HIGH-FREQUENCY TEMPERATURE VARIATIONS IN A FAULT-CONTROLLED HYDROTHERMAL SYSTEM

The surface expression of the Borax Lake Hydrothermal System comprises a group of approximately 175 thermal vents, located along the trace of a north-south trending normal fault in the Alvord Basin of southeast Oregon, USA. Temperatures in the Borax Lake hydrothermal springs demonstrate significant spatial and temporal variability: two springs separated by less than 1 m may differ in temperature by 40°C or more, and individual spring temperatures have been observed to change by ~35°C in one 24-hour period. We have been collecting temperature data at 15-minute intervals from selected springs in the Borax Lake system since May 2004, with the objective of relating the temporal variability of spring temperatures to particular physical phenomena.

In several instances we have identified what appear to be event-based changes in the thermal behavior of springs that are not related to surface phenomena such as diurnal forcing. These discrete events are characterized by an increase in high-frequency (greater than 0.25/hour) temperature fluctuations, followed by a brief interval of fluctuations above the Nyquist critical frequency (2/hour) that result in significant aliasing of the temperature date frequency spectrum. These events can occur simultaneously in several springs; in some instances spring temperatures return to a pre-event state, while in other cases temperatures stabilize at a new, post-event temperature significantly different from the pre-event temperature. We hypothesize that this behavior is the manifestation of changes in the internal stress state, and thus the permeability distribution, of the controlling fault; however, additional data on microseismic activity at the site will be required to confirm this relationship. We are unaware of published reports of rapid variations in spring discharge temperatures attributed to microseismic events, but other investigators have described changes in discharge rate or hydrochemistry in fault-controlled springs following significant tectonic events. If a relationship between spring temperature and microseismic activity can be confirmed, the Borax Lake data may provide important information on coupled hydro-mechanical processes in active faults.