Hideo Aochi

The 1999 İzmit, Turkey, Earthquake: Nonplanar Fault Structure, Dynamic Rupture Process, and Strong Ground Motion

We simulated dynamic rupture propagation along various nonplanar fault models of the 1999 Izmit, Turkey, earthquake using a boundary integral equation method. These models were inferred from geological and geodetic observations. Based on these results, we modeled seismic-wave propagation around the fault system using a finite difference method. We focused on the effect of different fault geometries on the rupture process and seismic-wave propagation. Numerical simulation results imply a rapid and continuous rupture propagation from the Izmit–Sapanca Lake segment to the Sapanca–Akyazi segment. The rupture under Sapanca Lake appears to have propagated not on a disconnected fault segment but along a smooth fault structure with a bend of only a few degrees. The observational complexity of the surface breaks, however, can be best simulated by a highly segmented fault model. This infers that fault geometric characters observed in the field reflect near-surface structure and that seismological and geodetic features are controlled by global fault structure at depth. Then we investigated the effect of frictional parameters and the initial stress field. In order to explain near-field seismograms at station SKR, located a distance of a few kilometers from the fault, we had to force the rupture to propagate at shallow depth close to the station. In order to obtain this, we had to introduce a finite cohesive force in the friction law that allows stress accumulation and release in the shallow crust. The external stress field had to be large enough for the rupture to propagate at very rapid speed. Our simulation results show that it is important to include detailed fault geometry in the numerical simulation, and to constrain frictional parameters and the initial stress field, for understanding of the full dynamic process of an earthquake.

Selectivity of spontaneous rupture propagation on a branched fault

We simulated spontaneous dynamic rupture propagation on a branched fault system using the boundary integral equation method (BIEM) in 3D homogeneous, unbounded elastic medium. From the numerical experiments, we found that slightly heterogeneous preload stress field on the branches caused selective rupture propagation. Dynamic rupture propagation spontaneously on both branches requires very delicate conditions for the initial stress field as well as for the fracture criterion on them.

A Probable Earthquake Scenario near Istanbul Determined from Dynamic Simulations

Abstract We perform numerical simulations of dynamic rupture processes along the North Anatolian fault in the Sea of Marmara, which poses a high risk to the nearby city of Istanbul. Several fault geometry models, nucleation points, and initial stress states are tested. The likelihood of each earthquake scenario is evaluated, and a probabilistic assessment of the ground‐motion estimation is proposed. The simulation results suggest that the fault geometrical configuration is not favorable for producing an earthquake of magnitude 6–7, as no scenario is found. On the contrary, the probability of occurrence of an earthquake of magnitude greater than 7 is high. Most of these large events are characterized by epicenters located in the central or eastern parts of the Sea of Marmara. However, the possibility of a western‐initiated rupture propagating eastward, the worst scenario for the Istanbul region, cannot be ruled out. Many simulations led to supershear ruptures, as observed for the nearby 1999 Izmit earthquake, and this significantly influences ground‐motion prediction in the region.

Exploiting Intensive Multithreading for the Efficient Simulation of 3D Seismic Wave Propagation

Parallel computing is widely used for large scale three-dimensional simulation of seismic wave propagation. One particularity of most of these simulations is to consider a finite computing domain whereas the physical problem is unbounded. Additional numerical conditions are then required to absorb the energy at the artificial boundaries, which introduces a different formulation and a load-imbalance. In the context of finite difference method, we study the use of thread overloading approach to alleviate the imbalance. We introduce a mixed-hybrid parallel implementation based on a classical cartesian partitioning at the MPI level and a self-scheduling algorithm at the thread level to handle more than 700 threads on 8 processors. We demonstrate the efficiency of our methodology on an example of regional modeling performed on 80 processors.

Finite Difference Simulations of Seismic Wave Propagation for the 2007 Mw 6.6 Niigata-ken Chuetsu-Oki Earthquake: Validity of Models and Reliable Input Ground Motion in the Near-Field

Finite difference simulations of seismic wave propagation are performed in the Niigata area, Japan, for the 2007 Mw 6.6 Niigata-ken Chuetsu-Oki earthquake at low frequencies. We test three 3D structural models built independently in various studies. First aftershock simulations are carried out. The model based on 3D tomography yields correct body waves in the near field, but later phases are imperfectly reproduced due to the lack of shallow sediment layers; other models based on various 1D/2D profiles and geological interpretation provide good site responses but generate seismic phases that may be shifted from those actually observed. Next, for the mainshock simulations, we adopt two different finite source models that differ in the near-field ground motion, especially above the fault plane (but under the sea) and then along the coastline. Each model is found to be calibrated differently for the given stations. For engineering purposes, the variations observed in simulated ground motion are significant, but for seismological purposes, additional parameter calibrations would be possible for such a complex 3D case.

Finite difference simulations of seismic wave propagation for understanding earthquake physics and predicting ground motions: Advances and challenges

Seismic waves radiated from an earthquake propagate in the Earth and the ground shaking is felt and recorded at (or near) the ground surface. Understanding the wave propagation with respect to the Earths structure and the earthquake mechanisms is one of the main objectives of seismology, and predicting the strong ground shaking for moderate and large earthquakes is essential for quantitative seismic hazard assessment. The finite difference scheme for solving the wave propagation problem in elastic (sometimes anelastic) media has been more widely used since the 1970s than any other numerical methods, because of its simple formulation and implementation, and its easy scalability to large computations. This paper briefly overviews the advances in finite difference simulations, focusing particularly on earthquake mechanics and the resultant wave radiation in the near field. As the finite difference formulation is simple (interpolation is smooth), an easy coupling with other approaches is one of its advantages. A coupling with a boundary integral equation method (BIEM) allows us to simulate complex earthquake source processes.

Influence of super-shear earthquake rupture models on simulated near-source ground motion from the 1999 Izmit, Turkey, earthquake

We numerically simulate seismic wave propagation from the 1999 Mw 7.4 earthquake in Izmit, Turkey, using a 3D finite difference method based on published finite-source models obtained by waveform inversions. This earthquake has been reported, based on observations at the near-fault station SKR, as an example of supershear rupture propagation toward the east. Although the modeled ground motion does show a characteristic Mach wave from the fault plane, it is difficult to identify any particular effects in terms of peak ground velocity (PGV), an important parameter in earthquake engineering. This is because the faults spatial heterogeneity is strong enough to mask the properties of supershear rupture, which has been reported through several numerical simulations mostly based on homogeneous fault conditions. This article demonstrates the importance of studying ground motions for known earthquakes through numerical simulations based on finite-fault source models.

Interactions between Topographic Irregularities and Seismic Ground Motion Investigated Using a Hybrid FD-FE Method

A hybrid method combining finite element and 4th-order finite difference techniques is developed to model SH and P-SV seismic wave propagation in a 2D elastic medium with irregular surface topography. Both the classic staggered grid finite difference scheme and the partially staggered grid scheme are tested. The accuracy of the hybrid method is studied by comparison with a semi-analytical and another numerical method. Subsequently, to study the amplification, numerical simulations of seismic wave propagation in a series of hills are carried out and compared with the single-hill case. Depending on the position of the source in relation to the topography, the ratio between the heights and lengths of the hills or the ratio between the lengths of the hills and the wavelength, the presence of several hills as opposed to a single one can increase the amplification effect due to topography. This study highlights the fact that, when evaluating topographic site effects, surrounding topography must be taken into account in addition to local topography.

Assessing components of ground-motion variability from simulations for the Marmara Sea region (Turkey)

Recent studies have shown that repeatable travel-path terms make a high contribution to the overall variability in earthquake ground motions. Having maps of such terms available for a given recording site would, theoretically, allow removal of this component from the aleatory variability of ground-motion models. The assessment of such travel path terms for a given site, however, relies on having recorded a rich set of earthquakes at that site. Given the relative youth of strong-motion networks the assessment of such terms from observations is currently difficult for most parts of the world. Ground-motion simulations, however, provide an alternative method to assess such terms. In this article many dozens of earthquakes, distributed in a grid, are simulated for the Marmara Sea region (Turkey), which borders the megacity of Istanbul and is an area of high seismic hazard. Ground motions are simulated within a detailed 3D velocity structure model using a finite-difference method at 70 recording sites in the area (200 x 120km). Horizontal peak ground velocities from these simulations are regressed to derive a ground-motion model. Next, residuals from these GMPEs are computed to assess repeatable source, site and path terms and various components of ground-motion variability. These components are similar to those derived from real strong-motion data, thereby lending support to those estimates as well as showing the worth of simulations for this type of exercise.

Towards Seismic Wave Modeling on Heterogeneous Many-Core Architectures Using Task-Based Runtime System

Understanding three-dimensional seismic wave propagation in complex media is still one of the main challenges of quantitative seismology. Because of its simplicity and numerical efficiency, the finite-differences method is one of the standard techniques implemented to consider the elastodynamics equation. Additionally, this class of modeling heavily relies on parallel architectures in order to tackle large scale geometries including a detailed description of the physics. Last decade, significant efforts have been devoted towards efficient implementation of the finite-differences methods on emerging architectures. These contributions have demonstrated their efficiency leading to robust industrial applications. The growing representation of heterogeneous architectures combining general purpose multicore platforms and accelerators leads to re-design current parallel application. In this paper, we consider Star PU task-based runtime system in order to harness the power of heterogeneous CPU+GPU computing nodes. We detail our implementation and compare the performance obtained with the classical CPU or GPU only versions. Preliminary results demonstrate significant speedups in comparison with the best implementation suitable for homogeneous cores.