Determination of P-Wave Velocity Structures, Earthquake Hypocenters, and Focal Mechanisms in the Morgan Hill Region of Central California

Cheng-Horng Lin,Steven W Roecker

doi:10.3319/tao.2000.11.4.929(t)

Abstract

P-wave velocity structure, earthquake hypocenters, and focal mechanisms are investigated by applying an analytic ray tracing technique to 2437 P-wave arrivals from 154 locally recorded earthquakes from the Morgan Hill region of central California. Taking as our reference the results of a previous investigation by Michael (1988) on the effects of laterally varying structure on these parameters of hypocenters and focal mechanisms, we found that to the first order the principal structures are preserved, but that features associated with the fault zone tend to he better focused and of higher contrast when the analytic ray tracer is used. In particular, the low velocities associated with the Madrone Springs and Calaveras faults are confined to a narrow zone of about 5 km width. Contrasts in velocities associated with this zone are 2-3% larger at shallow depth. These contrasts gradually decrease with depth, but are evident to depths of at least 12 km. Hypocenters relocated in this structure shift systematically to the east by 1-2 km and their depths generally increase by as much as 5 km. The combined effect of the epicenter and depth shift is to decrease the apparent dip of the seismic zone by 4° and to move the surface intercept of the seismic zone to the east by 0.8 km. Compared with those determined by approximate ray tracing, initial ray directions calculated with the analytic ray tracer are within+5° in azimuth but can be as much as 30° different in take-off angle. The danger of attempting focal mechanism analysis with only a few arrivals is therefore evident. The effects of these differences on the deduced focal mechanisms themselves appear to be on the order of 10° in the strike and dip of the probable fault plane.

Highlights

The existence of strong lateral variations in elastic wave velocity structure within the broad system of faults that make up the Pacific-North American plate boundary in central California has been documented in a number of studies using data from both explosive (e.g., Blumling et al 1985; Mooney and Colburn 1985; Walter and Mooney 1982) and earthquake (e.g., Wesson 1971; Healy and Peakel975; Thurber 1983) sources
The potentially dramatic effects on ray paths of laterally varying structure, such as may be found in central California have been demonstrated by several investigators (e.g., Engdahl and Lee 1976; Feng and McEvilly 1983)
In this study we examine the effects of using an analytic ray tracer in determining hypocenters and focal mechanisms in the region of the 24 April 1984 Morgan Hill earthquake, using as data the arrival times of P waves produced by the main and aftershocks of this event recorded by a network of local stations

Summary

INTRODUCTION

The existence of strong lateral variations in elastic wave velocity structure within the broad system of faults that make up the Pacific-North American plate boundary in central California has been documented in a number of studies using data from both explosive (e.g., Blumling et al 1985; Mooney and Colburn 1985; Walter and Mooney 1982) and earthquake (e.g., Wesson 1971; Healy and Peakel975; Thurber 1983) sources. In this study we examine the effects of using an analytic ray tracer in determining hypocenters and focal mechanisms in the region of the 24 April 1984 Morgan Hill earthquake, using as data the arrival times of P waves produced by the main and aftershocks of this event recorded by a network of local stations. Mechanisms produced with only a few observations, or which are controlled by only a few observations, should be regarded with suspicion

2.METHODOLOGY AND DATA

THREE-DIMENSIONAL VELOCITY STRUCTURE

RELOCATED EARTHQUAKES

Findings

RAY PATHS AND FOCAL MECHANISMS