HR: 08:45h
AN: G31A-04 INVITED     [Abstracts]
TI: GPS constraints on 35+ slow slip events within the Cascadia subduction zone, 1997- February, 2007
AU: * Melbourne, T
EM: tim@geology.cwu.edu
AF: Geodesy Lab Central Washington University, 400 University Way, Ellensburg, WA 98926, United States
AU: Santillan, M
EM: marcelo@geology.cwu.edu
AF: Geodesy Lab Central Washington University, 400 University Way, Ellensburg, WA 98926, United States
AU: Szeliga, W
EM: walter@geology.cwu.edu
AU: Miller, M
EM: meghan@geology.cwu.edu
AF: Geodesy Lab Central Washington University, 400 University Way, Ellensburg, WA 98926, United States
AB: Refinements to GPS analyses in which we factor geodetic time series to better estimate both reference frames and transient deformation resolve 35 slow slip events (SSE) located throughout the Cascadia subduction zone from 1997 through early 2007. Timing of transient onset is determined with wavelet-transformation of the geodetic time series. 30 continuous GPS stations are included in this study up through 2005, and over 70 stations for the 2007 event. Events are analyzed that range from northern California to southwestern British Columbia, with station density generally increasing towards the north. The improved analyses better resolves the largest creep and also identifies many smaller events. At 48.5N latitude, the 14-month average recurrence interval still applies, four events after first recognition. Elsewhere, such periodicity is not observed. Along central Vancouver Island to the north (49N), a host of smaller events distinct from the 14-month periodicity occur with no obvious periodicity. Sporadic smaller events also appear throughout the subduction zone to the south, including some within the region of the 14-month periodicity of larger events. In southern Washington State, some of the largest transient displacements are observed, but lack any obvious periodicity in their recurrence. Along central Oregon, an 18-month recurrence is evident, while in northern California (Yreka) the 11-month periodicity continues through 2005. To invert GPS offsets of the 12 best-recorded events for slip, we use a cross-validation scheme to derive optimal smoothing of non-negative thrust faulting along a plate interface divided into 40 along strike and 24 down-dip subfaults. Those events have equivalent moment magnitudes ranging from 6.3 (smallest resolvable with GPS) to 6.8, and typically 2-3 cm of slip. The largest spatial extent of all events resolved to date is just under 350 km along strike, with a maximum observed duration of seven weeks across the network; the majority last less than one month and show typically half the spatial extent. Unlike other subduction zones like the Middle America Trench, no longer-duration Cascadia events are observed, nor cumulative deformation greater than 0.6 cm. The frequency, size and style of the many newly resolved smaller transient deformation events show they occur frequently here, that GPS captures only the largest events, and that smaller SSE's routinely occur at much greater occurrence rate at levels not detectable with GPS. Moreover, the location of the slow slip events, and the general pattern of deformation, suggest that the characteristics of seismogenic locking beneath Cascadia remains enigmatic.
UR: http:www.geodesy.org/pnwdaily/GPS_Summary/gps_summary.html
DE: 6924 Interferometry (1207, 1209, 1242)
DE: 7223 Earthquake interaction, forecasting, and prediction (1217, 1242)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7240 Subduction zones (1207, 1219, 1240)
SC: Geodesy [G]
MN: 2007 Joint Assembly