Is the Mainshock Rupture and Aftershock Sequence of the 2011 Mineral, Virginia Earthquake Typical of Moderate Intraplate Shocks?

Session: Seismicity and Tectonics of Stable Continental Interiors
Type: Oral
Date: 4/30/2020
Time: 11:15 AM
Room: 240
Description: 

The Mw 5.65 Mineral, Virginia earthquake was a complex rupture. Relative locations of subevents indicate a rupture of roughly 2.5 km in diameter. This small rupture dimension, which extended from 8 to slightly less than 6 km depth, is in good agreement with the Brune stress drop of 67 MPa determined from the source corner frequency using empirical Green’s functions. This suggests that the Mineral mainshock represents near total stress drop and may involve the creation of a new fault. This view is supported by the spatial distribution of aftershocks, both those that occurred in the 3 months immediately following the mainshock and later aftershocks recorded by a recent (2017-2018) temporary network deployment. The aftershock sequence is both temporally and spatially stationary, and the mainshock rupture zone is a gap in aftershock activity. Both early and late aftershocks occur in a zone 10 km in length, but form two populations. Most of the aftershocks are at depths from 4 to 6 km and are spatially organized into a halo of events above and to the northeast of the mainshock rupture zone. Aftershocks at less than 4 km depth occur in several spatially isolated clusters and exhibit reverse motion on north to northwest trending nodal planes, whereas the mainshock was reverse on a N30E striking plane. The rate of aftershocks observed during the 2017-2018 period is within a factor of two of that predicted by Omori’s Law, using parameters determined from the early aftershock sequence in 2011.

If the Mineral shock was a typical intraplate shock, then those events can create new faults or else occur on high strength faults that have healed. The locations of the aftershocks cannot be used directly to infer the mainshock rupture dimensions. The aftershocks illuminate the periphery of the rupture zone and a much larger zone of positive Coulomb stress transfer, particularly at shallow depths. Finally, aftershock sequences can persist on time scales of at least a decade for moderate intraplate shocks like Mineral and perhaps for centuries for major shocks.

Presenting Author: Martin C. Chapman

Authors