Continental Tectonics from Dense Array Seismic Imaging: Intraplate Seismicity in Virginia and a Steep Cratonic Margin in Idaho

Davenport, Kathy

Continental Tectonics from Dense Array Seismic Imaging: Intraplate Seismicity in Virginia and a Steep Cratonic Margin in Idaho

dc.contributor.author	Davenport, Kathy	en
dc.contributor.committeechair	Hole, John A.	en
dc.contributor.committeemember	Zhou, Ying	en
dc.contributor.committeemember	Tikoff, Basil	en
dc.contributor.committeemember	Chapman, Martin C.	en
dc.contributor.committeemember	Spotila, James A.	en
dc.contributor.department	Geosciences	en
dc.date.accessioned	2016-09-22T08:00:28Z	en
dc.date.available	2016-09-22T08:00:28Z	en
dc.date.issued	2016-09-21	en
dc.description.abstract	Dense array seismic techniques can be applied to multiple types of seismic data to understand regional tectonic processes via analysis of crustal velocity structure, imaging reflection surfaces, and calculating high-resolution hypocenter locations. The two regions presented here include an intraplate seismogenic fault zone in Virginia and a steep cratonic margin in eastern Oregon and Idaho. The intraplate seismicity study in Virginia consisted of using 201 short-period vertical-component seismographs, which recorded events as low as magnitude -2 during a period of 12 days. Dense array analysis revealed almost no variation in the seismic velocity within the hypocentral zone, indicating that the aftershock zone is confined to a single crystalline-rock terrane. The 1-2 km wide cloud of hypocenters is characterized by a 29° strike and 53° dip consistent with the focal mechanism of the main shock. A 5° bend along strike and a shallower dip angle below 6 km points toward a more complex concave shaped fault zone. The seismic study in Idaho and Oregon was centered on the inversion of controlled-source wide-angle reflection and refraction seismic P- and S-wave traveltimes to determine a seismic velocity model of the crust beneath this part of the U.S. Cordillera. We imaged a narrow, steep velocity boundary within the crust that juxtaposes the Blue Mountains accreted terranes and the North American craton at the western Idaho shear zone. We found a 7 km offset in Moho depth, separating crust with different seismic velocities and Poisson's ratios. The crust beneath the Blue Mountains terranes is consistent with an intermediate lithology dominated by diorite. In the lower crust there is evidence of magmatic underplating which is consistent with the location of the feeder system of the Columbia River Basalts. The cratonic crust east of the WISZ is thicker and characterized by a felsic composition dominated by granite through most of the crust, with an intermediate composition layer in the lower crust. This sharp lithologic and rheologic boundary strongly influenced subsequent deformation and magmatic events in the region.	en
dc.description.abstractgeneral	Dense array seismic techniques involve using many instruments deployed closely together to record natural or man-made ground shaking. These techniques can be applied to different types of seismic data to understand the regional composition and behavior of the Earth’s crust, and identify locations where earthquakes have occurred. The two regions presented here include a zone in Virginia known to have small earthquakes and a location in eastern Oregon and Idaho where younger crust meets older crust across a very steep boundary. The seismicity study in Virginia consisted of using 201 instruments to record earthquake aftershocks with very small magnitudes during a period of 12 days. Dense array analysis techniques revealed almost no variation in the speed that seismic waves travel within the zone of aftershocks, indicating that the aftershocks are confined to a single crystalline-rock region. The 1-2 km wide zone of sub-surface aftershock locations is consistent with the rupture orientation of the main earthquake on 23 August 2011. The zone’s slightly concave shape indicates a complex region of rock movement. The seismic study in Idaho and Oregon was centered on analyzing seismic waves that are generated by explosions and travel through the crust, bending and reflecting when they pass through variations in the rock. We imaged a narrow, steep boundary that juxtaposes the younger Blue Mountains crust and the older North American craton at the western Idaho shear zone (WISZ). We found a sharp ~7 km step between the thicknesses of the two regions and different seismic velocities on either side of the boundary. The crust beneath the Blue Mountains terranes is consistent with an intermediate rock type dominated by diorite. In the lower crust there is evidence of a layer that is consistent with un-erupted material from the Columbia River Basalts. The continental crust east of the WISZ is thicker and dominated by granite through most of the crust, with an intermediate composition layer in the lower crust. This sharp boundary strongly influenced subsequent deformation and magmatic events in the region.	en
dc.description.degree	Ph. D.	en
dc.format.medium	ETD	en
dc.identifier.other	vt_gsexam:8960	en
dc.identifier.uri	http://hdl.handle.net/10919/72976	en
dc.publisher	Virginia Tech	en
dc.rights	In Copyright	en
dc.rights.uri	http://rightsstatements.org/vocab/InC/1.0/	en
dc.subject	Dense arrays	en
dc.subject	travel time tomography	en
dc.subject	western Idaho shear zone	en
dc.subject	Moho offset	en
dc.subject	Virginia earthquake	en
dc.subject	controlled-source seismic	en
dc.title	Continental Tectonics from Dense Array Seismic Imaging: Intraplate Seismicity in Virginia and a Steep Cratonic Margin in Idaho	en
dc.type	Dissertation	en
thesis.degree.discipline	Geosciences	en
thesis.degree.grantor	Virginia Polytechnic Institute and State University	en
thesis.degree.level	doctoral	en
thesis.degree.name	Ph. D.	en