Abstract
Finite-fault models for the 2010Mw8.8 Maule, Chile earthquake indicate bilateral rupture with large-slip patches located north and south of the epicenter. Previous studies also show that this event features significant slip in the shallow part of the megathrust, which is revealed through correction of the forward tsunami modeling scheme used in tsunami inversions. The presence of shallow slip is consistent with the coseismic seafloor deformation measured off the Maule region adjacent to the trench and confirms that tsunami observations are particularly important for constraining far-offshore slip. Here, we benchmark the method of Optimal Time Alignment (OTA) of the tsunami waveforms in the joint inversion of tsunami (DART and tide-gauges) and geodetic (GPS, InSAR, land-leveling) observations for this event. We test the application of OTA to the tsunami Green’s functions used in a previous inversion. Through a suite of synthetic tests we show that if the bias in the forward model is comprised only of delays in the tsunami signals, the OTA can correct them precisely, independently of the sensors (DART or coastal tide-gauges) and, to the first-order, of the bathymetric model used. The same suite of experiments is repeated for the real case of the 2010 Maule earthquake where, despite the results of the synthetic tests, DARTs are shown to outperform tide-gauges. This gives an indication of the relative weights to be assigned when jointly inverting the two types of data. Moreover, we show that using OTA is preferable to subjectively correcting possible time mismatch of the tsunami waveforms. The results for the source model of the Maule earthquake show that using just the first-order modeling correction introduced by OTA confirms the bilateral rupture pattern around the epicenter, and, most importantly, shifts the inferred northern patch of slip to a shallower position consistent with the slip models obtained by applying more complex physics-based corrections to the tsunami waveforms. This is confirmed by a slip model refined by inverting geodetic and tsunami data complemented with a denser distribution of GPS data nearby the source area. The models obtained with the OTA method are finally benchmarked against the observed seafloor deformation off the Maule region. We find that all of the models using the OTA well predict this offshore coseismic deformation, thus overall, this benchmarking of the OTA method can be considered successful.
Highlights
The February 27, 2010 Maule (Chile) Mw 8.8 earthquake is the third largest seismic event this century and it produced an extensive seismic sequence (e.g., Hayes et al, 2013)
We investigate the effect of Optimal Time Alignment (OTA) in the joint inversion using DART and tide-gauge tsunami waveforms separately to assess their respective resolving power on the slip distribution, and, as a consequence, optimize their relative weights for use during joint inversion
An additional sensitivity test is performed using an alternative set of tsunami Green’s functions computed with a bathymetric model around the tide-gauges with a lower spatial resolution (2 arc-min). This is tested against a checkerboard target model where the tsunami waveforms at the tide-gauges are produced using a bathymetry with a 0.5 arc-min spatial resolution; in this way, we do not add random delay to the tide-gauges because it is assumed to be intrinsic in the change of bathymetry, whereas we assume physics-like delays to the DARTs
Summary
The February 27, 2010 Maule (Chile) Mw 8.8 earthquake is the third largest seismic event this century and it produced an extensive seismic sequence (e.g., Hayes et al, 2013). Numerous studies have investigated the Maule earthquake rupture by inverting seismic (e.g., Lay et al, 2010; Koper et al, 2012; Hayes et al, 2013); geodetic (e.g., Tong et al, 2010; Pollitz et al, 2011; Vigny et al, 2011; Moreno et al, 2012); tsunami (Yoshimoto et al, 2016); joint seismic and geodetic (e.g., Delouis et al, 2010; Lin et al, 2013); joint geodetic and tsunami data (e.g., Lorito et al, 2011; Fujii and Satake, 2013; Yoshimoto et al, 2016); and joint seismic, geodetic and tsunami data (Yue et al, 2014) These studies all find along-strike bilateral rupture from the epicenter, but vary significantly in the along-dip placement of slip on the fault, with most models that emphasize geodetic data having little slip far-offshore. Seismic wave models sometimes resolve offshore slip (e.g., Lay, 2018), but lacking seafloor geodetic observations, coseismic slip in the shallow region of the megathrust is usually best-constrained by tsunami data (e.g., Lorito et al, 2016) provided that accurate bathymetric models are available
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