The sharp increase in seismicity over a broad region of central Oklahoma has raised concerns regarding the source of the activity and its potential hazard to local communities and energy-industry infrastructure. Efforts to monitor and characterize the earthquake sequences in central Oklahoma are reviewed. Since early 2010, numerous organizations have deployed temporary portable seismic stations in central Oklahoma to record the evolving seismicity. A multiple-event relocation method is applied to produce a catalog of central Oklahoma earthquakes from late 2009 into early 2015. Regional moment tensor (RMT) source parameters were determined for the largest and best-recorded earthquakes. Combining RMT results with relocated seismicity enabled determination of the length, depth, and style of faulting occurring on reactivated subsurface fault systems. It was found that the majority of earthquakes occur on near-vertical, optimally oriented (northeast-southwest and northwest-southeast) strike-slip faults in the shallow crystalline basement. In 2014, 17 earthquakes occurred with magnitudes of 4 or larger. It is suggested that these recently reactivated fault systems pose the greatest potential hazard to the region.
The Peru‐Chile subduction zone hosts M > 8 earthquakes as well as multiple ridges on the downgoing Nazca plate, making this region well suited for investigating the formation, evolution, and potential impacts of subducting features on seismogenesis. To evaluate the physical properties and structural variability of the Iquique Ridge offshore northern Chile, we present a P wave velocity model of the Nazca plate and Iquique Ridge outboard of the 2014 M 8.1 Iquique earthquake sequence. 2D tomographic inversions of P wave traveltime data from the 2016 PICTURES (Pisagua/Iquique Crustal Tomography to Understand the Region of the Earthquake Source) controlled‐source experiment indicate that the Iquique Ridge is characterized by significant crustal thickening relative to typical Nazca plate oceanic crust, with a maximum thickness of ∼13 km beneath the most prominent portion of the Iquique Ridge and outer rise. Reduced upper crustal seismic velocities extend from the ridge eastward to the trench, which may indicate increased fracturing and/or hydration during plate bending. Corresponding multi‐channel seismic reflection data show along‐profile structural variation as well, including an intermittent Moho reflector near the onset of crustal thickening and faulting along the landward slope. The structural heterogeneity entering the Peru‐Chile Trench has potential implications for slip behavior at depth assuming that the anomalous crustal structure continues beneath the forearc. We suggest that the increased buoyant normal force associated with discrete subducted Iquique Ridge seamounts similar to the zone of thick crust imaged here could lead to localized increased interplate coupling, while entrained sediment and fractured/altered crust could facilitate aseismic slip.
The Seattle fault zone (SFZ) is a north-directed thrust fault system that underlies the greater Seattle metropolitan area. Evidence of past land level changes, landslides, liquefaction, and a local tsunami indicate that this 70-km-long fault system can host up to M 7–7.5 earthquakes. Both the geometry and earthquake recurrence of the SFZ are debated and surveys of the shallow subsurface have not yet been incorporated into deeper crustal-scale structural interpretations, especially where the SFZ cuts across marine portions of the Puget Lowland. Here we use a new high-resolution marine seismic reflection dataset to image fault-related deformation in Quaternary sediments and Tertiary bedrock throughout Puget Sound and Lake Washington. We use this perspective of shallow geology as a link between existing crustal-scale geophysical insights into fault geometry at depth and paleoseismological observations of faulting at the surface and propose a refined structural model for the SFZ. We interpret that our new seismic reflection data in the Rich Passage area of Puget Sound images evidence of an inactive, south-dipping strand of the SFZ, which is overprinted by Quaternary folding and slip along north-dipping backthrusts within the hanging wall of a blind, south-dipping fault located 6 km farther north. To explain these results, we propose that the SFZ is a normal sequence fault propagation fold that has stepped northward through time, and we show the plausibility of this model through trishear forward modeling. Growth strata and faulting imaged in Quaternary sediments in Lake Washington and Rich Passage are consistent with the spatial distribution of folding and backthrusting that occurred during an M 7–7.5 earthquake in A.D. 900–930, corroborating existing evidence that the SFZ has been active throughout the Quaternary.
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