GEODETIC CONSTRAINTS ON THE JANUARY, 2007 CASCADIA SLOW SLIP EVENT

The 2007 Cascadia ETS started ~January 15 in the southwestern Puget Basin and appears, at the present time, to have terminated in the last two days ~200 km to the north beneath southern Vancouver Island on February 5. Unlike previous events in which geodetic and other instrumentation was relatively sparse, this event took place within a network consisting at the time of 70 GPS receivers, two long-baseline tiltmeters, several borehole strainmeters, seismometers from the EarthScope-funded CAFÉ experiment, and the Pacific Northwest Seismic Network. In terms of GPS, transient deformation appears on ~20 of the 70 GPS receivers and thus provides a relatively tight bracketing of non-zero deformation useful for inverting for slip. The transient deformation clearly corroborates seismic tremor migrating northwards from the southeastern Olympics to the NNW and the Strait of Juan de Fuca. Formal inversion of GPS-transients for slip through the course of this event will be presented at the meeting.

Two long-baseline tiltmeters were also operating throughout the event, of which one two-arm instrument, sampled every minute, shows clear transient tilting signals evolve during the event. Interestingly, there is a two-hour WSW-down tilting event of 20 nanoradians that occurs on Jan 10, approximately 5 days prior to the onset of nearby tremor that appears tectonic in origin. Over the next two weeks, the original WSW-down tilting event reverses into a ~50 nanoradian E-down tilting event that is varies in amplitude and rate over the course of the event. The transient tilting appears to stop near Feb 3 in the southern Puget Sound region. Data from the second long-baseline tiltmeter, located in Mt. Vernon, WA, has not yet been recovered but will be discussed at the meeting.

In summary, although the event has just terminated and all data has not been evaluated and modeled, it is clear that this is the best recorded ETS event to date, and that the measurements that we have for this event will allow us to test a number of hypotheses regarding the genesis of tremor and slow slip.