Dr. Gina Schmalzle is awarded an NSF Earth Science Postdoctoral Fellowship to carry out a program of research and education at the University of Washington. She will work on Slow Slip Events (SSEs) and Episodic Tremor and Slip (ETS) observed on Cascadia subduction zone. These events, observed in subduction zones worldwide, slowly release significant stress throughout the earthquake cycle in a matter of days to weeks. In contrast, earthquakes release stress in a matter of seconds to minutes. How SSE and ETS events redistribute stresses and contribute to large earthquakes in subduction zones is largely unknown. Recent advances in GPS and seismic networks have led to many interesting observations regarding SSE and ETS events. For example, SSE and ETS events in Cascadia have variable recurrence times along the coastline that spans from the Mendocino Triple Junction to north of Vancouver Island. Another interesting characteristic of ETS events is its sensitivity to lunar tides. This study will investigate these two observations: variable recurrence time along the coast and effects of lunar tides. This research will use existing geodetic and seismic data sets combined with an aggressive modeling approach to explain these important and fundamental observations that are critical to understanding the earthquake cycle of subduction zones and are crucial in seismic hazard assessment.
This project also includes teaching Research Methods of Seismology and GPS. This class covers theory and field methods of seismology and GPS, providing undergraduate and graduate students an opportunity to collect and analyze Cascadia field data.
In subduction zones around the world it has recently been discovered that slow slip over a period of weeks to months occurs along part of the plate boundary down-dip of megathrust earthquakes. These slow slip events have been reported at subduction zones in Japan, Mexico, Costa Rica, Kamchatka, Alaska, New Zealand and Cascadia. In Cascadia, slow slip events repeat every 10-20 months depending on location and are accompanied by seismic tremor. Such episodic tremor and slip (ETS) events release and redistribute part of the stresses built up in the subduction zone during the megathrust earthquake cycle. This project first explores how variations in lithology of the continental crust influences the continuation or termination of slip during ETS events. The second part of this project investigates how the continental crust deforms during ETS events, and how the deformation influences crustal faults. Influence of continental crust lithology on ETS events In Cascadia south of 46°N near the Washington/Oregon border the continental crust is comprised of an accreted basalt known as the Siletzia formation. North of 46°N the amount of Siletzia formation in the crust is significantly less. Intriguingly, ETS recurrence in Cascadia increases from once every two years in Oregon to every ~14 months in Puget Sound, Washington. Southward migrating ETS events that typically begin under Puget Sound, Washington terminate north of ~46°N. Among the four ETS events recorded that ruptured northernmost Oregon to date (June 2011, August 2009, July 2007 and November 2005 ETS events), only the August 2009 event ruptured southern Puget Sound through 46° N to central Oregon. Seismic data and geodetic displacements are used to inspect the August 2009 event. Displacements incurred by this event are large north of 46° N, but reduce by 50% to the south suggesting that the presence of the thick and rigid Siletzia terrane contributed in the reduction of slip during the ETS migration southward. The estimated moment magnitude for this event is 6.8. Influence of ETS on crustal faults Similar to megathrust earthquakes but on a much smaller scale, ETS produce significant deformation of the continental crust that includes subsidence and extension within the crust down-dip of slip and contraction and uplift up-dip. Crustal faults are susceptible to failure caused by deformation of the continental crust due to a subduction zone slip event. The August 2009 ETS event that spanned central Washington to central Oregon in Cascadia is used to investigate how the St. Helens Seismic Zone (SHZ), characterized by right lateral shear, was affected. The SHZ, which resides in a region of expansion and subsidence during the 2009 ETS event, unclamped, allowing for right lateral and west side down displacement that equated to a separate moment magnitude 5.0 slow slip event. This is the first study to show that ETS events influence crustal faults.
