Finite-Frequency Kernels for Pg Wavetrains

Abstract

ABSTRACT Pg waves, which propagate at high frequencies through the crust, are important for tomography and explosion monitoring, and can be among the most prominent P waves observed from small seismic events. Much of what we know about Pg propagation, however, comes from ray approximations, 1D Earth models, and other simplified treatments. For an improved understanding, we use full wave-equation modeling and the adjoint-state method to calculate 0.5 Hz Pg sensitivity kernels corresponding to a range of Earth models, source depths, and epicentral distances. The resulting Pg sensitivity kernels are in many cases dominated by diffraction effects, waveguide mode effects, and other nonray behavior. For a source at the surface observed at less than 4° epicentral distance, first-arriving Pg energy shows little sensitivity to the lower crust. Deeper events channel Pg sensitivity into whatever crustal layer the event originates. In all cases, the simple picture of Pg as a wave reverberating throughout the whole crust with similar sensitivity across upper, middle, and lower crustal layers is found to be inadequate. Other details of the sensitivity kernels can be understood in terms of reflections and conversions at the free surface, which have a greater overall effect on the Pg wavetrain than reflections or conversions at the Moho. Because P–P reflection coefficients at the surface are affected by changes in shear velocity, Pg travel times depend not only on compressional wave velocity, but also on shear-wave velocity. By comparing sensitivity kernels from 1D and 3D velocity models, we show that this strong dependence on near surface velocities, in turn, imparts strong importance to shallow 3D velocity variations. These results represent, to our knowledge, the first application of the adjoint-state method to regional Pg wavetrains.