Fragility Curves for Buildings Based on Damage Data in Uki City Due the 2016 Kumamoto Earthquake
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Fragility Curves for Buildings Based on Damage Data in Uki City due the 2016 Kumamoto Earthquake Kazuaki TORISAWA College of Science and Engineering, Kanto Gakuin University, Japan. E-mail: [email protected] Kei HORIE Corporate Planning Department, MS&AD InterRisk Research & Consulting, Inc., Japan. Ken KAWABE Corporate Planning Department, MS&AD InterRisk Research & Consulting, Inc., Japan. Masashi MATSUOKA Department of Architecture and Building Engineering, Tokyo Institute of Technology, Japan. Munenari INOGUCHI Faculty of Sustainable Design, Toyama University, Japan. Fumio YAMAZAKI National Research Institute for Earth Science and Disaster Resilience, Japan. A series of earthquakes hit Kumamoto Prefecture in Kyushu Island, Japan, on April 14 and 16, 2016. A large number of buildings, mostly wooden houses, were damaged. This study developed fragility curves of buildings using the damage survey data provided by the Uki City government. The damage ratios of buildings were investigated from the viewpoints of the structural material and the construction period. As the result, the damage ratio of wooden buildings was found to be larger than those of other structural materials, and the damage ratios of major, moderate+, and moderate- got smaller as the construction period became newer. Empirical fragility curves of buildings for Uki City were constructed based on the damage survey data and the estimated peak ground velocity (PGV). Compared with the result of the previous study for Mashiki Town due the 2016 Kumamoto earthquake, the major damage ratios of wooden buildings for Uki City was shown in a lower level than those for Mashiki Town in the same PGV. The fragility curves for Uki City were obtained using the damage survey data in the smaller PGV range than that for Mashiki Town, thus the regression results reflected the damage trend in the small PGVs. The fragility curves for Uki City and Mashiki Town due the 2016 Kumamoto earthquake were compared with those for Nishinomiya City and Nada Ward due the 1995 Kobe earthquake. As the result, it was observed that the fragility curves for the Kumamoto earthquake showed lower damage ratios than those for the Kobe earthquake. Keywords: The 2016 Kumamoto earthquake, Building damage, Construction period, Peak ground velocity, Major damage ratio, Fragility curve. 1. Introduction authors (National Institute for Land and Infrastructure Management 2016; Sugino et al. A series of earthquakes hit Kumamoto Prefecture 2016; Yamada et al. 2017; Yamazaki et al. 2019). in Kyushu Island, Japan, on April 14 and 16, Also, in other municipalities in Kumamoto 2016. A large number of buildings, mostly Prefecture many building damages occurred wooden houses, were damaged and some of them (Kumamoto Prefecture 2016). were totally collapsed. Building fragility curves (vulnerability Among various types of damage due to strong functions) have been used for damage assessment shaking, building damage is one of the most studies by the national and local governments in significant effects of earthquakes, especially for Japan to plan appropriate and efficient inland (crustal) earthquakes. Many building countermeasures against future earthquakes damages occurred especially in Mashiki Town (Yamazaki and Murao 2000; Yamaguchi and due the 2016 Kumamoto earthquake, and some Yamazaki 2000; Midorikawa et al. 2011; Cabinet analysis results of these data have already been Office of Japan 2013a). Fragility curves have reported by a lot of engineers and researchers for been created by combining the ground motion various organizations including the present distribution and the building damage survey data Proceedings of the 30th European Safety and Reliability Conference and the 15th Probabilistic Safety Assessment and Management Conference Edited by Piero Baraldi, Francesco Di Maio and Enrico Zio Copyright c ESREL2020-PSAM15 Organizers.Published by Research Publishing, Singapore. ISBN/DOI: 978-981-14-8593-0 Proceedings of the 30th European Safety and Reliability Conference and the 15th Probabilistic Safety Assessment and Management Conference by local governments. Most of the present damage Figure 2 compares the acceleration response assessment studies use empirical fragility curves spectra (the resultant of the two horizontal from the 1995 Kobe earthquake because there components, 5% damping ratio) of the recorded were no other earthquakes with abundant damage motions at the three seismic stations shown in Fig. data. For this reason, the reconstruction of 1 for the foreshock (April 14) and the mainshock fragility curves based on damage data in the (April 16). In the figure, the result for the recent damaging earthquakes is considered an important issue, and the large amount of data obtained in the 2016 Kumamoto earthquake should be properly analyzed for estimating and mitigating damages due to future seismic events. This study developed fragility curves of buildings using the damage survey data provided by the Uki City government. The damage ratios of buildings were investigated from the viewpoints of the structural material and the construction period. Empirical fragility curves of buildings for Uki City were constructed based on the damage survey data and the estimated peak ground velocity (PGV) distribution, and the results were compared with those of the previous studies for Mashiki Town due the 2016 Kumamoto earthquake, Nishinomiya City and Nada Ward due the 1995 Kobe earthquake. 2. Estimation of Ground Motion Distribution A Mw 6.2 earthquake hit Kumamoto Prefecture on April 14, 2016 at 21:26 (JST). The epicenter was located in the Hinagu fault with a shallow depth. On April 16, 2016 at 01:25 (JST), about 28 hours after the first event, another earthquake of Mw 7.0 occurred in the Futagawa fault, closely located with the Hinagu fault. Thus, the first event was called as the "foreshock" and the second one as the "mainshock". Figure 1 shows the location of Uki City and these causative faults of the 2016 Kumamoto earthquake. Uki City is located in the southwest direction of the causative faults. Also shown in the figure are the K-NET and KiK-net stations near Uki City. KMM006 Futagawa fault KMM008 Hinagu fault KMMH14 Uki City Fig. 2. Acceleration response spectra (the resultant of the two horizontal components, 5% damping ratio) of Fig. 1. Location of Uki City, K-NET and KiK-net the recorded motions at K-NET Kumamoto stations near Uki City and the causative faults of the (KMM006), K-NET Uto (KMM008) and KiK-net 2016 Kumamoto earthquake Toyono (KMMH14) Proceedings of the 30th European Safety and Reliability Conference and the 15th Probabilistic Safety Assessment and Management Conference connected time histories (April 14 and April 16 in The damage extent of each building was 2 minutes interval) is also plotted. It is seen for classified into five classes by its damage level (or each station that the April 16 event has a dominant monetary loss), as current damage (loss) class influence in the all period range. shown in Table 1. Note that this classification was We estimated the seismic ground motion carried out following the unified loss assessment distribution in the disaster area with high accuracy method of Japan (Cabinet Office of Japan 2013b). by collecting a total of 1,141 strong motion In the table, an approximate correspondence with records (K-NET and KiK-net: 698; Japan visual inspection methods (Grünthal ed. 1998; Meteorology Agency (JMA): 316; local Okada and Takai 2000) is also shown. Note that governments: 111; Saibu Gas Co.: 16) for the mainshock of the 2016 Kumamoto earthquake. The Kriging interpolation was applied to these peak ground motion values considering the attenuation relationship from the causative fault plane of the mainshock and soil amplification factors, following the method adopted by QuiQuake (Matsuoka and Yamamoto 2012). Note that the resultant PGVs of the two horizontal components were used in the calculation. Figure 3 shows the estimated PGV distribution in the 250-m grid cell units for Uki City. The estimated PGV is relatively larger in the central southern area than in the northeast area closer to the causative faults. Figure 4 shows the PGV amplification factor Fig. 3. Distribution of the estimated peak ground (National Research Institute for Earthquake velocity (PGV) in the 250-m grid cell units for Uki City Science and Disaster Resilience 2018) used in the Kriging interpolation for the study area. In the coastal area of this region, there are many reclaimed lands and deltas, and thus the distribution of the PGV amplification factor shows such tendency of the surface soil. 3. Analysis of Building Damage Soon after the 2016 Kumamoto earthquake, the municipality governments carried out building damage surveys in order to issue disaster-victim certificates. Building damage caused by the 2016 Kumamoto earthquake was serious in Mashiki Town and Minami-Aso Village, which are located close to the source region, but Uki City was also heavily damaged, next to those areas (Kumamoto Fig. 4. Distribution of the PGV amplification factor in Prefecture 2016). the 250-m grid cell units for Uki City Table 1. Earthquake loss assessment classes of buildings by local governments in Japan and schematic images of other damage classification methods (Source: Yamazaki et al. 2019) Proceedings of the 30th European Safety and Reliability Conference and the 15th Probabilistic Safety Assessment and Management Conference the result of loss assessment by local governments area less than 20 m2, those without ground floor is important for affected people to receive and those without location information. The financial support and property tax reduction. In remaining 28,486 data were used. Uki City, the first stage assessment, viewing from Figure 5 shows the spatial distribution of outside, was conducted for the all buildings. This damaged buildings in Uki City based on the city result was shown to the residents and in case, they government’s disaster-victim certificate data. It is did not accept it, the second stage assessment, seen that the major damage buildings are viewing the damage status of inside a building, distributed in the area where many buildings were was carried out.