Mapping the coupling properties of the subduction megathrust using seismic data

Summary: The properties of the subduction interface between plates vary in both space and time. In this project, you will develop techniques to map the properties of the subduction interface using remote observations of small-amplitude reflected seismic waves from intraplate earthquakes within the downgoing plate, which can be applied to subduction zones irrespective of local instrumentation, and in areas without the sub-aerial exposure needed for modern geodesy. This will allow us to probe the controls on spatio-temporal variations in the type of slip, in the presence of fluids along the subduction interface, and lead to an appraisal of hazard emanating from these subduction zones.

Full Description: The plate interfaces of subduction zones generate the largest, most damaging earthquakes on Earth. However, along with major earthquakes, the plate interface can also accommodate the motion between plates through stable sliding or through periods of enhanced aseismic “slow slip”. Understanding the properties of this subduction interface, and how these may vary in both space and time is critical for understanding its behaviour, and its potential to produce large earthquakes in the future. Classically, the behaviour of the subduction interface has been characterised using geodetic techniques, which measure the motion of the Earth’s surface above the plate interface, and use this to infer the slip behaviour of the interface. Established geodetic techniques are limited to terrestrial environments, limiting their resolving power in most submarine subduction zones, and leaving them unable to probe the behaviours of completely submarine subduction zones (e.g., Tonga-Kermadec, Bonin-Marianas). Sub-sea geodesy is in its infancy, and its prohibitively expensive, requiring extensive seafloor logistics and support.

However, remote seismic data offer a way to map the rheological properties of the subduction interface, without the need for extensive near-field instrumentation (e.g., Song et al., 2010). Using well-located earthquakes occurring within the downgoing plate beneath the subduction interface, detailed waveform analysis suggests that it is possible to detect small-amplitude reflections and conversions from the overlying interface, allowing its elastic properties to be determined, and from these, its slip behaviour to be inferred.

Figure 1: Sketch illustrating the generation of teleseismically-observed reflections from the subduction interface.

In this project, you will develop a set of seismological processing routines optimised for detecting these small- amplitude phases within the coda of off-interface earthquakes. This project will focus on technical development, and the construction of a test dataset, to span the Central American subduction zone. Central America is ideal for this technical development phase, as it is also geodetically well-instrumented, and known to display a range of interface behaviours. This will allow you to compare seismological results for the reflectivity of the subduction interface with other observational proxies, and allow you to determine what it is that a high-reflectivity subduction interface indicates – whether this is a proxy for interface coupling, or for the transient presence of fluids and/or slow slip, and thus what can be inferred about the slip behaviour of the interface using remote data only.

Following this, the study will expand to encompass other regions, starting the northern New Zealand/Hikurangi, where a similar level of local instrumentation will allow the verification of our conclusions from Central America, before moving to application of submarine subduction zones, starting with a northwards progression from Hikurangi up towards Tonga.

References:

• Song et al., (2009).  Subducting Slab Ultra-Slow Velocity Layer Coincident with Silent Earthquakes in Southern Mexico, Science, v324, pp502+506, doi:10.1126/science.1165795.
• Radiguet et al. ,(2011).  Spatial and temporal evolution of a long term slow slip event: the 2006 Guerrero Slow Slip Event, Geophysical Journal International, v184, doi:10.1111/j.1365-246X.2010.04866.
• Metois, Vigny, and Socquet (2016).  Interseismic Coupling, Megathrust Earthquakes and Seismic Swarms Along the Chilean Subduction Zone (38^o – 18^oS), Pure and Applied Geophysics, v173, pp1431-1449.
• Audet and Burgmann (2014).  Possible control of subduction zone slow-earthquake periodicity by silica enrichment, Nature, v510, pp389-392.