Evidence for Long-Time Scale ( > 103 years) Changes in Hydrothermal Activity Induced By Seismic Events

Abstract:

The pollen ¹⁴C 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 %o trend of decreasing over about a 5000 to 7000 year period following each earthquake is consistent with a single-pass, fault-controlled groundwater flow system that received increasing amounts of recharge of cool, unexchanged, meteoric water through time. Hydrothermal/isotopic model reconstructions of the Beowawe system suggests that each of the earthquakes must have produced at least a 10³ fold increase in fault permeability (from < 10^-14 m² to > 10^-11 m²). Observed overturned temperature profiles in geothermal wells close (300m) to the Malpais fault suggest the onset of thermal convection occurred over shorter time scales (200-1000 years). We suggest that individual Malpais fault segments become clogged on shorter time scales and new routes to the surface are taken.