Application of Mode-Adaptive Bidirectional Pushover Analysis to an Irregular Reinforced Concrete Building Retrofitted via Base Isolation

In this article, the applicability of mode-adaptive bidirectional pushover analysis (MABPA) to base-isolated irregular buildings was evaluated. The point of the updated MABPA is that the peaks of the first and second modal responses are predicted considering the energy balance during a half cycle of the structural response. In the numerical examples, the main building of the former Uto City Hall, which was severely damaged in the 2016 Kumamoto earthquake, was investigated as a case study for the retrofitting of an irregular reinforced concrete building using the base-isolation technique. The comparisons between the predicted peak response by MABPA and nonlinear time-history analysis results showed that the peak relative displacement can be properly predicted by MABPA. The results also showed that the performance of the retrofitted building models was satisfactory for the ground motion considered in this study, including the recorded motions in the 2016 Kumamoto earthquake.

Impact of crustal deformation detection by the DSI (difference of split-band interferograms) method with PALSAR-2 data: A case study on the 2016 Kumamoto Earthquake

Interferometric Synthetic Aperture Radar (InSAR) is a useful tool for detecting surface deformations at high spatial resolutions. When InSAR is applied to huge surface deformations, clear fringes with complicated phase gaps often appear in the interferograms. Although the surface deformations in such areas are important for understanding their mechanisms and for investigating disasters, it is difficult to convert the data on such fringes to surface deformation information because of difficulties associated with phase unwrapping. To resolve these difficulties, we created multiple SAR pairs with different frequencies using a band-pass filter and derived the difference of interferograms which are generated from these SAR pairs. Generally, its result corresponds to the result of SAR observations made with long-wavelength radar. Therefore, a phase wrap was less likely to occur, and phase unwrapping was easy to accomplish. We applied this method to the PALSAR-2 data pairs for the 2016 Kumamoto Earthquake and succeeded in identifying huge crustal deformations with complicated phase gaps in the vicinity of surface ruptures. Comparing these results with the crustal deformations observed from GNSS measurements, the root-mean-squares of the differences were found to be approximately 4 cm. Although this accuracy was poorer than that of conventional InSAR, it was nearly equivalent to that of the offsettracking method. Furthermore, its spatial resolution was significantly better than that of the offset-tracking method. However, the disadvantage of this method is that its detection accuracy is significantly degraded in zones with low coherence, due to noise amplification. The standard deviation of the noise component was approximately 2 cm for pixels with coherences above 0.7. However, for pixels with a coherence lower than 0.2, the standard deviation was greater than 10 cm, and the noise component occasionally exceeded 1 m. Despite the disadvantages of this method, it is effective for the detection of huge crustal deformations with high spatial resolution in areas where phase unwrapping methods for conventional InSAR are inappropriate.

Application of Mode-Adaptive Bidirectional Pushover Analysis to an Irregular Reinforced Concrete Building Retrofitted via Base Isolation

In this article, the main building of the former Uto City Hall, which was severely damaged in the 2016 Kumamoto earthquake, is investigated as a case study for the retrofitting of an irregular Reinforced Concrete building using the base-isolation technique. Its peak response is predicted via mode-adaptive bidirectional pushover analysis (MABPA), which was originally proposed by the authors. In the prediction step of MABPA, the peaks of the first and second modal responses are predicted considering the energy balance during a half cycle of the structural response. The numerical analysis results show that the peak relative displacement can be properly predicted by MABPA. The results also show that the performance of the retrofitted building models is satisfactory for the ground motion considered in this study, including the recorded motions in the 2016 Kumamoto earthquake.

Dynamic Rupture Study of Near-Field Velocity Pulses during the 2016 Kumamoto Earthquake, Japan

ABSTRACT Simulating the ground motions of future earthquakes requires a proper understanding and modeling of source, path, and site effects. Ground motions recorded during recent earthquakes very close to their ruptured faults provide new evidence of the importance of source effects and suggest that physics-based rupture modeling is critical to account for them. Here, we develop dynamic rupture models to simulate the near-fault ground motions generated by the 2016 Kumamoto, Japan, earthquake (Mw 7.0) at Nishihara village, which feature a large-amplitude velocity pulse. Comparison of mainshock and foreshock waveforms suggests that the source of the velocity pulse is on the Futagawa fault segment located very close to the site. Our dynamic models use the spectral element method and are built upon a previous kinematic description of the event via a so-called “characterized source model,” with three strong-motion generation areas (SMGAs) on the assumed fault plane. We first develop a reference model that reproduces the main features of the rupture process in agreement with previous results of kinematic source inversion. We then examine the sensitivity of the simulated near-fault ground motions to the frictional parameters (critical slip-weakening distance and stress drop) in the shallow part of the fault and to the geometrical properties of the shallow SMGA. Even assuming drastically different frictional properties in the shallow part of the fault, the amplitude of the simulated ground motions was affected little. On the other hand, changes of geometrical properties of the shallow SMGA generated large differences in simulated ground motions. The results indicate that geometrical features of the shallow SMGA played a more important role in generating near-fault ground motions with velocity pulses as observed at Nishihara village during the 2016 Kumamoto earthquake.