Journal of Earthquake and Tsunami

To validate the seismic retrofitting of buildings quantitatively, we observed microtremors in the Uji campus buildings at Kyoto University. We compared the dynamic characteristics derived from the resonant frequency of microtremors in a five-story steel-frame structure and a three-story reinforced-concrete (RC) building before and after retrofitting. We calculated the weight of the building from the retrofitting design plan and constructed a numerical model of the structure by estimating the building stiffness that fits the resonant frequency. Assuming the characteristics of the restoring force for each story of the numerical model from previous studies, we calculated the non-linear response of the building to the strong motions predicted for earthquakes occurring at the Obaku fault system that runs right in front of the Uji campus. We compared the response of the numerical models constructed before and after retrofitting to verify the effects of the seismic retrofitting. We found that the proposed method of dynamic response model construction using microtremor observations is effective when the reinforcement members are rigidly fixed to the existing structure, as is the case for RC buildings.

The objective of this study is to propose a framework for evaluating the seismic fragility of bridge systems analytically taking into account correlations between component behaviors. To include coupled effects of component behaviors on system behavior, the second-order reliability approach is adopted. The damage states of the combined fragility are proposed based on system behavior characteristics obtained by pushover analyses. The characteristics of the combined fragility information of two bridge models are illustrated using probabilistic seismic demand analysis based on the time history analysis results from several earthquake excitations. The fragility curves obtained from the second-order reliability model are compared with the first-order reliability model. It is shown that the proposed framework can provide more detailed fragility information rather than only providing bounds of system fragility as in the first-order reliability-based approach.

The ground is simultaneously subjected to both horizontal and vertical motions during earthquakes; however, the majority of existing studies on seismic compression are still limited to the horizontal earthquake motions only. In this work, the objective is therefore to experimentally investigate the effects of multidirectional nature of load application on seismic compression of sand. Multiple series of hollow cylindrical torsional shear tests were conducted on dry sand specimens with different relative densities, where different scenarios of stress conditions induced by earthquakes were simulated by applying various combinations of vertical and horizontal cyclic loads. Test results revealed that coupling with horizontal motions, the vertical component of seismic loads would significantly contribute to the development of both shear and vertical strains in the sand specimen. The increment in vertical strain under coupled motions could be up to over 70% compared to its counterpart for the sand specimen under horizontal cyclic stress alone. Nevertheless, it is also interestingly found that the contribution of superimposed vertical motion alone to the growth of vertical strain is limited. Besides, the effect of relative density on the change of vertical strain is nonlinear, and the decreasing rate of vertical strain diminishes with the increase of relative density.

The unconnected piles–caisson foundation (UPCF) consists of gravity caisson, mattress, and pile group. This foundation has been an attractive design for the mattress as it can reduce the seismic loads applied on the superstructure. However, a lack of understanding regarding its seismic behavior has hindered the application of this type of foundation. This paper reports on a series of slow cyclic tests (SCTs) conducted to investigate the seismic behavior of the UPCF. Six tests were conducted on model foundations on dry sand with the variables of mattress material, mattress thickness, vertical safety factor, and ground improvement. The experimental schemes, including the model design, the instrumentation, and the loading scheme, are illustrated in detail. The test results show that a large amount of energy was dissipated through the mattress during dynamic loading. With increasing loading displacement, the shear stiffness of these models decreased to a stable value. An empirical equation for the stiffness and loading displacement is developed. Below a drift ratio of 0.6%, there is no evident difference in the dynamic behavior of the foundation. After that, the aseismic behavior of the foundation depends on the mattress material, the mattress thickness, and the vertical pressure and is affected by the earthquake intensity and frequency.

In seismic hazard studies, simulation techniques are exploited relatively less compared to other statistical methods. The main purposes of their use were merely scientific: either to reduce uncertainty in seismic hazard or to test the consistency of a forecast. Its limited exploitation in the field does not mean that the method could not be exploited further. Given sufficient data and with the right models, a full-scale simulation is possible, which in fact becomes a forecast with future timing, location and magnitude of future events are simulated.

Within this context, the main purpose of this study is to layout a method to create a full scale simulation with all the parameters namely the time, location and magnitude of an earthquake is simulated. For that purpose, in addition to the main parameters defining past seismic patterns, namely the temporal, spatial, magnitude distributions, the combined spatio-magnitude distribution is also modeled. As another improvement compared to the current procedures, occurrence date is assigned to the simulated event according to the inter-event time distribution, which enhances the simulation procedure. The eventual temporal distribution and the associated earthquake rates are checked with the original earthquake rate for its consistency. In the end, a simulation procedure, which can also be used as a forecast model is developed to improve the existing procedures further.