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.
On the afternoon of 23 August, 2011, the eastern third of the United States was rocked by a M 5.8 earthquake located in central Virginia near the small town of Mineral where it locally caused significant structural damage (Fig. 1). The Mineral Earthquake, as the event came to be known, was one of the largest historic earthquakes east of the Mississippi River and was reported as being felt to the U. S. Geologic Survey by more people than any other earthquake in the U.S. (see http://earthquake.usgs.gov/earthquakes/eqinthenews/2011/se082311a/felt-comparisons.jpg for a graphic showing the "Did You Feel It?" questionnaire results). Geologists have learned a great deal in the past few decades as to why earthquakes are more common in some regions than others, but there remains much to be learned about how stresses build in the crust and when they are released abruptly at a fault, causing an earthquake. For region like California, where two great lithospheric plates are sliding past one another, geologists have developed advanced models to predict where the seismic hazards are the greatest. But on the east coast, which is located in the middle of a lithospheric plate, far away from plate boundaries, the locations, frequency, causes, and hazards associated with earthquakes is far less well understood. An effective way to begin to understand earthquakes and define their hazards is to collect real time data when earthquakes happen. The U.S. is a large country and it is not practical to blanket the entire nation with instruments at the density needed to capture useful geophysical data before, during, and after an earthquake. This is particularly true for settings like the east coast where large earthquakes are infrequent in comparison to the west coast. So, when a large earthquake happens in places like the east coast, the geologic and geophysical community mobilizes to send scientists and instruments to the site of the earthquake with the hopes of collecting data on ground deformation and aftershocks in the days and weeks following the main event. This is what was done during the Fall of 2011 in the wake of the Mineral earthquake. There are three main kinds of data that can be collected by instruments and detailed mapping of the earthquake area, which for Mineral, included most of Louisa County, VA. These data are (1) real-time ground motions measured by high-precision GPS receivers (Fig. 2), (2) ground shaking caused by aftershocks and measured by IRIS-RAMP YC-2011-2012 seismometers (Fig. 3), and geologic data like offset rocks, displaced stream deposits, and perturbed river channels (Fig. 4). Our research, supported by the EarthScope RAPID grant, included collection, public distribution, and archiving of all of these data. The preliminary results inspired and seeded follow-up studies funded by the USGS EDMAP program that corroborated and extended the initial data, and along with scholarship that emerged from complementary studies, is helping develop a more comprehensive understanding of the causes of east coast earthquakes. The GPS geodetic data is accessible from the UNAVCO data archive interface as stations VA01 and VA02: www.unavco.org/data/gps-gnss/data-access-methods/dai2/app/dai2.html#boundingBox=35.7707,-81.4111,40.9361,-74.3799;scope=Station;sampleRate=normal The seismological data are available from the IRIS DMC web interface: http://ds.iris.edu/ds/nodes/dmc/ We note that as of February 2015, approximately 500 gigabytes (0.5 terabytes) of the Virginia RAPID IRIS-RAMP YC-2011-2012 seismological data have been served out to 179 unique users at 123 different institutions in 23 countries. Key findings of our study are that the main earthquake nucleated at a depth of about 6 km on a steeply-dipping fault that was clearly illuminated by hundreds of aftershocks (Fig 4, inset). Field evidence strongly suggest that over long periods of geologic time, repeated earthquakes in this region, and perhaps on the Mineral fault, have accumulated deformation at the surface expressed as distorted segments of the South Anna River and its ancient river deposits. During the 2011 earthquake the fault did not reach the surface, but this does not mean that the surface was not permanently deformed during the earthquake. The slow rates of crustal deformation on the east coast means that the GPS receivers installed after the earthquake have yet to discern a surface deformation signal apart from the normal noise caused by seasonal changes in soil temperature, moisture, and weather. Dating of river deposits and the downward incision of the South Anna River into the land uplifted by many earthquakes over geologic time suggest that earthquakes similar in size to the Mineral event, statistically could occur in Louisa County over 10,000 to 30,000 year intervals.
