Evidence for long timescale (>10³ years) changes in hydrothermal activity induced by seismic events

Abstract

Abstract The pollen 14 C age and oxygen isotopic composition of siliceous sinter deposits from the former Beowawe geyser field reveal evidence of two hydrothermal discharge events that followed relatively low‐magnitude (<M5) earthquakes of Holocene and late Pleistocene age along the Malpais fault zone in Whirlwind Valley, Nevada, USA . The observed 20‰ trend of decreasing δ 18 O over about a 5000‐to‐7000‐year period following each earthquake is consistent with a fault‐controlled groundwater flow system that, following initial discharge of deep and hot groundwater, contains increasing amounts of cool meteoric water through time. Model simulations of this hydrothermal system can only match trends in the isotope data if we include a thousand‐fold increase in fault permeability (from <10 −14 m 2 to >10 −11 m 2 ) following each earthquake. However, the timescale for onset of thermal convection implied by an overturned temperature profile in a geothermal well 300 m from the Malpais fault is much shorter: 200–1000 years. We speculate that individual segments of the Malpais fault become clogged on shorter timescales and that upward flow of groundwater subsequently follows new routes to the surface.