Simultaneous inversion for hypocenters and lateral velocity variations: An iterative solution with a layered model

Abstract

An iterative inversion technique has been developed that uses the direct P and S wave arrival times from local earthquakes to compute simultaneously a three‐dimensional velocity structure and relocated hypocenters. Crustal structure is modeled by subdividing flat layers into rectangular blocks. An interpolation function is used to smoothly vary velocities between blocks, allowing ray trace calculations of travel times in a three‐dimensional medium. Tests using synthetic data from known models show that solutions are reasonably independent of block size and spatial distribution but are sensitive to the choice of layer thicknesses. Application of the technique to observed earthquake data from north‐central Utah shows the following: (1) lateral velocity variations in the crust as large as 7% occur over 30‐km distances, (2) earthquake epicenters computed with the three‐dimensional velocity structure were shifted an average of 3.0 km from locations determined assuming homogeneous flat layered models, and (3) the laterally varying velocity structure correlates with anomalous variations in the local gravity and aeromagnetic fields, suggesting that the new velocity information can be valuable in acquiring a better understanding of crustal structure.

Velocity Structure Beneath the Hidaka Mountains in Hokkaido, Japan

“…Methods for estimating the velocity structure from high density seismic array measurements have been discussed by CROSSON (1976) and AKI and LEE (1976). These and other similar methods have been widely used to estimate the velocity structures of several regions (AKI et al, 1977;SATO, 1979;HORIE and SHIBUYA, 1979;HIRAHARA, 1977;HAWLEY et al, 1981;HORIE and AKI, 1982;ROECKER, 1982). As is well known, these methods require many arrival time data observed at an array of seismic stations located over the area in question.…”

Nonlinear 3-D traveltime inversion of crosshole data with an application in the area of the Middle Ural Mountains

“…The controlling effect of ε on the inversion results has been discussed by various investigators (Jupp and Vozoff, 1975;Crosson, 1976;Hawley et al, 1981). The choice of ε depends upon the problem considered.…”

Theory and Observations - Seismic Tomography and Inverse Methods

“…Variations on the discrete block parameterization include laterally varying layers (Hawley et al, 1981) and a 3D grid of nodes (Thurber, 1983). In the approach of Hawley et al (1981), the model is divided into horizontal layers in which velocity is constant in the vertical direction, but velocity is obtained by interpolation in the horizontal directions among vertical nodal lines ( Figure 3(b)).…”

“…In the approach of Hawley et al (1981), the model is divided into horizontal layers in which velocity is constant in the vertical direction, but velocity is obtained by interpolation in the horizontal directions among vertical nodal lines ( Figure 3(b)). The spacing between nodal lines may vary from layer to layer.…”