Earthquake swarm in Long Valley Caldera, California, January 1983: Evidence for dike inflation

Abstract

The 1982–1983 deformation observed by trilateration and leveling surveys across the Long Valley caldera is apparently related to the 8.5‐km‐long by 8‐km‐deep vertical rupture surface defined by the January 1983 earthquake swarm that occurred in the south moat of the caldera. The observed deformation can be explained as follows. In late 1982, 0.03 km3 of magma was injected into a dike that dips 30° northward from the bottom of the rupture surface. The downdip dimension of this dike is 8 km. The dike inflation accounts for the uplift observed across the caldera as well as some of the horizontal deformation. Inflation of the dike generated a tension of about 3 bars across the vertical plane that was to become the rupture surface of the January swarm. This reduced the frictional stress on the rupture plane and perhaps triggered the slip that caused the January swarm. Right‐lateral slip (0.22 m) on the uppermost 2 km of the rupture plane during and after the January swarm accounts for the additional horizontal deformation observed. The model can be improved marginally if strike slip is admitted over the entire rupture surface and 0.006 km3 of magma is injected along that surface in the depth interval 3–8 km. The improvement in the model fit, however, is not sufficient to require shallow injection of magma. Thus we conclude that inflation of a dike at depth (8–12 km) dipping northward beneath the resurgent dome plus shallow right‐lateral slip on the rupture surface is a simple, but not unique, explanation of the observed deformation and seismicity.

The Interpretation of Gravity Changes and Crustal Deformation in Active Volcanic Areas

“…161, 2004 Interpretation of Gravity and Crustal Deformation 1457 south moat of the caldera (LANGBEIN et al, 1995). In addition, there is evidence for dike intrusion beneath the south moat (SAVAGE and COCKERHAM, 1984) and Mammoth Mt. .…”

Spatial distribution of crack structure in the focal area of a volcanic earthquake swarm at the Hakone volcano, Japan

“…Earthquake swarms sometimes occur in geothermal, or volcanic, areas, e.g., in the Long Valley Caldera of California (Savage and Cockerham, 1984) and the Yellowstone volcanic field of the western United States (Farrell et al, 2009). Hill (1977) proposed a model in which volcanic earthquake swarms are caused by faults arranged according to the local stress field, with magmatic/fluid intrusions in brittle rock.…”

Soil Radon and Elemental Mercury Distribution and Relation to Magmatic Resurgence at Long Valley Caldera

“…Cooler water forms of planktonic foraminifera (18) increasingly dominate in' the mid-Oligocene. Braarudosphaera, a coccolithophorid that is associated with crises, is common in the mid-Oligocene (19).…”

“…An Rn peak (>9x background) occurs in the southern moat, where deformation rates during 1982 and 1983 were most intense (18), over the surface projection of a dike postulated to have risen since 1978 (19). Smaller Rn peaks are generally related to faults active since 1978.…”

“…This suggests that ground gas anomalies are reflecting deep-seated perturbations and not near-surface hydrology (20). The magma bodies are postulated to reach to 4.5 km (average depth, approximately 8 km) below the caldera floor (17) and the dike to 3 km (19). The near-surface hydrologic system extends to depths of 0.5 to 1.5 km and is dominated by flow from northwest to southeast (20).…”