Systematic investigation of postseismic slip, from laboratory scale to satellite’s-eye view

Overview

In the hours to years following the seismic rupture of a fault, slow aseismic slip commonly occurs on the same fault or surrounding structures. For the first few months after moderate-small magnitude earthquakes, this ‘afterslip’ is commonly the most important of a host of different postseismic deformation mechanisms.

Measurements of the ratio of postseismic slip to earthquake slip magnitude, and the spatial relationship between the two types of slip, are critical both for understanding the mechanism of afterslip and for estimating how seismic hazard evolves after earthquakes. In addition, afterslip often probes the rheology of the region where seismic rupture halts, knowledge of which is essential for constraining models of the earthquake rupture process.

Recent studies have suggested that the ratio of afterslip to seismic slip depends on earthquake magnitude, with larger earthquakes experiencing relatively small afterslip, and small earthquakes experiencing relatively large afterslip (e.g. Fig. 1, Alwahedi & Hawthorne, 2019). However, it is not clear why this apparently simple relationship exists, or even if it is real; the trend could be an artefact due to sampling biases across a range of different earthquake magnitudes, depths and fault types.

This project aims to address these issues using a combination of satellite radar data, borehole strainmeter data, and measurements of laboratory earthquakes to investigate the relationship between coseismic and postseismic slip in earthquakes across a huge range of scales, from Mw -2 to Mw 7.

Satellite radar (InSAR) will be used to measure and model co- and postseismic ground deformation associated with earthquakes of Mw 5.5-7, borehole strainmeter observations will sample earthquakes of Mw 2-5, whilst measurements of laboratory-generated earthquakes will extend the sampled magnitude range down to ~Mw -2.

These complementary datasets will augment the existing data shown in Fig. 1 by filling in gaps and addressing sampling biases. Finally, joint analysis of the full augmented dataset will enable testing of data against competing afterslip models, along with estimation of how depth and fault type may affect the results.

Click on an image to expand

Image Captions

iapetus_fig1_walters.png: “Figure 1: Previous studies suggest that smaller earthquakes have more afterslip, but does the change in afterslip magnitude reflect fault type, earthquake dynamics, or the depth of afterslip? Data from 30 studies, listed by Alwahedi and Hawthorne (2019).”

iapetus_fig2_walters.png: “Figure 2: Temporal evolution of afterslip following the 2014 S. Napa earthquake in California (blue, green and red boxes and time-series), and comparison to the region of seismic slip (pink polygon), as estimated by satellite radar. Adapted from Floyd et al. (2016).”

Methodology

For several tens of Mw > 5.5 earthquakes since 2014, interferometric SAR (InSAR) will be used to analyse deformation signals over the epicentral region during the event and over the following 6 to 12 months . Atmospheric correction techniques will be used to mitigate the influence of atmospheric noise on the data and time-series inversion methods will be used to separate coseismic from postseismic slip (e.g. Fig 2, Floyd et al., 2016). We will analyse the postseismic period following each earthquake individually, as well as stacking normalised postseismic slip for multiple events to increase the signal-to-noise ratio.

The student will work with Dr. Jessica Hawthorne at the University of Oxford on analysis of borehole strainmeter data of much smaller earthquakes at one or more select sites, to supplement previous analyses (e.g. Alwahedi & Hawthorne, 2019).

Finally, coseismic and postseismic slip will be measured using strain gauges and piezo-electric sensors, on laboratory experimental faults under a range of conditions, including faults incorporating heterogeneity in initial stress and rheology.

Project Timeline

Year 1

Training will be provided in space geodesy techniques, in particular the processing and modelling of satellite radar data and in analysis of data from lab experimental earthquakes. Processing and analysis of radar data for selected earthquakes.

Year 2

Completion of radar data processing/analysis, and continuation of laboratory experiments. The work from Years 1 & 2 should lead to at least one publication.

Year 3

Strainmeter analysis, and completion of laboratory experiments. Analysis of completed catalogue of coseismic/postseismic deformation events. Comparison and modelling of remote sensing, strainmeter and laboratory results. This work should lead to an additional publication.

Year 3.5

Focus on combining the published outputs and associated material into a PhD thesis.

Training
& Skills

The student will receive training in space geodesy techniques, in particular the handling of satellite radar data, in modelling co- and postseismic deformation, and in setting up and running laboratory simulations of earthquakes and postseismic slip. Training in a wide range of essential skills (e.g. presentation skills, paper/thesis writing, and enterprise skills) important both for life as a PhD student and afterwards is provided by the Department of Earth Sciences at Durham University, and the student will also benefit from cross-disciplinary training provided as part of IAPETUS2.
The student will become a member of the UK’s Centre for the Observation and Modelling of Earthquakes, Volcanoes and Tectonics (COMET), benefitting from the shared expertise of Geosciences staff in several universities, and attending regular meetings where the research of these various groups is discussed.
The student will have opportunities to work with other partners in the UK and internationally and they are encouraged to travel to national and international scientific meetings to present results. We aim to see all students publish at least two papers in leading scientific journals during their PhD. Upon completion, the student will be well equipped for a career in academia or in a range of industries.

References & further reading

Floyd, M.A., Walters, R.J., Elliott, J.R., Funning, G.J., et al., 2016. Spatial variations in fault friction related to lithology from rupture and afterslip of the 2014 South Napa, California, earthquake. GRL, 43(13)
Alwahedi, M.A. and Hawthorne, J.C., 2019. Intermediateâ€magnitude postseismic slip follows intermediateâ€magnitude (M 4 to 5) earthquakes in California. GRL, 46(7)
Harbord, C.W.A., Nielsen S., De Paola N., Holdsworth, R.E., 2017. Earthquake nucleation on rough faults. Geology 45 (10)

Further Information

Dr Richard Walters
richard.walters@durham.ac.uk,
+44(0) 1913 341727

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