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A silent slip event on the deeper Cascadia subduction interface
1Geological Survey of Canada, Pacific Geoscience Centre, 9860 West Saanich Road, Sidney, British Columbia, Canada V8L 4B2. dragert@pgc.nrcan.gc.ca
Science (New York, N.Y.)
|April 21, 2001
Summary
Continuous Global Positioning System data revealed landward motion in British Columbia and Washington due to the locked Cascadia subduction fault. A 1999 event showed aseismic slip, indicating discrete pulses of movement on the fault interface.
Area of Science:
- Geophysics
- Tectonics
- Seismology
Background:
- Continuous Global Positioning System (cGPS) sites in southwestern British Columbia and northwestern Washington exhibit landward motion.
- This movement is attributed to the locked state of the offshore Cascadia subduction fault.
- The Cascadia subduction zone is a major tectonic boundary capable of generating large earthquakes.
Purpose of the Study:
- To investigate an anomalous event in 1999 where several cGPS sites reversed their direction of motion.
- To understand the mechanism behind sudden, aseismic displacements on the Cascadia subduction fault.
- To provide evidence for episodic slip on the deeper, hotter parts of the subduction interface.
Main Methods:
- Analysis of continuous Global Positioning System (cGPS) data from southwestern British Columbia and northwestern Washington.
- Geodetic measurements to detect ground displacement.
- Modeling of fault slip to explain observed surface motions.
Main Results:
- A cluster of seven cGPS sites briefly reversed their direction of motion in the summer of 1999.
- No associated seismicity was detected during this event.
- The displacements are explained by approximately 2 centimeters of aseismic slip over a 50 km by 300 km area on the subduction interface, downdip from the seismogenic zone.
- This slip event is equivalent to a moment magnitude 6.7 earthquake.
Conclusions:
- The study provides evidence for aseismic slip occurring on the deeper, plastic portion of the Cascadia subduction interface.
- These aseismic slip events can occur in discrete pulses.
- Such pulses contribute to stress loading of the megathrust earthquake zone, potentially influencing the timing and magnitude of future large earthquakes.
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