The magnitude 7.3 Landers earthquake of 28 June 1992 triggered a remarkably sudden and widespread increase in earthquake activity across much of the western United States. The triggered earthquakes, which occurred at distances up to 1250 kilometers (17 source dimensions) from the Landers mainshock, were confined to areas of persistent seismicity and strike-slip to normal faulting. Many of the triggered areas also are sites of geothermal and recent volcanic activity. Static stress changes calculated for elastic models of the earthquake appear to be too small to have caused the triggering. The most promising explanations involve nonlinear interactions between large dynamic strains accompanying seismic waves from the mainshock and crustal fluids (perhaps including crustal magma).

Two magnitude 3.8 earthquakes occurred south of the Wallowa Mountains in northeastern Oregon in August and September 1984. Locations of 15 aftershocks recorded on portable seismographs cluster around the Oregon-Idaho border near the mouth of the Powder River. A composite fault-plane solution indicates normal faulting on planes striking north or north-northwest, consistent with stresses observed in the Basin and Range Province to the east. The record of historical earthquakes in the region suggests periodicity with intervals of 11 to 21 yr between cycles of events consisting of several main shocks within a 3-yr time span. A magnitude 3.6 earthquake on 29 September 1981 may have begun the present cycle.

Small-magnitude earthquakes began beneath Mount St. Helens 40 days before the eruption of 20 March 1982. Unlike earlier preeruption seismicity for this volcano, which had been limited to shallow events (less than 3 kilometers), many of these earthquakes were deep (between 5 and 11 kilometers). The location of these preeruptive events at such depth indicates that a larger volume of the volcanic system was affected prior to the 20 March eruption than prior to any of the earlier dome-building eruptions. The depth-time relation between the deep earthquakes and the explosive onset of the eruption is compatible with the upward migration of magmatic gas released from a separate deep reservoir.

A dense network of portable seismographs was operated in the Ridgely area of western Tennessee in May of 1978. During 32 days of operation, 122 earthquakes were detected, of which 90 were located. The high quality P and S arrivals recorded exhibit travel time ratios (ts/tp) that decrease significantly with depth. This decrease is most rapid in the upper few kilometers and necessarily reflects similar changes in the velocity ratio (Vp/Vs). Based on laboratory and theoretical studies, such observations are consistent with a decrease in crack density with increasing confining pressure. A special study of Vp, Vs, and Vp/Vs revealed strong variations with depth that also correlate with rock type. Inversion of the data for both P and S velocity structures confirms the presence of a low‐velocity zone in the depth range 2 to 5 km; a high‐velocity lid, with high Vp/Vs and thus, high crack density; and a boundary at the near surface for S to P conversion, associated with the unconsolidated saturated sediments of the Mississippi embayment. Relocated hypocenters using independent velocity structures for P and S are systematically deeper and define a seismogenic layer between 4 and 13 km depth. The earthquakes near Ridgely show that most of the activity is occurring on a northwest striking reverse fault that dips to the southwest. Minor off‐fault activity within the hanging wall was also observed representing motion on a series of small normal faults dipping to the northeast. Other composite focal mechanisms were determined for nearby regions. These solutions are all in general agreement with a horizontal compressive stress field striking northeast to east, favoring right‐lateral strike‐slip motion on left‐stepping northeast trending faults, or reverse motion on faults trending northwest and north. These results are consistent with the stress field found in a large area interior to the United States, and imply a rotation of the regional stress field since the late Cretaceous when extensional faulting occurred in the area.

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