Observing ETS Evolution With Borehole Strainmeters
Abstract
The behaviour of along-strike propagation was recognized in the first report of the discovery of Cascadia slow slip [Dragert et al., 2001]. Even with very few and sparsely distributed GPS stations, it could be estimated that the slow slip in 1999 propagated in the northwest direction at ~6 km/day. It was later established that the occurrence of tremor at Cascadia tracks the along-strike propagation of the slips. Recent introduction of Gladwin borehole strainmeters (BSM) as well as the densification of GPS coverage under the Plate Boundary Observatory has enabled more detailed monitoring of the slip migration. In this study we analyze the behaviour of strategically located BSMs in ETS episodes and use the strain records to examine the along-strike migration of prolonged ETS in northern Cascadia. Consistent with the conclusion of a tidal calibration study [Roeloffs, 2010], we found that the BSM areal strains in this area are generally unusable, but the shear strains yield useful information. Although tidal calibration was conducted only for a subset of the BSMs, there appears to be a general correspondence between BSMs that yield good results in the tidal calibration and those that yield clear signals in multiple ETS episodes. BSMs are sensitive to many tectonic and nontectonic processes. Long-term trends caused by time-dependent adjustment of the surrounding formation and seasonal variations caused by surface and subsurface fluid pressure changes may be removed by analysis of long data records in conjunction with supplementary data, such as streamflow, from individual sites. Such corrections reduce uncertainties in the net strain remaining after a slow slip event. When detailed corrections are not feasible or possible, very simple processing of BSM data still provides useful information on the timing of the sudden change due to the slow slip and the sign of that change (increase or decrease). Despite various limitations, the BSM data have improved the characterization of the along-strike propagation of ETS slip. In a case study using the May 2008 ETS, the BSM data show remarkable consistency with GPS time series. The combination of BSM and GPS data reveals a pattern of bi-directional propagation of the slip from its initiation area west of northern Puget Sound. In a simple model, the speed of the northwest propagation varies from 8 km/day, to 2 km/day, and to 15 km/day, but that of the south propagation stays at 6 km/day. The tremors observed for this ETS episode show a similar bi-directional migration pattern and similar changes in northwest migration velocity, but the tremor migration front is slightly ahead of the slip propagation front. Finer details of the tremor migration such as rapid streaks in the dip direction and backward migration in the strike direction cannot be geodetically resolved and therefore must be limited in magnitude and/or areal extent. The results provide guidance for site selection of future BSM installations in this and other subduction zones. Dragert, H., K. Wang, and T. James (2001), Science 292, 1525-1528. Roeloffs (2010), JGR 115, B06405, doi:10.1029/2009JB006407.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.S31G..01W
- Keywords:
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- 1207 GEODESY AND GRAVITY / Transient deformation;
- 7209 SEISMOLOGY / Earthquake dynamics;
- 8004 STRUCTURAL GEOLOGY / Dynamics and mechanics of faulting;
- 8170 TECTONOPHYSICS / Subduction zone processes