International audienceWe analyzed the acceleration time histories recorded by the K-NET and KiK-net stations of the Mw 9 Tohoku Earthquake in order to investigate site response issues related to near-source effects. Time-frequency analysis of K-NET stations in the Miyagi prefecture, closest to the rupture area, show that sites having a Vs30 < 400 m/s present a combination of deamplification at frequencies higher than 5 to 10 Hz and cyclic mobility (high acceleration peaks riding over a low frequency carrier). This suggests strong nonlinear site response at these stations. Furthermore, using KiK-net data we are able to compute borehole transfer functions from the mainshock and events having small PGA values from the local dataset. The ratio between weak-motion and strong-motion borehole transfer functions constitutes an indicator of nonlinear site response. This ratio reveals strong dependence on Vs30 and shows that widespread nonlinear behavior took place during this large earthquake

International audienceThe 2011 moment magnitude (M w) 9.0 Tohoku-Oki Japan earthquake occurred in a region where giant megathrust earthquakes were not expected. This earthquake proved the difficulty in assessing seismic hazard by relying mainly on information from historical and instrumental seismicity. To help improve the seis-mic-hazard assessment for such rare events, we propose a methodology to estimate the slip distribution of future megathrust earthquakes based on a model of interseismic coupling distribution in subduction margins, as well as information of historical earth-quakes, and apply the method to the central Peru region, Lima. The slip model ob-tained from geodetic data represents the large scale features of asperities within the megathrust, which is appropriate for simulation of long-period waves and tsunami modeling. For the simulation of a broadband strong ground motion, we add small scale heterogeneities to the source slip to be able to simulate high frequencies. To achieve this purpose, we propose broadband source models constructed by adding short-wavelength slip distributions obtained from a Von Karman power spectral den-sity function, to the slip model inferred from interseismic geodetic data. Using these slip models and assuming several hypocenter locations, we calculate a set of strong ground motions for Lima and incorporate site effects obtained from microtremors sur-veys and geotechnical data. Our simulated average pseudospectral accelerations (period 0.3 s) are above 1:5g for wide areas in Lima, which may be critical in terms of damage of low-to midrise masonry and reinforced concrete buildings, which char-acterize the majority of buildings in Lima

In the present study we investigate the high-frequency (HF) radiation mechanism of the 2000 Tottori earthquake in Japan based on a 3D spontaneous fault rupture dynamic model. We generalize the model of HF radiation of a suddenly stop- ping circular crack (Madariaga, 1977; Boatwright, 1982) to the radiation from a gen- eral 3D rupture in a planar fault, where HF is radiated during gradual changes of rupture velocity at the rupture front. Local rupture velocity changes are expressed as the divergence of local rupture velocity vectors that are derived from gradients of rupture times from the dynamic model. Our numerical model of the Tottori earth- quake indicates that rupture velocity changes are largely induced by barriers (locally stronger fault sections) across the fault plane and that HF radiation mainly originates within asperities (large stress-drop regions) in areas where the product of dynamic stress drop and rupture velocity changes is maximum. We develop a strong ground motion simulation methodology that incorporates HF radiation inferred from a dy- namic fault rupture model. Using this methodology we investigate the HF radiation of the Tottori earthquake by inverting observed near-source acceleration envelopes of the earthquake. Our inversion results corroborate that HF radiation originates within asperities and show that significant HF radiation represents no more than a 20% of the total asperity area. Our results show that the incorporation of a directivity factor, on the basis of a well-defined physical rupture model to the radiation pattern leads to a sig- nificant improvement in fitting of observed ground motions. Our simulated near- source strong ground motions of the Tottori earthquake are also able to reproduce the ω� 2 radiation theoretically predicted in 2D dynamic fault rupture models.