P wave attenuation of the Yellowstone Caldera from three‐dimensional inversion of spectral decay using explosion source seismic data

Abstract

Using explosion source, seismic refraction data, recorded in the 1978 and 1980 Yellowstone‐Snake River Plain seismic experiments, a three‐dimensional inversion of differential P wave attenuation was used to assess the relative variations in Q−1 in and around the volcanically active, 45 km by 70 km, Yellowstone caldera, northwestern Wyoming. Differential attenuation was derived from spectral decay of upper crustal Pg phases, observed from six explosions and recorded at 90 temporary stations. Because of the relatively short time windows used to determine the spectral content, a maximum entropy technique was employed to estimate the spectra that yielded an optimally small variance. Differential P wave attenuation was calculated from least squares determinations of the spectral ratios corrected for source and path effects. The observed differential attenuation parameters were then inverted using a weighted least squares technique for a discretized, 70×105 km, three‐dimensional surface and upper crustal Q−1 model of the Yellowstone caldera and surrounding region. Results showed that the surface layer, to depths of 2 km within the Yellowstone caldera, is characterized by relatively high attenuation with low Q values less than 30, compared to values of 38 to 50 outside the caldera. The higher attenuation in the caldera's surface layer is thought to be associated with Quaternary lake sediments, highly altered rhyolites, and the possible influence of steam in areas of hydrothermal activity. In the crystalline upper crust, at depths of 2 km to 12 km, Q values of 40 to 70 were observed in areas of thick sedimentary fill northwest of the caldera and in areas of hydrothermal activity. Within the caldera, upper crustal attenuation generally corresponded to Q of 200 in areas that are interpreted to be associated with hot but now solidified granitic material. In comparison, relatively high attenuation, Q = 40, was observed in the upper crust of the northeastern Yellowstone caldera in an area that also corresponds to a 20% reduction in Pg velocity, a prominent negative Bouguer gravity low of −20 mGal, near Yellowstone's largest area of hydrothermal activity and near a Quaternary, volcanic resurgent dome. On the basis of constitutive models of velocity and density, this zone of high attenuation is likely a product of unusual hydrothermal activity, highly altered upper‐crustal rocks, a shallow magmatic source or a highly porous, steam‐saturated body. Outside the caldera, the upper crust is relatively unaffected by high temperatures and thermal alteration, and the corresponding attenuation is low with Q values greater than 200.