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)