Martin van Driel

The Earthquake‐Source Inversion Validation (SIV) Project

Finite-fault earthquake source inversions infer the (time-dependent) displacement on the rupture surface from geophysical data. The resulting earthquake source models document the complexity of the rupture process. However, multiple source models for the same earthquake, obtained by different research teams, often exhibit remarkable dissimilarities. To address the uncertainties in earthquake-source inversion methods and to understand strengths and weaknesses of the various approaches used, the Source Inversion Validation (SIV) project conducts a set of forward-modeling exercises and inversion benchmarks. In this article, we describe the SIV strategy, the initial benchmarks, and current SIV results. Furthermore, we apply statistical tools for quantitative waveform comparison and for investigating source-model (dis)similarities that enable us to rank the solutions, and to identify particularly promising source inversion approaches. All SIV exercises (with related data and descriptions) and statistical comparison tools are available via an online collaboration platform, and we encourage source modelers to use the SIV benchmarks for developing and testing new methods. We envision that the SIV efforts will lead to new developments for tackling the earthquake-source imaging problem.

Strain rotation coupling and its implications on the measurement of rotational ground motions

Spatial derivatives of the seismic wave field are known to be sensitive to various site effects (e.g., cavity effects, topography, and geological inhomogeneities). In this study, the focus is on strain rotation coupling that can cause significant differences between point measurements compared to array-derived rotational motions. The strain rotation coupling constants are estimated based on finite element simulations for inhomogeneous media as well as for the 3D topography around Wettzell, Germany (the location of the G ring laser). Using collocated array and ring laser data, the coupling constants of the ring laser itself are shown to be small. Several examples are shown to illustrate the order of magnitude that strain-induced rotation might have on the seismograms in the near field of volcanoes as well as in the far field and in the low-frequency spectrum (free oscillations).

On-demand custom broadband synthetic seismograms

ABSTRACT We present a new webservice, Syngine, running at the Incorporated Research Institutions for Seismology Data Management Center (IRIS‐DMC), that offers on‐demand and custom‐tailored seismograms served over HTTP. The free service produces full seismic waveforms, including effects like attenuation and anisotropy, that are calculated in commonly used spherically symmetric Earth models (preliminary reference Earth model [PREM], ak135‐f, IASP91). Users can freely adjust sources and receivers, retrieve seismograms from finite sources, convolve with arbitrary source time functions, and download Green’s functions suitable for moment tensor inversions. Syngine extracts and processes seismograms in as fast as fractions of a second, making it suitable for applications demanding short iteration times and a large number of waveforms. For the first time, researchers without large computational resources or specialized knowledge can easily access high‐quality, custom, broadband seismograms. In this article, we present the rational and basic principles of our method, including its limitations. Additionally, we demonstrate the features of Syngine and the included Earth models, showcase several applications, and discuss future possibilities.

Seismic Wave Propagation in Icy Ocean Worlds

Seismology was developed on Earth and shaped our model of the Earths interior over the 20th century. With the exception of the Philae lander, all in situ extraterrestrial seismological effort to date was limited to other terrestrial planets. All have in common a rigid crust above a solid mantle. The coming years may see the installation of seismometers on Europa, Titan and Enceladus, so it is necessary to adapt seismological concepts to the setting of worlds with global oceans covered in ice. Here we use waveform analyses to identify and classify wave types, developing a lexicon for icy ocean world seismology intended to be useful to both seismologists and planetary scientists. We use results from spectral-element simulations of broadband seismic wavefields to adapt seismological concepts to icy ocean worlds. We present a concise naming scheme for seismic waves and an overview of the features of the seismic wavefield on Europa, Titan, Ganymede and Enceladus. In close connection with geophysical interior models, we analyze simulated seismic measurements of Europa and Titan that might be used to constrain geochemical parameters governing the habitability of a sub-ice ocean.

Efficient Methods in Global Seismic Wave Propagation

Despite the ongoing exponential growth of computational power available on supercomputers and significant advances in numerical seismology in the recent decades, the global 3D seismic forward problem remains a computational challenge. Particularly for high frequency body waves, 3D simulations are prohibitively expensive in many applications. For that reason, approximations with respect to the model or the physics of seismic wave-propagation are commonly used. This thesis deals with a numerical method to simulate seismic wave-propagation within the entire Earth that, by assuming axial symmetry of the structural model, reduces the computational burden by orders of magnitude. The two main components of the efforts presented in this thesis are: 1) theoretical developments of the methods that are partly also applicable to the full 3D problem, 2) their numerical implementation resulting in the release of two community codes. This work enables users to study a broad range of effects in global seismic wave-propagation using the full solution of the wave equation where only approximate solutions were previously available. The theoretical part of this thesis starts with the decomposition of the elastic wave equation into a series of uncoupled equations for its multipole components. While previous mathematical developments of this decomposition where limited to spherically symmetric models in spherical domains, we generalize the proof to general axisymmetric domains and models comprising full triclinic anisotropy and attenuation. By abstracting the problem to symmetry properties of the equation, the contribution of this thesis is directly applicable to other physical problems. The second important theoretical topic is the incorporation of viscoelastic dissipation into time-domain seismic wave-propagation methods with a focus on parameter regime representative for the global case. Since the axisymmetric approach enables the simulation of wave-propagation beyond 1000 wavelengths, special care has to be taken with respect to common physical approximations. This work argues for a criterion to choose material parameters such that the resulting error in the seismograms is minimized. An efficient means of finding these parameters in the case of weak to moderate attenuation in large scale applications is outlined. Furthermore, the ’coarse grained’ memory variable approach is adapted for the spectral element method leading to a speed up of a factor five in the anelastic part of the code and