Magnitude, Location, and Ground Motion Estimates Derived From the Community Internet Intensity Maps

As is typical for stable continental region events, the 2002 Au Sable Forks, NY, and Evansville, IN, earthquakes had a dearth of ground motion recordings. In contrast, the USGS collected over 9,300 and 6600 Internet responses for these two events, respectively, through the Community Internet Intensity Map (CIIM) Web pages providing a valuable collection of intensity data. CIIM is an automatic system for rapidly generating seismic intensity maps based on shaking and damage reports collected from Internet users immediately following felt earthquakes in the United States. These intensities (CII) have been shown to be comparable to USGS Modified Mercalli Intensities (MMI). Given the CII for an event, we have developed tools to make it possible to generate ground motion estimates in the absence of data from seismic instruments. We compare both mean ground motion estimates based on the ShakeMap instrumental intensity relations with values computed from a Bayesian approach, based on combining probabilities of ground motion amplitudes for a given intensity with those for regionally-appropriate attenuation relationships. We also present a method for deriving earthquake magnitude and location automatically, updated as a function of time, from online responses based on the algorithm of Bakun and Wentworth. We perform a grid search centered on the area with the highest intensity responses, treat each node as a `trial epicenter', and determine the magnitude and intensity centroid that best fits the CII observation points according a region-dependent intensity-distance attenuation relation. We use the M4.9 2002 Gilroy, CA, event to test all these new tools since it was well recorded by strong motion instruments and had an impressive CIIM response. We show that the epicenter and ground motions determined from the CIIM data correlate well with instrumentally derived parameters. We then apply these methods to the Au Sable Forks, NY, and Evansville, IN, events. To show the feasibility of this procedure for automatic determination of magnitude and location, we present time histories of CIIM responses and histories of these parameters determined at intervals after the event. The accuracy of course increases with time as more responses are received. CIIM can be found online at http://earthquake.usgs.gov under "Did You Feel It?"