An Adjoint Sensitivity Method Applied to Time Reverse Imaging of Tsunami Source for the 2009 Samoa Earthquake

Abstract

We have previously developed a tsunami source inversion method based on “Time Reverse Imaging” and demonstrated that it is computationally very efficient and has the ability to reproduce the tsunami source model with good accuracy using tsunami data of the 2011 Tohoku earthquake tsunami. In this paper, we implemented this approach in the 2009 Samoa earthquake tsunami triggered by a doublet earthquake consisting of both normal and thrust faulting. Our result showed that the method is quite capable of recovering the source model associated with normal and thrust faulting. We found that the inversion result is highly sensitive to some stations that must be removed from the inversion. We applied an adjoint sensitivity method to find the optimal set of stations in order to estimate a realistic source model. We found that the inversion result is improved significantly once the optimal set of stations is used. In addition, from the reconstructed source model we estimated the slip distribution of the fault from which we successfully determined the dipping orientation of the fault plane for the normal fault earthquake. Our result suggests that the fault plane dip toward the northeast.

Resolving Complicated Faulting Process Using Multi‐Point‐Source Representation: Iterative Inversion Algorithm Improvement and Application to Recent Complex Earthquakes

“…Duputel, Kanamori, et al () performed a two‐subevent W ‐phase MPS solution for this event (see the Supplement in their paper), also finding a thrust event with M W 8.1 located to the southwest of the normal faulting. Hossen et al () performed adjoint time reverse imaging of the tsunami source, obtaining similar results. Additional modeling of long‐period waves performed by Fan et al () is also consistent with this compound faulting, while their rather ambitious interpretation of short‐period BPs suggests a complex connection between the outer rise faulting and the megathrust.…”

Effects of Source Faulting and Fringing Reefs on the 2009 South Pacific Tsunami Inundation in Southeast Upolu, Samoa

“…In the aftermath of the event, extensive investigation of the tsunami source and wave characteristics were undertaken based on seismic and GPS records, field surveys, and numerical modeling (e.g., Beavan et al., 2010; Dilmen et al., 2018; Fan et al., 2016; Goff & Dominey‐Howes, 2011; Han et al., 2019; Hossen et al., 2017; Lay et al., 2010; E. A. Okal et al., 2010; Roeber et al., 2010; Williams, 2019; Wood et al., 2018; Yamazaki et al., 2011; Zhou et al., 2012). Previous efforts to reconstruct tsunami characteristics in Samoa were limited to simulating wave initiation and propagation up to the reef edge (e.g., Franchello & Annunziato, 2012; Fritz et al., 2011; E. Okal et al., 2011; Zhou et al., 2012).…”

“…In the aftermath of the event, extensive investigation of the tsunami source and wave characteristics were undertaken based on seismic and GPS records, field surveys, and numerical modeling (e.g., Beavan et al, 2010;Dilmen et al, 2018;Fan et al, 2016;Goff & Dominey-Howes, 2011;Han et al, 2019;Hossen et al, 2017;Lay et al, 2010;E. A. Okal et al, 2010;Roeber et al, 2010;Williams, 2019;Wood et al, 2018;Yamazaki et al, 2011;Zhou et al, 2012).…”

“…We assess whether current understanding of the rupture mechanism and propagation explains the extreme runup and flow depths observed in this region. We test rupture source mechanisms compiled by Fan et al (2016), Hossen et al (2017), and Han et al (2019) against the Deep Ocean Assessment and Reporting of Tsunami (DART) data closest to the source, Apia tide gauge measurements, and runup measurements from post-disaster surveys (e.g., Dudley et al, 2011;E. A. Okal et al, 2010Reese et al, 2011).…”

Adaptive Tsunami Source Inversion Using Optimizations and the Reciprocity Principle

“…Consequently, the tsunami propagation model should be run multiple times until the expected solution is achieved. To overcome those difficulties, Gusman et al () and Hossen et al () proposed a two‐step tsunami source inversion method, in which the initial sea surface displacement is first estimated from observed tsunami waveforms, then the slip distribution is inferred from the estimated sea surface displacement. In this case, the Green's functions in the second step can be developed simply by using Okada's model (Okada, ), without generating synthetic tsunami waveforms.…”