Structure of Long Valley Caldera, California, from a seismic refraction experiment

Two seismic refraction profiles crossing the Long Valley caldera in approximately east and north directions indicate that the crystalline basement with P wave velocities of 6.0±0.4 km/s has been downdropped by 2.5–3 km across normal faults along the north and northwest sides of the caldera and 1–2 km along the south and east sides. Basement depths beneath the caldera floor range from between 3 and 4 km in the north and east sections to about 2 km in the central and south sections. Relief on the basement within the caldera suggests that the caldron block was partially disrupted during collapse, although a steplike offset in the eastern part of the basement may be due in part to precollapse displacement along the Hilton Creek fault. The distribution of P wave velocities in the caldera fill suggests that the Glass Mountain rhyolite and Bishop tuff have velocities of 4.0–4.4 km/s and the postcollapse rhyolite, rhyodacite, and basalt flows have velocities of 2.7–3.4 km/s. Domelike relief on the 4.0- to 4.4-km/s horizon indicates that postcollapse resurgence elevated the west central part of the caldera by about 1 km. Evidence for the roof of the magma chamber is contained in later arrivals tentatively identified as reflections from a low-velocity horizon at a depth of 7–8 km. Evidence for anomalous scattering or absorption properties associated with the region of shallow hydrothermal alteration and hot spring activity is contained in relative attenuation of high frequencies in a guided wave propagating through this region.