This project will characterize the next slow slip earthquake on the Cascadia Subduction zone interface using uplift measured by a dense network of tide gauges. 12 temporary tide gauges are being deployed on the Olympic Peninsula of Washington, the portion of the Cascadia subduction zone expected to experience a M ~6.5 slow slip earthquake in ~August of 2010. Because the recurrence interval of slow slip on this part of the subduction zone is ~14 months and the ocean is an order of magnitude quieter during the summer, the ~August 2010 event provides an opportunity to catch an event at the calmest time of year that will not recur for at least 7 years. Data from this temporary network and the existing sparse NOAA network of tide gauges are providing a detailed image of the vertical deformation associated with the 2010 event, which is poorly resolved by GPS or seismic techniques that have been used to study the last 12 of these events. The vertical deformation, combined with horizontal deformation from GPS and source location and timing information from seismology, allows us to accurately determine the distribution of slip and the release of strain on the subduction interface. This improved understanding of slip on the deep subduction zone interface will provide the necessary data to build better models that link the time-dependent release of strain along the subduction zone, including the timing and extent of rupture of M~9 megathrust earthquakes that occur on average 300-500 years on the Cascadia subduction zone. It has been speculated that a future slow slip earthquake will trigger the next megathrust earthquake and tsunami, so it is critically important to understand these events and how they relate to damaging seismogenic earthquakes.
Undergraduate students from NSF's UCORE program, who are participating in a summer research at the University of Oregon, are carrying out most of the instrument deployments. The UCORE program brings community college students to the University of Oregon to learn to carry out scientific research by actively participating in ongoing projects. The UCORE students are installing and calibrating the tide gauges, downloading and interpreting the data, and will present the results of their work in a formal poster session at the end of the program.
In addition to the primary goal of characterizing the 2010 slow slip earthquake, our deployment allows us to calculate the long-term relative sea level change at each site, because most of our sites have been occupied intermittently over the past century. This means we can compare the long term uplift rate to the uplift associated with individual slow slip earthquakes and thus determine how much of the strain provided by plate tectonics is available for megathrust rupture and permanent landscape development. Finally, our project is allowing us to improve our methodology for finding tectonic transients in tidal records, and potentially to apply this method to generate a catalog of slow slip events that extends back ~100 years in this area.
Slow slip events on the Cascadia subduction zone represent an enigmatic and significant (~Mw 6) release of tectonic strain that is not accompanied by strong ground shaking. Measuring the vertical surface deformation from these events is problematic given the noise levels of standard instrumentation (i.e. GPS). To better measure the vertical uplift of the ground during one of these slow slip events, we deployed a network of 12 temporary tide gauges in the portion of the Cascadia subduction zone expected to experience a slow slip event in ~August of 2010. Combined with the sparse existing NOAA network of tide gauges, the data set provided an image of the vertical deformation associated with the 2010 event in northwest Washington. The tidal deployment also allowed us to infer the long-term uplift spanning many slow slip cycles because most of these sites have been occupied historically. We developed and constructed a set of tide gauges at low cost. This project utilized community college undergraduates from the UCORE program as field assistants, and provided them a unique field experience. Our deployment was timed to take advantage of the low ocean noise levels that exists in the summer. Given the 14 month repeatability of these slow slip events in northwest Washington, we were also able to time our deployment in order to capture the entire event. This project has allowed us to refine our methodology for removing ocean and atmospheric noise in the tidal time series and demonstrate the feasibility of using tidal time series to resolve the deformation from slow slip. This methodology can ultimately be used by investigators to perform a similar analysis on tidal time series at other subduction zones where similar records are readily available.
