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Detailed observations of seismicity, stress drop and directivity on a complex fault structure in Mogul Nevada

Rachel E. Abercrombie, Christine J. Ruhl, & Ken D. Smith

Published August 15, 2017, SCEC Contribution #7808, 2017 SCEC Annual Meeting Poster #056

The exceptionally well-recorded Mogul earthquake sequence, near Reno NV in 2008 allows detailed investigation of seismicity migration, and propagation through complex structures. Following two months of swarm-like earthquakes an increase in both seismicity rates and event magnitudes over several days culminated in an Mw 4.9 dextral strike-slip earthquake. Although very shallow, the mainshock had a different sense of slip than mapped dip-slip surface faults. We precisely relocate 7,549 earthquakes and calculate 1,100 focal mechanisms to reveal an internally clustered sequence; foreshocks evolved on multiple structures surrounding the eventual mainshock rupture on the previously unknown Mogul fault. The seismicity volume (2-6 km depth) expands before the mainshock, consistent with pore-pressure diffusion, and the aftershock volume is larger than is typical for a M4.9 earthquake. We identify highly clustered events that are largely mainshock-aftershock sequences, without evidence for migration, occurring within the diffuse background seismicity.

We use the EGF approach developed by Abercrombie et al. (2017) to estimate spectral ratios, source time functions (STFs), and stress drops (Δσ) for 148 earthquakes using both P and S waves. Spatiotemporal variation along the mainshock fault plane is distinct and larger than measured uncertainties. High-Δσ foreshocks cluster around a region of low seismicity ruptured only by low-Δσ foreshocks and not re-ruptured in the aftershock period, suggesting a difference in rheology along the fault plane.
Azimuthal source directivity can be retrieved for earthquakes with multiple EGFs, recorded at stations over a wide range of azimuth.

Following Prieto et al. (2017) for each event, we stretch and correlate the STFs at pairs of stations to quantify the azimuthal variation of the source duration. We model this variation in duration using bilateral and unilateral line sources to find which fits best, and the best fitting orientation. We were able to apply this method to over 80 earthquakes, and found that the majority were best fit with unilateral rupture. The rupture velocities are relatively low but this is an artifact of the limited frequency range of the signal. The line source orientations are very stable enabling the fault plane to be identified. They agree well with the lineations of earthquakes in the different clusters, confirming slip on a mesh-like structure.

Key Words
earthquake, directivity, stress drop, fault structure

Abercrombie, R. E., Ruhl, C. J., & Smith, K. D. (2017, 08). Detailed observations of seismicity, stress drop and directivity on a complex fault structure in Mogul Nevada. Poster Presentation at 2017 SCEC Annual Meeting.

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