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Finite-fault source inversion using adjoint methods in 3D heterogeneous media

Somala, Surendra Nadh and Ampuero, Jean-Paul and Lapusta, Nadia (2018) Finite-fault source inversion using adjoint methods in 3D heterogeneous media. Geophysical Journal International, 214 (1). pp. 402-420. ISSN 0956-540X. doi:10.1093/gji/ggy148.

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Accounting for lateral heterogeneities in the 3-D velocity structure of the crust is known to improve earthquake source inversion, compared to results based on 1-D velocity models which are routinely assumed to derive finite-fault slip models. The conventional approach to include known 3-D heterogeneity in source inversion involves pre-computing 3-D Green’s functions, which requires a number of 3-D wave propagation simulations proportional to the number of stations or to the number of fault cells. The computational cost of such an approach is prohibitive for the dense data sets that could be provided by future earthquake observation systems. Here, we propose an adjoint-based optimization technique to invert for the spatio-temporal evolution of slip velocity. The approach does not require pre-computed Green’s functions. The adjoint method provides the gradient of the cost function, which is used to improve the model iteratively employing an iterative gradient-based minimization method. The adjoint approach is shown to be computationally more efficient than the conventional approach based on pre-computed Green’s functions in a broad range of situations. We consider data up to 1 Hz from a Haskell source scenario (a steady pulse-like rupture) on a vertical strike-slip fault embedded in an elastic 3-D heterogeneous velocity model. The velocity model comprises a uniform background and a 3-D stochastic perturbation with the von Karman correlation function. Source inversions based on the 3-D velocity model are performed for two different station configurations, a dense and a sparse network with 1 and 20 km station spacing, respectively. These reference inversions show that our inversion scheme adequately retrieves the rise time when the velocity model is exactly known, and illustrates how dense coverage improves the inference of peak-slip velocities. We investigate the effects of uncertainties in the velocity model by performing source inversions based on an incorrect, homogeneous velocity model. We find that, for velocity uncertainties that have standard deviation and correlation length typical of available 3-D crustal models, the inverted sources can be severely contaminated by spurious features even if the station density is high. When data from thousand or more receivers is used in source inversions in 3-D heterogeneous media, the computational cost of the method proposed in this work is at least two orders of magnitude lower than source inversion based on pre-computed Green’s functions.

Item Type:Article
Related URLs:
URLURL TypeDescription
Ampuero, Jean-Paul0000-0002-4827-7987
Lapusta, Nadia0000-0001-6558-0323
Additional Information:© The Author(s) 2018. Published by Oxford University Press on behalf of The Royal Astronomical Society. Accepted 2018 April 11. Received 2017 November 22; in original form 2018 April 4. Published: 12 April 2018.
Group:Seismological Laboratory
Subject Keywords:Inverse theory; Numerical modelling; 3D Waveform inversion; Computational seismology; 3D Wave propagation
Issue or Number:1
Record Number:CaltechAUTHORS:20180418-152250720
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Official Citation:Surendra Nadh Somala, Jean-Paul Ampuero, Nadia Lapusta; Finite-fault source inversion using adjoint methods in 3-D heterogeneous media, Geophysical Journal International, Volume 214, Issue 1, 1 July 2018, Pages 402–420,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:85950
Deposited By: George Porter
Deposited On:18 Apr 2018 22:32
Last Modified:15 Nov 2021 20:34

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