SHALLOW AND PERIPHERAL VOLCANIC SOURCES OF INFLATION REVEALED BY MODELING 2-COLOR GEODIMETER AND LEVELING DATA FROM LONG-VALLEY CALDERA, CALIFORNIA, 1988-1992

We refined the model for inflation of the Long Valley caldera near Mammoth Lakes, California, by combining both geodetic measurements of baseline length and elevation changes. Baseline length changes measured using a two-color geodimeter with submillimeter precision revealed that the resurgent dome started to reinflate in late 1989. Measurements between late 1989 and mid-1992 revealed nearly 13 cm of extension across the resurgent dome. Geodetic leveling surveys with approximately 2-mm precision made in late 1988 and in mid-1992 revealed a maximum of about 8 cm of uplift of the resurgent dome. Two ellipsoidal sources satisfy both the leveling and two-color measurements, whereas spherical point sources could not. The model's primary inflation source is located 5.5 km beneath the resurgent dome with the two horizontal axes being nearly equal in size and the vertical axis being 4 times the length of the horizontal axes. A second ellipsoidal source was added to improve the fit to the two-color measurements. This secondary source is located at a depth between 10 and 20 km beneath the south moat of the caldera and has the geometry of an elongated ellipsoid or pipe that dips down to the northeast. In addition, the leveling data suggest dike intrusion beneath Mammoth Mountain during the 1988-1992 interval, which is likely associated with an intense swarm of small earthquakes during the summer of 1989 at that location. Our analysis shows the dike intrusion to be the shallowest of the three sources with a depth range of 1-3 km below the surface to the top of the intrusion.