Investigating the Radiation Pattern of Earthquakes in the Central and Eastern United States and Comments on Seismic Hazard

Abstract:

The radiation pattern emitted from earthquakes is not currently considered in many seismic hazard assessments. This may be due to the fact that the focal mechanisms of potential ruptures are not well studied or are assumed to be random. In this case, all mechanisms are given equal likelihood, and the effect of radiation pattern is essentially averaged. But for about a dozen earthquake sources in the central and eastern United States (CEUS), faults with known mechanism are incorporated into the hazard assessment, but the radiation pattern is not included. In this study, we investigate the radiation pattern from larger CEUS earthquakes, one of which, the 2011 M5.7 Prague earthquake, was sampled by the relatively uniform and broad coverage of USArray. The radiation pattern from this event is readily apparent below about 1 Hz out to several hundred kilometers from the epicenter and decays with increasing frequency and distance, consistent with the effects of scattering attenuation. This decay is modeled with an apparent attenuation that is 5–­10 times greater than the attenuation of Lg waves for the CEUS. We consider the radiation pattern of potential sources in the New Madrid seismic zone to show the effect of radiation pattern on the seismic hazard assessment of major metropolitan areas in the region including Memphis, Tenn., Evansville, Ind., St Louis, Mo., and Little Rock, Ark. For the scenarios we choose, earthquakes with expected mechanisms within the seismic zone, both strike-slip and thrust, tend to focus energy to the southwest towards Little Rock and to the northeast towards Evansville. Eastern Memphis and St Louis, on the other hand, tend to be in lobes of reduced seismic shaking. This can have a significant impact on seismic hazard assessment for these cities, increasing hazard for the former and decreasing it for the latter, particularly for larger structures that are sensitive to longer shaking periods. It is more complicated, however, when considering that this effect would be coupled with smaller aleatory ground motion variability, channeling of seismic energy to the northeast and southwest by the Reelfoot rift, and potential site amplification and resonance by soft sediments in the Mississippi embayment.