Geological, chronological, and structural studies of the Long Valley-Mono/Inyo Craters area document a long history of related volcatfic eruptions and earthquakes controlled by regional extensional tectonics of the Basin and Range province. This activity has persisted for hundreds of thousands of years and is likely to continue. The Long Valley magma chamber had a volume approaching 3000 km 3 prior to its climatic caldera-forming •ruption 0.7 m.y. ago but has been reduced to less than a third of this volume by cooling, eruption, and crystallization. Seismic evidence indicates that the main mass of the present Long Valley magma chamber is about 10 km in diameter and that its roof is 8-10 km deep with smaller cupolas as shallow as 4-5 km. Although a chamber of this size is probably capable of producing an eruption approaching 30 km 3 of lava, [he record over the past 0.5 m.y. suggests that eruptions of 1 km 3 or less are far more likely. Models proposed for the current ground uplift and seismicity within the caldera require inflation of 0.1-0.2 km 3 by additional magma since mid-1979, and some models suggest that inflation was accompanied by injection of a thin dike or dikes (probably of silicic magma) into the ring fracture zone beneath the south moat. Several of the M 5.8-6.2 earthquakes that occurred in the region beginning in 1978 had non-double-couple focal mechanisms. Whether these unusual mechanisms indicate injection of mafic (low-viscosity) magma at midcrustal depths in the Sierra Nevada block south of the caldera remains debatable. Studies of calderas of various ages throughout the world indicate that episodes of unrest are relatively common and do not invariably culminate in eruptions. Although current unrest is concentrated in the south moat of Long Valley caldera, the Inyo/Mono Craters probably hold a greater potential for producing an eruption in the foreseeable future. The Inyo/Mono Craters have erupted at 500-year intervals over the past 2000-3000 years, whereas the Long Valley magma chamber has erupted at about 200,000-year intervals over the past 700,000 years. In either case, a major earthquake near the caldera could strongly influence the course of volcanic activity. along the Inyo/Mono Craters chain [Miller, 1985; Sieh, 1984]. These are the most recent eruptions within the chain of Quaternary volcanic centers that extends northward through eastern California from the Salton Trough in the south to the south end of the Cascade Range in northern California. With the exception of the active Caõcade volcanoes these are the most recent eruptions in the conterminous United States. Long Valley, Yellowstone• and Valles are the only silicic calderas in the conterminous United States to produce major, caldera-forming, ash flow eruptions in roughly the last 1 m.y.. The Long Valley region, which is on the boundary between the Sierra Nevada and Basin and Range provinces, illustrates

Intensive microearthquake swarms with the appearance of volcanic tremor have been observed in the southwest part of Long Valley caldera, southeastern California. This activity, possibly associated with magma injection, began 6 weeks after several strong (magnitude 6+) earthquakes in an area south of the caldera and has continued sporadically to the present time. The earthquake sequence and magmatic activity are part of a broad increase in tectonic activity in a 15,000-square-kilometer region surrounding the "White Mountains seismic gap," an area with high potential for the next major earthquake in the western Great Basin.

For most areas in the western Great Basin, focal mechanisms show a consistent pattern of primarily strike‐slip motion for shallow events and oblique or normal slip for deeper events. However, orientation of the axis of least principal stress (T axis) is different for different areas: NW–SE for western Nevada and the Mono Lake region, and NE–SW for the Mammoth Lakes area. Along the remainder of the Sierra Nevada frontal fault zone, T axes show both orientations. In general the change in mechanism with depth is interpreted as being caused by increasing overburden pressure, resulting in rotation of the maximum compressive stress (P axis) from horizontal at depths less than about 6 km to vertical at depths greater than about 9 km. The absence of normal‐slip events at depths greater than 10 km in one area (Mono‐Excelsior‐Luning zone) may be explained by a larger horizontal compressive stress than exists in areas that do have normal faulting at such depths. In some areas conjugate right‐ and left‐lateral shear on nearly vertical fractures may be associated with the formation of clusters of magma‐filled dikes at shallow depths. Assuming strike‐slip faulting to be characteristic of earthquakes with depth less than 6 km and normal faulting for events deeper than 10 km, extrapolation of measured shear stress in the upper few kilometers of the crust provides a rough estimate of the maximum and minimum principal stress as a function of depth. At about 3 km depth we find that S1 and S3 are both horizontal, with values of about 104 and 55 MPa, respectively. At depth of 20 km, S1 is vertical and equal to the overburden pressure, about 530 MPa; S3 is horizontal and about 260–300 MPa.

Based on the results of a seismic refraction profile from Hilo on the northeast coast of the island of Hawaii to Kalae on the south the earth's crust in the vicinity of Kilauea volcano is found to consist of three layers. The uppermost of these layers varies in thickness from 1.2 to 2.5 km along the profile, is characterized by a P‐wave velocity of about 3 km/sec, and probably represents a series of fractured vesicular lava flows. The second layer, with a velocity of 5.3 km/sec, is much less even in thickness and probably represents the principal volcanic layer of the crust. It is 4 km thick at the Hilo end of the profile, 6 km thick at Kalae, and elsewhere fills in the crustal section between the lavas at the top of the column and the denser rock below. The third layer is 6–7 km thick, has a P‐wave velocity of about 7 km/sec, and corresponds to the principal layer of the oceanic crust. Under the summit of the volcano, this layer is faulted, with a resulting vertical component of offset equal to about 7500 meters. Depth to the Mohorovicic discontinuity is 12 km at the two ends of the profile, and the maximum crustal thickness is about 17 km at a point midway between Hilo and the Kilauea summit area. To relate this crustal profile to the over‐all structure and volcanism of Hawaii, the seismic results were compared with gravity and surface geology features on the island. This comparison indicates that the main rift zones of Mauna Loa and Kilauea are north‐dipping transcrustal fracture zones, closely related to fundamental processes of uplift and volcanism in the upper mantle. It is concluded that, although most of the island of Hawaii is subsiding, owing to the response of the crust to the load of the volcanos, the southeastern flank, which includes Kilauea volcano, is being inflated and uplifted by magmatic intrusions.

Shallow earthquakes around the southwest boundary of Long Valley caldera, west of the Hilton Creek fault, are characterized by lack of S‐waves at regional seismic network stations to the northwest, north and northeast, and P‐waves for these same station‐event combinations are deficient in frequencies higher than about 2‐3 Hz. Earthquakes east of the Hilton Creek fault and southeast of the caldera have normal P‐ and S‐wave signatures at the same stations. These effects are explained by propagation through a magma chamber in the south‐central part of Long Valley caldera, at depth greater than 7‐8 km.