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

Abstract

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.

Microearthquakes in and near Long Valley, California

“…This study produced negative results, as the ray paths involved did not penetrate to sufficient depth in the caldera to encounter significant low-velocity material, other than that in the upper 3 or 4 km described in the refraction study of Hill [1976]. The ray paths in the caldera were too shallow to detect the low-velocity material described by Steeples and lyer [1976] The observation that large numbers of microearthquakes are often found within but not outside of geothermal areas during short periods of field recording suggests that seismic activity within the geothermal areas may be a far more continuous process than seismic activity in most other areas, even though all the seismic activity may be along the same fault system and in response to the same regional stresses.…”

“…Sixteen of these systems were operated at 20 different locations during the recording period. The array was centered on the eastern half of the caldera, and six locations from a previous study by Pitt and Steeples [1975] were reoccupied (Table 1), CRUSTAL MODEL A shallow crustal model for the immediate Long Valley area was described by Hill [1976]. The microearthquakes recorded (discussed later) were mostly outside the caldera, so the local Hill model was bypassed in favor of a 'regional model derived from the reversed re&action profile between M OhO Lake and China Lake, California, described by Eaton [1966] S arrivals were only used to locate events when the P-only solution produced S residuals suggesting poor quality locations.…”

Long Valley Caldera Lake and reincision of Owens River Gorge

“…5) penetrated ~700 m of ash-rich lake sediments before entering intracaldera Bishop Tuff, which is itself at least 1,500 m thick (Smith and Rex, 1977;Fournier, 1989). Seismic refraction profiles (Hill, 1976) and gravity contours (Carle, 1988) suggest that the postcaldera fill is substantially thicker to the north and east of the Republic well site. The lake-sediment section is therefore likely to be even thicker than 700 m elsewhere-principally in the northeast part of the caldera.…”

Mineral resources of the Minarets Wilderness and adjacent areas, Madera and Mono counties, California

“…The volume of this chamber above an arbitrary depth of 15 km is estimated to have been about 3,000 km 3 • Thermal calculations (Lachenbruch and others, 1976) and my own unpublished petrologic studies suggest that after eruption of the Bishop Tuff, most, if not all, of the magma remaining in the chamber crystallized during the ensuing 0.7 m.y. However, teleseismic studies of the Long Valley caldera (Steeples and lyer, 1976) and seismic-refraction studies (Hill, 1976) indicate that a low-velocity zone, 14 km in diameter and extending from 7-8 to possibly 15 km depth, presently exists beneath the caldera. On · the basis of presently available data, it is not known whether this low-velocity zone is due to the presence of partially molten residual magma or to a still-hot crystalline core.…”

“…The report draws heavily on the results of my own geologic mapping (Bailey, 1974;Bailey and others, 1976;Bailey and Koeppen, 1977), • as well as on gravity and aeromagnetic studies by Kane, Mabey, and Brace (1976) and Williams, Berkman, and Mankinen (1977), seismic-refraction studies by Hill · (1976), heat-flow studies by Lachenbruch, Sass, Monroe, and Moses (1976), hydrology and water chemistry by Sorey and Lewis (1976) and Mariner and Willey (1976), and teleseismic studies by Steeples and Iyer (1976 …”