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Abstract.The eastern Snake River Plain of southern Idaho poses a paradoxical problem because it is nearly aseismic and unfaulted although it appears to be actively extending in a SW-NE direction continuously with the adjacent block-faulted Basin and Range Province. The plain represents the 100-kmwide track of the Yellowstone hotspot during the last -16-17 m.y., and its crust has been heavily intruded by mafic magma, some of which has erupted to the surface as extensive basalt flows. Outside the plain's distinct topographic boundaries is a transition zone 30-100 km wide that has variable expression of normal faulting and magmatic activity as compared with the surrounding Basin and Range Province. Many models for the evolution of the Snake River Plain have as an integral component the suggestion that the crust of the plain became strong enough through basaltic intrusion to resist extensional deformation. However, both the boundaries of the plain and its transition zone lack any evidence of zones of strike slip or other accommodation that would allow the plain to remain intact while the Basin and Range Province extended around it; instead, the plain is coupled to its surroundings and extending with them. We estimate strain rates for the northern Basin and Range Province from various lines of evidence and show that these strains would far exceed the elastic limit of any rocks coupled to the Basin and Range; thus, if the plain is extending along with its surroundings, as the geologic evidence indicates, it must be doing so by a nearly aseismic process. Evidence of the process is provided by volcanic rift zones, indicators of subsurface dikes, which trend across the plain perpendicular to its axis. We suggest that variable magmatic strain accommodation, by emplacement and inflation of dikes perpendicular to the least principal stress in the elastic crust, allows the crust of the plain to extend nearly aseismically. Dike injection releases accumulated elastic strain but generates only the small earthquakes associated with dike propagation. The rate of dike emplacement required to accommodate the estimated longitudinal strain rate of the plain is roughly a Copyright 1998 by the American Geophysical Union.Paper number 98TC00463. 0278-

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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