Imaging earthquakes and tectonic strain build-up in 3-D from space

This project aims to take advantage of a new satellite mission to extract 3-D displacement maps for earthquakes and slowly-deforming tectonic regions, by combining the new data with data from existing missions. These measurements will be used to better understand the physics of earthquakes and improve estimates of seismic hazard.

Radar Interferometry (InSAR)

Radar Interferometry (InSAR) is a technique that provides measurements of surface displacement from space, with centimetre-to-millimetre accuracy. These measurements are used in the natural hazards community for measuring tectonic strain rates, estimation of fault slip during earthquakes, and monitoring of volcanoes and landslides. They are also used for monitoring of anthropogenic activities such as oil and gas extraction and drawdown of underground water storage.

Mapping tectonic strain

The COMET Centre of Excellence, led from Leeds, has an ongoing effort to use long time series of radar acquisitions to measure tectonic strain rates with sufficient accuracy to improve earthquake hazard maps. Since 1900, between 1.4 and 1.7 million people have been killed follwing earthquakes in continental interiors. In contrast to the narrow boundaries on the edges of oceanic plates, continental seismic belts span broad regions and earthquakes often occur on unidentified faults. Although most earthquakes appear to have no recognisable short-term precursor, all are preceded by a long, slow build-up of strain around the causative faults. Maps of tectonic strain can therefore inform models of seismic hazard.

From 1-D to 2-D displacements

InSAR measurements are intrinsically 1-D; they only give the displacement of the ground towards, or away from, the satellite.  As radar satellites do not point directly downwards, but off to the right side, there is a component of both horizontal and vertical motion in the measurements. Therefore, by combining measurements made when the satellite is moving approximately south-to-north with those made when it is moving approximately north-to-south, it is possible to extract a 2-D displacement field. This gives the components of the displacement that lie in a northward-dipping vertical plane striking east-west (see Figure 1 for an example of one component in this plane). The limitation of this is that northward displacement cannot be separated from vertical motion, and motion perpendicular to this plane cannot be measured at all.

E-W velocities for Turkey
Figure 1. East-west velocities for Turkey, primarily from InSAR, modified from Weiss et al. (2020).

From 2-D to 3-D displacements

In early 2022, NASA and the Indian Space Agency will launch a new satellite mission called NISAR. Unlike other radar satellites, it will point to the left side, and hence be sensitive to displacement in a southward-dipping east-west plane. When combined with measurements from right-looking satellites, such as Sentinel-1, this will enable us to extract the full 3-D displacement field. However, there are challenges to overcome. The radio wavelength used by NISAR is longer than that of Sentinel-1 and the noise characteristics are different; the satellites have different sensitivities to phase change during propagation through the ionosphere and to changes in soil moisture and ground scattering characteristics. The measurements are also made at different times, which makes combination non-trivial when the rate of ground motion varies.

Project aims

In this project, the student will develop an optimal approach for combining data from left- and right-looking satellites to estimate 3D deformation. This will be applicable both to steady-state processes, such as tectonic strain build-up, and also for non-steady-state processes, such as earthquakes followed by postseismic slip and relaxation. The student will apply these algorithms to a region with north-south trending strike-slip faults, such as the West-Lut fault in eastern Iran, and estimate the strain rate and seismic hazard for the region. The student will also apply the approach to a recent earthquake to estimate the slip on the associated fault(s), and to constrain the postseismic processes.

References

Weiss, J.R., Walters, R.J., Morishita, Y., Wright, T.J., Lazecky, M., Wang, H., Hussain, E., Hooper, A.J., Elliott, J.R., Rollins, C. and Yu, C., 2020. High‐resolution surface velocities and strain for Anatolia from Sentinel‐1 InSAR and GNSS data. Geophysical Research Letters, 47(17), p.e2020GL087376.