Strong Motions on Land and Ocean Bottom: Comparison of Horizontal PGA, PGV, and 5% Damped Acceleration Response Spectra in Northeast Japan and the Japan Trench Area

ABSTRACT A large-scale permanent ocean-bottom seismograph network, named S-net, has been established in the Japan Trench area and consists of 150 observatories equipped with seismometers and tsunamimeters. Most stations at water depths <1500 m were buried to a depth of about 1 m while they were sited freely on the seafloors at greater water depths. To understand the characteristics of strong ground motions on the offshore area, we compared the horizontal vector peak ground accelerations (PGA), peak ground velocities (PGVs), and acceleration response spectra (ARS) between the land and S-net sites for nine earthquakes (5.3≤Mw≤7.1) using ground-motion prediction equations developed for Japan. We found that the observed values of PGAs and short-period (<0.5 s) ARS were generally similar between the land and S-net sites, whereas the PGVs and ARS for the periods longer than 0.5 s were apparently larger at the S-net sites. These results based on data covering a wide area on the seafloors were generally similar to the previous results based on limited ocean-bottom stations. However, analysis of the residuals, within the source-to-site distance of 200 km, revealed that the residual values were smaller in the shallow water region compared to those toward the Japan Trench, which is characterized by proximity to high Qs in the Pacific plate, the presence of thick unconsolidated sediments on the upper crust, and increasing heights of water columns. The difference of station settings in the shallow and deep water regions may also have contributed to the biased distribution of residuals at the short periods. Quantifications of these results are expected to contribute to the predictions of ground motions for earthquake early warning and seismic demand analysis of offshore facilities and await further analysis of a larger data set.

Dynamical similarity of multi-block catenary arches and rocking blocks subjected to horizontal base excitation

In this paper, the dynamical similarity of multi-block catenary arches and columns is discussed, which can be used for the simplified design of rocking arches. The basic hypothesis is that the dynamic response of multi-block arches and columns is similar when the shape of the arch follows the thrust line, i.e. it is a catenary arch. It is validated numerically that the responses of slender catenary arches are safe and reliable approximations of those of not slender arches and then that the overturning acceleration (response) spectra of rigid, slender monolithic blocks can be directly applied for catenary arches. The similarity is based on two parameters, on the limit peak ground acceleration (under which the structure will not move at all) and on the frequency parameter (defined by Housner for rigid blocks).

A practice-oriented method for estimating elastic floor response spectra

A practice-oriented modal superposition method for setting elastic floor acceleration response spectra is proposed in this paper. The approach builds on previous contributions in the literature, making specific recommendations to explicitly consider floor displacement response spectra and accounts for uncertainty in modal characteristics. The method aims to provide reliable predictions which improve on existing code methods but maintain simplicity to enable adoption in practical design. This work is motivated by recent seismic events which have illustrated the significant costs that can be incurred following damage to secondary and nonstructural components within buildings, even where the structural system has performed well. This has prompted increased attention to the seismic performance of nonstructural components with questions being raised about the accuracy of design floor acceleration response spectra used in practice. By comparing floor acceleration response spectra predicted by the proposed method with those recorded from instrumented buildings in New Zealand, it is shown that the proposed approach performs well, particularly if a good estimate of the building’s fundamental period of vibration is available.

Effects of Earthquake Source, Path, and Site Conditions on Damping Modification Factor for the Response Spectrum of the Horizontal Component from Subduction Earthquakes

Damping modification factors (DMFs) are important for estimating the response spectrum for the design of structures with different damping ratios. This study investigated the effects of earthquake source parameters (magnitude, source depth, and source categories), source distance (the closest distance from a site to a fault plane for large earthquakes and hypocentral distance for the other events), and site conditions on DMFs for the displacement and acceleration response spectra of the horizontal components of the records from Japan. This study used a total of 14,713 strong‐motion records from the KiK‐net and K‐NET to compare the DMFs from three earthquake category groups, namely shallow crustal and upper mantle, subduction interface, and subduction slab earthquakes. Statistical tests were carried out to determine whether the DMFs from these three types of earthquakes differ significantly from each other. The test results show that, between each pair of the three types of earthquakes, the DMFs for both displacement and acceleration response spectra differ significantly in terms of statistical tests and practically for engineering applications at many spectral periods, with the largest difference over 40%. The effects of earthquake category and site conditions for acceleration spectrum are similar to those of the displacement spectrum at short periods up to about 0.3 s but are much larger than those of the displacement spectrum at long spectral periods. The effects of magnitude and earthquake depth are also significant. Therefore, separate DMF models for the response spectrum of the horizontal component should be derived for each type of earthquake and should account for the effects of earthquake source and path parameters and site conditions.