Paleoseismology and seismic hazards evaluations in extensional volcanic terrains

Abstract

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...

Final Report - Assessment of Testing Options for the NTR at the INL

“…Over the last 4 m.y. the post-hot spot track volcanism has been dominated by low intensity effusive basalt volcanism from 100's of widely scattered, overlapping, monogenetic shield volcanoes and northwest trending volcanic rift zones (Kuntz et al 1992;Smith et al 1996), and emplacement of scattered rhyolite cryptodomes and lava domes (e.g., McCurry et al 2008). Basalt lavas incrementally accumulated to a thickness of up to 2 km thick in the center of the ESRP.…”

Development Report on the Idaho National Laboratory Sitewide Three-Dimensional Aquifer Model

“…The intrusion of dikes in these rifts is a proposed mechanism to accommodate the calculated NE-SW extension of the eastern Snake River Plain aseismically (Parsons, Thompson, and Smith 1998;Smith, Jackson, and Hackett 1996). Eastern Snake River Plain rifts therefore should consist of NW-SE-trending linear rift features with multiple dikes in the subsurface aligned perpendicular to the axis of the plain, where these dikes may or may not reach the surface.…”

Development of Earthquake Ground Motion Input for Preclosure Seismic Design and Postclosure Performance Assessment of a Geologic Repository at Yucca Mountain, NV

“…Ruptures along dike-induced normal faults and fissures occur incrementally, as opposed to the sudden catastrophic release of strain along tectonic faults. The faults and fissures develop at shallow depths (less than 5 km) above the dike top limiting down-dip rupture widths (Smith et al 1996 [DIRS 101020], p. 6,284), in contrast to typical tectonic faults that have rupture widths extending to midcrustal levels near the brittleductile transition.…”