Preliminary Investigation of an Aseismic Doughnut Hole Region in the Northwest Geysers, California

Abstract

The Geysers Geothermal Reservoir experiences thousands of seismic events each month; some of these events are associated with recent coldwater injection and steam production within the Geysers basin. The greatest injection volume rate occurs in the Northern Geysers, and it is here that a flattened spheroidal region of relatively low seismic density, called the Doughnut Hole, has become visible within the last 20 years. The Doughnut Hole is preliminarily defined as a region of low seismic density (normalized number of earthquakes per unit volume) surrounded on all sides by a region of higher seismic density. Seismic density gridblock size is preliminarily determined by the mean RMS travel-time residual of the entire year’s catalog. This study is in its preliminary stages, but seeks to determine the true 3-D extent of the Doughnut Hole, to image concurrent changes in the local velocity structure, and to describe its origins. The dataset is comprised of over 87,000 events detected in the Geysers by the Lawrence Berkeley National Laboratory seismic array, and located using a new automated phase-picking, locating, and processing system.