TIME-DEPENDENT STRAIN ACCUMULATION AND RELEASE AT ISLAND ARCS: IMPLICATIONS FOR THE 1946 NANKAIDO EARTHQUAKE by

Underthrusting and the elastic-rebound theory are consistent with the gross static deformations after earthquakes for island-arc regions such as Japan, Alaska, and Chile. Yet anomalous, time-dependent post-earthquake adjustments suggest additional processes. Here the asthenosphere or mantle becomes the element that both determines the post-seismic deformation and controls the accumulation of strain. The lithosphere and asthenosphere represent a coupled system. A large earthquake strains the entire system; stress relaxation in the viscous asthenosphere follows and allows the post-seismic readjustments. The convergence zone is first considered as a semiinfinite, elastic plate overlying a viscoelastic foundation. Analytic solutions for short-period deformations place bounds on the behavior for the surface deformations, boundary conditions on the fault interface, and stress-propagation following an earthquake. Detailed models are then considered using a novel, time-dependent, finite element solution for the convergence zone. The solution avoids propagation of errors in time and readily accommodates inversion theory. The method clearly defines the behavior for realistic models. Thus, scaling with the fault depth and lithospheric thickness controls the shape for simple, planar fault models, while the time scale depends on the asthenospheric viscosity. Different boundary conditions imposed on the fault, whether a constant dislocation or a constant stress with time, strongly affect the resulting deformations and stresses. Finally, stresses introduced by thermal density anomalies within the descending lithosphere are compared to earlier models and focal mechanisms near Hokkaido. Generalized-matrix inversion theory now places bounds on the effective viscosity and fault parameters using geodetic data, focal mechanisms, and tectonic setting for the 1946 Nankaido earthquake (M 8.2) in southwest Japan. Using the assumption of stress relaxation in the asthenosphere, the data constrains the fault geometry to a shallow 150 dip, followed by a 60* dip from 26 km depth to the base of the lithosphere at 60 km depth. Near the hypocenter the slip is 3 meters, while the maximum slip is less than 15 meters. Other models with constant dip or shallower dip beyond 25 km do not satisfy these constraints. The viscosity of the asthenosphere now becomes 1020 poise. The results suggest segmentation of the lithosphere and deformations that generate the embayed shoreline, sedimentary basins off southwest Japan, seismicity, and focal mechanisms. Thesis Supervisor: M. Nafi Toks8z Title: Professor of Geophysics ACKNOWLEDGEMENTS To my adviser, Professor M. Nafi Toks6z, goes my appreciation for his support and encouragement. During my discussions with Professor Keiiti Aki and Dr. Katsuyuki Abe I found many fruitful paths and notions. Professor T. Pian of the Department of Aeronautics and Astronautics gave useful feedback on the finite element method, while FEABL (Finite element analysis basic library) introduced a computational framework. Carl Johnson inspired exotic uses of the computer, and Sara Brydges has gone above and beyond the call of duty during the typing and organization. Frank Richter was always a challenging colleague and reassuring friend. Finally, I thank all my friends for their patience and love. A Fannie and John Hertz Foundation Fellowship allowed extensive opportunities for research; I am grateful for their generous and continuing support. The research has been supported by the Advanced Research Projects Agency, monitored by the Air Force Office of Scientific Research under contract F44620-71-C-0049, by the Air Force Cambridge Research Laboratories, Air Force Systems Command under contract F19628-74-C-0072, and by the Earth Sciences Section, National Science Foundation, grant GA-36132X. TABLE OF CONTENTS PAGE