Currently,
most of my research involves modeling earthquake-related deformation
over time scales between seconds (relevant time scales for seismology)
and thousands of years (paleoseismological time scales). This
deformation that can be characterized at the surface with GPS
measurements, inSAR, and other kinds of surveying data. Tying together
deformation over very short, intermediate, and very long time scales, with
a single model of plate-boundary lithosphere, is my ultimate goal.
I use both semi-analytical codes and the finite-element method to model
deformation.
The models I develop are based on as much relevant
geological and geophysical information as I can find. This information
is used both to construct the model and to test its performance. Elastic
properties of the lithosphere and deep rupture geometry come from seismic
studies; surface fault geometry from geological mapping, and rheology
of material below the brittle-ductile transition is inferred from density,seismic
velocities, the geothermal gradient (heat flow data), geological studies
of exhumed faults and lower crust, and laboratory deformation experiments.
To evaluate model performance, I compare predicted surface displacements
(orientations and magnitudes, as a function of position and time)
with observations (typically from ultra-precise GPS measurements,
but also from coastal uplift studies, leveling surveys, and whatever
else I can get my hands on). For models of longer-term deformation, I compare
modeled fault slip rates with geological slip rate estimates. I
plan to evaluate future models in terms of their ability to match other
geophysical observables, such as microgravity data or seismicity rate
changes. Here are some details on the modeling I do, divided according
to deformation time scale:
Instantaneous deformation of the Earth’s lithosphere due to an earthquake.
That is, permanent elastic deformation after
shaking has stopped, but before significant inelastic deformation
(creep) has occurred. This modeling is useful for estimating
how faults slip kinematically (how deep? patchy slip distribution
[high stress drop]? does slip scale with the fault length or magnitude,
as paleoseismologists assume when estimating magnitudes of pre-historic
earthquakes?). Static deformation modeling also shows how elastic
stresses change on local faults in response to a large earthquake,
which is of use in estimating changes in the likelihood that these
faults will fail in the near future. So far, I have modeled stress
changes due to the 1999 Izmit, Turkey earthquake, and have looked at strike-slip
earthquakes in general, to see how heterogeneous elasticity influences
deformation. Since faults are heterogeneous, coseismic fault slip and
postseismic fault creep (next topic) are interrelated: often it's hard
to tell where one ends and the other begins.
Transient deformation.
I also model accelerated deformation of the
Earth's surface after large earthquakes (or spontaneous, short-lived
creep events) in an effort to characterize the structure and rheology
of fault zones (or the lithosphere as a whole) below the brittle-ductile
transition. It’s also useful to look at interseismic surface deformation
near a fault if we know the recurrence interval and how long it’s
been since the last large earthquake: surface deformation may vary
significantly with time between earthquakes, and how it varies is
controlled by the rheology of the plate boundary at depth. Right now,
the $64,000 question for most plate boundary regions is: is the upper
mantle gooey and incapable of supporting large deviatoric stresses,
or is it essentially rigid over “long” time periods? That is, how far
can we push the classic ideas of plate tectonics, in which the relative
motion of rigid plates (lithosphere) is permitted only by slip along
fault surfaces? This must be known before we can understand how
stresses build up along active fault systems between earthquakes. The
answer could vary for different plate boundaries. So far, the Anatolia-Eurasia
boundary looks pretty plate-like (PDF files: 1,
2).
Secular deformation.
(kinematics and dynamics) Another group of projects involves modeling
the deformation of plate boundary regions containing many faults,
over time intervals spanning many "earthquake cycles" on each fault.
For these models, variation in deformation patterns between earthquakes
are ignored. Such models are constrained by geological and paleoseismic
estimates of fault slip rates. If something about the time-dependence
of surface deformation between earthquakes (so transient deformation
can essentially be filtered out), GPS data can also provide fault slip
rates. Regional models can provide estimates of slip rates on
faults which are not well understood, and can illustrate spatial patterns
of surface deformation with transient, earthquake-cycle effects removed.
(PDF file: Regional-scale kinematic model for eastern California (the
Walker Lane Belt). Analysis of these patterns can elucidate the relative
contributions of tractions along plate edges, basal tractions, and gravitational
forces, within a deforming region. I am also interested in the
development of plate boundary fault systems and the related seismicity.
Why are some plate boundaries simple (a single fault) while others comprise
a complex network of faults?
Some links to like-minded people and groups:
Southern California Earthquake Center
Earthscope
GSC-Pacific Geodynamics
Program