Duration and dynamic stress drop during the initial rupture “breakaway” stage of Ridgecrest earthquakes

Chen Ji

Submitted September 11, 2022, SCEC Contribution #12531, 2022 SCEC Annual Meeting Poster #063

It was well known that the initial rise of P wave velocity seismograms generated by earthquakes often starts with a relatively low rate for a short time before growing quickly in an approximately ramp-like shape (Iio, 1992; Ellsworth and Beroza, 1995). Researchers have attempted to model such observations with either a continuous function tn (2<n<4 and t is the time measured from the onset) (Iio, 1995; Meier et al., 2016), or “nucleation” and “breakaway” two stages (Ellsworth and Beroza, 1995). Here, we adopt the latter representation and study the ramp-like “breakaway” phases of moderate and large California earthquakes using the self-similar dynamic circular growth model (Kostrov, 1964). For a given earthquake, we attempt to measure its breakaway duration and breakaway dynamic stress drop using Boatwright's (1980) method. The result of the Boatwright method is sensitive to the near-surface velocity structure at individual stations; we constrain this structure 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). The “breakaway” phases of the moderate and large earthquakes that we examined are generally well recorded by multiple local seismic stations, allowing us to assess the measurement uncertainties. Our pilot analysis for 17 selected 4<Mw<5.4 earthquakes during the 2019 Ridgecrest earthquake sequence revealed that these earthquakes have similar breakaway durations (~0.09 s) and breakaway stress drop, independent of source depth and the earthquake’s final magnitude. However, confirming its independence of magnitude will need a larger magnitude range than we have currently analyzed.

Key Words
Duration, dynamic stress drop, "Breakaway"

Ji, C. (2022, 09). Duration and dynamic stress drop during the initial rupture “breakaway” stage of Ridgecrest earthquakes. Poster Presentation at 2022 SCEC Annual Meeting.

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