sustainability

Article Seismic Performance of Ancient Masonry Structures in Rediscovered in 2016 M 5.8 Earthquake

Heon-Joon Park 1,* , Jeong-Gon Ha 2, Se-Hyun Kim 3 and Sang-Sun Jo 3,*

1 Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Korea 2 Korea Atomic Energy Research Institute (KAERI), Daejeon 34057, Korea; [email protected] 3 National Research Institute of Cultural Heritage (NRICH), Daejeon 34122, Korea; [email protected] * Correspondence: [email protected] (H.-J.P.); [email protected] (S.-S.J.); Tel.: +82-52-217-2850 (H.-J.P.); +82-42-860-9216 (S.-S.J.)  Received: 30 January 2019; Accepted: 6 March 2019; Published: 14 March 2019 

Abstract: The , which include the millennium-old capital of the Kingdom, are located in the region most frequently affected by seismic events in the Korean peninsula. Despite the numerous earthquakes documented, most of the stone architectural heritage has retained their original forms. This study systematically reviews and categorises studies dealing with the seismic risk assessment of the architectural heritage of the historic areas. It applies research methodologies, such as the evaluation of the engineering characteristics of subsoil in architectural heritage sites, site-specific analysis of the ground motions in response to earthquake scenarios, geographic information system (GIS)-based seismic microzonation according to the geotechnical engineering parameters, reliability assessment of dynamic centrifuge model testing for stone masonry structures and evaluation of seismic behaviour of architectural heritage. The M 5.8 earthquake that hit Gyeongju on 12 September 2016 is analysed from an engineering point of view and the resulting damage to the stone architectural heritage is reported. The study focuses on Cheomseongdae, an astronomical observatory in Gyeongju, whose structural engineering received considerable attention since its seismic resistance was reported after the last earthquake. Dynamic centrifuge model tests applying the Gyeongju Earthquake motions are performed to prove that it is not a coincidence that Cheomseongdae, a masonry structure composed of nearly 400 stone members, survived numerous seismic events for over 1300 years. The structural characteristics of Cheomseongdae, such as the well-compacted filler materials in its lower part, rough inside wall in contrast to the smooth exterior, intersecting stone beams and interlocking headstones are proven to contribute to its overall seismic performance, demonstrating outstanding seismic design technology.

Keywords: Gyeongju Historic Areas; 2016 Gyeongju Earthquake; site characterisation; site-specific ground response analysis; stone architectural heritage; dynamic centrifuge test; Cheomseongdae; ancient seismic design technique

1. Introduction Architectural heritage is a precious cultural asset that provides insight into a nation’s historical background. As such, the current generation has the duty to preserve them well for future generations. Since architectural heritage is exposed to the external environment, measures should be taken to protect them from both natural and manmade disasters. Systematic approaches based on scientific and engineering applications should be adopted to protect them from natural disasters such as earthquakes. Unlike seismic performance and design assessments of general buildings, the main focus of seismic risk

Sustainability 2019, 11, 1565; doi:10.3390/su11061565 www.mdpi.com/journal/sustainability Sustainability 2019, 11, 1565 2 of 13 assessments for architectural heritage should be on the maintenance of their unique values, authenticity and engineering performance. There are many interesting ways for geotechnical engineers to contribute to conservation issues for historic sites and monuments [1]. Geotechnical engineering also has a very important role to play because there are quite a number of historic sites and monuments affected by geotechnical risks of different types [2]. As all architectural heritages are standing on ground, the seismic risk assessments for architectural heritage can be initiated from geological and geotechnical approaches to understand the composition and the material property distribution of the beneath and surrounding ground, local site effect, and seismic zonation [3–6]. For the structural and architectural engineering perspective, the seismic behaviour of historic monument and architectural heritage composed of stone block or masonry structure have been evaluated analytically, numerically [7–11] and experimentally [12–15]. In Korea, the National Research Institute of Cultural Heritage [16] conducted a multidisciplinary research planning project on disaster prevention for architectural heritages. Detailed mid- and long-term research tasks were derived from this project, and various studies were conducted to develop an overarching cultural heritage disaster-prevention system. Among them, studies on seismic risk assessments were conducted on architectural heritage in historic cities from 2009 to 2012. In addition, a seismic risk assessment methodology for historic areas was proposed based on aspects of geotechnical earthquake engineering. A seismic risk assessment for architectural heritage at a historical site begins with an understanding of the engineering characteristics of the subsoil and architectural structure. First, an investigation of the subsoil and seismic response analysis are performed at an architectural heritage site to predict the level of ground shaking for each earthquake scenario. Furthermore, an information system based on a geographic information system (GIS) for determining the geotechnical earthquake engineering parameters can be established to estimate the damage potential for each historic area. Architectural heritage has different structural forms, systems and load transfer characteristics. Hence, the seismic behaviour of each architectural heritage must be analysed using an empirical or analytical approach through experimental or numerical modelling. To improve the efficiency of such research, major architectural heritage, each representative of their specific epochs and styles, should be selected and their seismic behaviour characteristics should be evaluated. In this research project, seismic risk assessments were carried out for the subsoils of historical sites in Gyeongju, Buyeo and Seoul in 2009, 2010 and 2011, respectively. A geotechnical earthquake engineering database of over 70 architectural heritage sites in these three historical cities was established by means of in situ tests [17]. This database served as the basis for setting up a GIS-based geotechnical earthquake engineering information system for Gyeongju and Buyeo [18]. Stone architectural heritage exhibits dynamic seismic behaviour due to the friction between the stone members. This behaviour can be evaluated through centrifuge testing using scaled-down models [19]. The three-story stone ( (Shakamuni Stupa)) of Temple and the five-story stone pagoda of Jungnimsa Temple, which are representative of the architectural style of the time, were evaluated along with their seismic behaviours using multiple dynamic centrifuge tests with a shaking table [20]. The dynamic centrifuge tests were conducted for scaled models of Cheomseongdae, an astronomical observatory in Gyeongju that has unique structural features [21]. The results of this study were used in 2012 as basic data to establish an architectural heritage seismic performance and risk assessment method and to propose standards for seismic risk assessments. The M 5.8 earthquake that hit Gyeongju on 12 September 2016 is analysed in this study from an engineering point of view and the related damages to the stone architectural heritage are reported with a focus on Cheomseongdae. Dynamic centrifuge model tests that apply the Gyeongju Earthquake motions are performed to prove that it is not a coincidence that Cheomseongdae, a masonry structure composed of nearly 400 stone members, survived numerous seismic events for over 1300 years. Based on the assessment of the damage to the stone architectural heritage and the results of earthquake Sustainability 2019, 11, 1565 3 of 13 monitoring and dynamic centrifuge model tests for Cheomseongdae, the efforts before and after the Gyeongju earthquake and its reliability are verified from an engineering viewpoint.

2. Gyeongju Historic Areas and Cheomseongdae The Gyeongju Historic Areas, the millennium-old capital of the Silla Kingdom (57 B.C.–935 A.D.) and UNESCO World Heritage Sites, are the representative heritage areas, as they are the cradle and pivot of the development of Buddhism in the Korean peninsula. The vibrant Buddhist culture in the area resulted in the creation of numerous stone heritages such as , lanterns, bridges and revetments. Many of the existing stone heritage have structures that are seemingly susceptible to collapse under lateral loads such as those produced during earthquakes. However, despite the numerous earthquakes documented, most of the aforementioned architectural heritage remained intact for over a millennium. The Gyeongju Historic Areas are adjacent to the Yangsan Fault, and many seismic events in this area have been documented. In Samguksagi (a historical record of the Three Kingdoms), there is a record of an earthquake in 779 that killed more than 100 people and is the largest documented estimation of casualties due to an earthquake. The scroll, whose deciphered content began to be released in 2007, was discovered in 1966 while dismantling the three-story stone pagoda in the Bulguksa Temple. It contains records of great damage to the Bulguksa Temple facilities and Seokgatap pagoda due to earthquakes in 1013 and 1036. The scroll also describes the entire process from dismantling to rebuilding of the Seokgatap pagoda. Park et al. [18] summarised and systemised the pre-existing site investigation databases of 50 sites, the results of in situ experiments conducted at 32 architectural heritage sites in the Gyeongju area and the results of the site-specific ground response analyses for scenario earthquakes. Using these data and site-specific response analysis results, they generated a GIS-based visualisation of the bedrock depths, natural ground periods, peak ground accelerations and ground amplification ratios for an earthquake scenario. Gyeongju is located in a basin of plains and low hilly areas with settlements and paddies/fields surrounded by mountains, through which the Hyeongsan River flows. The river is fed by various tributaries, such as Bukcheon (north stream) and Namcheon (south stream). The ground here is predominantly composed of a gravelly alluvial soil layer below a ~1–4 m-deep gravelly fill layer in which weathered soils, residual soils, weathered rocks and soft rock layers are locally distributed. The depth of the soft rocks varies greatly according to the drilling location and are distributed 8 to 50 m below the ground surface. Located between the Bukcheon and Namcheon streams in the centre of the Gyeongju Historic Areas, Cheomseongdae (National Treasure 31) stands on a typical sedimentary basin terrain covered with a thick flood-induced alluvial deposit layer. Core samples taken from the subsoils near Cheomseongdae revealed a vertical soil profile (1–16 m) of sandy gravel and boulders. The depth-dependent stiffness distribution was derived from in situ down-hole and surface wave tests performed; these results were used to determine the seismic motions considering the site amplification factors. In other words, the surface motion at Cheomseongdae ground in each earthquake scenario was analysed. As a result, the ground around Cheomseongdae was found to incur a peak ground acceleration amplification of approximately 1.85 times. The Cheomseongdae observatory was constructed during the reign of Queen Seondeok (632–646). Its original form survived over 1300 years of exposure to the natural and manmade disasters such as earthquakes. This stone masonry structure is comprised of nearly 400 stone members. Its materials and structure are maintained in their original state, and it is a monument of high artistic value with a mixture of harmoniously contrasting curves, straight lines, circles and squares. Cheomseongdae’s smoothly curved cylindrical main body consists of 27 layers of stone members piled up on a square stylobate and pedestal as shown in Figure1[ 21]. Two layers of interlocking headstones are placed on top of the main body. There is a small three-layer-high (13th to 15th layer) entrance or window facing south. The inside of the wall from the first to the 12th layer is densely Sustainability 2019, 11, 1565 4 of 13 packed with irregularly shaped and sized rubble and soil, whereas the inside is empty from the 13th layer onwards (Figure1a). Unlike the smooth exterior of the main body, the interior is composed of roughly touching stone members up to the 18th layer (Figure1b). The 19th layer comprises two stone beams with rectangular cross-sections that are placed in the east-west direction, piercing the cylindrical plane with their ends protruding. The 20th layer comprises two identically shaped stone Sustainability 2019, 11, x FOR PEER REVIEW 4 of 13 beams placed in the north-south direction across the stone beams below (19th layer) without piercing the wallshaped (Figure stone1c). beams This placed structure in the is considerednorth-south to direction enhance across the frictional the stone andbeams adhesive below ( forces19th layer) between the members.without piercing The 21st—24th the wall (Figure layers comprise1c). This structure stone members is considered that are to enhance piled in th thee frictional same manner and as in theadhesive 12th—18th forces layers. between The the 25thmembers. and The 26th 21st layers—24th show layers the comprise same patternstone members as in the that 19th are piled and 20th layers,in butthe same in the manner opposite as in direction, the 12th— i.e.18th with layers. the The two 25th stone and beams 26th layers piercing show the the cylindrical same pattern plane as in in the north-souththe 19th directionand 20th layers, in the but 25th in layer the opposi and anotherte direction, two i.e. beams with placedthe two across stone beams them inpiercing the 26th the layer. The 27thcylindrical layer hasplane a stonein the slabnorth slanted-south direction to the east. in the The 25th top layer part and of Cheomseongdaeanother two beams is placed square. across The 28th them in the 26th layer. The 27th layer has a stone slab slanted to the east. The top part of layer has two stone beams with quasi-square cross-sections that are placed in the east-west direction Cheomseongdae is square. The 28th layer has two stone beams with quasi-square cross-sections that and another two identically shaped beams that are placed in the north-south direction inserted into the are placed in the east-west direction and another two identically shaped beams that are placed in the groovesnorth made-south on direction the members inserted underneath into the grooves them. made The sameon the interlocking members underneath pattern is them. applied The tosame the 29th layerinterlocking with the reverse pattern directions, is applied to i.e. the the 29th east-west layer with members the reverse are directions, inserted into i.e. the the east north-south-west members members usingare a half-lapinserted jointinto the (Figure north1-southd). members using a half-lap joint (Figure 1d).

(d)

(a) (c) (a) (b) (c)

(b) (d)

Figure 1. The photos of exterior and interior of Cheomseongdae: (a) Filler beneath the 13th layer; (b) InnerFigure irregular-shaped 1. The photos of exterior stones; and (c) Long-shapedinterior of Cheomseongdae: horizontal tie (a) stones Filler beneath of 19th, the 20th, 13th25th layer; and (b) 26th layers;Inner and irregular (d) Grid-shaped of interlocking stones; (c stones) Long- andshaped half-lap horizontal joint tie[21 ]. stones of 19th, 20th, 25th and 26th layers; and (d) Grid of interlocking stones and half-lap joint [21]. In a previous study [21], dynamic centrifuge tests were performed on two Cheomseongdae In a previous study [21], dynamic centrifuge tests were performed on two Cheomseongdae models to compare their seismic responses. One model was an exact duplicate of the actual structure models to compare their seismic responses. One model was an exact duplicate of the actual structure and theand other the other was was a duplicate a duplicate without without the the interlocking interlocking headstonesheadstones and and intersecting intersecting stone stone beams. beams. As As a result,a theresult, roles the of roles the interlockingof the interlocking headstones headstones and stoneand stone beams beams in enhancing in enhancing Cheomseongdae’s Cheomseongdae’s seismic performanceseismic performance were verified. were Additionally, verified. Additionally, the rough interior the rough members interior in members contrast toin the contrast smooth to theexterior, the denselysmooth packedexterior, fillerthe densely materials packed in thefiller space materi belowals in the the space window, below the the shapeswindow, of the the shapes stone of members the fromstone the 19th members layer from onward the 19th and layer theshape onward of and the the joints shape lead of tothe the joints belief lead that to the the belief Cheomseongdae that the structureCheomseongdae is resistant tostructure dynamic is resistant lateral loadingto dynamic such lateral as earthquakes. loading such as In earthquakes. the dynamic In centrifugethe dynamic model tests oncentri thefuge Cheomseongdae model tests models, on the the Cheomseongdae headstones underwent models, the lateral headstones displacements underwent of up lateralto 1.46 mm displacements of up to 1.46 mm from a 1 s earthquake excitation in a 15 g centrifugal gravitational from a 1 s earthquake excitation in a 15 g centrifugal gravitational field under a seismic wave similar field under a seismic wave similar to that in the Ofunato earthquake with a return period of 1000 to that in the Ofunato earthquake with a return period of 1000 years (0.333 g of surface peak ground years (0.333 g of surface peak ground acceleration). By applying this result to the ground response acceleration).analysis, a By lateral applying displacement this result of 21.9 to the mm ground was estimated response to analysis,occur under a lateral a 15 s earthquake displacement excitation of 21.9 mm was estimatedbased on scaling to occur factors under from a 15thes dynamic earthquake centrifuge excitation test. based on scaling factors from the dynamic centrifuge test. 3. Gyeongju Earthquake on September 12, 2016 and Damage to Architectural Heritage There are 75 records of seismic events from 64 to 932 when Gyeongju was the capital of the Silla Dynasty. The Korea Meteorological Administration set up an analogue seismic monitoring network in 1978 after the Hong Sung earthquake (M 5.0), and a nationwide digital seismic monitoring network Sustainability 2019, 11, 1565 5 of 13

3. Gyeongju Earthquake on 12 September 2016 and Damage to Architectural Heritage There are 75 records of seismic events from 64 to 932 when Gyeongju was the capital of the Silla Dynasty. The Korea Meteorological Administration set up an analogue seismic monitoring network in 1978 after the Hong Sung earthquake (M 5.0), and a nationwide digital seismic monitoring network in

1997Sustainability after the Gyeongju2019, 11, x FOR Earthquake PEER REVIEW (M 4.2). A total of 21 earthquakes were recorded in the5 Gyeongjuof 13 area between 1978 and 2016 prior to the 2016 Gyeongju earthquake. On 12 September 2016, a M 5.1 foreshockin 1997 hit after a region the Gyeongju 8.2 km south-south-west Earthquake (M 4.2). of Gyeongju A total of 21at 19:44, earthquakes followed were by recorded a M 5.8 earthquakein the at 20:32.Gyeongju It was area the between largest 1978 earthquake and 2016 afterprior theto the creation 2016 Gyeongju of the seismic earthquake. observation On September network 12, 2016, in Korea, and vibrationsa M 5.1 foreshock were felt hit ina region most parts8.2 km of south the country-south-west [22 ].of Gyeongju at 19:44, followed by a M 5.8 earthquakeDespite being at 20:32. the largest-scale It was the largest earthquake earthquake ever after recorded the creation in the Korean of the seismic peninsula, observation the damage network in Korea, and vibrations were felt in most parts of the country [22]. of the 9–12 Gyeongju Earthquake was not as extensive as expected because it lasted only 5–7 s, Despite being the largest-scale earthquake ever recorded in the Korean peninsula, the damage withof an the approximately 9–12 Gyeongju 1–2Earthquake s strong was motion not as duration, extensive andas expected mostly because high-frequency it lasted only components 5–7 s, with were observed.an approximately The majority 1–2 of s strong the damage motion was duration, incurred and by mostly medium-high high-frequency and components low structures, were stone fencesobserved. and stone The majority structures, of the including damage was architectural incurred by medium heritage,-high which and low were structures, more susceptible stone fences to the high-frequencyand stone structure components.s, including architectural heritage, which were more susceptible to the high- frequencyAt the MKL components. station in Gyeongju Myeonggye-ri (35.7322 ◦N, 129.2420 ◦E), the earthquake monitoringAt station the MKL with station rock-outcrop in Gyeongju condition Myeonggye closest-ri to (35.7322 the epicentre °N, 129.2420 (epicentral °E), the distance: earthquake 5.68 km), the peakmonitoring ground station acceleration with rock-outcrop values condition measured closest within to the the epi timecentre domain (epicentral sampled distance: at 5.68 100 km), Hz were 0.346the g forpeak the ground E-W componentacceleration values and 0.275 measured g for within the N-S the component. time domain sampled At USN at station 100 Hz in were Ulsan 0.346 Bokan-ri g for the E-W component and 0.275 g for the N-S component. At USN station in Ulsan Bokan-ri (35.7024 (35.7024 ◦N, 129.1232 ◦E), the earthquake monitoring station with soil condition closest to the epicentre, °N, 129.1232 °E), the earthquake monitoring station with soil condition closest to the epicentre, the the measuredmeasured peak acceleration acceleration was was 0.400 0.400 g for g forthe theE-W E-W component component and 0.430 and g 0.430 for the g forN-S the component. N-S component. TheThe 9–12 9– Gyeongju12 Gyeongju Earthquake Earthquake damaged damaged 52 52 designated designated cultural cultural heritage heritage sites. sites. Bulguksa Bulguksa Temple Temple sufferedsuffered considerable considerable damage. damage For.example, For example, roof tiles roof fells tiles from fells the Main from Buddha the Main Hall Buddha (Daewoongjeon), Hall walls(Daewoongjeon), in the west corridor walls in were the west cracked, corridor the were fence cracked, of Guanyeumheon the fence of Guanyeumheon was destroyed was and destroyed the banister jointsand in the banister Pagoda joints were in Dabotap displaced. Pagoda In Cheomseongdae, were displaced. In the Cheomseongdae, interlocking headstones the interlocking moved and wereheadstones partially displaced.moved and Figurewere partially2 illustrates displaced. the typicalFigure 2 damages illustrates tothe the typical stone damages pagoda to caused the stone by the 9–12pagoda Gyeongju caused Earthquake. by the 9– Table12 Gyeongju1 provides Earthquake. an overview Table of 1 the provides seismic an damage overview to of the the stone seismic pagodas due todamage the Gyeongju to the stone Earthquake pagodas due (modified to the Gyeongju from Kim Earthquake et al. [23 (modified]). Most of from the Kim damaged et al. [23]). stone Most pagodas of the damaged stone pagodas have three stories and a main body composed of massive stones. have three stories and a main body composed of massive stones. Various parts of the pagodas suffered Various parts of the pagodas suffered damage including the base, main body and steeple. Serious damagestructural including damage the base, was main not observed, body and and steeple. most Serious damage structural included damage cracking, was separation not observed, and and mostdisplacement damage included of non- cracking,structural members. separation and displacement of non-structural members.

(a) (b) (c) (d)

Figure 2. Typical damages to stone pagoda caused by 9–12 Gyeongju Earthquake (Photo by NRICH): (a) SeparatedFigure 2. Typical third-story damages body to stone, stone pagoda three-story caused Stone by 9 Pagoda–12 Gyeongju at Cheollyongsa Earthquake (Photo Temple by Site NRICH): in Namsan (a) Separated third-story body stone, three-story Stone Pagoda at Cheollyongsa Temple Site in Namsan Mountain; (b) Rotated stone member, displaced third-story body stone, three-story Stone Pagoda of Mountain; (b) Rotated stone member, displaced third-story body stone, three-story Stone Pagoda of Yongjanggye Jigok Valley in Namsan Mountain; (c) Rotated body and roof stones, three-story Stone Yongjanggye Jigok Valley in Namsan Mountain; (c) Rotated body and roof stones, three-story Stone Pagoda of Bipagok Valley in Namsan Mountain; and (d) Displaced third-story body and roof stones, Pagoda of Bipagok Valley in Namsan Mountain; and (d) Displaced third-story body and roof stones, rhree-storyrhree-story Stone Stone Pagoda Pagoda of of Jiamgok Jiamgok Valley in in Namsan Namsan Mountain. Mountain.

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Table 1. Seismic damage incurred by stone pagodas due to the 9–12 Gyeongju Earthquake.

SustainabilityClassification 2019, 11, x FOR PEER REVIEW Name of Cultural Heritage Seismic Damage Height (m)6 of 13 National Treasure 20 Dabotap Pagoda of Bulguksa Temple Displaced banister joints 10.3 NationalTable Treasure 1. Seismic 30 damageStone incurred Brick Pagoda by of Bunhwangsastone pagodas Temple due to the Cracked9–12 Gyeongju lower part Earthquake. East and West Three-story Stone Pagodas in Destroyed steeples of the east and East 6.73, Treasure 168 Classification NameCheongun-dong of Cultural Heritage westSeismic pagodas Damage WestHeight 7.72 (m) National Treasure 20 Dabotap Pagoda of Bulguksa Temple Displaced banister joints 10.3 East and West Three-story Stone Pagodas of Loosen east pagoda roof stone, tilted NationalTreasure Treasure 678 30 Stone Brick Pagoda of Bunhwangsa Temple Cracked lower part Unmunsa Temple west pagoda, cracked main body East and West Three-story Stone Pagodas in Cheongun- Destroyed steeples of the east and west East 6.73, Treasure 168 Treasure 908 Three-story Stone Pagodadong in Yongmyeong-ri Chipped upper endpagodas of main body 5.6West 7.72 Three-storyEast and West Stone Three Pagoda-story at Stone Cheollyongsa Pagodas of Temple Unmunsa Loosen east pagoda roof stone, tilted TreasureTreasure 1188 (Figure678 2a) Separated third-story body stone 7.16 Site in NamsanTemple Mountain west pagoda, cracked main body Treasure 908 EastThree and-story West Stone Three-story Pagoda Stone in Yongmyeong Pagodas at-ri Chipped upper end of main body 5.6 Treasure 1429 Three-story Stone Pagoda at Cheollyongsa Temple Site in Broken east pagoda roof stone 6.04 Treasure 1188 (Figure 2a) Wonwonsa Temple Site Separated third-story body stone 7.16 Namsan Mountain Three-story Stone Pagoda at Changnimsa Temple Cracked base, separated main body, Treasure 1867 East and West Three-story Stone Pagodas at Wonwonsa 7.0 Treasure 1429 Site of Namsan Mountain partiallyBroken broken east roofpagoda stone roof stone 6.04 Temple Site Treasure 1935 ThreeThree-story-story Stone Stone Pagoda Pagoda at ofChangnimsa Yongjanggye Temple Jigok Site of RotatedCracked stone base, member, separated displaced main body, Treasure 1867 7.0 (Figure2b) Valley inNamsan Namsan Mountain Mountain third-storypartially body broken stone roof stone Treasure 1935 ThreeArchaeological-story Stone Pagoda Area ofof NamsanYongjanggye Mountain Jigok Valley in Rotated stone member, displaced third- Historic Site 311 Loosened third-story body stone 7.0 (Figure 2b) (Three-story PagodaNamsan of Yeonbulsa Mountain Temple Site) story body stone Province-Designated Tangible Archaeological Area of Namsan Mountain (Three-story Historic Site 311 Three-story Stone Pagoda of Bipagok Valley in RotatedLoosened body and third roof-story stones body stone 7.0 Cultural Heritage 448 Pagoda of Yeonbulsa Temple Site) Namsan Mountain (counter clockwise) Province-Designated(Figure2c) Tangible Three-story Stone Pagoda of Bipagok Valley in Namsan Rotated body and roof stones (counter Cultural Heritage 448 Province-Designated Tangible Mountain clockwise) Three-story Stone Pagoda of Jiamgok Valley in Displaced third-story body and Cultural(Figure Heritage 2c) 449 Province-Designated Tangible Namsan Mountain roof stones (Figure2d) Three-story Stone Pagoda of Jiamgok Valley in Namsan Displaced third-story body and roof Cultural Heritage 449 Mountain stones (Figure 2d) In the case of Cheomseongdae, continuous monitoring has been carried out since 2009 with regard to memberIn the separation case of Cheomseongdae, and displacement, continuous separation monitoring of interlocking has headstones been carried due out to destroyedsince 2009 joints with andregard tilting to of member the structure separation [24]. Before and displacement, the earthquake, separation the centre of axis interlocking of Cheomseongdae headstones was due tilted to towardsdestroyed the joints north andby ~200 tilting mm, of and the the structure southeast [24]. and northwest Before the corner earthquake, of headstones the centre had partially axis of lostCheomseongdae their original was joint. tilted towards the north by ~200 mm, and the southeast and northwest corner of headstonesAs predicted had in partially the previous lost their model original tests [joint.21], the seismic damage of Cheomseongdae during the GyeongjuAs predicted Earthquake in the on previous 12 September model tests 2016 [21], was the more seismic intense damage in the of upper Cheoms parteongdae than in during the lower the partGyeongju filled withEarthquake filler materials, on September and the 12, separation 2016 was and more detachment intense in of the the upper stone part members than in thethe upperlower partpart andfilled the with displacement filler materials, of the and southeast the separation headstones and detachment was clearly of visible. the stone A precisemembers analysis in the ofupper the resultspart and of the a 3D displacement survey conducted of the immediatelysoutheast headstones after the Gyeongjuwas clearly Earthquake visible. A showedprecise analysis that the upperof the headstonesresults of a 3D were survey twisted conducted and rotated immediately clockwise, after as shown the Gyeongju in Figure E3arthquake[25]. showed that the upper headstones were twisted and rotated clockwise, as shown in Figure 3 [25].

(a) (b) (c)

FigureFigure 3. DisplacementDisplacement of of Cheomseongdae Cheomseongdae headstones headstones caused caused by by the the 9–12 9–12 Gyeongju Gyeongju Earthquake Earthquake:: (a) (Displacementa) Displacement and and rotation rotation of headstonesof headstones;; (b) (b Displacement) Displacement measurement measurement of ofheadstones headstones;; and and (c) (Separationc) Separation of headstones of headstones on onsoutheast southeast corner corner..

TheThe southernsouthern upperupper headstoneheadstone movedmoved 38 mmmm toto thethe northnorth afterafter jointjoint failurefailure causedcaused byby thethe M4.6M4.6 aftershockaftershock onon SeptemberSeptember 19,19, 2016.2016. TheThe overalloverall behaviourbehaviour of the structure waswas evaluated byby analysinganalysing thethe extentextent ofof membermember separationseparation andand tilting.tilting. NoNo damagesdamages werewere reportedreported otherother thanthan thethe crackingcracking andand separationseparation ofof thethe upperupper headstoneheadstone andand separationseparation ofof somesome stonestone members.members.

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4. Dynamic Centrifuge Tests for Cheomseongdae Model As mentioned, a seismic performance assessment for architectural heritage sites begins with obtaining an understanding of the engineering characteristics of the subsoil and architectural structure. First, a site investigation and seismic response analysis are performed at an architectural heritage site to predict the level of ground shaking for each earthquake scenario. Seismic loading exerted on an architectural heritage triggers different seismic behaviours and performances depending on the structure type and style and the load transfer characteristics. Therefore, an empirical or analytical approach based on experimental or numerical modelling of each architectural heritage site is necessary to understand its seismic behaviour. In the previous study [19], a dynamic centrifuge test was proposed to evaluate the seismic behaviour of stone block structures. If appropriate scaling factors are applied and the model material has the same density as the real structure, the seismic behaviour can be observed under the same stress conditions as reality. Based on the study, the dynamic centrifuge model test was proposed for seismic performance assessment to evaluate how the stone architectural heritage resists seismic loads and whether it can resist damage from future earthquakes [20]. Table2 shows the scaling factors for the dynamic centrifuge model test.

Table 2. Scaling factors of dynamic centrifuge test for stone block and architectural heritage model.

Quantities Scaling Factors (Prototype/Model) Displacement, Length N Acceleration, Gravity N−1 Mass N3 Density 1 Stress 1 Strain 1 Time (Dynamic) N

The Cheomseongdae ground response was analysed based on an actual seismic waveform measured on 12 September 2016, at MKL station. Dynamic centrifuge model tests were performed on the Cheomseongdae model using the surface response obtained from the site-specific ground response analysis, which enabled Cheomseongdae’s seismic performance to be re-assessed.

4.1. Ground Response Analysis and Generation of Input Motion for Shaking Table The ground response for the Gyeongju Earthquake accelerograms measured at the MKL station was analysed using the shear-wave velocity profile and the modulus reduction and damping curve of each ground material at Cheomseongdae determined in a previous study [18]. The peak acceleration value of the E-W component measured at the MKL station was higher than that of the N-S component. The E-W component contained a higher proportion of high-frequency components whose frequency exceeded 25 Hz than the N-S component. Given that components with frequencies lower than 25 Hz are more important in ground engineering and structural engineering and considering the excitation frequency band of the shaking table, the N-S component of MKL was used in this study [26]. Figure4 shows the results of the Cheomseongdae ground response analysis for the Ofunato Earthquake motion and the Gyeongju MKL (N-S component) motion. The waveforms of the latter had a shorter duration of strong motion than those of the former with a similar peak acceleration value. Based on a threshold acceleration of 0.05 g, the bracketed durations Td were 11.35 s and 3.23 s for the Ofunato and Gyeongju earthquakes, respectively. Moreover, the Arias intensity Ia values [27] used for a quantitative calculation of the duration of strong motion were 0.0335 and 0.0039 m/s for the Ofunato and Gyeongju Earthquake motions, respectively. Sustainability 2019,, 11,, 1565x FOR PEER REVIEW 88 of of 13

(a) (b)

Figure 4. GroundGround response response analysis analysis for for Cheomgeongdae Cheomgeongdae site: site: (a) ( aOfunato) Ofunato earthquake earthquake motion; motion; and and (b) (Gyeongjub) Gyeongju MKL MKL motion. motion.

The frequency frequency range range of of the the shaking shaking table table mounted mounted on onthe thecentrifuge centrifuge was was30–300 30–300 Hz for Hz the for scale the model.scale model. Hence, Hence, the tests the were tests performed were performed at a centrifugal at a centrifugal gravitational gravitational field of 15 field g using of 15 a g 2 using–20 Hz a prototype2–20 Hz prototype-scale-scale component. component. The time The history time history from from the the ground ground response response analysis analysis results results was T converted using the bandpass filterfilter of the 2–202–20 Hz component. As a result, the Tdd values of the input seismic wave used for actual testing were 11.15 s and 2.46 s s for for the the Ofunato Ofunato and and Gyeongju Gyeongju E Earthquakearthquake motions, respectively. 4.2. Preparation of Cheomseongdae Models and Centrifuge Model Testing 4.2. Preparation of Cheomseongdae Models and Centrifuge Model Testing To analyse Cheomseongdae’s seismic behaviour characteristics, 1/15-scale models were created, To analyse Cheomseongdae’s seismic behaviour characteristics, 1/15-scale models were created, and dynamic centrifuge model testing was performed using a centrifuge device and shaking table from and dynamic centrifuge model testing was performed using a centrifuge device and shaking table the KAIST Geo-Centrifuge Testing Center [28,29]. The scaling factors were considered by applying from the KAIST Geo-Centrifuge Testing Center [28,29]. The scaling factors were considered by a 15 g centrifugal force vertically, which is 15 times the gravitational acceleration, to the 1/15-scale applying a 15 g centrifugal force vertically, which is 15 times the gravitational acceleration, to the Cheomseongdae model. Hwangdeung granite was used for the model members because its density 1/15-scale Cheomseongdae model. Hwangdeung granite was used for the model members because and stiffness are closest to those of the material at Cheomseongdae. This enhances the plausibility of its density and stiffness are closest to those of the material at Cheomseongdae. This enhances the reproducing the actual seismic behaviour of Cheomseongdae. plausibility of reproducing the actual seismic behaviour of Cheomseongdae. All the stone members of the model were fabricated manually to produce models as accurately as All the stone members of the model were fabricated manually to produce models as accurately possible. Cheomseongdae is composed of over 380 stone members, of which the long members of the as possible. Cheomseongdae is composed of over 380 stone members, of which the long members of 19th, 20th, 25th and 26th layers, and the two headstone layers were proven to contribute to bearing the 19th, 20th, 25th and 26th layers, and the two headstone layers were proven to contribute to larger lateral loads. Shaking table tests were performed within centrifuge to compare the seismic bearing larger lateral loads. Shaking table tests were performed within centrifuge to compare the behaviour of the Cheomseongdae models with and without the four headstones that are supposed to seismic behaviour of the Cheomseongdae models with and without the four headstones that are enhance the seismic performance, as shown in Figure5. supposed to enhance the seismic performance, as shown in Figure 5.

Sustainability 2019, 11, 1565 9 of 13 Sustainability 2019, 11, x FOR PEER REVIEW 9 of 13

(a) (b)

Figure 5. Cheomseongdae model: (a) Cheomseongdae model with headstones; and (b) Cheomseongdae Figuremodel without 5. Cheomseongdae headstones. model: (a) Cheomseongdae model with headstones; and (b) Cheomseongdae model without headstones. Accelerometers were installed at eight heights on the east side of the Cheomseongdae model, andAccelerometers the model was were subjected installed to Gyeongju at eight heights Earthquake on the motions east side in of the the east-west Cheomseongdae direction. model, The andaccelerometers the model werewas placed subjected in the to vibrationGyeongju shakerEarthquake of the centrifugemotions in model the east tester,-west Cheomseongdae direction. The accelerometersbase, 12th layer were (on the placed filler in surface), the vibration 13th layer shaker (southern of the windowcentrifuge above mode thel tester, filler), Cheomseongdae 19th layer (stone base,beam), 12th 25th layer layer (on (stone the filler beam), surface), 26th layer13th layer (stone (southern beam), 28th window layer above (lower the headstone) filler), 19t andh layer 29th (stone layer (upperbeam), 25th headstone). layer (stone The 28thbeam), and 26th 29th layer layers (stone were beam), used 28th for the layer prototype (lower headstone) model only. and The 29th Ofunato layer (upperand Gyeongju headstone) Earthquake. The 28th motions and 29th were layers applied were duringused for the the tests, prototype starting model from onl a lowy. The acceleration Ofunato andthat Gyeongju was gradually Earthquake increased. motions For bothwere Cheomseongdaeapplied during the models, tests, starting seismic wavesfrom a werelow acceleration applied six thattimes was per gradually waveform. increased. The acceleration For both results Cheomseongdae were processed models, after seismic converting waves them were into applied the actual six timesCheomseongdae per waveform. prototype The acceleration unit and taking results the were scale processed factor into after account. converting These werethem usedinto the to explain actual Cheomseongdaethe actual phenomena prototype that may unit occurand taking at Cheomseongdae the scale factor during into account. an earthquake, These were considering used to the explain scale thefactor actual for thephenomena 1/15 scale that and may 15 goccur conditions at Cheomseongdae with respect dur to size,ing an duration earthquake, and frequency. considering the scale factor for the 1/15 scale and 15 g conditions with respect to size, duration and frequency. 4.3. Acceleration Amplification With Height at Cheomseongdae 4.3. Acceleration Amplification With Height at Cheomseongdae Figure6 shows the peak acceleration values measured at various heights for the two CheomseongdaeFigure 6 shows models the (with peak and acceleration without headstones) values exposedmeasured to Ofunato at various and heightsGyeongju for Earthquake the two Cheomseongdaemotions. Almost no models acceleration (with amplification and without was headstones) measured exposed in the lower to Ofunatoparts under and the Gyeongju window, Ei.e.arthquake the part motions. packed withAlmost filler no materials,acceleration and amplification different acceleration was measured values in werethe lower measured parts under in the theupper window, part depending i.e. the part on the packed absence with or filler presence mater ofials, headstones. and different Neither acceleration model collapsed, values were with measuredonly slight in headstone the upper displacement part depending occurring on the at absence the 0.5 gor level presence of strong of headstones. motion. The Neither movement model of collapsed,headstone with members only slight was similar headstone to that displacement reported after occurring the actual at the Gyeongju 0.5 g level Earthquake. of strong motion. A similar The movementtendency was of head demonstratedstone members when was the similar model to was that exposed reported to after Gyeongju the actual Earthquake Gyeongju motion Earthquake. with a Ahigher similar proportion tendency of was high-frequency demonstrated components, when the model and considerablewas exposed differencesto Gyeongju were Earthquake observed motion in the withamplification a higher proportion characteristics of high of the-frequency upper part components, depending and on theconsiderable absence or difference presences of were headstones. observed in theDuring amplification the dismantling characteristics of themodels of the upon upper the part completion depending of testing, on the a absence stone beam or presence in the 19th of headstones.layer was found to be broken in the centre, as shown in Figure7. This proves that the stone beams playDuring an important the dismantling role in enhancing of the models seismic upon performance the completion by functioning of testing, as tiea stone bars. beam in the 19th layer was found to be broken in the centre, as shown in Figure 7. This proves that the stone beams play an important role in enhancing seismic performance by functioning as tie bars. Sustainability 2019, 11, 1565 10 of 13 Sustainability 2019, 11, x FOR PEER REVIEW 10 of 13 Sustainability 2019, 11, x FOR PEER REVIEW 10 of 13

. .

(a) (a) (b) (b)

(c) (c) (d) (d) FigureFigure 6. Peak 6. Peak acceleration acceleration values values measured measured at various at various heights heights in the in two the twoCheomseongdae Cheomseongdae models: models: Figure 6. Peak acceleration values measured at various heights in the two Cheomseongdae models: (a) Ofunato(a) Ofunato earthquake earthquake motion motion in original in original model; model; (b) Ofunato (b) Ofunato earthquake earthquake motion motion in Cheomseongdae in Cheomseongdae (a) Ofunato earthquake motion in original model; (b) Ofunato earthquake motion in Cheomseongdae modelmodel without without headstones; headstones; (c) Gyeongju (c) Gyeongju Earthquake Earthquake motion motion in original in original model; model; and and(d) Gyeongju (d) Gyeongju model without headstones; (c) Gyeongju Earthquake motion in original model; and (d) Gyeongju EarthquakeEarthquake motion motion in Cheomseongdae in Cheomseongdae model model with withoutout headstones. headstones. Earthquake motion in Cheomseongdae model without headstones.

(a) (b) (a) (b)

Figure 7. Broken stone beam of 19th layer after seismic excitations: (a) Before testing; and Figure 7. Broken stone beam of 19th layer after seismic excitations: (a) Before testing; and (b) After Figure(b) After 7. testing.Broken stone beam of 19th layer after seismic excitations: (a) Before testing; and (b) After testing. testing. 4.4. Frequency Response of Structure During Earthquake Excitation 4.4. Frequency Response of Structure During Earthquake Excitation 4.4. FrequencyFigure8 shows Response the of frequency Structure response During Earthquake of the structure Excitation exposed to the Ofunato earthquake motion withFigureFigure a 0.137 8 shows 8 g-level shows the surface the frequency frequency peak response acceleration. response of the of A the structure peak structure acceleration exposed exposed of to 0.660 the to the Ofunato g was Ofunato measured earthquake earthquake at the motionmotionupper with headstone with a 0.137 a 0.137 g (29th-level g-level layer),surface surface and peak partialpeak acceleration. acceleration. amplification A peak A peak occurred acceleration acceleration in the of 2.4–3.90.660 of 0.660 g Hzwas g andwas measured 5.1–6.3measured Hz at theatbands. theupper upper The headstone amplification headstone (29th (29th layer), profile layer), and was and partial not partial clearly amplification amplification discernible. occurred Thisoccurred may in the in be the2.4 due– 2.43.9 to– theHz3.9 complexHzand and 5.1– 5.16.3 mode– 6.3 Hz bandHz bands. Thes. The amplification amplification profile profile was was not notclearly clearly discernible discernible. This. This may may be due be due to the to complexthe complex mode mode shapeshape of the of thestructure, structure, which which was was composed composed of nearly of nearly 400 400members. members. Figure Figure 9 shows 9 shows the thefrequency frequency Sustainability 2019, 11,, 1565x FOR PEER REVIEW 1111 of of 13 Sustainability 2019, 11, x FOR PEER REVIEW 11 of 13 response of the structure exposed to the Gyeongju Earthquake motion with a 0.126 g-level surface responsepeakshape acceleration. of theof the structure, structure A peak which exposed acceleration was composedto the Gyeongju of 0.329 of nearly gE arthquake occurred 400 members. motion at the Figure topmostwith9 a shows 0.126 layer, g the- level and frequency surface partial peakamplificationresponse acceleration. of the occurred structure A peakat exposed 2.7 accelerationand to4.9 the Hz. Gyeongju of 0.329 Earthquake g occurred motion at the with topmost a 0.126 g-level layer, surface and partial peak amplificationacceleration. Aoccurred peak acceleration at 2.7 and 4.9 of 0.329Hz. g occurred at the topmost layer, and partial amplification occurred at 2.7 and 4.9 Hz.

Figure 8. FrequencyFrequency response of structure exposed to Ofunato earthquake motion.motion. Figure 8. Frequency response of structure exposed to Ofunato earthquake motion.

Figure 9. FrequencyFrequency response of structure exposed to Gyeongju EarthquakeEarthquake motion.motion. It is difficultFigure to9. Frequency determine response the natural of structure frequency exposed of to Cheomseongdae, Gyeongju Earthquake which motion has. almost 400 It is difficult to determine the natural frequency of Cheomseongdae, which has almost 400 members [21]. In the Cheomseongdae prototype, acceleration amplification was measured in the membersIt is [21]. difficult In the to Cheomseongdae determine the natural prototype, frequency acceleration of Cheomseongdae, amplification was which measured has almost in the 3.23 400 3.23 Hz and 7.1–11.07 Hz bands under excitation in the east-west direction. From the test results, it was membersHz and 7.1 [21].–11.07 In the Hz Cheomseongdae bands under excitation prototype, in the acceleration east-west amplificationdirection. From was the measured test results, in the it 3.23was concluded that Cheomseongdae has a high seismic performance with a harmony between the local Hzconcluded and 7.1 –that11.07 Cheomseongdae Hz bands under has excitation a high seismic in the eastperformance-west direction. with a Fromharmony the testbetween results, the it local was behaviour of each member and the overall behaviour of the structure. Additionally, the behaviour of concludedbehaviour ofthat each Cheomseongdae member and the has overall a high behaviour seismic performance of the structure. with Additionally, a harmony between the behaviour the local of the lower part under the window level packed with filler materials exhibited rigid motion, and the behaviourthe lower partof each under member the window and the l overallevel packed behaviour with fillerof the materials structure. exhibited Additionally, rigid themotion, behaviour and the of intersecting long beams and interlocking headstones were found to enhance the seismic performance theintersecting lower part long under beams the and window interlocking level packed headstones with werefiller foundmaterials to enhance exhibited the rigid seismic motion, performance and the of the upper part with a large empty space. intersectingof the upper long part beamswith a andlarge interlocking empty space. headstones were found to enhance the seismic performance of5. Conclusionsthe upper part with a large empty space. 5. Conclusions This paper summarised and categorised studies conducted between 2009 and 2012 on the seismic 5. ConclusionsThis paper summarised and categorised studies conducted between 2009 and 2012 on the risk assessment of architectural heritage in Korea’s historic areas in relation to the evaluation of the seismic risk assessment of architectural heritage in Korea’s historic areas in relation to the evaluation engineeringThis paper characteristics summarised of architectural and categorised heritage studies sites, conducted the site-specific between ground 2009 response and 2012 analysis on the of the engineering characteristics of architectural heritage sites, the site-specific ground response seismicfor earthquake risk assessment scenarios, of the architectural GIS-based heritage seismic microzonationin Korea’s historic according areas in to relation geotechnical to the earthquakeevaluation analysis for earthquake scenarios, the GIS-based seismic microzonation according to geotechnical ofengineering the engineering parameters, characteristics the reliability of architectural assessment ofheritage the dynamic sites, the centrifuge site-specific model ground testing respo of stonense earthquake engineering parameters, the reliability assessment of the dynamic centrifuge model analysismasonry for structure earthquake and thescenarios, seismic the performance GIS-based seismic of architectural microzonation heritage. according The M to 5.8 geotechnical earthquake testing of stone masonry structure and the seismic performance of architectural heritage. The M 5.8 earthquakethat hit Gyeongju engineering on 12 September parameters, 2016 the wasreliability analysed assessment from an engineering of the dynamic point centrifuge of view and model the earthquake that hit Gyeongju on September 12, 2016 was analysed from an engineering point of view testingrelated of damages stone masonry to stone architecturalstructure and heritage the seismic were performanc categorisede according of architectural to the typeheritage. of damage. The M The5.8 and the related damages to stone architectural heritage were categorised according to the type of earthquakereliability and that importance hit Gyeongju of theon September research achievements 12, 2016 was presentedanalysed from in this an studyengineering were highlighted point of view in damage. The reliability and importance of the research achievements presented in this study were andthe Cheomseongdaethe related damages damage to stone report architectural written after heritage the 2016 Gyeongjuwere categorised Earthquake. according The reliability to the type of theof damage.highlighted The in reliabili the Cheomseongdaety and importance damage of the report research written achievements after the 2016 presented Gyeongju in Earthquake. this study were The highlightedreliability of inthe the seismic Cheomseongdae performance damage assessment report method written for after architectural the 2016 Gyeongjuheritage b asedEarthquake. on dynamic The reliability of the seismic performance assessment method for architectural heritage based on dynamic Sustainability 2019, 11, 1565 12 of 13 seismic performance assessment method for architectural heritage based on dynamic centrifuge model testing was also proven using a Gyeongju Earthquake records that have rarely been used in Korea thus far. The findings of the dynamic centrifuge tests for the Cheomseongdae models using actual Gyeongju Earthquake motion were (1) the different acceleration values with height in the upper part depending on the absence or presence of headstones; (2) the observation of movement of headstone members similar to that reported after the actual 2016 Gyeongju Earthquake; and (3) a broken stone beam in the 19th layer during dynamic centrifuge tests. These findings show that it is not a coincidence that Cheomseongdae, a masonry structure composed of nearly 400 stone members, survived numerous seismic events for over 1300 years. The structural characteristics of Cheomseongdae, such as the well-compacted filler materials in its lower part, the rough inside wall in contrast to the smooth exterior, intersecting stone beams and interlocking headstones, were shown to contribute to its overall seismic performance based on the results of dynamic centrifuge model testing. It is interesting that Cheomseongdae maintained its original form given the ground conditions since the seventh century. It is a great cultural heritage in which we can still feel our ancestors’ breaths and rediscover their outstanding seismic design technology.

Author Contributions: Conceptualization, H.-J.P. and J.-G.H.; Formal analysis, J.-G.H.; Funding acquisition, H.-J.P.; Investigation, S.-H.K.; Methodology, H.-J.P., S.-H.K. and S.-S.J.; Resources, S.-H.K. and S.-S.J.; Software, J.-G.H.; Supervision, H.-J.P.; Validation, S.-S.J.; Writing—original draft, H.-J.P.; Writing—review & editing, H.-J.P. Funding: This research was also supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (grant number NRF-2014R1A6A3A04056405). Acknowledgments: This study, which forms a part of the project, has been achieved with the support of national R&D project, which has been hosted by National Research Institute of Cultural Heritage of Cultural Heritage Administration. The authors would like to thank Kyung-Sik Seo, Sung-Bok Lee, Tae-Sic Oh, Moon-Gyo Lee, Yeong-Hoon Jeong, Kil-Wan Ko, Hye-Lim Lee, and Sunji Park for their advice and support for centrifuge model tests. Conflicts of Interest: The authors declare no conflict of interest.

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