Investigating Coseismic vs. Interseismic Uplift of Marine Terraces at the Southern Terminus of the Cascadia Subduction Zone: Cape Mendocino to Punta Gorda, Petrolia, CA

Abstract:

The Cascadia subduction zone of the Pacific Northwest terminates in the south at the Mendocino Triple Junction, a region of elevated seismic activity. Here, tectonically driven uplift is likely responsible for the formation of Holocene-aged marine terraces. In 1992, a M 7.1 thrust mainshock and two ~M 6.5 aftershocks occurred offshore of Cape Mendocino, resulting in 1.4 meters of uplift, measured from stranded intertidal species on uplifted wave-cut platforms. However, it is unknown whether these marine terraces formed due solely to large episodic coseismic uplift caused by moderately large to large magnitude earthquakes, orwhether interseismic deformation plays a role in their formation. Moreover, detailed mapping of these terraces has not been done since Merits (1996). For this study, we map a suite of four terraces and bedrock wave cut platforms between Cape Mendocino and Punta Gorda on the northern California coast near the town of Petrolia. By mapping both exposed and buried Holocene marine terraces we aim to discern the pattern of deformation causing their uplift, and determine whether uplift is related solely to episodic coseismic events, or both coseismic and interseismic deformation. Detailed mapping of the extent and elevation of both exposed and buried terraces is achieved by using real time kinematic (RTK) GPS surveying on base maps compiled using the 2009-2011 California Coastal Conservancy LIDAR. Preliminary results indicate formation predominantly from coseismic uplift in moderate magnitude subduction earthquakes. This work offers insight into understanding the upper plate crustal response to seismic events and interseismic periods in tectonically complex areas like the southern end of the Cascadia Subduction zone.