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 form, andapplied immediately to the Gyeoungju applied to Earthquake the Gyeoungju (ML Earthquake 5.8, 12 September (ML 5.8, 122016) September and 2017 2016) Pohang and earthquake 2017 Pohang (M earthquakeL 5.4, 15 November (ML 5.4, 152017). November 2017).

Figure 2.2. Post-earthquakePost-earthquake survey survey framework framework for for rapid rapi earthquaked earthquake damage damage estimation estimation correlated correlated with seismicwith seismic site effects. site effects. 2. Seismic Site Effects in Pohang Based on Geo-Data 2. Seismic Site Effects in Pohang Based on Geo-Data In general, the term “site amplification” refers to the increase in the amplitude of seismic In general, the term “site amplification” refers to the increase in the amplitude of seismic waves waves during their propagation through soft soil layers. Accounting for such effects is critical in during their propagation through soft soil layers. Accounting for such effects is critical in the the formulation of seismic regulations, land-use planning, and the seismic design of critical facilities. formulation of seismic regulations, land-use planning, and the seismic design of critical facilities. Therefore, the amplified seismic response related to the intrinsic characteristics of regions and sites must Therefore, the amplified seismic response related to the intrinsic characteristics of regions and sites be determined from a quantitative assessment of the geotechnical and environmental characteristics. must be determined from a quantitative assessment of the geotechnical and environmental Comprehensive information regarding the geotechnical seismic response datasets or results as spatial characteristics. Comprehensive information regarding the geotechnical seismic response datasets or information for the wide target area [8] should be produced using advanced GIS (geographic results as spatial information for the wide target area [8] should be produced using advanced GIS information system) methods applied to the seismic response classification or spatial zonation of (geographic information system) methods applied to the seismic response classification or spatial the wide target area [9]. In this study, the quantitative regional seismic response characteristics of zonation of the wide target area [9]. In this study, the quantitative regional seismic response Pohang were assessed using the optimized GIS platform. Pohang is the largest coastal urban area in characteristics of Pohang were assessed using the optimized GIS platform. Pohang is the largest Gyeongsangbuk-do and is located on the south-eastern edge of the Korean Peninsula. An extended area coastal urban area in Gyeongsangbuk-do and is located on the south-eastern edge of the Korean (22.7 km west to east × 26.1 km north to south) encompassing the study area (Figure3) was considered. Peninsula. An extended area (22.7 km west to east × 26.1 km north to south) encompassing the study Kriging interpolation based on a geostatistical analysis was expected to produce more reliable area (Figure 3) was considered. predictions of unknown geotechnical data from known geotechnical data than extrapolation in the Kriging interpolation based on a geostatistical analysis was expected to produce more reliable spatial domain. We introduced the concept of an extended area encompassing the study area [4,9]. predictions of unknown geotechnical data from known geotechnical data than extrapolation in the Figure3 shows the geographic information for Pohang and the selected areas (extended area and spatial domain. We introduced the concept of an extended area encompassing the study area [4,9]. study area within the territory of Pohang city) with the satellite image. Based on the geo-data, a spatial Figure 3 shows the geographic information for Pohang and the selected areas (extended area and zonation map of the site-specific seismic response parameters was generated and visualized for use study area within the territory of Pohang city) with the satellite image. Based on the geo-data, a spatial zonation map of the site-specific seismic response parameters was generated and visualized for use in a regional seismic strategy. We chose the target study area as the entire territory, focusing on the epicenters and damaged buildings of the 2017 Pohang earthquake.

ISPRS Int. J. Geo-Inf. 2018, 7, 375 4 of 15 in a regional seismic strategy. We chose the target study area as the entire territory, focusing on the epicenters and damaged buildings of the 2017 Pohang earthquake. ISPRS Int. J. Geo-Inf. 2018, 7, 375 4 of 15

FigureFigure 3. 3. ExtendedExtended and and target target areas areas for for spatial spatial modeling modeling of of geo-data geo-data inin Pohang. Pohang.

2.1. Construction Construction of of Geo-Data Geo-Data in in the the Pohang Pohang Target Target Area Area TheThe geo-data geo-data included included geotechnical geotechnical investigation investigation data, data,a DEM a (digital DEM elevation (digital elevationmodel, 5 × model, 5 m mesh5 × 5 msize),mesh digital size), digitalnumerical numerical information information (e.g., (e.g.,watershed watershed and and administrative administrative boundaries), boundaries), infrastructureinfrastructure information, information, and and geological geological maps maps (scale (scale 1:250,000) 1:250,000) (Figure (Figure 4).4). The The target target area area was was first first separated into into 10-m-mesh 10-m-mesh areas areas based based on on the the DEM, DEM, yielding yielding 2,296,288 2,296,288 spatial spatial grids. grids. Component Component mesh-unit datasets datasets were were created created for for each each spatial spatial gr grid.id. First, First, we we gathered gathered existing existing borehole borehole data data and and conducted site site visits visits across across the the study study area area to acquire to acquire surface surface geo-knowledge geo-knowledge data. data.The subsurface The subsurface soil layerssoil layers identified identified from from the theborehole borehole data data were were classified classified into into five five categories: categories: fill, fill, alluvial alluvial soil, soil, weathered soil, weathered weathered rock, rock, and and bedrock bedrock [8]. [8]. The The surface surface geo-knowledge geo-knowledge datasets datasets (bedrock (bedrock outcrop data) data) were were established established with with a a geotechnical geotechnical grou groundnd survey survey (e.g., (e.g., using using a simple a simple cone cone test, test, GPS) GPS) at grid-type locations,locations, andand cross-checkedcross-checked with with the the geotechnical geotechnical layers layers from from neighboring neighboring borehole borehole data. data.Spatial Spatial estimates estimates for the for five the categories five categories of geotechnical of geotechnical layer acrosslayer across the extended the extended area were area collected were collectedfrom approximately from approximately 2562 existing 2562 boreholeexisting borehole datasets anddatasets approximately and approximately 150 surface 150 geo-knowledge surface geo- knowledgedatasets. In addition,datasets. aIn 1:250,000-scale addition, a geological1:250,000-scale map withgeological lithofacies, map geologic with lithofacies, boundaries, geologic and fault boundaries,information and was fault obtained information from the was geologic obtained information from the geologic system of information the Korea Institute system of of the Geoscience Korea Instituteand Mineral of Geoscience Resources and (Figure Mineral4c) Resources [ 2]. The Pohang(Figure 4c) Basin [2]. consistsThe Pohang of Miocene Basin consists (~20 millionof Miocene years (~20ago) million non-marine years toago) deep non-marine marine sedimentary to deep marine strata sedimentary with bedrocks strata [10 with]. A post-earthquakebedrocks [10]. A post- impact earthquakeevaluation ofimpact damaged evaluation buildings of damaged was also buildings conducted was using also structure conducted information, using structure which information, was collected whichbased onwas the collected registered based building on the dataregistered (structure building type, da numberta (structure of stories, type, etc.) number in Pohang. of stories, etc.) in Pohang.

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(a) (b)

(c)

FigureFigure 4. 4.Geospatial Geospatial grid grid information information of theof th representativee representative surface surface datasets: datasets: (a) digital (a) digital elevation elevation model; (b)model; slope; ( (bc) slope; geological (c) geological map. map.

ToTo estimate estimate the the geo-layer geo-layer spatiallyspatially forfor thethe targettarget area in Pohang, we we applied applied the the optimized optimized site-specificsite-specific geostatistical geostatistical modeling modeling method method using using ordinary ordinary kriging kriging [ 11[11]] to to the the extended extended areaarea (Figure(Figure5 ). Ordinary5). Ordinary kriging kriging was expectedwas expected to produce to produce more more reliable reliable predictions predictions of unknown of unknown geotechnical geotechnical data fromdata known from known geotechnical geotechnical data than data extrapolation than extrapol ination the spatial in the domain.spatial domain. Accordingly, Accordingly, the optimum the geostatisticaloptimum geostatistical interpolation interpolation method was determinedmethod was as ordinarydetermined kriging as ordinary using the kriging maximum using zonation the methodmaximum at unit zonation grid-cells method (10 × at10 unit m) forgrid-cells the extended (10 × 10 Pohang m) for area. the extended Figure5 presents Pohang spatialarea. Figure zonation 5 informationpresents spatial for identifying zonation information the thickness for distributionidentifying the of thethickness fill, alluvial distribution soil and of the weathered fill, alluvial layer (weatheredsoil and weathered soil and layer rock) (weathered as the main soil layers and rock) on top as the of main the bedrock. layers on Thetop of fill the layer bedrock. was The spatially fill layer was spatially distributed, concentrated on the reclaimed housing complex within 7.3 m. The distributed, concentrated on the reclaimed housing complex within 7.3 m. The plains including the plains including the coasts and rivers, and the thick alluvial soil (maximum thickness of ~20.5 m) on coasts and rivers, and the thick alluvial soil (maximum thickness of ~20.5 m) on some hills was some hills was modeled in the study area. In addition, weathered soil and rock exposed to modeled in the study area. In addition, weathered soil and rock exposed to long-term weathering, long-term weathering, was also found in the target area and evenly distributed up to 20.2 m. Such was also found in the target area and evenly distributed up to 20.2 m. Such zones involving thick zones involving thick soil and the bedrock at a large depth are susceptible to ground motion soil and the bedrock at a large depth are susceptible to ground motion amplification. To determine amplification. To determine reliable site-specific criteria for geotechnical layer classification reliableconsidering site-specific the local criteria site effects, for geotechnical the cross-validation-based layer classification root consideringmean square the error local (RMSE) site effects, for

ISPRS Int. J. Geo-Inf. 2018, 7, 375 6 of 15 ISPRS Int. J. Geo-Inf. 2018, 7, x FOR PEER REVIEW 6 of 15 thegeo-layers cross-validation-based was calculated root as mean 1.2 squarem on erroraverage (RMSE) [12]. forFor geo-layers comparison, was calculatedthe RMSE as of 1.2 mthe on averagecross-validation [12]. For result comparison, was the thesquare RMSE root ofof thethe cross-validationaverage squared distance result was of a the data square point rootfrom ofthe the averagefitted line, squared as calculated distance with of a datathe following point from equation: the ﬁtted line, as calculated with the following equation:

s n 11 ^ 2 = (yˆ − y ) RMSERMSE = ∑ ii , ,(1) (1) n i=1 where yi and yî are^ the measured and estimated values of the ith data point, respectively, and n is the where and are the measured and estimated values of the ith data point, respectively, and n is total number of data points. As the RMSE approaches zero, the estimation becomes more accurate. the total number of data points. As the RMSE approaches zero, the estimation becomes more The coefficient of variation is the ratio of the RMSE to the mean of the dependent variable [13]. accurate. The coefficient of variation is the ratio of the RMSE to the mean of the dependent variable The RMSE for four representative geo-layers was evaluated and the results are presented in Figure5. [13]. The RMSE for four representative geo-layers was evaluated and the results are presented in Thus, the geospatial grid map can provide intuitive information for solving geotechnical engineering Figure 5. Thus, the geospatial grid map can provide intuitive information for solving geotechnical problems and making decisions. In this study, the geospatial grid information, constructed by the engineering problems and making decisions. In this study, the geospatial grid information, collectedconstructed geo-data, by the was collected used asgeo-data, the base was value used for as seismic the base site value classification. for seismic site classification.

(a) (b)

FigureFigure 5. 5. GeospatialGeospatial grid information information of of representative representative geo-layers: geo-layers: (a) fill (a) layer; ﬁll layer; (b) alluvial (b) alluvial soil; ( soil;c) (c)weathered weathered soil; soil; (d ()d weathered) weathered rock. rock.

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2.2. Site Classification Framework Based on Multivariate Site Response Parameters Multivariate-based site classification was proposed based on empirical correlations with site response parameters, and it was performed for the regional zonation of seismic site effects to identify the amplification of soil characteristics in South Korea depending on the site-specific geo-spatial conditions [9,14]. To optimize the classification criteria by considering the geotechnical uncertainty and local site effects in South Korea, a dual framework for a site classification system with site response parameters and geo-proxies was established (Table2). In this study, to secure the practical seismic utilization of the fundamental site period, a site classification system according to the site period was introduced, as listed in Table2, from the results of the previous studies on improvement methods of the geotechnical classification system [2]. In addition, the corresponding fundamental frequency (f 0) was also classified according to TG. Site classification systems use seismic response parameters related to the geotechnical characteristics of the study area as the classification criteria. The current site classification systems in South Korea and the United States recommend VS30, which is the average VS up to 30 m underground. This criterion uses only the dynamic characteristics of the site without considering its geometric distribution characteristics [14]. Conversely, the bedrock depth (H), which has also been considered an empirical brief index, reflects only the geometric characteristics of the site without considering VS, which is the soil stiffness. Additionally, the site period (TG) has been recently considered by many researchers [15,16], and is presented as a reference indicator that reflects both the geotechnical dynamic and geometric characteristics of the target site. In this study, spatial zonation was performed for three geotechnical response parameters (VS30, H, and TG).

Table 2. Site classiﬁcation criteria for multivariate site response parameters (modiﬁed after [9]).

Geotechnical Criteria Geo-Proxy-Based Criteria Generic Site Class H V Slope Elevation Description S30 T (s) f (Hz) Geology (m) (m/s) G 0 (%) (m) Rock B <6 >760 <0.06 >16.67 >5.6 >80 Plutonic/metamorphic rocks Cretaceous ﬁne-grained Weathered C1 <10 >620 <0.10 >10.00 >3.5 >60 Rock and sediments Very Stiff Soil C Sheared/weathered crystalline C2 <14 >520 <0.14 >7.14 >2.0 >45 rocks Oligocene–Cretaceous Intermediate C3 <20 >440 <0.20 >5.00 >1.1 >31 sedimentary rocks Stiff Soil Coarse-grained younger C4 <29 >360 <0.29 >3.45 >0.62 >22 material D1 <38 >320 <0.38 >2.63 >0.23 >18 Miocene ﬁne-grained sediments Deep Stiff D D2 <46 >280 <0.46 >2.17 >0.08 >14 Coarse younger alluvium Soil D3 <54 >240 <0.54 >1.85 >0.023 >11 Holocene alluvium Fine-grained alluvial/estuarine D4 <62 >180 <0.62 >1.61 >0.006 >9 deposits Deep Soft E ≥62 ≤180 ≥0.62 ≤1.61 ≤0.006 ≤9 Inter-tidal mud Soil

The equations for the site effect parameters can be summarized as follows. First, VS30, which is primarily applied in the current site classiﬁcation system, can be calculated as:

n di Vs30 = 30/ ∑ (2) i=1 VSi di and VSi represent the thickness and average VS of the ith layer up to 30 m underground, respectively. In this case, the sum of di is 30 m. In addition to the existing VS30, various site response parameters are examined and identiﬁed, including TG, VS, DS (DS < 30 m) and VS,soil. TG can be calculated as: ISPRS Int. J. Geo-Inf. 2018, 7, 375 8 of 15

ISPRS Int. J. Geo-Inf. 2018, 7, x FOR PEER REVIEW n 8 of 15 di TG = 4 ∑ (3) VSi i=1 Oligocene–Cretaceous C3 <20 >440 <0.20 >5.00 >1.1 >31 Intermediate sedimentary rocks Additionally, the site period (TG) rises as the bedrock depth (H = ∑Di) increases. TG is a useful Stiff Soil Coarse-grained younger indicator of the periodC4 of<29 vibration, >360 during <0.29 which >3.45 the most >0.62 significant amplification >22 is expected. material In addition, by considering the application of seismic design criteria andMiocene the site-specificfine-grained D1 <38 >320 <0.38 >2.63 >0.23 >18 characteristics of South Korea, a geo-proxy-based empirical site classification system wassediments applied to the Deep Stiff D2 <46 >280 <0.46 >2.17 >0.08 >14 Coarse younger alluvium target area in Pohang.D The slope and elevation information of the surface terrain based on DEM, which maySoil correspond to theD3 major<54 terrain >240 factors <0.54 related >1.85 to the geotechnical >0.023 geo-layer >11 formation, Holocene alluvium were first Fine-grained regarded as topographicalD4 <62 indicators >180 and <0.62 defined >1.61 as geo-proxies >0.006 [9]. Additionally,>9 a geology map alluvial/estuarine deposits wasDeep used Soft to indicate the geological indicators for the geo-proxy to determine the surficial geological E ≥62 ≤180 ≥0.62 ≤1.61 ≤0.006 ≤9 Inter-tidal mud characteristicsSoil in Pohang.

2.3.2.3. Site-SpecificSite-Specific ZonationZonation ofof SeismicSeismic SiteSite EffectsEffects inin PohangPohang TheThe seismicseismic zoneszones dependingdepending onon thethe groundground motionmotion werewere determineddetermined byby performingperforming sitesite classificationclassification andand subsequentlysubsequently calculatingcalculating thethe sitesite coefficientscoefficients usingusing aa sitesite classificationclassification systemsystem thatthat includesincludes geotechnical geotechnical classification classification criteria criteria (Figure (Figure6) and 6) geo-proxy-based and geo-proxy-based criteria (Figure criteria7), (Figure according 7), toaccording Table2. Theto Table study 2. areaThe study had a area maximum had a maximum bedrock depth bedrock of approximately depth of approximately 28 m, and 28 a bedrockm, and a atbedrock a depth at ofa 20depth m was of 20 primarily m was distributedprimarily distri in thebuted central in the plain central (Heunghae plain (Heunghae basin) and basin) reclaimed and downtownreclaimed downtown (Jangseong-dong) (Jangseong-dong) near the coast near (Figure the coast6a). (FigureVS30 ranges 6a). V fromS30 ranges approximately from approximately 320 m/s to 520320 m/sm/s into the520 areasm/s in including the areas the including plains near the theplains coast, near where the coast, residential where and residential industrial and facilities industrial are concentratedfacilities are (Figureconcentrated6b). Additionally, (Figure 6b).T Additionally,G ranges from T approximatelyG ranges from0.20–0.38 approximately s in most 0.20–0.38 plains nears in themost coast plains (Figure near6c). the The coast earthquake (Figure vulnerability 6c). The earthquake is based on vulnerability the structure is resonance based on possibility, the structure and byresonance using the possibility, site-period and distribution by using information, the site-period it can distribution be predicted information, for facilities withit can 2–4 be floorspredicted in plain for regionsfacilities that with include 2–4 floors several in residentialplain regions and that industrial include facilities. several This residential estimation and considers industrial the facilities. natural periodThis estimation of a building considers floor as 0.1the snatural [17]; however, period theof Ta Gbuildingof major floor target as sites 0.1 is s approximately [17]; however,0.20–0.38 the TG of s. Consideringmajor target thatsites the is studyapproximately area is not 0.20–0.38 only a residential s. Considering but also that an industrial the study and area commercial is not only area, a itresidential is likely that but seismic also an performance industrial assessmentand commercial and seismic area, it reinforcement is likely that are seismic required performance for a large numberassessment of buildings and seismic and reinforcement structures. are required for a large number of buildings and structures.

(a) (b)

Figure 6. Cont.

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(c)

FigureFigure 6.6. SeismicSeismic zonationzonation forfor geotechnicalgeotechnical response-parameter-based,response-parameter-based, site-speciﬁcsite-specific effects:effects: ((a)) HH;(; (bb)) VVSS30;;( (cc)) TTGG. .

FigureFigure7 a7a presents presents the the spatial spatial distribution distribution of of elevation-based elevation-based site site classes. classes. Site Site class class E (elevation E (elevation of lessof less than than 9 m) 9 was m) spatiallywas spatially categorized categorized to represent to represent most of most the coastal of the and coastal plain and regions, plain excluding regions, theexcluding mountainous the mountainous area. Similar spatialarea. Similar trends werespatial noted trends in the were zonation noted of thein slope-basedthe zonation site of class the (Figureslope-based7b), except site class in parts (Figure of the 7b), plain except areas. in Theparts average of the and plain standard areas. deviationThe average of theand slope standard was 0.013deviation m/m of and the 0.0015 slope m/m,was 0.013 respectively. m/m and The 0.0015 site classes m/m, respectively. D1–D3, which The had site an averageclasses D1–D3, slope of which 0.005 m/m,had an were average concentrically slope of 0.005 distributed m/m, inwere the concentr central andically southeast distributed plain in areas. the central Using geology-basedand southeast siteplain class areas. determination, Using geology-based D1 and D2 site were class categorized determination, to represent D1 and theD2 centralwere categorized plain and coast,to represent which werethe central covered plain with and Quaternary coast, which alluvium were depositscovered underlainwith Quaternary by thick alluvium Tertiary sedimentsdeposits underlain (Figure7 c).by Thethick central Tertiary and sediments south plains, (Figure in which 7c). The alluvium central areas and were south located plains, at in an which elevation alluvium of less areas than 10were m, werelocated classified at an elevation as D2–E; of the less slope than in this10 m, case were was classified 0.0008 m/m. as D2–E; the slope in this case was 0.0008 m/m.To consider local geotechnical characteristics and topographic effects, the proposed TG-based and slope-basedTo consider (withlocal thegeotechnical highest resolution) characteristics site classification and topographic was determined effects, the asproposed a replacement TG-based of theand more slope-based conservative (with criteriathe highest for seismicresolution) design site andclassification performance was evaluation.determined Theas a sitereplacement class of the of slope-basedthe more conservative zonation was criteria categorized for seismic as a design more vulnerable and performance grade in evaluation. the partially The plain site andclass coastal of the areas,slope-based where zonation borehole was datasets categorized were lacking. as a more Consequently, vulnerable grade the complementary in the partially siteplain classification and coastal consideringareas, where geotechnical borehole datasets and geometrical were lacking. characteristics Consequently, was reasonable the complement becauseary the site closest classification borehole datasetsconsidering of the geotechnical downtown plainand geometrical area in the studycharacteristics area were spatiallywas reasonable interpolated because to compute the closest the siteborehole response datasets parameters. of the Indowntown this study, plainTG, whicharea in estimated the study the area intensified were spatially amplification interpolated property to influencingcompute the the site seismic response fragility parameters. of the structure, In this was study, selected TG, aswhich the representativeestimated the criterion intensified for comparisonamplification of theproperty earthquake influencing impact usingthe seismic a post survey,fragility in of place the of structure, the more conservativewas selected criterion as the followedrepresentative in seismic criterion design for and comparison performance of the evaluation. earthquake impact using a post survey, in place of the more conservative criterion followed in seismic design and performance evaluation.

ISPRS Int. J. Geo-Inf. 2018, 7, 375 10 of 15 ISPRS Int. J. Geo-Inf. 2018, 7, x FOR PEER REVIEW 10 of 15

(a) (b)

(c)

FigureFigure 7.7.Seismic Seismic zonation zonation for for geo-proxy-based, geo-proxy-based, site-speciﬁc site-specific effects: effects: (a) elevation; (a) elevation; (b) slope; (b) ( cslope;) geology (c) (geologicalgeology (geological description). description).

3.3. Post-EarthquakePost-Earthquake ImpactImpact EvaluationEvaluation ofof DamagedDamaged BuildingsBuildings ToTo identifyidentify thethe quantitativequantitative severityseverity ofof thethe earthquakeearthquake damage,damage, aa post-earthquakepost-earthquake assessmentassessment waswas conductedconducted consideringconsidering thethe fourfour daysdays immediatelyimmediately followingfollowing thethe occurrenceoccurrence of thethe mainshock,mainshock, inin accordanceaccordance with the the field field guidelines guidelines [18,19]. [18,19]. Clear Clear guidelines guidelines regarding regarding the the quantification quantification of the of theapparent apparent physical physical damage damage cannot cannot be established, be established, because because the thesensibility sensibility of the of the surveyor surveyor plays plays an anobvious obvious major major role role in inthis this aspect. aspect. However, However, indicators indicators (Table (Table 3)3) were defineddefined toto representrepresent thethe criticalitycriticality ofof thethe apparentapparent damagedamage dependingdepending onon thethe manifestationmanifestation ofof geotechnicalgeotechnical andand structurestructure hazardshazards [[20,21].20,21]. TheThe geotechnicalgeotechnical hazardhazard criterioncriterion (classified(classified intointo 1010 gradesgrades fromfrom 0–9)0–9) includesincludes thethe descriptiondescription of the morphology of of the the site site where where the the building building is is located, located, and and the the possible possible presence presence of ofvisible visible soil soil instabilities, instabilities, with with a adistinction distinction betw betweeneen unstable unstable slopes slopes and settlements affectingaffecting thethe buildingbuilding foundations.foundations. As the structural damage classificationclassification is based on crack type (shear, flexural)flexural) andand failurefailure mode mode (in (in plane, plane, overturning), overturning), a damage a damage pattern pattern categorization categorization with ten with grades ten wasgrades defined. was defined.

ISPRS Int. J. Geo-Inf. 2018, 7, 375 11 of 15

Table 3. Criteria for geotechnical hazard and structure hazard for the post-earthquake survey.

Criteria for Geotechnical Hazard Criteria for Structure Hazard Grade of Grade of Structure Geotechnical Damage Structure Damage Geotechnical Hazard Hazard Negligible 0 Negligible 0 Liquefaction settlements 1 Hairline crack at in fill/column joints 1 < 50 mm Liquefaction settlements 2 Hairline cracks in structure & in-fill 2 50–200 mm Liquefaction settlements Some frame elements yielded. Larger 3 3 > 200 mm cracks in in-fill Vertical foundation Larger flexural cracks and spalling. Some 4 4 movement < 50 mm crushing of in-fill at corners Vertical foundation Some failure to non-ductile elements. Most 5 5 movement 50–100 mm in-fill exhibits large cracks, minor falls Vertical foundation Many failures to non-ductile elements. 6 6 movement > 100 mm Some in-fill fallen or bulged Most non-ductile elements failed. Severe Slight horizontal spreading 7 deformation. Most in-fill fallen or severely 7 < 25 mm damaged Moderate horizontal 8 Full structure in danger of collapse 8 spreading 25 to 100 mm Severe horizontal spreading 9 Destruction 9 > 100 mm

Accordingly, the major information from the field assessment was reported for each damaged building (Table4), and the peak ground acceleration (PGA), TG-based site class, earthquake manifestation, and grades of hazards were summarized. The earthquake exposure grade was determined by integrating the grade of geotechnical and structural hazards and involved the five damage state categories—‘Safe’, ‘Slight’, ‘Moderate’, ‘Extensive’, ‘Complete’—considered as fragility and vulnerability indicators in the building-damage-state descriptions derived by Hazus [22] (Figure8). Most buildings that suffered damage to structural or non-structural elements were spatially distributed within 5 km of the epicenter of the mainshock; the damage due to subsidence that may be caused by liquefaction at the central plain near the epicenters was not considered. However, the liquefaction potential and induced subsidence in the study area were not clearly identified and have been studied in further research.

Table 4. Example of the post-earthquake impact of the 11.15 Pohang earthquake.

Grade of Grade of Grade of Structure Category of PGA T -Based Geotechnical Geotechnical Structure Structural Earthquake G Type Structure (g) Site Class Manifestation Hazard Manifestation Hazard Exposure (Stories) (A) (B) (A + B) Concrete Major failures to Elementary Subsidence 8 0.21 C3 Moment 1 non-ductile 7 school (2~7 cm) (Extensive) Frame (3) element Some failures to Steel Moment Subsidence 8 Apartment 0.2 C4 2 non-ductile 6 Frame (5) (15~25 cm) (Extensive) element Concrete Larger ﬂexural 4 University 0.2 C3 Moment - 1 4 cracks (Moderate) Frame (6) Concrete Some failures to 5 Row house 0.21 D1 Moment - 0 non-ductile 5 (Extensive) Frame (3) element Concrete Hairline cracks at 2 Bus terminal 0.12 C1 Moment - 0 2 in ﬁll (Slight) Frame (2) ISPRS Int. J. Geo-Inf. 2018, 7, 375 12 of 15

Table 4. Example of the post-earthquake impact of the 11.15 Pohang earthquake.

Grade of Grade of Grade of Structure Category of PGA T -Based Geotechnical Geotechnical Structure Structural Earthquake G Type Structure (g) Site Class Manifestation Hazard Manifestation Hazard Exposure (Stories) (A) (B) (A + B) Unreinforced Masonry Hairline cracks at 2 Row house 0.17 C1 - 0 2 Bearing Walls in fill (Slight) (5) Concrete Hairline cracks at 1 Hospital 0.13 C2 Moment - 0 1 ISPRS Int. J. Geo-Inf. 2018, 7, x FOR PEER REVIEW in fill (Slight) 12 of 15 Frame (5) Unreinforced (5)Masonry Larger flexural 5 Market 0.22 C3 Unreinforced - 0 5 Bearing Walls Largercracks flexural (Extensive)5 Market 0.22 C3 Masonry Bearing(2) - 0 5 cracks (Extensive) Walls (2) Unreinforced Unreinforced Masonry HairlineHairline cracks cracks at at in 22 RowRow house house 0.21 0.21 C3 C3 Masonry Bearing - -0 0 22 Bearing Walls infill fill (Slight)(Slight) Walls (4)(4) Unreinforced Detached Unreinforced Hairline cracks at in 3 Detached 0.18 C3 MasonryMasonry Bearing - 0 Hairline cracks at 3 3 house 0.18 C3 - 0 fill 3 (Moderate) house WallsBearing (2) Walls in fill (Moderate) (2)

Figure 8.8. SpatialSpatial distributiondistribution of of the the earthquake earthquake damage damage categories categories for for majorly majorly damaged damaged buildings buildings and imagesand images of buildings. of buildings. 4. Spatial Correlations between Earthquake Damage and Seismic Site Effect 4. Spatial Correlations between Earthquake Damage and Seismic Site Effect

The spatial correlations between the TG-based zonation and the surveyed earthquake damage The spatial correlations between the TG-based zonation and the surveyed earthquake damage state were evaluatedevaluated by overlayingoverlaying GIS-basedGIS-based layerlayer informationinformation (Figure9 9).). AlthoughAlthough therethere waswas nono distinct correspondence between the site-effect-dependentsite-effect-dependent vulnerability and the reported buildingbuilding damage, mostmost ofof the the severely severely damaged damaged buildings buildings (damage (damage classiﬁed classified as ‘moderate’ as ‘moderate’ or ‘extensive’) or ‘extensive’) were concentricallywere concentrically distributed distributed in site in classes site classes C2–D1, C and2–D1, downtown and downtown in the central in the plaincentral and plain near and the near east coast.the east Further, coast. Further, the response the response spectra spectra of the horizontal of the horizontal components components of PGA, ofand PGA, 5% and damped 5% damped linear elasticlinear elastic response response spectra spectra obtained obtained from the from rock the (free-ﬁeld) rock (free-field) seismic seismic station station and soil and seismic soil seismic station closeststation toclosest the epicenters, to the epicenters, were examined. were examin The energyed. The of theenergy Pohang of earthquakesthe Pohang wasearthquakes concentrated was concentrated in the high (or middle-high) frequency band (period of less than 1 s) of the response spectra. Thus, primarily, slight or moderate damage to low-rise structures (between 2–4 floors) in the plains, and non-structural failures were noted. A higher severity of seismic amplification due to site effects means that the site class changed from C2 to D1, leading to the earthquake exposure for buildings becoming more critical; however, a lower number of buildings were damaged due to the earthquake (Figure 10a). Besides the major earthquake, the site-specific soil amplification in low-to-moderate seismic regions significantly affects the amplitude, frequency, and duration of earthquake-induced ground shaking, and thereby influence the occurrence and degree of damage to buildings and other structures. Thus, the dominant frequency is more concentrated in the high frequency band (approximately 0.15), and buildings (between 2–3 floors) on the corresponding site classified as C2

ISPRS Int. J. Geo-Inf. 2018, 7, x FOR PEER REVIEW 13 of 15 ISPRS Int. J. Geo-Inf. 2018, 7, 375 13 of 15 were more influenced by the amplified ground motion. Specifically, higher amplification was also (approximatelyobserved for low 0.15), intensity and buildings ground (betweenmotions, 2–3particularly floors) on in the site corresponding class C2 in the site low classified period as (high C2 werefrequency more influencedrange). It is by possible the amplified to conclude ground that moti exposureson. Specifically, to low-rise higher structures amplification are focused was on also the observedharder site for class low dueintensity to the ground short duration motions, and particularly high frequency in site rangeclass ofC2 thein themainshock. low period Moreover, (high ISPRS Int. J. Geo-Inf. 2018, 7, 375 13 of 15 frequencythe damaged range). stories It is of possible a building to conclude were approximat that exposuresively proportional to low-rise structures to the ratio are of focused 10 times on the the T G harder(Figure site 10b). class Correspondently, due to the short duration this is andbased hi ghon frequency the natural range period of the of mainshock. 0.1 s according Moreover, to eachthe damagedinbuilding the high storiesfloor (or middle-high)[9]. of aThe building earthquake frequency were approximativvulnerability band (period elyis of basedproportional less thanon the 1 s) tostructure of the the ratio response resonance of 10 spectra. times possibility the Thus, TG (Figureprimarily,obtained 10b). slightusing Correspondently, orthe moderate site-period damage this distribution is based to low-rise on information, the structuresnatural andperiod (between thus, of 0.1the2–4 s earthquakeaccording ﬂoors) in to thevulnerability each plains, building and of floornon-structuralfacilities [9]. The with earthquake failurestwo to five were vulnerability stories noted. in A plains higher is based with severity on se theveral of structure seismic residential ampliﬁcation resonance and indust possibility duerial to facilities, site obtained effects couldmeans using be thethatpredicted. site-period the site class distribution changed from information, C2 to D1, and leading thus, to the the earthquake earthquake vulnerability exposure for of buildings facilities becomingwith two tomore five critical; stories however,in plains awith lower several number residential of buildings and wereindustrial damaged faciliti duees, tocould the earthquakebe predicted. (Figure 10a).

Figure 9. Spatial comparison between TG-based seismic zonation and earthquake damage category of buildings. Figure 9. SpatialSpatial comparison comparison between between TTG-basedG-based seismic seismic zonation zonation and and earthquake earthquake damage damage category category of buildings.of buildings.

(a) (b) Figure 10. Correlations based on seismic site class: (a) grade of earthquake exposure(b) and number of Figure 10. Correlations(a based) on seismic site class: (a) grade of earthquake exposure and number of damaged buildings; (b) building stories. Figuredamaged 10. Correlationsbuildings; (b based) building on seismic stories. site class: (a) grade of earthquake exposure and number of damagedBesides thebuildings; major ( earthquake,b) building stories. thesite-specific soil amplification in low-to-moderate seismic 5. Conclusions regions significantly affects the amplitude, frequency, and duration of earthquake-induced ground 5.shaking, ConclusionsOwing and therebyto the higher influence levels the of occurrence seismic amplification and degree of due damage to site-specific to buildings site and effects other structures.during the Thus,2017Owing thePohang dominant to theearthquakes, higher frequency levels it iswas moreof seismic expected concentrated amplification that ina thehigher due high intensityto frequency site-specific would band site (approximatelycorrespond effects during to 0.15),lower the 2017and buildingsPohang (betweenearthquakes, 2–3 floors)it was onexpected the corresponding that a higher site classifiedintensity aswould C2 were correspond more influenced to lower by the amplified ground motion. Specifically, higher amplification was also observed for low intensity ground motions, particularly in site class C2 in the low period (high frequency range). It is possible to conclude that exposures to low-rise structures are focused on the harder site class due to the short duration and high frequency range of the mainshock. Moreover, the damaged stories of a building ISPRS Int. J. Geo-Inf. 2018, 7, 375 14 of 15

were approximatively proportional to the ratio of 10 times the TG (Figure 10b). Correspondently, this is based on the natural period of 0.1 s according to each building ﬂoor [9]. The earthquake vulnerability is based on the structure resonance possibility obtained using the site-period distribution information, and thus, the earthquake vulnerability of facilities with two to ﬁve stories in plains with several residential and industrial facilities, could be predicted.

5. Conclusions Owing to the higher levels of seismic amplification due to site-specific site effects during the 2017 Pohang earthquakes, it was expected that a higher intensity would correspond to lower amplification if the site experienced more non-linear behavior. A survey of extensively damaged structures in residential areas near the epicenter, and the geological alluvium areas where the depth to the bedrock was, on average, 35 m, showed that the proportion of damaged buildings in the softer site (from class C2 to D1) was much higher. It can be concluded that low-rise structures with slight or moderate earthquake damage were concentrated on softer sites owing to their low focal depth (4 km), topographical effects, and high (or middle-high) frequency range of mainshocks. Finally, the geospatial zonation and correlations between post-earthquake impacts and seismic amplification should be analyzed considering multi-dimensional subsoil characteristics to establish a site-specific earthquake recovery plan. In order to identify the comparative correlations and spatial coincidence in earthquake exposure, the source and path effects of the 2017 Pohang Earthquake should be considered with multiple seismic zonation depending on the seismic site effects. Especially as the depth of the epicenters of the 2017 Pohang Earthquake was shallower than that of the 2016 Gyeongju Earthquake (Table1), the source effects corresponding to the focal mechanism and velocity structure of the bedrock should be analyzed in future research. Moreover, comparative analysis is limited without the seismic fragility of the structure, which influences the earthquake damage depending on the structure type, seismic design, and performance. Accordingly, the seismic fragility function of the damaged buildings can be derived by numerical modeling and compared with the severity of the exposure to damage.

Author Contributions: Conceptualization: H.-S.K. and C.-G.S.; Supervision: C.-G.S.; Data Curation: H.-I.C.; Writing: H.-S.K. Funding: This research received no external funding. Acknowledgments: The authors wish to express their gratitude for the support from the Basic Research Project of the Korea Institute of Geoscience and Mineral Resources (KIGAM). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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