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Geospatial Assessment of the Post-Earthquake Hazard of the 2017 Pohang Earthquake Considering Seismic Site Effects

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Geospatial Assessment of the Post-Earthquake Hazard of the 2017 Pohang Earthquake Considering Seismic Site Effects International Journal of Geo-Information Article Geospatial Assessment of the Post-Earthquake Hazard of the 2017 Pohang Earthquake Considering Seismic Site Effects Han-Saem Kim 1 , Chang-Guk Sun 2,* and Hyung-Ik Cho 1 1 Earthquake Research Center, Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, Korea; [email protected] (H.-S.K.); [email protected] (H.-I.C.) 2 Geological Research Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, Korea * Correspondence: [email protected]; Tel.: +82-42-868-3265 Received: 15 July 2018; Accepted: 5 September 2018; Published: 10 September 2018 Abstract: The 2017 Pohang earthquake (moment magnitude scale: 5.4) was South Korea’s second strongest earthquake in decades, and caused the maximum amount of damage in terms of infrastructure and human injuries. As the epicenters were located in regions with Quaternary sediments, which involve distributions of thick fill and alluvial geo-layers, the induced damages were more severe owing to seismic amplification and liquefaction. Thus, to identify the influence of site-specific seismic effects, a post-earthquake survey framework for rapid earthquake damage estimation, correlated with seismic site effects, was proposed and applied in the region of the Pohang earthquake epicenter. Seismic zones were determined on the basis of ground motion by classifying sites using the multivariate site classification system. Low-rise structures with slight and moderate earthquake damage were noted to be concentrated in softer sites owing to the low focal depth of the site, topographical effects, and high frequency range of the mainshocks. Keywords: geo-data; seismic site effects; post-earthquake survey; earthquake impact; 2017 Pohang earthquakes 1. Introduction The 2017 Pohang earthquake occurred on 15 November 2017, in Heunghae, Pohang in the North Gyeongsang Province in South Korea [1]. The earthquake measured 5.4 on the Richter magnitude scale (ML), and is considered to be South Korea’s second strongest earthquake in decades, nearly equaling the 2016 Gyeongju Earthquake (ML 5.8; 12 September 2016). The earthquake caused severe damage to infrastructure, injured 82 people, and left approximately 1500 homeless. The epicenters of the Pohang earthquake, which involved one mainshock and six aftershocks, as shown in Table1[ 1], were spatially distributed along the NNE–SSW direction (Figure1)[ 2]. The clusters of epicenters were located along an unknown branch of the fault system as well as distributed across the Heunghae basin [2]. Although the degree of seismic amplification of the 2017 Pohang earthquake was lower than that of the 2016 Gyeongju earthquake, the Pohang earthquake damage was more severe as its epicenters were spatially concentrated on unconsolidated Quaternary sediments (alluvial fans and granite wash) [2]. Earthquake ground motion amplification is affected predominantly by site-specific geotechnical characteristics. The amplification of ground motion and changes in the surface or underground terrain due to the surface geological conditions are deeply related to the local seismic site effects [2–4], which indicate the seismic ground motion expressed by the acceleration or velocity of each period (or frequency), and directly affect the dynamic response characteristics of near-surface structures [5]. Local site effects related to geological and geotechnical conditions have been observed in many ISPRS Int. J. Geo-Inf. 2018, 7, 375; doi:10.3390/ijgi7090375 www.mdpi.com/journal/ijgi ISPRS Int. J. J. Geo-Inf. Geo-Inf. 2018,, 7,, 375 375 22 ofof 1515 Local site effects related to geological and geotechnical conditions have been observed in many earthquake events, including including the the 1985 1985 Mexico Mexico City, City, 1989 Loma Prieta, 1994 Northridge, 1995 1995 Kobe, Kobe, 2008 WenChuan,WenChuan, 2010 2010 Haiti, Haiti, 2011 2011 Tohoku, Tohoku, and 2016and Kumamoto2016 Kumamoto earthquakes. earthquakes. A fairly goodA fairly agreement good agreementbetween the between zone characterized the zone characterized by the highest by the building highest damage building ratio damage and the ratio localization and the localization of the site ofeffect the wassite effect also reported. was also Seismicreported. shaking Seismic in shakin thick sedimentsg in thick sediments is extremely is extremely strong, leading strong, to leading extensive to extensivedamage due damage to the due site to effect. the site effect. The post-event days are when preliminary macroseismic intensity assessments (2–3 days) and reconnaissance surveys (2–3 weeks) are performed. Generally, Generally, post-earthquake usability and damage evaluations are the major major source source of of damage damage collection collection in in high high seismicity seismicity regions regions such such as as Italy, Italy, Turkey, Turkey, and Japan. Japan. Post-earthquake Post-earthquake usability usability assessment assessment is commonly is commonly aimed aimed at evalua at evaluatingting the possible the possible short- termshort-term use of use a building of a building [6,7]. [During6,7]. During the assessment, the assessment, the buildings the buildings that thatcan canbe safely be safely used used in case in case of aftershocksof aftershocks are are determined, determined, together together with with the the emergency emergency measures measures to tobe beadopted. adopted. Figure 1. Epicenters of major events of the 2017 Pohang earthquake [[1].1]. Table 1. Major events of the 2017 Pohang earthquake [[1].1]. DateDate andand TimeTime of of EpicenteEpicenterr DepthDepth EarthquakeEarthquake MML L OccurrenceOccurrence (KST)(KST) LatitudeLatitude Longitude Longitude (km)(km) MainshockMainshock 2017-11-152017-11-15 14:29:31 14:29:31 36.11 36.11 129.37 129.37 7.0 7.0 5.4 5.4 Aftershock1Aftershock1 2017-11-152017-11-15 16:49:30 16:49:30 36.12 36.12 129.36 129.36 10.0 10.0 4.3 4.3 Aftershock2Aftershock2 2017-11-162017-11-16 09:02:42 09:02:42 36.12 36.12 129.37 129.37 8.0 8.0 3.6 3.6 Aftershock3Aftershock3 2017-11-192017-11-19 23:45:47 23:45:47 36.12 36.12 129.36 129.36 9.0 9.0 3.5 3.5 Aftershock4Aftershock4 2017-11-202017-11-20 06:05:15 06:05:15 36.14 36.14 129.36 129.36 12.0 12.0 3.6 3.6 Aftershock5Aftershock5 2017-12-252017-12-25 16:19:22 16:19:22 36.11 36.11 129.36 129.36 10.0 10.0 3.5 3.5 Aftershock6Aftershock6 2018-02-112018-02-11 05:03:03 05:03:03 36.08 36.08 129.33 129.33 9.0 9.0 4.6 4.6 Thus, the estimation of the earthquake earthquake damage mechanism with consideration of its correlations correlations with the seismic source and site effects isis essentialessential toto understandunderstand the comprehensivecomprehensive engineering seismological characteristics and establish appropriate site-specificsite-specific earthquake recovery plans. In this study, methodical solutions for post-earthquake assessments were proposed and applied to the epicenter area of the Pohang earthquake in order to understand the earthquake hazards induced by the seismic site effects. effects. To To underst understandand the site-specific site-specific earthquake impact correlated with seismic site effects, the framework for the post-earthquake damage survey was established on the basis of the ISPRS Int. J. Geo-Inf. 2018, 7, 375 3 of 15 ISPRS Int. J. Geo-Inf. 2018, 7, x FOR PEER REVIEW 3 of 15 effects,established the frameworkon the basis for of the the post-earthquake seismic site effect damages, and survey involved was four established steps: (a) on construction the basis of theof seismicgeo-data; site (b) effects, site-classification and involved of four seismic steps: site (a) constructioneffects; (c) post-earthquake of geo-data; (b) site-classificationsurvey; (d) seismic of seismicimpact siteanalysis effects; based (c) post-earthquake on the site effects survey; (Figure (d) seismic 2). Co impactnsidering analysis the effects based onof thesurface site effectsand subsurface (Figure2). Consideringtopography theincluding effects ofthe surface basin andeffect subsurface in the epic topographyenter area including of the Pohang the basin earthquake, effect in the epicenterfirst, the areageo-data of the was Pohang constructed earthquake, by collecting first, the multiple geo-data sources was constructedof geospatial by inform collectingation. multiple Second, sourcesthe site ofclassification geospatial information.system was proposed Second, theon sitethe classificationbasis of the systemmulti-variated was proposed site response on the parameters. basis of the multi-variatedThird, the impact site responseof the earthquake parameters. on Third, a buil theding impact in Pohang of the earthquake was estimated on a building according in Pohangto the wasempirical estimated criteria according for geotechnical to the empirical and structural criteria forhazards geotechnical on the basis and structuralof the post-earthquake hazards on the survey. basis ofFourth, the post-earthquake the spatial correlations survey. Fourth, of the theearthquake-i spatial correlationsnduced, site-specific of the earthquake-induced, site effects and site-specificearthquake sitedamage effects were and analyzed earthquake for damagethe representative were analyzed damaged for the building. representative Thus, a damaged generic post-earthquake building. Thus, adamage generic survey post-earthquake for geotechnical damage assessment survey for geotechnical was established assessment as a version was established of the proposed as a version field of theinvestigation proposed fieldform, investigation and immediately
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