Aftershock Sequence of the Mw = 7.9 Little Sitkin, Alaska Earthquake of June 23, 2014

Abstract:

On June 23, 2014 at 20:53:07 GMT, a Mw = 7.9 earthquake struck the Rat Islands region of Alaska in the central Aleutians. This is one of the most seismically active areas on earth, where the Pacific plate is subducted beneath the North American plate at a rate of nearly 6 centimeters per year. Prior to June 23, the most recent significant earthquake in this region was a Mw = 7.7 earthquake that ruptured up-dip on the Aleutian interface in 2003. Great earthquakes ruptured most of the megathrust in this region in 1957 and 1965. Unlike these events, the June 23 earthquake appears to have been intraslab, rupturing the down-going Pacific plate at moderate depth with a normal sense of faulting. By mid-July 2014, the Alaska Earthquake Center had located in excess of 1500 aftershocks from this event using broad-band instruments from the regional network as well as short-period sensors from local volcano monitoring arrays. More than 40 aftershocks were larger than ml = 4, with the largest being an Mw = 6.6 event that occurred in the over-riding North American plate. We used a double-difference relocation method and a regional velocity model to investigate the spatial distribution of aftershocks. Relocated seismicity delineates a narrow plane dipping to the northwest, the dimensions of which suggest that a large stress drop may have been necessary to achieve the moment release reported by the Global Centroid Moment Tensor Project. Comparing the relocated seismicity distribution to preliminary slip models from the United States Geological Survey indicated that aftershock locations correlate well with areas between asperities, where there is a transition from areas of high slip to areas of low slip. We also computed the Coulomb stress change induced by the coseismic slip in order to explore the possibility of static triggering of aftershocks. Preliminary indications are that aftershocks do not appear to be preferentially located in areas of enhanced stress.