Emanuele Casarotti

CUBIT and seismic wave propagation based upon the Spectral-Element Method: An advanced unstructured mesher for complex 3D geological media

Unstructured hexahedral mesh generation is a critical part of the modeling process in the Spectral-Element Method (SEM). We present some examples of seismic wave propagation in complex geological models, automatically meshed on a parallel machine based upon CUBIT (Sandia Laboratory), an advanced 3D unstructured hexahedral mesh generator that offers new opportunities for seismologist to design, assess, and improve the quality of a mesh in terms of both geometrical and numerical accuracy. The main goal is to provide useful tools for understanding seismic phenomena due to surface topography and subsurface structures such as low wave-speed sedimentary basins. Our examples cover several typical geophysical problems: 1) “layer-cake” volumes with high-resolution topography and complex solid-solid interfaces (such as the Campi Flegrei Caldera Area in Italy), and 2) models with an embedded sedimentary basin (such as the Taipei basin in Taiwan or the Grenoble Valley in France).

Global postseismic stress diffusion and fault interaction at long distances

Abstract By means of a new theoretical model of global postseismic deformation we compute the time-depending postseismic stress field associated with eight of the greatest events of the century on an area extending for almost half of the Earth’s surface. We evaluate the stress transferred by these big earthquakes to all the seismogenic structures of the Pacific belt that have generated earthquakes with M ≥5 in the last years. We discuss the effect of this stress field on the state of the faults: the distribution of favoured and not favoured events is not uniform. The modeling suggests the existence of a physical connection among the patterns of release of seismic moment by the different plate margins of the Pacific area and the possibility of a self-organised geometrical configuration of the tectonic plates system.

New insights on long distance fault interaction

Abstract We investigated the plausibility of remote fault triggering on a global scale as a result of postseismic stress transfer by large earthquakes. Previous studies have shown that the postseismic stress field generated by eight of the largest events that have occurred in the Pacific area promoted the rupture of 53.6% of all the M≥5 events recorded in the last century in the circumpacific ring. We tried to assess the statistical significance of this result by performing a set of new statistical simulations involving very intensive computational tasks. To this aim we implemented a new numerical procedure based on parallel codes. We found that our simulations did not show strong statistical evidence, but there was a clear indication that as we applied more realistic geometrical constraints to the synthetic distributions, we tended to reproduce more closely the observed quantities. These results support the hypothesis of some kind of physical connection of the configuration of a plate margin and its activity with those of all the other plate margins, more than the possibility of remote fault interaction in the ‘classical’ sense.

VERCE Delivers a Productive E-science Environment for Seismology Research

The VERCE project has pioneered an e-Infrastructure to support researchers using established simulation codes on high-performance computers in conjunction with multiple sources of observational data. This is accessed and organised via the VERCE science gateway that makes it convenient for seismologists to use these resources from any location via the Internet. Their data handling is made flexible and scalable by two Python libraries, ObsPy and dispel4py and by data services delivered by ORFEUS and EUDAT. Provenance driven tools enable rapid exploration of results and of the relationships between data, which accelerates understanding and method improvement. These powerful facilities are integrated and draw on many other e-Infrastructures. This paper presents the motivation for building such systems, it reviews how solid-Earth scientists can make significant research progress using them and explains the architecture and mechanisms that make their construction and operation achievable. We conclude with a summary of the achievements to date and identify the crucial steps needed to extend the capabilities for seismologists, for solid-Earth scientists and for similar disciplines.

Complex Fault Geometry and Rupture Dynamics of the MW 6.5, 30 October 2016, Central Italy Earthquake

We study the October 30th 2016 Norcia earthquake (MW 6.5) to retrieve the rupture history by jointly inverting seismograms and coseismic GPS displacements obtained by dense local networks. The adopted fault geometry consists of a main normal fault striking N155°and dipping 47° belonging to the Mt. Vettore-Mt. Bove fault system (VBFS) and a secondary fault plane striking N210° and dipping 36° to the NW. The coseismic rupture initiated on the VBFS and propagated with similar rupture velocity on both fault planes. Up-dip from the nucleation point, two main slip patches have been imaged on these fault segments, both characterized by similar peak-slip values (~3 m) and rupture times (~3 s). After the breakage of the two main slip patches, coseismic rupture further propagated southeastward along the VBFS, rupturing again the same fault portion that slipped during the August 24th earthquake. The retrieved coseismic slip distribution is consistent with the observed surface breakages and the deformation pattern inferred from InSAR measurements. Our results show that three different fault systems were activated during the October 30th earthquake. The composite rupture model inferred in this study provides evidences that also a deep portion of the NNE-trending section of the Olevano-Antrodoco-Sibillini (OAS) thrust broke co-seismically, implying the kinematic inversion of a thrust ramp. The obtained rupture history indicates that, in this sector of the Apennines, compressional structures inherited from past tectonics can alternatively segment boundaries of NW-trending active normal faults or break co-seismically during moderate-to-large magnitude earthquakes.

Rapid Estimation of Damage to Tall Buildings Using Near Real‐Time Earthquake and Archived Structural Simulations

This article outlines a new approach to rapidly estimate the damage to tall buildings immediately following a large earthquake. The preevent groundwork involves the creation of a database of structural responses to a suite of idealized ground‐motion waveforms. The postevent action involves (1) rapid generation of an earthquake source model, (2) near real‐time simulation of the earthquake using a regional spectral‐element model of the earth and computing synthetic seismograms at tall building sites, and (3) estimation of tall building response (and damage) by determining the best‐fitting idealized waveforms to the synthetically generated ground motion at the site and directly extracting structural response metrics from the database. Here, ground‐velocity waveforms are parameterized using sawtoothlike wave trains with a characteristic period (T), amplitude (peak ground velocity, PGV), and duration (number of cycles, N). The proof‐of‐concept is established using the case study of one tall building model. Nonlinear analyses are performed on the model subjected to the idealized wave trains, with T varying from 0.5 s to 6.0 s, PGV varying from 0.125  m/s, and N varying from 1 to 5. Databases of peak transient and residual interstory drift ratios (IDR), and permanent roof drift are created. We demonstrate the effectiveness of the rapid response approach by applying it to synthetic waveforms from a simulated 1857‐like magnitude 7.9 San Andreas earthquake. The peak IDR, a key measure of structural performance, is predicted well enough for emergency response decision making.

Towards Addressing CPU-Intensive Seismological Applications in Europe

Advanced application environments for seismic analysis help geoscientists to execute complex simulations to predict the behaviour of a geophysical system and potential surface observations. At the same time data collected from seismic stations must be processed comparing recorded signals with predictions. The EU-funded project VERCE ( http://verce.eu/ ) aims to enable specific seismological use-cases and, on the basis of requirements elicited from the seismology community, provide a service-oriented infrastructure to deal with such challenges. In this paper we present VERCE’s architecture, in particular relating to forward and inverse modelling of Earth models and how the, largely file-based, HPC model can be combined with data streaming operations to enhance the scalability of experiments. We posit that the integration of services and HPC resources in an open, collaborative environment is an essential medium for the advancement of sciences of critical importance, such as seismology.