GPS constraints on 34 slow slip events within the Cascadia subduction zone, 1997–2005

Abstract

Refinements to GPS analyses in which we factor geodetic time series to better estimate both reference frames and transient deformation resolve 34 slow slip events located throughout the Cascadia subduction zone from 1997 through 2005. Timing of transient onset is determined with wavelet transformation of geodetic time series. Thirty continuous stations are included in this study, ranging from northern California to southwestern British Columbia. Our improvements in analysis better resolve the largest creep events and also identify many smaller events. At 48.5°N latitude, a 14‐month average recurrence interval has been observed over eight events since 1997. Farther north along Vancouver Island a host of smaller events with a distinct 14‐month periodicity also occurs. 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 an 11‐month periodicity continues through 2005. We invert GPS offsets of the 12 best recorded events for thrust slip along the plate interface using a cross‐validation scheme to derive optimal smoothing parameters. These 12 events have equivalent moment magnitudes between 6.3 and 6.8 and have 2–3 cm of slip. Unlike other subduction zones, no long‐duration events are observed, and cumulative surface deformation is consistently less than 0.6 cm. The many newly resolved smaller transient events in Cascadia show that slow slip events occur frequently with GPS best capturing only the largest events. It is likely that slow slip events occur more frequently at levels not detectable with GPS.

Slow slip events in Costa Rica detected by continuous GPS observations, 2002–2011

“…We detected five significant events during the period of 2002 to 2011, indicating an average SSE recurrence interval of 21 AE 6 months. The recurrence interval we identified in this area is longer than SSE recurrence interval in Cascadia [Rogers and Dragert, 2003;Szeliga et al, 2008] and short term SSE in southwest Japan [Obara, 2010]. However, given the significant difference between the 2009 event and earlier events, we question the validity of a uniform recurrence interval model for the Nicoya segment of the Middle America Trench.…”

“…These characteristics may influence frictional conditions, such that SSE events in Nicoya require a longer time to "load," but subsequently release a larger seismic moment. The moment magnitude from inversion of both campaign and continuous GPS data of the five significant SSEs in Costa Rica are between M w 6.6 to M w 7.2 [Outerbridge et al, 2010;Jiang et al, 2011] larger than the GPS detected Cascadia SSE events (Mw 6.2-Mw 6.8) [Szeliga et al, 2008] and shortterm SSE in Japan [Obara, 2010]. Another possibility that potentially contributes to the apparently long recurrence interval here could be due to the fact that our GPS observations are not sensitive to small SSE.…”

“…In Cascadia, such events typically last 1-4 weeks, have centimeters of slip on the plate interface and have recurrence intervals of 11 to 18 months [Szeliga et al, 2008;Schmidt and Gao, 2010;Holtkamp and Brudzinski, 2010]. In Japan, episodic tremors and slip events vary in duration and distribution [Obara, 2010;Liu et al, 2010].…”

Teleseismic constraints on the geological environment of deep episodic slow earthquakes in subduction zone forearcs: A review

“…Rogers and Dragert (2003) characterized the recurrence of aseismic slow slip in conjunction with non-volcanic tremor beneath southern Vancouver Island that firmly established the presence of slow episodic megathrust fault rupture downdip of the seismogenic zone. These so-called episodic tremor and slip (ETS) events were later found to occur all along Cascadia (Brudzinski and Allen, 2007) from the mapping of either or both slow slip and tremor (e.g., Kao et al, 2008;Szeliga et al, 2008;Wech et al, 2009). At the time the lack of precise tremor hypocenters fuelled the debate regarding the nature of the tremor signal, whether it was occurring within the overriding plate as the slow slip was progressing and changing the stress field to generate hydraulic fracturing (Kao et al, 2005;Rogers and Dragert, 2003), or via direct shear slip on the plate interface during slow slip (Shelly et al, 2006(Shelly et al, , 2007.…”

HTDP 3.0: Software for Coping with the Coordinate Changes Associated with Crustal Motion

“…Unlike secular velocities they are limited in duration. Such slow slip events have been observed along the coastal regions of Oregon and Washington ͑Dragert et al 2004;Szeliga et al 2008͒ and along the San Andreas fault in central California ͑Linde et al 1996͒. Since the surface displacements observed to date from these events have been small ͑Ͻ5 mm͒, they are not specifically modeled in HTDP 3.0.…”