This dissertation focuses on two different types of responses of Earth; that is, seismic and electromagnetic, and aims to better understand Earth processes at a wider range of scales than those conventional approaches offer. Electromagnetic responses resulting from the subsurface diffusion of applied electromagnetic fields through heterogeneous geoelectrical structures are utilized to characterize the underlying geology. Geology exhibits multiscale hierarchical structure which brought about by almost all geological processes operating across multiple length scales and the relationship between multiscale electrical properties of underlying geology and the observed electromagnetic response has not yet been fully understood. To quantify this relationship, the electromagnetic responses of textured and spatially correlated, stochastic geologic media are herein presented. The modelling results demonstrate that the resulting electromagnetic responses present a power law distribution, rather than a smooth response polluted with random, incoherent noise as commonly assumed; moreover, they are examples of fractional Brownian motion. Furthermore, the results indicate that the fractal behavior of electromagnetic responses is correlated with the degree of the spatial correlation, the contrasts in ground electrical conductivity, and the preferred orientation of small-scale heterogeneity. In addition, these inferences are also supported by the observed electromagnetic responses from a fault zone comprising different lithological units and varying wavelengths of geologic heterogeneity. Seismic signals generated by aftershocks are generally recorded by local aftershock networks consisted of insufficient number of stations which result in strongly spatially-aliased aftershock data. This limits aftershock detections and locations at smaller magnitudes. Following the 23 August 2011 Mineral, Virginia earthquake, to drastically reduce spatial aliasing, a temporary dense array (AIDA) consisting of ~200 stations at 200-400 m spacing was deployed near the epicenter to record the 12 days of the aftershocks. The backprojection imaging method is applied to the entire AIDA dataset to detect and locate aftershocks. The method takes advantage of staking of many seismograms and improves the signal-to-noise ratio for detection. The catalog obtained from the co-deployed, unusually large temporal traditional network of 36 stations enabled a quantitative comparison. The aftershock catalog derived from the dense AIDA array and the backprojection indicates event detection an order of magnitude smaller including events as small as M–1.8. The catalog is complete to magnitude –1.0 while the traditional network catalog was complete to M–0.27 for the same time period. The AIDA backprojection catalog indicate the same major patterns of seismicity in the epicentral region, but additional details are revealed indicating a more complex fault zone and a new shallow cluster. The b-value or the temporal decay constant were not changed by inclusion of the small events; however, they are different for two completeness periods and are different at shallow depth than greater depth. / Ph. D. / This dissertation revolves around two ends of geophysics: seismology and electromagnetics. The electromagnetic method of exploration geophysics aims to characterize the underlying geology by evaluating the electromagnetic responses resulting from the interaction between the applied electromagnetic fields and the subsurface electrical properties. In case of rough geology comprising heterogeneity at every scale, the electromagnetic responses are more complicated than the response of a piecewise smooth Earth structure. Most analyses treat the responses of small-scale heterogeneities as random, uncorrelated noise. Here, more realistic geologic models comprising spatially-correlated, fine-scale heterogeneities are incorporated into electromagnetic modeling to better understand the relationship between the causative multiscale geoelectrical heterogeneities and the electromagnetic responses. The numerical results indicate that these electromagnetic responses are not random as commonly assumed, in contrast, they are repeatable and fractally distributed presenting spatial fluctuations that appear on all length scales. Moreover, the numerical results indicate that the fractal behavior of electromagnetic responses is correlated with the degree of the spatial correlation, the contrasts in ground electrical conductivity, and the preferred orientation of small-scale heterogeneity. In addition, the analysis of the observed electromagnetic responses from a fault zone comprising multiscale heterogeneity also support these inferences. Seismic signals generated by aftershocks are generally recorded by local aftershock networks consisted of insufficient number of stations which result in strongly spatially-aliased aftershock data. This limits aftershock detections and locations at smaller magnitudes. Following the 23 August 2011 Mineral, Virginia earthquake, to drastically reduce spatial aliasing, a temporary dense array (AIDA) consisting of ∼200 stations at 200-400 m spacing was deployed near the epicenter to record the 12 days of the aftershocks. The backprojection imaging method is applied to the entire AIDA dataset to detect and locate aftershocks. The method takes advantage of summing of many seismograms and improves the signal-to-noise ratio for detection. The catalog obtained from the co-deployed, unusually large temporal traditional network of 36 stations enabled a quantitative comparison. The aftershock catalog derived from the dense AIDA array and the backprojection indicates event detection an order of magnitude smaller. The AIDA backprojection catalog indicate the same major patterns of seismicity in the epicentral region, but additional details are revealed indicating a more complex fault zone and a new shallow cluster. The decay of aftershock rate and the distribution of earthquakes with respect to the magnitude do not show a significant change by inclusion of the small events; however, they differ at shallow and greater depth, and for different completeness periods.

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. / Ph. D. / 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.