Double-difference attenuation tomography method and pilot applications

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Seismologists use seismic waves to construct images of the Earth's internal structure, termed seismic tomography, analogous to the way doctors use X-rays to construct images of the human brain with CAT scans (Computerized Axial Tomography). In seismology, tomography is most often done using measured travel times of seismic waves to infer the seismic wave velocity structure within the Earth. Another possibility is to use the amplitudes of seismic waves at different frequencies to infer the Earth's attenuation structure, that is, to identify regions where seismic waves lose energy more or less than normal. In this project, the researchers will develop and test a new method to image attenuation structure, with fine spatial resolution, using measured attenuation differences between waves from pairs of nearby earthquakes. The effectiveness of the method will be evaluated on earthquake data from northern Honshu, Japan, and The Geysers geothermal field, California. In the former case, the researchers will endeavor to image three-dimensional attenuation variations deep within the Earth, which will provide insight into the zone where magmas originate that erupt at "Ring of Fire" volcanoes and also into the nature of deep earthquakes. In the latter case, the goal is to detect changes in attenuation structure within the geothermal system over time, which can be used to monitor fracturing changes due to geothermal exploitation or stimulation.This project will develop a new method termed ?double-difference attenuation tomography?, that will be tested on synthetic data, and applied to two regions of interest. The concept is to do for seismic attenuation imaging what double-difference location has done for hypocenter locations and double-difference tomography has done jointly for hypocenter locations and seismic velocity imaging. The key development is event-pair differential t* analysis, which contrasts with previous differential t* analysis methods, which have generally utilized either a phase-pair or a station-pair approach. A spectral ratio approach for nearby event pairs will be used to obtain the event-pair differential t* values, analogous to how waveform cross-correlation is used to obtain differential times for double-difference location and double-difference tomography. This method will be thoroughly tested on both synthetic and real data. Two areas have been selected for applying the new double-difference attenuation tomography method: The Geyser geothermal field, California, and Northern Honshu, Japan. These areas were chosen both for their intrinsic interest and for the availability of results from previous studies to which our results can be compared. This method has the potential for obtaining very high resolution images of seismic attenuation structure in the regions where earthquakes occur. Seismic attenuation is a strong complement to seismic velocity in efforts to distinguish lithology, saturation, fracturing, and temperature in the subsurface, and these factors can affect velocities and attenuation differently. In some of the applications that we envision, high-resolution detection of temporal changes in attenuation is a realizable goal. Many of the possible applications are to problems of high societal importance, such as geothermal energy, mining, wastewater disposal, fracking, CO2 sequestration, and volcanoes. The project will support a female early career scientist and a graduate student.