Crustal Tilt Fields and Propagation Velocities Associated with Earthquakes†

Abstract

Tilt steps similar to strain steps have been observed for earthquakes with magnitudes ranging from 2 to 8 and distances ranging from 10 to 12 O00 km. Tilt-step propagation velocities from the hypocentre to the station of 1-2-3.8 kms-1 have been observed in addition to those corresponding to the arrival time of teleseismic S waves. Tilt directions, amplitudes, and velocities observed at several stations simultaneously for the same earthquake are internally consistent and are likely to depend on the tectonic environment of the station in addition to the focal mechanism. The low velocity of 1-2 kms-1 is consistent with plastic-wave propagation. Other velocities are close to the Rayleigh wave short-period group velocity over continental paths.Theoretical tilt amplitudes for fixed epicentral distance, focal depth, and stress system depend logarithmically on the magnitude, with a possible cut-off. However, observed tilt amplitudes are too large when compared to existing theoretical dislocation models for a single-layered homogeneous isotropic elastic crust. Alternative solutions in terms of more realistic crustal layering and including plastic deformation under preexisting tectonic stress probably could account for a number of observations.