Применение онтологий для решения задач геофизики

The paper covers an intelligent support system that allows to describe and construct solutions to various scientific problems. In this study, in particular, we consider geophysical problems. This system is being developed at the Institute of Computational Mathematics and Mathematical Geophysics of the Russian Academy of Sciences (ICMMG SB RAS) and Institute of Informatics System of the Russian Academy of Sciences (IIS SB RAS). The system contains a knowledge base, the core of which is a set of several interconnected ontologies such as the ontology of supercomputer architectures, the ontology of algorithms and methods. Ontology can be viewed as a set of concepts and how those concepts are linked. As the result, the authors present an ontological description of two geophysical problems via the means of the intelligent support system: 1) the seismic wavefield simulation and 2) the reconstruction of a seismic image through pre-stack time or depth migration. For a better visual understanding of the system described and the results obtained, the paper also contains several schematic diagrams and images. В статье рассматривается система интеллектуальной поддержки, позволяющая описывать и выстраивать решения различных научных задач. В данной работе рассматриваются геофизические задачи. Система разрабатывается в Институте вычислительной математики и математической геофизики Российской академии наук (ИВМГ СО РАН) и Институте систем информатики Российской академии наук (ИИС СО РАН). Система содержит базу знаний, ядром которой является набор из нескольких взаимосвязанных онтологий, таких как онтология суперкомпьютерных архитектур, онтология алгоритмов и методов. Онтологию можно рассматривать как набор концепций и связей между ними. В результате авторы представляют онтологическое описание двух геофизических задач с помощью средств системы интеллектуальной поддержки: 1) моделирование сейсмического волнового поля и 2) реконструкция сейсмического изображения посредством временной или глубинной миграции до суммирования. Для лучшего визуального понимания описанной системы и полученных результатов в работе также есть несколько схематических диаграмм и изображений.

Assessment of seismic tomographic models of the contiguous United States using intermediate-period 3D wavefield simulation

Summary The contiguous United States has been well instrumented with broadband seismic stations due to the development of the EarthScope Transportable Array. Previous studies have provided various 3D seismic wave speed models for the crust and upper mantle with improved resolution. However, discrepancies exist among these models due to differences in both data sets and tomographic methods, which introduce uncertainties on the imaged lithospheic structure beneath North America. A further model refinement using the best data coverage and advanced tomographic methods such as full-waveform inversion (FWI) is expected to provide better seismological constraints. Initial models have significant impacts on the convergence of FWIs. However, how to select an optimal initial model is not well investigated. Here, we present a data-driven initial model selection procedure for the contiguous US and surrounding regions by assessing waveform fitting and misfit functions between the observations and synthetics from candidate models. We use a data set of waveforms from 30 earthquakes recorded by 5,820 stations across North America. The results suggest that the tested 3D models capture well long-period waveforms while showing discrepancies in short-periods especially on tangential components. This observation indicates that the smaller-scale heterogeneities and radial anisotropy in the crust and upper mantle are not well constrained. Based on our test results, a hybrid initial model combining S40RTS or S362ANI in the mantle and US.2016 for Vsv and CRUST1.0 for Vsh in the crust is compatible for future FWIs to refine the lithospheric structure of North America.

Tectonic tremors in the Northern Mexican subduction zone remotely triggered by the 2017 Mw8.2 Tehuantepec earthquake

Surface waves from the 2017 Mw8.2 Tehuantepec earthquake remotely triggered tectonic tremors in the Jalisco region, approximately 1000 km WNW in the northern Mexican subduction zone. This is the first observation of tremor triggering in this region and one of the largest known examples of a triggered tremor in the world. Although prior studies have found tectonic tremors triggered by teleseismic waves in subduction zones and plate boundaries, further investigation of tremor triggering is crucially important for understanding the causative mechanism. We calculate the stress and strain changes across the three-dimensional plate interface attributable to seismic waves from the earthquake by full wavefield simulation. The maximum magnitude of the dynamic strain tensor eigenvalues on the plate interface, where tremors likely occur, is approximately 10–6. The subducting slab geometry effectively amplifies triggering waves. The triggering Coulomb failure stress changes resolved for a thrust fault plane consistent with the geometry are estimated to be approximately 10–40 kPa. The relationship between the triggering stress and triggered tremor amplitude may indicate that the aσ of the rate–state-dependent friction law is 10–100 kPa.

Tectonic Tremors in the Northern Mexican Subduction Zone Remotely Triggered by the 2017 Mw8.2 Tehuantepec Earthquake

Surface waves from the 2017 Mw8.2 Tehuantepec earthquake remotely triggered tectonic tremors in the Jalisco region, approximately 1000 km WNW in the northern Mexican subduction zone. This is the first observation of tremor triggering in this region and one of the largest known examples of a triggered tremor in the world. Although prior studies have found tectonic tremors triggered by teleseismic waves in subduction zones and plate boundaries, further investigation of tremor triggering is crucially important for understanding the causative mechanism. We calculate the stress and strain changes across the three-dimensional plate interface attributable to seismic waves from the earthquake by full wavefield simulation. The maximum magnitude of the dynamic strain tensor eigenvalues on the plate interface, where tremors likely occur, is approximately 10 -6 . The subducting slab geometry effectively amplifies triggering waves. The triggering Coulomb failure stress changes resolved for a thrust fault plane consistent with the geometry are estimated to be approximately 10-40 kPa. The relationship between the triggering stress and triggered tremor amplitude may indicate that the [[EQUATION]] of the rate-state-dependent friction law is 10 to 100 kPa.

Full wavefield simulation versus measurement of microwave scattering by a complex 3D-printed asteroid analogue

Context. The small bodies of the Solar System, and especially their internal structures, are still not well-known. Studies of the interior of comets and asteroids could provide important information about their formation and also about the early Solar System. Aims. In this paper, we investigate the possibility of obtaining information about their inner structure from their response to an incident electromagnetic field in preparation for future space radar missions. Our focus is on experimental measurements concerning two analog models with the shape of 25143 Itokawa, a small rubble pile asteroid monitored by the Japanese space agency’s (JAXA) Hayabusa mission in 2005. Methods. The analog models prepared for this study are based on the a priori knowledge of asteroid interiors of the time. The experimental data were obtained by performing microwave-range laboratory measurements. Two advanced in-house, full-wave modelling packages – one performing the calculations in the frequency domain and the other one in the time domain – were applied to calculate the wave interaction within the analog models. Results. The electric fields calculated via both the frequency and time domain approach are found to match the measurements appropriately. Conclusions. The present comparisons between the calculated results and laboratory measurements suggest that a high-enough correspondence between the measurement and numerical simulation can be achieved for the most significant part of the scattered signal, such that the inner structure of the analog can be observed based on these fields. Full-wave modeling that predicts direct and higher order scattering effects has been proven essential for this application.

Tectonic Tremors in the Northern Mexican Subduction Zone Remotely Triggered by the 2017 Mw8.2 Tehuantepec Earthquake

Surface waves from the 2017 Mw8.2 Tehuantepec earthquake remotely triggered tectonic tremors in the Jalisco region, approximately 1000 km WNW in the northern Mexican subduction zone. This is the first observation of tremor triggering in this region and one of the largest known examples of triggered tremor in the world. While prior studies found tectonic tremors triggered by teleseismic waves in subduction zones and plate boundaries, further investigation of tremor triggering is crucially important for understanding the causative mechanism. We calculate the stress and strain changes across the three-dimensional plate interface attributable to seismic waves from the earthquake by full wavefield simulation. The maximum magnitude of the dynamic strain tensor eigenvalues on the plate interface, where tremors likely occur, is approximately 10-6. The subducting slab geometry effectively amplifies triggering waves. The triggering Coulomb failure stress changes resolved for a thrust fault plane consistent with the geometry are estimated at approximately 10-40 kPa. The relationship between the triggering stress and triggered tremor amplitude may indicate that the aσ of the rate-state-dependent friction law is 10 to 100 kPa.

An immersed boundary method with iterative symmetric interpolation for irregular surface topography in seismic wavefield modelling

The irregular free surface topography has a significant impact on simulations of seismic wave propagation. Therefore, an accurate representation of the irregular free surface is required for an accurate wavefield simulation. We propose an immersed boundary method used in fluid dynamics calculation to simulate acoustic waves with finite-difference in media with irregular surfaces. First, we set the number of ghost layers to half the length of the finite-difference stencil. Then, we define mirror points by orthogonally projecting the ghost points to fractional points below the free surface. We calculate the wavefield at these mirror points using an iterative symmetric interpolation method. Finally, we set the wavefield at the ghost points to the negative value of the wavefield of their corresponding mirror points. The proposed iterative symmetric interpolation method allows computing the wavefield at the mirror points more accurately and stably than the conventional immersed boundary methods. Numerical examples validate the accuracy and stability of this method in seismic forward modelling with strongly varying topography.

The asymptotic local finite-difference method of the fractional wave equation and its viscous seismic wavefield simulation

Viscous seismic wave propagation simulation using the fractional order equation has attracted much recent attention. However, conventional finite-difference (FD) methods of the fractional partial difference equation adopt a global difference operator to approximate the fractional derivatives, which reduces the computational efficiency dramatically. To improve the efficiency of the FD method, we have developed a reasonable truncated stencil pattern by strict mathematical derivation and adopted an asymptotic local FD (ALFD) method. Theoretical analysis and numerical results indicate that the ALFD method is accurate and efficient. In fact, our numerical results illustrate that the numerical solution solved by the ALFD method has a maximum relative error not exceeding 0.014% compared to the reference solution (applied to a finely meshed computational domain). The computation speed of ALFD is also significantly faster than that of the original FD method. The computational time of the three ALFD methods satisfying a different preset accuracy is only approximately 2.71%, 1.26%, and 0.78% of that of the original fractional wave equation FD method. The ALFD method provides a useful tool for viscoelastic seismic wavefield propagation simulation.