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Engineering Applications Using Probabilistic Aftershock Hazard Analyses: Aftershock Hazard Map and Load Combination of Aftershocks and Tsunamis

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Engineering Applications Using Probabilistic Aftershock Hazard Analyses: Aftershock Hazard Map and Load Combination of Aftershocks and Tsunamis geosciences Article Engineering Applications Using Probabilistic Aftershock Hazard Analyses: Aftershock Hazard Map and Load Combination of Aftershocks and Tsunamis Byunghyun Choi 1,*, Akemi Nishida 1, Tatsuya Itoi 2 ID and Tsuyoshi Takada 3 1 Japan Atomic Energy Agency, Kashiwa, Chiba 277-0871, Japan; [email protected] 2 Department of Nuclear Engineering and Management, The University of Tokyo, Bunkyo, Tokyo 113-8656, Japan; [email protected] 3 Department of Architecture, The University of Tokyo, Bunkyo, Tokyo 113-8656, Japan; [email protected] * Correspondence: [email protected]; Tel.: +81-80-9645-8489 Received: 31 October 2017; Accepted: 18 December 2017; Published: 22 December 2017 Abstract: After the Tohoku earthquake in 2011, we observed that aftershocks tended to occur in a wide region after such a large earthquake. These aftershocks resulted in secondary damage or delayed rescue and recovery activities. In addition, it has been reported that there are regions where the intensity of the vibrations owing to the aftershocks was much stronger than those associated with the main shock. Therefore, it is necessary to consider the seismic risk associated with aftershocks. We used the data regarding aftershocks that was obtained from the Tohoku earthquake and various other historically large earthquakes. We investigated the spatial and temporal distribution of the aftershocks using the Gutenberg–Richter law and the modified Omori law. Subsequently, we previously proposed a probabilistic aftershock occurrence model that is expected to be useful to develop plans for recovery activities after future large earthquakes. In this study, the probabilistic aftershock hazard analysis is used to create aftershock hazard maps. We propose a hazard map focusing on the probability of aftershocks on the scale of the main shock for use with a recovery activity plan. Following the lessons learned from the 2011 Tohoku earthquake, we focus on the simultaneous occurrence of tsunamis and aftershocks just after a great subduction earthquake. The probabilistic aftershock hazard analysis is used to derive load combination equations of the load and resistance factor design. This design is intended to simultaneously consider tsunamis and aftershocks for tsunami-resistant designs of tsunami evacuation buildings. Keywords: probabilistic; aftershock; seismic hazard; subduction earthquake; recovery activity plan; load combination; tsunami 1. Introduction An earthquake with a moment magnitude of 9.0 struck Tohoku by the coast near the Pacific Ocean on 11 March 2011. Subsequently, thousands of aftershocks occurred and the distribution of their hypocenters ranged from the Iwate Sea to the Ibaraki Sea. Therefore, it is clear that it is necessary for emergency workers to consider the occurrence of aftershocks so as to be prepared to face such a scenario in the future. The importance of including recovery activities in contingency plans has increasingly been recognized. The reports of damages due to the aftershocks caused by the 2011 Tohoku earthquake included collapses of buildings that were damaged by the main shock, delays in recovery activities owing to water cutoffs or no-passing zones, and delays in rescue activities. However, the Central Disaster Management Council of Japan [1] does not consider the effect of Geosciences 2018, 8, 1; doi:10.3390/geosciences8010001 www.mdpi.com/journal/geosciences Geosciences 2018, 8, 1 2 of 22 Geosciences 2018, 8, 1 2 of 22 aftershocks.methods to evaluate Probabilistic the hazards methods of toaftershocks evaluate thehave hazards been deemed of aftershocks to be particularly have been effective deemed toin beimproving particularly decision-making effective in improving and planning decision-making due to various and planning uncertainties due to variousthat are uncertainties associated with that areearthquakes associated in with terms earthquakes of the location, in terms scale, of and the location,frequency scale, of aftershocks and frequency [2,3]. of aftershocks [2,3]. Multiple studies of aftershock hazard analyses were conductedconducted after thethe NiigataNiigata ChuetsuChuetsu earthquake in 2004 2004 [4–7]. [4–7]. Standard Standard analysis analysis methods methods were were proposed proposed by bythe theHeadquarters Headquarters for forEarthquake Earthquake Research Research Promotion Promotion to design to designprobabil probabilisticistic aftershock aftershock occurrenc occurrencee models [8]. models However, [8]. However,aftershocks aftershocks with a magnitude with a greate magnituder than 7.0 greater after the than occurrence 7.0 after of the a main occurrence shock with of a a main magnitude shock withof 9.0 awere magnitude not reviewed. of 9.0 werePreviously, not reviewed. aftershock Previously, risk analyses aftershock were conducted risk analyses immediately were conducted after the immediatelyoccurrence of after a main the shock occurrence [9]. However, of a main it is shock diffic [9ult]. However,to evaluate it the is difficult hazards to of evaluate an aftershock the hazards before ofthe an main aftershock shock due before to various the main uncertainties. shock dueto For various possible uncertainties. great earthquakes, For possible we must great make earthquakes, decisions webased must on makesuch uncertainties, decisions based and on it suchis important uncertainties, to quantify and itthe is importantvarious uncertainties. to quantify Therefore, the various a uncertainties.probabilistic modelTherefore, that takes a probabilistic into accountmodel the variou that takess uncertainties into account is necessary the various to review uncertainties the safety is necessarymeasures before to review the occurrence the safety measuresof an earthquake. before theWe occurrencepreviously proposed of an earthquake. a probabilistic We previouslyaftershock proposedhazard analysis a probabilistic method aftershockfor trench-type hazard mega-earthquakes analysis method [10]. for trench-type mega-earthquakes [10]. InIn thisthis paper,paper, engineeringengineering applicationsapplications ofof thethe proposedproposed approachapproach forfor probabilisticprobabilistic aftershockaftershock hazard analysis are shownshown forfor demonstrationdemonstration purposes.purposes. One application is to use aftershock hazard maps to plan recoveryrecovery activities.activities. Another application is to derive load combination equations of the loadload andand resistanceresistance factorfactor designdesign (LRFD)(LRFD) consideringconsidering thethe simultaneoussimultaneous occurrenceoccurrence ofof tsunamistsunamis andand aftershocks for the tsunami-resistanttsunami-resistant designdesign ofof tsunamitsunami evacuationevacuation buildings.buildings. 2. Probabilistic Aftershock Hazard Analysis Model 2.1. Aftershock DefinitionDefinition The classification classification of of aftershocks aftershocks [11] [11 used] used in inour our study study is illustrated is illustrated in Figure in Figure 1. In1 general,. In general, many manyearthquakes earthquakes subsequently subsequently occur occurnear the near hypocenter the hypocenter of a large of a earthq large earthquakeuake after its after occurrence. its occurrence. If the Ifsubsequent the subsequent earthquakes earthquakes are smaller are smaller in magnitude in magnitude than the than initial the initialearthquake, earthquake, the initial the one initial is oneclassified is classified as the main as theshock main and shockthe subsequent and the ones subsequent are classified ones as are aftershocks. classified Conversely, as aftershocks. if a Conversely,subsequent ifearthquake a subsequent is largerearthquake in magnitude is larger than in magnitude the initial thanearthquake, the initial the earthquake, initial earthquake the initial is earthquakeclassified as is a classified foreshock as aand foreshock the largest and thesubsequent largest subsequent earthquake earthquake is classified is classified as the main as the shock. main shock.Typically, Typically, subduction subduction earthquakes earthquakes stronger stronger than a thanRichter a Richter magnitude magnitude of 7.0 ofare 7.0 assumed are assumed to be main to be mainshocks shocks because because the majority the majority of such of earthquakes such earthquakes are historically are historically observed observed to be tomain be mainshocks. shocks. The area where thethe aftershocksaftershocks occur is the aftershock region, which contains the fault plane as wellwell asas the the expanded expanded fault fault plane. plane. The distributionThe distribu oftion aftershocks of aftershocks resulting resulting from the Tohokufrom the earthquake Tohoku asearthquake per the Japan as per Meteorological the Japan Meteorological Agency [12] is Agency depicted [12] in Figureis depicted2. Aftershocks in Figure that 2. Aftershocks occur at a large that distanceoccur at a from large the distance fault plane, from the i.e., fault induced plane, earthquakes i.e., induced (e.g., earthquakes on 12 March (e.g., 2011, on 12 or March 15 March 2011, 2011), or 15 areMarch not 2011), included are innot this included study. in this study. Figure 1. Aftershock classification.classification. Geosciences 2018, 8, 1 3 of 22 2.2. Probabilistic Aftershock Occurrence Model 2.2.1. Reasenberg and Jones Model In this study, we assume that the relationship between the magnitude and frequency of aftershocks can be expressed using the Gutenberg–Richter (GR) law [13]. Further, we observe that the relationship between the time elapsed
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