Effects of three‐dimensional crustal structure and smoothing constraint on earthquake slip inversions: Case study of the Mw6.3 2009 L'Aquila earthquake

Abstract

Earthquake slip inversions aiming to retrieve kinematic rupture characteristics typically assume 1-D velocity models and a flat Earth surface. However, heterogeneous nature of the crust and presence of rough topography lead to seismic scattering and other wave propagation phenomena, introducing complex 3-D effects on ground motions. Here we investigate how the use of imprecise Green's functions-achieved by including 3-D velocity perturbations and topography-affect slip-inversion results. We create sets of synthetic seismograms, including 3-D heterogeneous Earth structure and topography, and then invert these synthetics using Green's functions computed for a horizontally layered 1-D Earth model. We apply a linear inversion, regularized by smoothing and positivity constraint, and examine in detail how smoothing effects perturb the solution. Among others, our tests and resolution analyses demonstrate how imprecise Green's functions introduce artificial slip rate multiples especially at shallow depths and that the timing of the peak slip rate is hardly affected by the chosen smoothing. The investigation is extended to recordings of the 2009 Mw6.3 L'Aquila earthquake, considering both strong motion and high-rate GPS stations. We interpret the inversion results taking into account the lessons learned from the synthetic tests. The retrieved slip model resembles previously published solutions using geodetic data, showing a large-slip asperity southeast of the hypocenter. In agreement with other studies, we find evidence for fast but subshear rupture propagation in updip direction, followed by a delayed propagation along strike. We conjecture that rupture was partially inhibited by a deep localized velocity-strengthening patch that subsequently experienced afterslip.

Rupture process of the 2014 Cephalonia, Greece, earthquake doublet (Mw6) as inferred from regional and local seismic data

“…To obtain the model of rupture propagation along the fault plane we adopted the linear slip inversion method of Gallovič et al (2015). It has been extended, for the purpose of the present work, to perform the inversion simultaneously on multiple fault segments.…”

A New Strategy to Compare Inverted Rupture Models Exploiting the Eigenstructure of the Inverse Problem

“…We perform the first tests on the inversion results obtained with the method of Gallovič et al (2015), a linear-inversion method with multitime window parameterization, unconstrained rupture speed and rise time, constrained total rupture duration (12 s or less), and two regularization constraints: a k −2 slip covariance function (k being the wavenumber) and positivity of slip rate (see also Sokos et al, 2015). The degree of smoothing is controlled by a parameter σ D defined by Gallovič et al (2015).…”

“…The degree of smoothing is controlled by a parameter σ D defined by Gallovič et al (2015). A value of σ D 0:1 m is adequate for realdata applications with imperfect GFs, but in this controlled experiment we use weaker smoothing with σ D 0:01 m. Exact GFs are used in this test: we apply the inversion procedure to data evaluated using our own version of the target model and our own computed GFs (the disseminated waveforms are not used at this point).…”

“…We now consider a case in which the GFs used in the forward and inverse modeling are not identical: we invert the disseminated benchmark data using the method of Gallovič et al (2015) based on our GFs. The obtained model is denoted as Gallovic0.01 (see Ⓔ Table S1).…”

“…We consider two additional crustal models: a halfspace and a smooth 3D tomographic velocity model by Di Stefano et al (2011). Details on the velocity model and the GF calculations are provided by Gallovič et al (2015). In both cases, the conditioning of G worsens, but it is more severe in the former case.…”