We investigated dynamic rupture scenarios of anticipated megathrust earthquakes on the Nankai‐Tonankai subduction zone, southwest Japan. To improve the scenario reliability, the model parameters should be constrained by available data, or derived from their analysis. We employed the three‐dimensional plate interface geometry and the slip‐deficit rate on the interface. Accumulated slip‐deficit was used to obtain the stress drop distribution of anticipated earthquakes. The estimated stress drop distribution is consistent with the seismogenic asperity locations known from the analysis of past earthquakes. Fault friction constitutive parameters, however, had to be assumed from indirect observations because they cannot be constrained directly by the data. Based on various geophysical observations, we defined three regions where larger fracture energy is required. These are the eastern edge of the Tonankai area, the western edge of the Nankai area, and the region between the Tonankai and Nankai areas (beneath the Kii peninsula). Such lateral heterogeneity promoted the segmented rupture along the Nankai trough. With predefined stress drop and constitutive parameters, various rupture scenarios for Tonankai and Nankai asperities were obtained for different initiation locations. In some cases, a single segment is ruptured, while in other cases, all the segments are broken due to dynamic linkage at the segment boundary, causing a giant earthquake. The initiation location is a critical parameter that controls the rupture propagation across the segment boundary. These scenarios will be extremely useful to evaluate deterministically the strong ground motions and tsunami hazards caused by the next major earthquakes in southwest Japan.

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[1] We study the effects of a plastic behavior of the volume around the fault on in-plane and anti-plane 2D rupture dynamics. Both rupture modes exhibit similar answer to off-fault yielding, in terms of modification of the kinematics of the rupture front, and in terms of energy lost outside the fault plane. We then compare the ability of the rupture to propagate through a barrier on the interface. The plastic behavior, responsible for a linear increase of the global fracture energy during dynamic crack growth, enhances the rupture front sensitivity to a static resistance increase on the fault. Consequently, the rupture arrest is more easily provoked in heterogeneous models that include a plastic yielding, even with relatively small variations of frictional resistance along the fault plane. Citation: Hok, S., M. Campillo, F. Cotton, P. Favreau, and I. Ionescu (2010), Offfault plasticity favors the arrest of dynamic ruptures on strength heterogeneity: Two-dimensional cases, Geophys. Res. Lett., 37, L02306,

S U M M A R YWe developed a code for dynamic rupture propagation on arbitrarily shaped faults embedded in half-space using boundary integral equation method (BIEM). It is based on an existing code for the full-space case, in which virtual free surface elements were introduced. Computations in half-space are definitely necessary for the modelling of shallow dipping fault ruptures. First, we describe the implementation and conduct some validation tests that demonstrate the accuracy of the code. The validation tests are carried out by comparing the BIEM results both with the analytical static solution and with the seismic wavefield obtained by a discrete wavenumber method. Then, we apply the code to the study of the 2008 Iwate-Miyagi, Japan, earthquake (M w 6.9) rupture dynamics. The average stress drop was estimated to be rather large (16 MPa) in the asperity area, while the average strength excess was 5 MPa. The average fracture energy inside the asperity was found to be 36 MJ m −2 . Finally, the free surface effect examination showed that the stress drop and the fracture energy were overestimated at shallow depth in the full-space computation.

On November 11, 2019, a Mw 4.9 earthquake hit the region close to Montelimar (lower Rhône Valley, France), on the eastern margin of the Massif Central close to the external part of the Alps. Occuring in a moderate seismicity area, this earthquake is remarkable for its very shallow focal depth (between 1 and 3 km), its magnitude,  and the moderate to large damages it produced in several villages. InSAR interferograms indicated a shallow rupture about 4 km long reaching the surface and the reactivation of the ancient NE-SW La Rouviere normal fault in reverse faulting in agreement with the present-day E-W compressional tectonics. The peculiarity of this earthquake together with a poor coverage of the epicentral region by permanent seismological and geodetic stations triggered the mobilisation of the French post-seismic unit and the broad French scientific community from various institutions, with the deployment of geophysical instruments (seismological and geodesic stations),  geological field surveys, and field evaluation of the intensity of the earthquake. Within 7 days after the mainshock, 47 seismological stations were deployed in the epicentral area to improve the Le Teil aftershocks locations relative to the French permanent seismological network (RESIF), monitor the temporal and spatial evolution of microearthquakes close to the fault plane and temporal evolution of the seismic response of 3 damaged historical buildings, and to study suspected site effects and their influence in the distribution of seismic damage. This seismological dataset, completed by data owned by different institutions, was integrated in a homogeneous archive and distributed through FDSN web services by the RESIF data center. This dataset, together with observations of surface rupture evidences, geologic, geodetic and satellite data, will help to unravel the causes and rupture mechanism of this earthquake, and contribute to account in seismic hazard assessment for earthquakes along the major regional Cévenne fault system in a context of present-day compressional tectonics.

Abstract-In an attempt to explain the large shallow slip that occurred near the trench during the 2011 Tohoku-Oki earthquake, numerical simulations of earthquake dynamic rupture were carried out using a fault model with a subduction interface containing a bump-shaped asperity, which might result from subduction of an old submarine volcano or seamount. It was assumed that during the interseismic period, slip only occurs outside the bump area and that stress accumulates inside the bump, creating a seismogenic asperity. We roughly evaluated the amount of slip outside the bump during the interseismic period, assuming a constant long-term subduction rate. Then we could estimate the accumulated stress inside the bump. We constructed the initial stress distribution based on the stress change caused by the slip-deficit distribution. A constitutive relation was constructed based on a slip-weakening friction law and was used to compute spontaneous ruptures. The results indicate that a large slip can occur between the trench and the bump, even though a very small amount of stress is accumulated there before the rupture. This is due to an interaction between the free surface and the fault that causes slip overshoot. On the region of the fault below the bump, such overshoot cannot occur because the fault is pinned by the deeper un-slipped zone. However, on the shallower side, the edge of the fault becomes free when the rupture approaches the free surface. In this region, such a large slip can occur without releasing a large amount of stress.

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