1. Introduction

Microearthquakes with magnitude (M < ˜3) occur naturally or due to fluid injection and production in geothermal, CO2 sequestration, hydrocarbon, and natural gas reservoirs. In geological environments where sufficient numbers occur, they are a source of energy that provides information about reservoirs not often achieved by other methods. Recordings of microearthquakes can provide 3D tomographic solutions of isotropic velocity (Vp and Vs) and attenuation quality factors (Q p and Qs). These results can be used to also obtain solutions for Poisson's ratio, lambda, and Young's and bulk moduli. Furthermore, analysis of the microearthquakes provides locations, moment tensors and stress drops. These results can be combined with basic source theory and rock physics principles to interpret reservoir properties including regions of fluids, fractures, porosity, and permeability. We show that a relatively high density of recording stations, about one each per km<sup>2</sup> , and relatively few earthquakes (˜300) can provide solutions at sufficient resolution and in a relatively short amount of time to be useful for exploration and reservoir management. We provide a case study for The Geysers, California, USA.

We do not attempt to obtain absolute values from tomography results, as this would require an excessively large number of events. We are interested in anomalies—changes in tomography results for areas with the same statistical resolution, depth, and comparable geology. Anomalies can indicate alterations and possible reservoir conditions. We make extensive use of automation to process the large amount of data collected [1, 2] and a tomographic inversion code designed to work with the output of the automated data processing (SimulCR; [3]).
