Strain steps associated with earthquakes

Abstract Strain steps have been observed following earthquakes ranging in magnitude from 3.0 to 8.5. An empirical relationship is derived which indicates that the magnitude, M, of the smallest earthquake from which a strain step of the order of 10 −9 may be expected at a distance of Δkm is : M = 1.1 + 1.7 log ⁡ Δ 10 . Fault length is related empirically to earthquake magnitude by the equation M = 3.3 + 1.7 log ⁡ L 10 Where L is fault length in km. Comparison of these two equations shows that strains of the order of 10 −9 may be expected at distances of approximately 20 fault lengths. The fault length versus magnitude equation presented is shown to be compatible with a previously published energy-magnitude relationship log ⁡ 10 E = A + 1.8 M . Strain steps are shown to have two characteristics which are similar to those of surface waves. The speed of propagation is nearly constant over a wide range of epicentral distances. Depending upon whether the travel paths are continental or oceanic, the speeds are, respectively, 3.0 ± 0.3 km/sec or near 3.6 km/sec. The strain step amplitude dependence upon distance is like R -3/2 .