potential uses of a community fault model in geodetic research
TRANSCRIPT
Potential uses of a community fault model in
geodetic research
Laura Wallace, Ian Hamling, Charles Williams
GNS Science
Needs for geodetic models of:
Interseismic deformation
Slow slip events
Coseismic deformation
GNS Science
We often use elastic block models to interpret the three
dimensional interseismic GPS velocity field in NZ. These
require definition of block boundaries and the faults that
coincide with the boundaries
These earlier versions have largely used the definition of the major faults from the NSHM.
We define the plate boundary by nodes at common depths. They can be widely spaced,
but we create much smaller patches between the nodes that approximate rectangles
GNS Science
Time dependent inversions of cGPS timeseries
for Slow Slip events
Charles Willams et al., in
prep
For the Network Inversion Filter we define the
interface by a series of interlocking triangles.
This allows for more complex geometries
without simple rectangles
Time-dependent inversions using TDefnode—
we define the fault surfaces in the same way
as for the interseismic problem
Inversion for 2019 east coast SSE
Katie Woods, VUW PhD student
GNS Science
We are now collecting seafloor geodetic data, so good
offshore fault geometries are important, especially for
Hikurangi subduction zone
Numerous recent active source
seismic experiments offshore
(SHIRE, NZ3D, others) mean that we
now have data to constrain the
offshore geometry well
Wallace et al., 2016
Williams and Wallace, 2018
GNS Science
Coseismic slip models
Hamling et al., 2017
Grassmere EQ, Hamling et al., 2014
An easy-to-use, comprehensive CFM would
greatly speed up production of fault slip models
from GPS and InSAR following large events.
Faults for Ian’s coseismic slip models use
rectangular patches to define the fault surface.
This can pose problems for faults where there
are changes in strike, or geometry. Ian is
planning to use triangles in the future (rather
than rectangles).
A CFM will also be useful for evaluating
Coulomb stress changes on other nearby
faults (from earthquakes or SSEs).
GNS Science
Summary• Interseismic deformation models: Right now, block models (Defnode/TDefnode)
are the primary approaches we use for this. Due to complexity of NZ tectonics,
interseismic deformation models must account for independent block motion and
complex fault intersections, and along-strike variation in fault geometries. Fault
definition done via more widely spaced nodes, which are then discretized into very
small fault patches (quadrilaterals) between nodes to approximate Okada
rectangles. Very flexible
• Coseismic slip models: A comprehensive model of NZ active faults would help
greatly in defining fault geometries to use in coseismic slip models. Current models
by Ian assume rectangular fault sources, although he hopes to move towards using
triangular patches, which will enable definition of more realistic fault geometries. A
CFM composed of triangular patches would likely help move this forward
• Slow slip event models: Hikurangi subduction interface models important here.
Given the increasing availability of seafloor geodetic data for SSEs, a good model
for the offshore Hikurangi subduction zone that also integrates the latest
information from recent active source seismic experiments is critical. Currently
using triangles to define fault patches (for NIF), and nodes (TDefnode)
GNS Science
Thoughts…
• It seems like triangles might be needed for meshing the
faults if you want to capture any detailed or complex
geometry (rectangles cause problems). Triangles will
work well for most geodetic models.
• For us, the offshore faults are just as important as
onshore (and may be easier to get accurate geometries
since there is more offshore seismic reflection data)
• A key for any CFM should be flexibility, so people can
tailor it to their needs. Perhaps a few different levels of
resolution? Most geodetic needs are fairly coarse (with
the exception of coseismic fault models)