Modeling long-term tectonic deformation recorded by corals
Our GPS networks are providing a wealth of information about tectonic processes, but they only cover a decade of observations. This is a small snapshot in time when our goal is to understand multiple earthquake cycles. We are therefore working in collaboration with Aron Meltzner, Belle Philibosian and Kerry Sieh to extend our investigations back in time with the help of data from coral microatolls. These corals preserve information on the height of relative sea level over their lifetimes, and thus the height of the land.
PhD student Louisa Tsang has been working on modeling the coral data. Here is a glimpse of her two recent papers:
A 15-year slow slip event on the Sunda megathrust
At the Banyak Islands of Sumatra, coral microatolls recorded a 15-year reversal of interseismic vertical displacement between 1966 and 1981. At this location interseismic deformation results in subsidence, but for this period the corals recorded uplift. Louisa's results show that these observations likely reflect a very long duration slow-slip event (SSE). SSEs release strain on tectonic faults at slip rates faster than plate convergence rates, but slowly enough that seismic shaking is not generated. This 15-year event is the longest SSE recorded so far.
Tsang, L. L. H., A. J. Meltzner, B. Philibosian, E. M. Hill, J. T. Freymueller, and K. Sieh;
A 15-year slow slip event on the Sunda megathrust offshore Sumatra; Geophys. Res. Lett., 42, doi:10.1002/201GL064928, 2015.
(Click to link to an Open Access copy of the paper.)
Time-varying interseismic rates on the Sunda megathrust
An ~1100-year-long paleogeodetic record of land-height change along the Simeulue section of the Sumatra subduction zone reveals significant variations in vertical motion rates. Louisa examined the data for a ~267-year-long segment of these data, and developed models to explain rate variations in the decades before the 1861, 2005 and 2005 great Sumatran earthquakes. One explanation for the rate changes is a significant change in the depth of interseismic coupling on the Sunda megathrust. Rates accelerated by a factor of 4-10x in the decades before the 1861 earthquake, and were different prior to the 1861 and 2005 earthquakes. These results highlight the need to treat coupling maps derived from GPS data as only reflecting a small snapshot of fault conditions that are spatially variable.