Microgravity Effects of Earthquakes in the Cascadia Subduction Zone

Jeong Woo Kim1, Juergen Neumeyer1, Joseph Alan Henton2, Byung-Gon Chae3, Hojjat Kabirzadeh1, Michael G Sideris4, Ik Woo5, Ricky Kao1,6 and Junghae Choi3, (1)University of Calgary, Geomatics Engineering, Calgary, AB, Canada, (2)Pacific Geoscience Center, Sidney, BC, Canada, (3)Korea Institute of Geoscience and Mineral Resources, Geologic Hazards Department, Daejon, Korea, Republic of (South), (4)University of Calgary, Calgary, AB, Canada, (5)Kunsan National University, Dept. of Marine Engineering, Kunsan, Korea, Republic of (South), (6)National Chiao Tung University, Dept. of Civil Engineering, Hsinchu, Taiwan

Contact First Author: Jeong Woo Kim; jw.kim@ucalgary.ca

Previously Published Material: The preliminary results were presented at the meeting as a poster, but never been published. This presentaiton contains the updated results with recent data with advanced methodologies.

Abstract ID#: 36281

 

English Abstract:
An iGrav superconducting gravimeter (SG) and an A10 absolute gravimeter (AG) have been deployed at Pacific Geoscience Centre (PGC) on Vancouver Island, Canada, since 2012. The PGC is situated in the forearc of the northern Cascadia Subduction Zone (CSZ). In this area, a transient surface deformation accompanied by tremor-like seismic signals has been documented with a recurrence interval of 13 to 16 months. This phenomenon, named episodic tremor and slip (ETS), has been interpreted to be associated with slow slip events (silent earthquakes) in the CSZ. To detect the ETS events, the continuous microgravity recordings from the SG were reduced for all known environmental effects including Earth and ocean tides, polar motion, atmospheric pressure and soil moisture. The residual gravity effects were then compared with the GPS-detected ETS.

The gravity effect of the Haida Gwaii earthquake was analyzed. The earthquake occurred near the plate boundary between the Pacific and North America plates on 28 October 2012 with a magnitude 7.8. A large co-seismic gravity change of -2.6 microGal was recorded at the onset of the earthquake. In addition, a significant decrease of gravity was observed for the 15 days prior to the eartquake, and the decrease lasted for 11 days after the earthquake. The distance from the SG to the earthquake center is about 780 km. Compared to the observed co-seismic gravity change of 0.58 microGal whinin a distance of 3.4o by Imanish et al. [2004] that was confirmed by a dislocation model, it is a significant gravity change. This gravity change can be associated with both subsurface mass redistribution and surface height displacement. For further interpretation, it has to be proved with straightforward seismic and dislocation models. Another earthquake that occurred in a similar region was the southwestern Alaska earthquake (5 January 2013, m7.5), and although it triggered a tsunami warning, it did not show any sign of co-seismic gravity change in the SG recordings.

These studies have shown that identification and assessment of geohazards and geodynamics such as active fault movement, as well as mass changes in geological CO2 storage, may be possible with continuous monitoring of microgravity with combined SG and AG when the environmental gravity effects and surface displacements are effectively reduced.

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