Researchers: M.A. Alwahedi, University of Leeds and J.C. Hawthorne, University of Oxford
Written by Linda Rowan
31 May 2019
Examination of borehole strainmeter data for medium size earthquakes (magnitude 4 to 5) in California shows a postseismic moment that is intermediate between small and large earthquakes. This suggests there is a size dependence on the dynamics of earthquakes or on the properties of the fault area around the earthquakes.
Although an earthquake may feel like a sudden lurching and shaking of the Earth’s surface, the motion around the fault can continue to deform the area for hours to years after the event and this motion is considered to be postseismic deformation. Deformation within hours to months is commonly called afterslip. Medium to large earthquakes do not show any systematic relationship in previous studies, between the size of coseismic moment and the size of the postseismic moment; that is the amount of afterslip seems to be independent of the magnitude of the earthquake. This is consistent with a self-similar model of earthquakes where large events are just scaled up versions of small events. Studies of small earthquakes near San Juan Bautista, CA show large afterslip for small events, indicating that the self-similar model does not hold perhaps because the size of the event is too small to drive more rapid slip or because the afterslip is confined to the coseismic deformation area.
Here the authors use borehole strain data from UNAVCO’s and USGS’s networks of borehole strainmeters to examine postseismic slip for magnitude 4 to 5 earthquakes in central and northern California.
Assessment of the afterslip (occurring between 20 minutes and 1.5 days after each event) for 11 medium-size earthquakes shows a median postseismic moment of 0.45 times the coseismic moment. The ratio is intermediate for medium size earthquakes, falling between small to large earthquakes. Thus the postseismic moment appears to be magnitude dependent and does not fit a self-similar model for earthquakes. The postseismic slip may be dependent on the dynamics of the earthquakes or on the properties of the faults in the earthquake areas.
Alwahedi, M. A., & Hawthorne, J. C. (2019). Intermediate-magnitude postseismic slip follows intermediate-magnitude (M 4 to 5) earthquakes in California. Geophysical Research Letters, 46, 3676–3687. doi: 10.1029/2018GL081001.
postseismic slip, borehole strainmeter, self-similarity
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