Dynamic Strains at Regional and Teleseismic Distances

We develop formulas for the size of dynamic strains caused by seismic waves from an earthquake of given magnitude and distance. These formulas include peak strain, peak dissipated power, and total dissipated energy, and they are applicable at regional and teleseismic distances. The formulas are fits to data from 89 large (6.5 <= M-w <= 9.0) shallow earthquakes, with source distances between 500 and 16,000 km, recorded between 1977 and 2013 by three long-base laser strainmeters at Pinon Flat Observatory in southern California; these strainmeters provide uniquely well-calibrated measurements of tensor strain. The residuals to the fits suggest that strain values can usually be predicted to within a factor of 2. These data also show that the strain tensor can be substantially different from that expected for plane waves: in particular, the extension perpendicular to the back azimuth, which should be zero, is always 20% or more of the extension along that azimuth. How much the strains resemble those for plane waves depends on their path, perhaps because inhomogeneities along different paths produce different amounts of multipathing. The observed strains are systematically 10%-30% larger at nearby laser strainmeter sites in the Salton trough, suggesting local amplification from inhomogeneous crustal structure.