Evidence for long timescale (> 10(3) years) changes in hydrothermal activity induced by seismic events

Abstract

The pollen C-14 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 parts per thousand trend of decreasing O-18 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 300m from the Malpais fault is much shorter: 200-1000years. 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.