Accurate modeling of rockfalls and pyroclastic flows is still an open issue, partly due to a lack of measurements related to their dynamics. Using seismic data from the Soufrière Hills Volcano, Montserrat, and granular flow modeling, we show that the power laws relating the seismic energy Es to the seismic duration ts and relating the loss of potential energy ΔEp to the flow duration tf are very similar, like the power laws observed at Piton de la Fournaise, Reunion Island. Observations showing that tf≃ts suggest a constant ratio Es/ΔEp≃10−5. This similarity in these two power laws can be obtained only when the granular flow model uses a friction coefficient that decreases with the volume transported. Furthermore, with this volume‐dependent friction coefficient, the simulated force applied by the flow to the ground correlates well with the seismic energy, highlighting the signature of this friction weakening effect in seismic data.

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In this study we present evidence that nonlinearity can be directly observed in acceleration time histories such as those recorded at the Wildlife Refuge and Kushiro Port downhole arrays from the 1987 Superstition Hills, California, and the 1993 Kushiro-Oki, Japan, earthquakes, respectively. These accelerograms and others compiled in this study present a characteristic waveform composed of intermittent high-frequency peaks riding on a low-frequency carrier. In addition, soil amplification of the surface records is strongly observed compared to their downhole counterpart; this is contrary to the expected amplification reduction produced by the nonlinear soil behavior. Laboratory studies show that the physical mechanism that produces such phenomena is the dilatant nature of cohesionless soils, which introduces the partial recovery of the shear strength under cyclic loads. This recovery translates into the ability to produce large deformations followed by large and spiky shear stresses. The spikes observed in the acceleration records are directly related to these periods of dilatancy and generation of pore pressure. These results are significant in strong-motion seismology because these spikes produce large if not the largest acceleration. They are site related, not source related. Using the in situ observations from the Kushiro Port downhole array, we have modeled the 1993 Kushiro-Oki earthquake. The synthetic accelerograms show the development of intermittent behavior-high frequency peaks-as observed in the recorded acceleration time histories. Shear modulus degradation due to pore pressure produces large strains in the soil with large amplification in the low-frequency band of the ground motion. We also modeled data from the 1987 Superstition Hills earthquake recorded at the Wildlife Refuge station. The results show the importance of better soil characterization when pore pressure may develop and the effects of dilatancy in the understanding of nonlinear site response.

Knowledge of the acceleration spectral shape is crucial to various applications in engineering seismology. Spectral amplitude decays rapidly at high frequencies. Anderson and Hough (1984) introduced the empirical factor κ to model this attenuation. This is the first time κ is studied in a vertical array consisting of more than two stations. We use 180 earthquakes recorded at a downhole array with five stations in soils and rock to investigate the effect of soil conditions on κ. Given that κ computation processes vary across literature when following the classic AndersonHough method, we investigate its variability with the different assumptions that can be made when applying the method. The estimates of κ 0 range between 0.017 and 0.031 s at the surface and between 0.004 and 0.024 s at rock. This variability due to the assumptions made is larger than the error of each estimate and larger than the average difference in values between sediment and rock. For this data set, part of it can be attributed to the type of distance used. Given this variability, κ 0 values across literature may not always be comparable; this may bias the results of applications using κ 0 as an input parameter, such as ground-motion prediction equations. We suggest ways to render the process more homogeneous. We also find that κ at rock level is not well approximated by surface records from which we deconvolved the geotechnical transfer function. Finally, we compute κ on the vertical component and find a dependence of the vertical-to-horizontal κ ratio on site conditions.

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