EarthScope RAPID: Geodetic and seismological response to the Mineral, VA Earthquake, 23 August, 2011

Lehigh University, Bethlehem PA

Investigators

Abstract

The overall goal of this research is to instrument the central Virginia seismic zone (CVSZ) in the region surrounding the Mineral, VA earthquake with seismometers and global positioning system (GPS) receivers that are designed to record aftershocks and subtle ground movements respectively for a year. These measurements allow geoscientists to visualize the fault that ruptured on 23 August 2011 and better understand the state of stress in the crust that lead to the earthquake. Eastern North America has a long history of infrequent, but sometimes large and damaging earthquakes. Unlike the seismic activity that is concentrated along plate boundaries, earthquakes in the passive margin of eastern North America are not clearly identified with active faults nor are the processes that drive them well understood. Earthquakes in eastern North America are clustered into well-defined seismic zones, like the CVSZ where small to moderate-sized events including the Mineral, VA earthquake seem to occur every few decades. The seismic zones are interspersed with seismic gaps, one of which surrounds the nation's capitol, where there is little history of earthquake activity. One possible explanation of these observations is that the earthquakes cluster in areas because there is a localized, underlying geologic weakness in the crust that periodically releases accumulated stresses. Alternatively, the observed earthquakes, including the Mineral, VA event are all aftershocks of pre-historic, infrequent, but very large earthquakes and represent a crustal fracture healing over a period of several centuries. Examples of such earthquakes with estimated magnitudes of 7.0 or greater are known from the Charleston, SC (1886) and Grand Banks (1929) events and both of these locations are now recognized seismic zones characterized by frequent small to moderate-sized earthquakes. The tools that are being employed in this research are seven seismometers available from the Incorporated Research Institutions for Seismology (IRIS) instrument pool and two new GPS receivers that will be constructed following Plate Boundary Observatory (PBO) specifications. The small earthquakes that are now occurring in the Mineral, VA area are all aftershocks of the 23 August, 2011 main event that are migrating away from that rupture and traveling up along the fault plane towards the surface. As this happens, precise pinpointing of their location with a seismometer network builds a three-dimensional picture of the fault plane. At the same time, the GPS receivers are being positioned to record, with millimeter accuracy, the horizontal and vertical position of the ground surface on either side of the suspected fault plane. Together, the seismological and GPS geodesy data document how the crust and land surface deforms in the year following a moderate-sized earthquake. These data inform geological thinking on the state of stress in the crust that is used to constrain models of earthquake recurrence and hazards. Eastern North America requires more of this kind of research because many old faults inherited from the construction of the Appalachian Mountains and opening of the Atlantic Ocean are embedded in the crust and are evidently loaded and capable of generating earthquakes that cause structural damage and threaten human life. The ability of the geologic community to inform infrastructural engineers, planners, and communities as to the relative hazards associated with eastern North American faults and earthquakes will be greatly aided by a close and detailed study of the Mineral, VA event.