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 1984 November 23 ML 5.8 Round Valley earthquake is one in the.series of moderate earthquakes to have occurred in the Bishop-Mammoth Lakes, California area since 1978. This event and its aftershock sequence are particularly well recorded in that they occurred within a dense, local high frequency seismic network, and strong motion accelerograms, and regional and teleseismic digital seismograms are available for the main shock. We have derived the fault plane solution from the local and regional first motion data, inverted the teleseismic body wave data for the moment tensor solution, and fit regional surface waves for the low frequency moment. Unlike results for several of the earlier moderate events in the Bishop-Mammoth Lakes region, all results for the Round Valley main shock data sets give solutions consistent with left-lateral strike slip faulting on a near vertical N30"E striking plane. The seismic moment of the main shock from the regional surface wave analysis is 7.9 X 101'Nt m. Measurements of the seismic moment from regional surface waves and teleseismic body waves have been converted to spectral level and combined with the acceleration spectra to give a composite source spectrum for the main shock. The spectrum shows a lower corner frequency at 0.2Hz associated with the overall faulting event, and a higher comer frequency at 4.0 Hz which may be associated with the subevents composing the but at a lower slope of w -l between the low and high frequency corners as postulated in models for complex or partial stress drop events.The Round Valley main shock was followed by a widespread and prolonged aftershock sequence. Because of the dense local, high gain permanent seismograph network operated jointly by the University of Nevada-Reno and the United States Geological Survey, accurate locations and fault plane solutions could be determined for the aftershocks beginning immediately after the main shock, and the temporal and spatial growth of the aftershock sequence could be followed in detail. The Round Valley sequence is characterized by the development of two conjugate planes of aftershock activity; one, a near vertical plane striking N30"E associated with the main shock, and another which developed in the first 24 h of activity striking N40"W and dipping 55"NE. This shallow plane conforms to an extension of the Hilton Creek fault postulated by Malcome Clark, and may be the first indication of activity on this major Holocene structure since the recent period of earthquake activity began in 1978. Aftershocks defining the deeper near vertical plane tend to concentrate around the periphery of a 36-49 km2 area which appears to represent the slip surface of the main shock. Relating the seismic moment determined from regional surface waves to this slip area results in an average dislocation of 0.54-0.73 m and a stress drop of 1.5-2.3 MPa. Nearly 800 fault plane solutions for aftershocks were determined. A smaller subset which samples the aftershock zone both spatially and temporally shows that during the initial...

Annual surveys of trilateration and leveling networks in and around Long Valley caldera in the 1982–1985 interval indicate that the principal sources of deformation are inflation of a magma chamber beneath the resurgent dome and right‐lateral strike slip on a vertical fault in the south moat of the caldera. The rate of inflation of the magma chamber seems to have been roughly constant (0.02 km3/yr) in the 1982–1985 interval, but the slip rate on the south moat fault has decreased substantially. In addition, there is evidence for a shallow source of dilatation (possibly dike intrusion) beneath the south moat of the caldera in 1983 and less certain evidence for a deep source (possibly magma chamber inflation beneath Mammoth Mountain) in the western caldera in 1983–1985. Deformation in the 1985–1986 interval as inferred from trilateration alone seems to be similar to that observed in 1984–1985.

The Morgan Hill, California, earthquake (magnitude 6.1) of 24 April 1984 ruptured a 30-kilometer-long segment of the Calaveras fault zone to the east of San Jose. Although it was recognized in 1980 that an earthquake of magnitude 6 occurred on this segment in 1911 and that a repeat of this event might reasonably be expected, no short-term precursors were noted and so the time of the 1984 earthquake was not predicted. Unilateral rupture propagation toward the south-southeast and an energetic late source of seismic radiation located near the southeast end of the rupture zone contributed to the highly focused pattern of strong motion, including an exceptionally large horizontal acceleration of 1.29g at a site on a dam abutment near the southeast end of the rupture zone.

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Paleomagnetic and magnetic property studies have been made on sediments and basalts recovered from holes 315A (northern Line Island chain) and 317A (Manihiki Plateau) during leg 33 of the Deep‐Sea Drilling Project (DSDP). The purpose of this investigation is twofold: (1) to investigate the magnetic properties of basalts from holes 315A and 317A and (2) to determine the origin of the inclination discrepancy between hole 317A carbonate and hole 317A basalt as reported by Cockerham and Jarrard [1976]. Basalts cored at hole 315A, DSDP leg 33, have irreversible Js‐T curves (typical of submarine basalts) with Curie temperatures of approximately 300°C. When they are viewed with reflecting light, large (30–100 μ ) anhedral homogeneous titanomagnetite grains of class 1 oxidation are observed. These basalts have acquired a large viscous remanence. The six flow units cored yield an average inclination of −30.9° (σ = 14.4). Hole 317A basalts have reversible Js‐T curves and Curie temperatures of about 570°C. Some 317A basalts have a secondary Curie temperature at about 360°C. The two Curie temperatures in a sample are believed to represent two compositionally different titanomagnetites. Under reflected light, class 3–4 deuterically oxidized titanomagnetite grains are observed with abundant ilmenite exsolution lamellae. These basalts are very resistant to AF demagnetization (median destructive field (MDF) values of ∼250 Oe). A mean inclination of −65.2° (σ = 6.1) has been calculated for these basalts, while the overlying carbonate sediments have a 20° shallower inclination (Ī = 46.7, σ = 9.0). Volcaniclastic sediments, 250 m in thickness, separate the basalts from the carbonates. The mean inclination of the volcaniclastics is −62.2° (σ = 2.4); however, the lowest samples of the carbonates together with the uppermost volcaniclastic samples have inclinations that systematically span the 20° difference. The difference in inclinations of the carbonates and the basalts is believed to be a result of tectonic tilting. The remains of shallow water molluscan fauna, volcaniclastics derived from hyaloclastite eruptions, and the magnetic properties of 317A basalts all suggest a subaerially or very shallow water (<200m)eruption of the 317A basalts.

Stable remanent magnetizations were found in dominantly Cretaceous sediments from Holes 315A and 317A and in basalts from Hole 317A. Except for uppermost Cretaceous and Paleocene sediments from Hole 315A, virtually all samples were of normal polarity. Highly consistent inclinations of magnetization from the sediments at each site indicate about 15° of northward motion for each site since the Late Cretaceous. However, inclinations from 317A basalts are consistently about 20° steeper than inclinations of the overlying sediments.

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