Earth Planets Space, Vol. 54 (No. 11), pp. 1195-1205, 2002
Bruce E. Hobbs1, Hidemi Tanaka2, and Yoshihisa Iio3
1CSIRO, Perth 6102, Australia
2Department of Earth and Planetary Science, The University of Tokyo, Tokyo, Japan
3Earthquake Research Institute, The University of Tokyo, Tokyo, Japan
(Received April 16, 2002; Revised September 30, 2002; Accepted September 30, 2002)
Abstract: This paper addresses concepts presented in Session 7 of the conference on "Slip and Flow Processes Near the Base of the Seismogenic Zone" held at Sendai, Japan in November, 2001. The questions raised in this session were associated with the downward extensions of seismic faults into the lower crust. The important issue is whether asiesmic slip accelerates on downward extensions prior to large earthquakes on the upper, seismic part. If this is the case, then such movement may be measurable as a precursor to large seismic events as accelerated tilt and/or distortion at the surface. Associated issues involve the geometry of downward extensions of faults, the mechanisms of localisation in the lower crust, and the mechanisms for earthquake generation near the base of the upper crust. The outcomes from this session are that aseismic slip in the lower crust could be generated by several mechanisms of localisation including yield of an elastic-viscous-plastic material, softening (including, in particular, thermal softening) of an elastic-viscous material and ductile fracture. Fine scale modelling of localisation in the lower crust is still required to resolve the issue whether accelerated motion precedes seismic slip in the upper crust. Such modelling also demands a better understanding of crustal rheology than we have at present.