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Use of Borehole Strainmeter Data to Study Subduction-Related Tremor and Slow Slip: Wendy McCausland


Project Title: Use of Borehole Strainmeter Data to Study Subduction-Related Tremor and Slow Slip
Mendenhall Fellow: Wendy McCausland, (360) 993-8985, wmccausland@usgs.gov
Duty Station: Cascades Volcano Observatory, Vancouver, WA
Start Date: January 7, 2007
Education: Ph.D. (2006), Earth and Space Sciences, University of Washington, Seattle
Research Advisors: Evelyn Roeloffs, (360) 993-8937, evelynr@usgs.gov; Seth Moran, (360) 993-8934, smoran@usgs.gov
Project Description:

Two important factors for assessing the seismic hazards in the Cascadia subduction zone include how and where stresses caused by the process of subduction are released. An interesting recent discovery in seismology and geodesy is the cyclic concurrence of slow slip earthquakes and deep tremor along the Cascadian subduction zone (Rogers and Dragert, 2003; McCausland and others, 2005; Szeliga, and others, 2005). The slow slip events manifest as reversals to plate convergence on regional GPS networks along the subduction zone. These events were the first indication that inter-seismic subduction is not a steady-rate process and that not all of the stresses resulting from subduction are released by earthquake activity or permanent deformation. Despite what we have learned from existing data, we do not yet know if the tremor and slow slip events are two different manifestations of the same source process on different time-scales, always occurring together, or if they are separate processes that can be excited by similar source condition(s) and therefore are capable of occurring individually as well as concurrently. We have observed significant tremor in Cascadia unaccompanied by a measurable slow slip event; however, given the current resolution of GPS networks, it is possible that a slow slip event did occur that was below the resolution of the network.

To answer these questions, we require data that not only has greater sensitivity to ground deformation than is possible with current GPS technology but that can also bridge the gap between the temporal resolution of GPS (minutes to days to decades) and seismic data (hundredths of a second to minutes). The objective of this research is to use the Plate Boundary Observatory (PBO) strainmeters in conjunction with seismic and GPS data to study the accumulation, redistribution and release of strain associated with episodic tremor and slip events in Cascadia; to further observe the temporal and spatial occurrence of and the relationship between slow slip and tremor; to better constrain the slip surface(s) through modeling of the strainmeter with the GPS data; and to understand how the phenomena should change our current assessment of large earthquake hazards along the Cascadia subduction zone.

Before the strainmeters can be used to model the slow slip events, they must be calibrated to reflect the regional strain field, not just the strain of the instrument. Furthermore, other physical processes can be recorded on strain instrumentation, i.e. tidal and atmospheric loading or changes in the surrounding strata or aquifer, and the effects of each must be well understood so that they are not mistaken for the processes of strain accumulation and stress release associated with the tremor and slow slip events.

Map of the Cascadia subduction zone

Figure 1. (Left) Map of the Cascadia subduction zone showing the locked and transitions zones and the modeled slip surface for the 1999 slow slip event (modified from Hyndman and Wang, JGR, 1995 and Dragert et al, Science, 2001). (Right) Map showing the location of the PBO borehole strainmeters including the two shown in figure 2.

Linearized, detrended strainmeter time series for January-February 2007 slow slip event on stations B018 and B004
Figure 2. Linearized, detrended strainmeter time series for January-February 2007 slow slip event on stations B018 and B004. Blue line represents the strainmeter data before the tides and atmospheric pressure are removed; red, orange and yellow lines represent the data after they have been removed. The left-hand vertical axis is instrument strain; and the right-hand vertical axis is calibrated to formation strain. The strain event hardly overlaps in time and has a different sense of shear on the two instruments.

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Last modified: 16:08:31 Thu 13 Dec 2012