WHY WAS THE 23 AUGUST 2011 M 5.8 VIRGINIA EARTHQUAKE SO WIDELY FELT? INFERENCES FROM COMPARISONS OF EASTERN/CENTRAL US AND WESTERN US INTENSITY AND INSTRUMENTAL DATA

Differences in seismic energy propagation within intraplate versus interplate settings have been recognized for decades. This difference was evident from the 23 August 2011 M 5.8 Virginia (intraplate) earthquake, which was reportedly felt from Florida to Canada (~2000 km) and as far west as Chicago. Similar magnitude interplate earthquakes in California are rarely felt more about 500 to 700 km from the source. We compared median values of “Did You Feel It” (DYFI) intensities from the Virginia earthquake and the 6 November 2012 M 5.6 Oklahoma (intraplate) earthquakes with similar-magnitude (interplate) earthquakes in California. Because major contributions to shaking from intensity data are difficult to determine due to variations in building quality and quantity, source effects, and human sensitivities, we also compared controlled-source data from intraplate and interplate settings, which lack those variables. The intensity data comparisons infer that intraplate propagation distances are more than 5 times greater for comparable magnitude earthquakes, but maximum epicentral distances for reported damage varies by about 2 to 3 times. Similar comparisons of peak ground velocities (PGV) from controlled-source instrumental data show similar patterns. Within the first 100 km, seismic energy propagation (PGV & DYFI intensities) is similar for the intraplate and interplate settings, but beyond 100 km, PGV and DYFI intensities are significantly higher in the intraplate setting. Beyond about 200 km, however, there is a significant decrease in seismic energy for the intraplate setting. For our instrumental data, the only differences affecting propagation are attenuation and Earth structure, so other factors can be eliminated. The propagation similarities within the first 100 km suggest crustal attenuation may not be the primary factor affecting propagation distances. Our modeling suggests direct and reflected body waves and crustal thickness account for much of the differences in felt shaking beyond 100 km. For the Virginia earthquake, the DYFI data suggest the strongest shaking beyond the epicenter occurred (1) in a NE-SW direction along the structural trend (faults) due to a guided-wave effect and (2) at critical distances for Moho reflections.