ALBERT

Description: The wealth of accelerometric recordings collected by the K-NET and KiK-net networks in Japan since 1996 provides a unique opportunity to improve our understanding of many important seismological research questions. Subsets of these data have been used for many case studies, most of them, however, not focusing specifically on the best practices for data selection and giving relatively little attention to the properties and peculiarities directly observable from the data. Yet for many applications, these steps are an important prerequisite for successful and reliable analysis. For this reason, we devote this article to the extraction of a large data set of surface and borehole recordings from the K-NET and KiK-net databases with strong emphasis on data quality and reliability. The final data set available for subsequent work consists of 78,840 records from 2201 earthquakes covering the Japan Meteorological Agency (JMA) magnitude range 2.7-8, observed at 1681 sites throughout Japan. We explain how this data set has been compiled, including automatic phase picking and relocation of events. We also present an overview of the general features of the data set, providing important information for subsequent analysis. Strong amplification effects at high frequencies are immediately visible on the surface recordings. Furthermore, there is a clear presence of downgoing waves in the borehole records, as deconvolution of borehole/surface recording pairs indicates.

Description: In this study we apply a nonparametric spectral inversion scheme to a data set of accelerograms recorded by the K-NET and KiK-net networks in Japan in order to derive attenuation characteristics, source spectra, and site response. For this purpose, we use a total of more than 67,000 S-wave records from 2178 earthquakes (MJMA 2.7-8) obtained at 1555 stations at the Earth's surface and more than 29,000 records from 1826 events recorded at 637 borehole stations at depths of 100 to 3000 m. Attenuation characteristics are investigated in five separate regions, showing that crustal Q depicts lower values in central compared to southern Japan, and a significant frequency dependence is observed in every region. The source spectra follow the{omega} 2 model with higher stress drops for subcrustal earthquakes as compared with crustal ones. While strong amplification effects dominate the site contributions for the surface sensors, those for the borehole sensors are characterized by smaller variability. Nevertheless, consistent with observations from deconvolution of borehole/surface recording pairs, downgoing wave effects are visible in the site contributions for many borehole stations. Finally, the site amplification functions obtained at the surface are compared with surface-to-borehole (S/B) and horizontal-to-vertical (H/V) spectral ratios, showing that the S/B ratios generally provide better estimates of the horizontal amplification than the H/V ratios due to amplification of the vertical component of ground motion.

Description: Recent studies have focused on the analysis of the nature of κ0⁠, as well as its application for improving seismic hazard assessment. It follows that a better understanding of κ0 (i.e., what exactly it represents) and of its correct application for improving the estimation of local shaking is mandatory. In this study, by means of the numerical simulation of vertically incident SH waves, we set out to clarify the contribution of the intrinsic attenuation and transmission properties of a media to the different portions of the seismic signal passing through it. Numerical simulations were carried out using models with constant or increasing depth velocities that are perturbed with random vertical impedance contrasts. The analysis allows us to quantify the apparent attenuation related to the transmission part for a given situation and to outline the variability of the results, depending on the chosen window of the signal. We showed that the attenuation due to transmission effects, even in strongly perturbed media, is generally much lower than that due to the intrinsic attenuation that is generally associated with near‐surface material. Finally, an example of this method’s application to real data that was collected from a borehole sensor array is used. We demonstrate that κ0 may be used to approximate the high‐frequency trend in the moduli of the transfer function of the medium, due to the contribution of different propagation effects. However, although the model used to describe the wave propagation is oversimplified, when the intrinsic attenuation is strong, κ0 could be used to differentiate sites with different subsurface attenuation structures. On the other hand, because κ0 might not be able to capture the absolute intrinsic attenuation correctly in cases in which it is not very strong, it should be used with care for the numerical simulation of ground motion.

Description: Expansion of urban areas in Central Asia increases their exposure to seismic hazard, but at present no earthquake early warning (EEW) systems exist in the region. Such systems, successfully implemented in other regions, aim to provide warning of the order of tens of seconds about impending disasters, enabling the first rapid‐response steps to be taken. The feasibility of such systems for Bishkek, Kyrgyzstan, has been demonstrated. This study investigates how the use of the spectral content, instead of just ground‐motion thresholds, can be used to improve the performance of proposed regional warning systems. We find that using the spectral content of the first few seconds after the P‐wave arrival can provide timely warning for events closer to the target city than was possible with the threshold systems. It is further shown that for events less than 60 km from the target, any regional system needs to be complemented with an onsite one to provide a comprehensive EEW system.

Description: Knowledge of the acceleration spectral shape is important for the prediction of ground motion. At high frequencies, the rapid decrease of the spectral amplitude, which controls the peak values, has been modeled by the spectral decay factor k, allowing an estimate of the apparent attenuation and which currently constitutes a basic input parameter for the generation of stochastic ground motion and the calibration of ground‐motion prediction equations. Based on numerical simulations of ground motion, we investigate the role of intrinsic and scattering attenuation in determining the high‐frequency decay of earthquake‐induced ground motion. We show that the attenuation term related to scattering depends nonlinearly on the intrinsic term, meaning that the commonly used explanation for the high‐frequency decay spectrum parameter might not be appropriate when analyzing signal windows of several seconds’ width.