Analysis of borehole seismograms from Long Valley, California: Implications for caldera structure

Abstract

A three‐component seismometer was emplaced in a borehole in Long Valley caldera for a period of 2 months to record local earthquakes to the south, mostly in the Sierra Nevada. The seismic records were plotted in depth versus time sections to produce “vertical hypocentral profiles” for both the vertical and horizontal components. The data collected and displayed in this way provide good resolution of the crustal structure at depth and avoid the attenuation and complications introduced by the near‐surface caldera fill. The record sections were then modeled with two‐dimensional ray tracing to match the observed travel times. Although nonreversal of the profiles and uncertainties in hypocentral locations introduce ambiguities, the final interpretation reveals two separate low‐velocity bodies which we have tentatively identified as magma beneath the resurgent dome in Long Valley. The first of these is a small (2 km by 6 km) body with a depth to top of approximately 3.7 km beneath the surface at the southern end of the resurgent dome. The other body appears larger and lies at a depth of 5.5 km beneath the northern end of the resurgent dome.

The Geophysics of a Restless Caldera Long Valley, California

“…Centered around the same well used by Elbring & Rundle (1986) on the northwest corner of the resurgent dome, the investigation was a coordinated attempt to image the refl ector f i rst near LM on Figure 1. As described by Rundle et al (1985) and Deemer The principal problem in attempting to image subsurface refl ectors in Long Valley is that strong, near-surface velocity variations ("statics") obscure the underlying deeper structure by destroying the coherence of the wave trains [see Telford et al (1976) for a discussion].…”