Exploiting the demographics of "Did You Feel It?" responses to estimate the felt area of moderate earthquakes in California

We combine "Did You Feel It?" (DYFI) responses with the populations of reporting and nonreporting ZIP codes to improve estimates of the lowest intensities (modified Mercalli intensity; 1< or =MMI< or =3) and the overall felt area for moderate earthquakes. We presume that participation in the DYFI website is sufficiently widespread to interpret shaking in nonreporting ZIP codes as "not felt." For most moderate earthquakes, no nonreporting ZIP codes are located near the earthquake; reporting and nonreporting ZIP codes overlap over intermediate distances; and few reporting ZIP codes are located at regional distances. We combine the intensities from reporting and nonreporting ZIP codes by weighing the average intensity in each reporting ZIP code by the number of reports and by weighing the "not felt" intensity in each nonreporting ZIP code by the population/5000. We reduce the average intensities for underreporting ZIP codes, where the number of reports is less than the population/5000, to account for the nonreporting population. We refer to the combination of intensities from reporting, underreporting, and nonreporting ZIP codes as the revised community decimal intensity (CDI*) to distinguish it from the standard CDI of Wald et al. (1999) This revision significantly improves estimates of the lowest intensities. We contour the revised intensity distributions and interpret the CDI (super *) =1.5 contour as the limit of the felt area. This contour is well determined in densely populated areas and adequately determined in sparsely populated areas.