A New Adjoint-state Full Waveform Tsunami Source Imaging Method

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

Tsunamis are oceanic gravity waves generated by the displacement of a large volume of water in the ocean, typically resulting from complex geophysical processes including shallow earthquakes that break the oceanic crust and seafloor landslides. Tsunamis can cause large number of casualties and economic loss in coastal regions, which are home to over a third of the world population. Tsunami warning is thus a crucial system to reduce and mitigate tsunami hazards. In order to perform a more rapid and accurate tsunami early warning as well as better understanding the complexity of tsunami source, the investigators propose to apply the adjoint-state method to solve for the initial seafloor deformation of the tsunami source. The method improves the imaging resolution of the earthquake source process and produces little artifacts with a relatively small computational cost. This project takes full advantage of increasing tsunami instrumentations including ocean-bottom pressure sensors and coastal tide gauges and supports the Ph. D work of a female student. The research results are being shared via conferences and journals, as well as outreach to public schools and in undergraduate classes. Traditional source inversion using tsunamis waves is based on either the finite-fault slip modeling or the time-reversal imaging. Such inversion methods suffer from the uncertainty of fault parameters or crustal rigidity. Moreover, the heavy computational burden of calculating Green's functions result in limited spatial resolution and hinders the real-time applicability of the traditional methods to tsunami early warning. In this work, we transplant the state-of-art adjoint-state full-waveform inversion method from exploration seismology to tsunami source imaging. The adjoint-state method solves the initial-water-elevation pattern with less computational cost, which potentially can improve the speed of tsunami early warning and reduces the blind zone. Our new method does not rely on pre-defined fault parameters and is suitable for tsunami-generating earthquakes with unknown fault geometry. This method also efficiently handles dense mesh grid and is capable of resolving small-scale secondary tsunami sources, such as the seafloor landslide or secondary ruptures on splay faults. Our results will advance tsunami science and earthquake source dynamics and set the stage to improve real-time applications such as tsunami early warning. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.