ishii, 2018) and empirical relations derived from laboratory observations (e.g., Boore, 2016). We find breakaway durations for 3.9<Mw<5.5 Ridgecrest earthquakes are about 0.1 s, 5-30% of the total rupture duration. The “breakaway” stress drop is nearly independent of magnitude and the “breakaway” duration, but it is positively correlated with Brune’s stress drop of the entire rupture. The measures are proportional to (V_R/V_s)^-3 where V_R and V_s are rupture velocity and S wave speed. Assuming V_R/V_s=0.7, the geometric mean of the “breakaway” stress drop is 21 MPa with a factor of 1.8 uncertainty. This mean changes from 112 MPa to 10 MPa when V_R/V_s increases from 0.4 to 0.9. It is 4.5-51 times the mean Brune’s stress drop (~2.2 MPa) of these earthquakes. The estimates of “breakaway” stress drop statistically significantly increase with source depth at a rate of 1.9 MPa/km. A conceptual rupture model is proposed to explain the big difference between the “breakaway” stress drop and Brune’s stress drop.

Session: Earthquake Source Parameters: Theory, Observations and Interpretations

Type: Oral

Date: 4/18/2023

Presentation Time: 08:45 AM (local time)

Presenting Author: Chen Ji

Student Presenter: No

Invited Presentation: 

Authors