SCEC Award Number 18221 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Toward physics-based models of fault loading in CRM
Investigator(s)
Name Organization
Yuri Fialko University of California, San Diego
Other Participants
SCEC Priorities 1a, 1e, 3b SCEC Groups CXM, FARM, SDOT
Report Due Date 03/15/2019 Date Report Submitted 05/16/2019
Project Abstract
 One of the central questions in continental tectonics is the relative strength of the brittle and ductile parts of the lithosphere. This question in turn
depends on the degree of strain localization in the ductile substrate which controls whether deformation is accommodated primarily by discrete weak
zones bounding relatively rigid fault blocks, or
by diffuse viscous flow throughout the entire lower crust and upper
mantle. These deformation styles have important implications for how
seismogenic faults are loaded across temporal and spatial scales. Existing models typically assume a constant velocity boundary
condition applied either on a deep continuation of a fault,
or on a far lateral side of a computational domain. Models that assume a constant slip rate on a deep continuation of a fault may be considered unphysical because they assume elastic behavior below the brittle-ductile transition, and because a constant slip rate in itself must be a consequence (rather than cause) of the relative plate motion. The side-driven models do take into account visco-elastic deformation of the lower crust and upper mantle, but may suffer from unrealistic behavior of surface velocities in the far field. In this project we explored how basal tractions may affect the far-field asymptotic behavior of surface velocities throughout the earthquake cycle. Results of our simulations show that the non-zero basal tractions are required to produce vanishing strain rates at the Earth surface several locking depths away from a fault, as suggested by geodetic observations of deformation throughout the earthquake cycle.
Intellectual Merit The degree to which strain is localized in the ductile part of the
lithosphere below major faults is a major unresolved question in
continental tectonics. Two classes of models have been proposed: one
postulating a broadly distributed viscous deformation in the lower
crust and upper mantle (the ``thin lithosphere'' model), and another
one postulating extension of localized shear well below the
brittle-ductile transition (the ``thick lithosphere''
model). Understanding the mechanics of lithospheric shear zones is
essential for a number of problems in continental tectonics, including
the long-term strength of the Earth's crust and upper mantle, stress
transfer from the relative plate motion to seismogenic faults, and,
ultimately, seismic hazards. In this project we investigated the
long-term deformation and strain evolution due to major strike-slip
faults in the continental crust. In particular, we used numerical
models to evaluate the efficiency of various strain-softening
mechanisms, such as thermo-mechanical coupling, grain-size reduction,
and mylonitic fabric, and assess the degree to which they promote or
inhibit strain localization, individually and in combination, in
response to long-term fault slip, and their effect on surface
deformation throughout the earthquake cycle.
Broader Impacts Realistic models of long-term deformation informed by the
experimentally determined rheologies are essential for integrating various kinds of the SCEC community models, including CTM, CRM, CGM, SCM, and USR. The result also bear on the long-standing
debates such as the block-like versus diffuse deformation in the
continental interiors, the effective strength of the continental
lithosphere, and the mechanisms of transient deformation following
large earthquakes. Modeling results generated under the auspices
of this project have been used in classes taught by the PI at SIO/UCSD.
Exemplary Figure Figure 2
Model of a strike-slip fault loaded by a constant velocity in the far field that represents the relative plate velocity. A velocity boundary condition is also applied at the bottom of the domain. The model assumes an elastic upper crust underlain by viscoelastic lower crust and upper mantle. The seismogenic fault extends from the surface to a depth of 17 km. The top surface is stress-free. Colors denote the temperature, in degrees K.