Aftershock Imaging with Dense Arrays (AIDA) for the 23 August 2011, Mw 5.8, Virginia earthquake aftershock sequence

Larry Brown (Cornell University)
John Hole (Virginia Tech)
Walter Mooney (U. S. Geological Survey, Menlo Park)
Martin Chapman (Virginia Tech)
Rowena Lohman (Cornell University)

Students working on the data as part of their graduate studies:
Diego Quiros (Cornell University)
Kathy Davenport (Virginia Tech)

People who did the field work:
Cornell University: Diego Quiros, Chen Chen, plus several shorter-term volunteers
Virginia Tech: Kathy Davenport, Liang Han, plus several shorter-term volunteers
IRIS-PASSCAL: Patrick Bastien
U. S. Geological Survey: Alex Ferguson, Neil Fenning
James Madison University: several short-term volunteers
Washington and Lee University: several short-term volunteers

Project funded by the National Science Foundation and the U. S. Geological Survey
NSF logo USGS logo

The central Virginia seismic zone, which has a history of minor to moderate seismicity, was the location of a magnitude 5.8 earthquake on 23 August 2011. While several such seismic zones exist in the eastern USA, specific faults responsible for the earthquakes have not been identified due to relatively low strain rates and sparse seismic networks. The 2011 earthquake occurred in an area where the geology is well mapped and a 1980's seismic reflection line illuminates deep crustal structure. The seismic zone occurs within the crystalline Piedmont province and the seismic reflection data indicate that previous seismicity occurs above an Appalachian detachment in the upper crust. Preliminary hypocenters from the 2011 earthquake are also in the upper crust. The 2011 earthquake provides a rare opportunity to try to understand a seismically active fault at depth in the eastern USA.

Traditional earthquake aftershock deployments consist of at most a few dozen stations at spacings that are badly spatially aliased at frequencies relevant for waveform analysis. The research community has been pursuing a "Flexi-RAMP" instrument pool to deploy an order of magnitude more stations to vastly improve the spatial resolution of aftershock and local geologic images. The Aftershock Imaging with Dense Arrays (AIDA) deployment in Virginia represents a first effort to field such a large number of seismographs. The objectives were to record wavefields at sufficiently dense spacing to allow the use of array methods to locate the aftershocks with high precision and to image geologic structure with resolution on par with typical controlled-source crustal surveys. Single-channel short-period "Texan" seismographs were deployed from the EarthScope Flexible Array. These instruments are designed for controlled-source studies and supported by IRIS-PASSCAL. On 27 August, 103 stations were deployed at the epicenter in two 7-11 km linear arrays at 200 m spacing. On 1 September, an additional 30 three-component "triple-Texan" stations were deployed in a 60-km linear array outward from the epicenter. On 3 September, the original 103 seismographs plus 14 new seismographs were re-deployed at 200 or 400 m spacing on three new 7-12 km lines at the epicenter. All stations were removed on 9 September due to the effort of battery swapping.

Initial data analysis efforts will focus on a) evaluating the impact of AIDA on locating aftershocks, b) imaging seismic velocity structure of the epicentral volume, c) imaging structure using both aftershocks and ambient noise as sources, and d) quantifying seismic wave attenuation along the 3-component 60-km-long array.

Texan being deployed Texan in ground
Deployment of "Texan" seismographs is simple compared to traditional earthquake seismographs, allowing the rapid deployment of large arrays. However, as the instruments were not designed for long-term recording, they require more frequent maintenance. Technology exists today to build seismographs with the best attributes of both systems, enabling large Flexi-RAMP deployments.

aftershock network
Seismograph stations deployed to record aftershocks.
Green: "Texan" stations deployed as part of this project, AIDA-VA. See figures below for station density along the green lines.
Red: stations deployed by the Virginia Tech Seismological Observatory (Martin Chapman).
Blue: stations deployed by other groups: U. S. Geological Survey, IRIS, University of Memphis, and Lamont-Doherty Earth Obseratory.
Magenta star is the epicenter of the magnitude 5.8 event on 23 August 2011.

Texan deployment, phase 1
Map of Texans deployed August 27 until September 2. Symbols are the same as the previous plot.

Texan deployment, phase 2
Map of Texans deployed September 1st or 3rd until September 9. Symbols are the same as the previous plot.

Aftershock deployments (triangles) and aftershocks (circles) up to September 1.
Station symbols are the same as the previous plot.
Magenta aftershocks were located by the Virginia Tech Seismological Observatory (Martin Chapman).
Red aftershocks are mostly the same events located by the U. S. Geological Survey (Bill Ellsworth).
Differences between the red and magenta locations are representative of errors due to using different stations to locate the events. These datasets are being merged and the event locations will improve.

To Larry Brown's home page
To John Hole's home page
To Martin Chapman's home page
To Rowena Lohman's home page