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).

Extensional structures in volcanic terrains are the surface expression of shallow dike intrusion and can be misinterpreted as structures associated with major tectonic faults. Dike-induced structures can be distinguished from their tectonic counterparts by their association with cogenetic volcanic rocks and by several geometric relationships. Tensile fissures with little or no vertical displacement, fissure swarms, flexural monoclines, and normal faults are commonly symmetrically distributed about a central eruptive fissure, sometimes forming a graben above shallow dikes. The structures typically occur within broad zones, not narrow belts near a main fault zone, reflecting their origin by repeated dike injection. Colluvial wedges containing records of single large earthquakes generally do not form; instead, fault scarps with several-meter vertical displacements in volcanic bedrock may reflect the cumulative effects of many decimeter-scale displacement events from several dike injection episodes. The mechanism of dike intrusion and the nature of cointrusive seismicity have important implications for determination of the maximum magnitude and recurrence of earthquakes in extensional volcanic terrains. Observational seismicity from volcanic rift zones worldwide suggests the maximum magnitudes of dikeinduced earthquakes are 3.8 _+ 0.8. Earthquakes are generally small to moderate because downdip extents of faults and fissures are controlled by the depth to the top of the associated dike (usually <5 km), permitting only small rupture areas. Also, rupture and displacement on faults and fissures migrate incrementally at about the velocity of propagating dikes (0.5 m/s) as dike dilation stresses the zone above and ahead of the dike. Earthquake recurrence is tied to recurrence of volcanic cycles based on the geochronology of the associated volcanic materials. Based on these concepts, innovative approaches have led to estimates of the maximum magnitude and recurrence for magma-induced seismicity in eastern Snake River Plain (ESRP) volcanic rift zones. The most conservative approach uses surface length, fault width (downdip extent), and rupture areas of normal faults and fissures produced by dike injection to estimate a maximum M5.5 earthquake for an episode of rift zone volcanism. Chronology of volcanic rocks suggests annual probabilities of 10 -4 to 10 -5 for volcanic rift zones near Idaho National Engineering Laboratory facilities. Probabilistic assessments show that ground motion hazard due to volcanic rift zone seismicity is lower than the hazard from other earthquake sources. This is because of the relatively low magnitudes and long recurrence intervals of cointrusive earthquakes in volcanic rift zones. Introduction Magma intrusion is an important component of worldwide crustal extension [Parsons and Thompson, 1991; Gans, 1987; Coney, 1987; Forslund and Gudmundsson, 1991; Lepine and Him, 1992; Rubin and Pollard, 1988]. Seismicity, surface faulting, magma intrusion, and volcanism are expressed within many tectonic setti...

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SUMMARYThirteen seismic reflection lines were processed and interpreted to determine the southern terminations of the Lost River and Lemhi faults along the northwest boundary of the eastern Snake River Plain (ESRP). The southernmost terminations of the Arco and Howe segments were determined to support characterization of the Lost River and Lemhi fault sources, respectively, for the INL probabilistic seismic hazard analysis.Four commercial seismic reflection lines (Arco lines 81-1 and 81-2; Howe lines 81-3 and 82-2) were obtained from the Montana Power Company. The seismic data were collected in the early 1980's using a Vibroseis source with station and shot point locations that resulted in 12-fold data. Arco lines 81-1 and 81-2 and Howe lines 81-3 and 82-2 are located within the basins adjacent to the Arco and Howe segments, respectively.Seven seismic lines (Arco lines A1, A2, A3, and A4 and Howe lines H1, H2, and H3) were acquired by EG&G Idaho, Inc. Geosciences for this study using multiple impacts with an accelerated weight drop source. Station and shot point locations yielded 12-fold data. The seismic reflection lines are oriented perpendicular to and at locations along the projected extensions of the Arco and Howe fault segments within the ESRP.Two seismic lines (Arco line S2 and Howe line S4) were obtained from Sierra Geophysics. In 1984, they acquired seismic reflection data using an accelerated weight drop source with station and shot point locations that yielded 6-fold data. The two seismic reflection lines are oriented perpendicular to and at locations along the projected extensions of the Arco and Howe fault segments within the ESRP. In 1992 for this study, Geotrace Technologies Inc. processed all of the seismic reflection data using industry standard processing techniques.