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 dike-induced 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. ¿ American Geophysical Union 1996