WHAT CAN THE STUDY OF MODERN SEISMICITY TELL US ABOUT PAST AND FUTURE LARGE SHOCKS?
In 2011, an 8-station temporary network was deployed in the Summerville, SC area. The resulting 134 hypocenter locations define a south-striking tabular seismogenic zone dipping 43 degrees to the west in the upper 13 km. The majority of the 48 well-constrained focal mechanisms exhibit reverse slip on N to NW trending planes. Those observations and our analyses of reflection profiles and the potential field in the Summerville area support the hypothesis that the source of the Charleston earthquake was compressional reactivation of a south-striking Mesozoic extensional fault. At Mineral and Summerville hypocenters define tabular seismogenic zones, with the majority of events at shallow depth (1 to 6 km). The two sets of focal mechanisms show similar diversity. For Mineral, more than half of the solutions exhibit reverse mechanisms with P-axis trends differing by more than 15 degrees from that of the mainshock. The same is true for Summerville, in terms of the difference between focal mechanism B-axis trend and the N186E strike of the seismogenic zone. This suggests that seismicity following the two mainshocks is largely controlled by the spatial geometry of Coulomb stress transfer caused by the respective, mostly-reverse, mainshocks acting on minor faults with diverse orientation. The resulting hypocenter locations define the orientation of both mainshock damage zones. This is certainly the case for the Mineral, Virginia aftershocks. It appears that this is also the case for the on-going seismicity in the epicentral area of the 1886 Charleston earthquake, despite the passage of 130 years since the mainshock.