Studies in Fluid Dynamics as Applied to Seismology and Volcanology

This thesis addresses three problems in seismology and volcanology by applying fluid dynamical theories that have been developed for engineering applications. Fault zones are proposed to operate analogously to journal bearings. A quantitative assessment of the physical regimes in which faults behave as lubricated systems is made using elastohydrodynamic theory. Elastohydrodynamic lubrication with typical parameters explains the following observable phenomenon: (1) a reduction in the frictional stress by 50% during large earthquakes, (2) a decrease in high-frequency (>1 Hz) radiation above a critical slip distance of a few meters and (3) a two orders of magnitude variation in scaled radiated energy between small (Mw < 4) and large earthquakes (Mw > 6). Regionally triggered seismicity often occurs in geothermal areas. It is documented here that the 1999 Mw=7.4 Izmit, Turkey, earthquake was followed by widespread seismicity in Greece over a study region extending from 400 km to nearly 1000 km away from the epicenter. The increase in cataloged earthquakes is statistically significant at the 95% level. A related phenomenon is the regional triggering of volcanic eruptions. A model for triggering eruptions based on rectified diffusion is formulated and evaluated. The excess pressure from rectified diffusion in a typical basaltic system following a regional M≥8 earthquake is between 0.001 and 0.02 MPa. Strong constraints on the porosity, size of the bubbly region, velocity structure and permeability must be imposed for rectified diffusion to be effective. A fluid dynamical model based on supersonic nozzle flow is used to link observed seismic waves with the mass discharge rate of an explosive volcanic eruption. The method is tested by calculating the vertical mass discharge rate from Mount St. Helens for the beginning of the May 18, 1980 eruption. The observed seismic sources are modeled as thrusts due to a combination of the momentum flux of the erupted products and the pressure of the eruptive jet. The momentum discharge rate is converted to a mass discharge rate. The calculated mass ejected in the first 100 s is 1.6x10¹¹-4.6x10¹¹ kg. Since the total blast deposit is ~3.2x10¹¹-4.1x10¹¹ kg, one interpretation is that the directed blast had a significant (≥40%) vertical component.