Investigating the high-frequency, weak-motion nucleation phase initiating the June 10th, 2016 Mw 5.2 Borrego Springs Earthquake

Adam Arce, Mareike N. Adams, & Chen Ji

Submitted August 15, 2016, SCEC Contribution #7001, 2016 SCEC Annual Meeting Poster #057

A nucleation phase, when present, offers insight into an earthquake’s rupture mechanics and may corroborate the “cascade” rupture model. In cascade rupture, slip initiates on a relatively small patch progressing into slip on a larger patch and so on, as accumulated strain continues to exceed the frictional stress along the fault. The nucleation phase is recorded as weak ground motion preceding the strong ground motion of a given event and has been observed with earthquakes of moment magnitude (Mw) ranging 2.1 – 8.1 (Beroza and Ellsworth, 1996).
On June 10th 2016 a Mw 5.2 earthquake occurred 20 kilometers NNW of Borrego Springs with moderate shaking reported (USGS) more than 80 km away. Strong motion data from 15 stations, ranging ~15 – 50 km from the source with good azimuthal coverage, recorded a high-frequency, weak-motion signal during the 1.70 – 1.76 seconds preceding the arrival of strong-motion phases. Ellsworth and Beroza (1995) observed a scaling relationship between nucleation duration and seismic moment, likely dependent on a constant stress-drop. The scaling law predicts a nucleation duration of ~ 0.12 s for the Borrego Springs event, an order of magnitude smaller than what is observed. A finite fault source inversion is being performed on the velocity seismograms of the strong-motion data. Further investigation is being done to understand the anomalous duration and what role this nucleation phase may have played in rupture of the Borrego Springs event.

Citation
Arce, A., Adams, M. N., & Ji, C. (2016, 08). Investigating the high-frequency, weak-motion nucleation phase initiating the June 10th, 2016 Mw 5.2 Borrego Springs Earthquake. Poster Presentation at 2016 SCEC Annual Meeting.


Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)