Summary of Physics of the earth doctoral thesis: Urban on seismic risk assessment for Hanoi city

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Upgradation and enhancement of the urban seismic risk assessment tool in Vietnam based on the inherent results of the previous studies and the cutting-edge methodology of seismic risk assessment in the world; development of a realistic portrayal of seismic risk for the Hanoi urban area based on the state-of-the-art methodologies for seismic risk assessment and updated dataset.

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MINISTRY OF EDUCATION VIETNAM ACADEMY OF AND TRAINING SCIENCE AND TECHNOLOGY GRADUATE UNIVERSITY SCIENCE AND TECHNOLOGY ---------------------------- URBAN ON SEISMIC RISK ASSESSMENT FOR HANOI CITY Major: Physics of the earth code: 9 44 01 11 SUMMARY OF PHYSICS OF THE EARTH DOCTORAL THESIS HANOI – 2020
This thesis has been completed at Graduate University of Science and Technology – Vietnam Academy of Science and Technology Supervisors: 1. Assoc.Prof. Dr. Nguyen Hong Phuong 2. Prof. Dr. Wen Kuo-Liang Peer Reviewer 1: … Peer Reviewer 2: … Peer Reviewer 3: …. The dissertation will be defended in front of the Institute of Doctoral Dissertation Assessment Council, taking place at the Academy of Science and Technology - Vietnam Academy of Science and Technology at ... hour .... ', day ... month ... year 202.... This thesis will be stored in: - Library of Graduated University of Science and Technology - Library of Institute of Geophysics - Vietnam National Library
LIST OF PUBLICATIONS 1) 01 published in the SCI-E list 1. Hong Phuong Nguyen, The Truyen Pham, Ta Nam Nguyen, 2019. Investigation of long-term and short-term seismicity in Vietnam. Journal of Seismology, Doi: 10.1007/s10950-019-09846-x (SCI-E) 2) 05 papers published in the national journals 1. Pham The Truyen, Nguyen Hong Phuong, 2019. Probabilistic seismic hazard assessment for Hanoi city. Vietnam Journal of Earth Sciences, 41(4), 321-338 2. Bui Thi Nhung, Nguyen Hong Phuong, Pham The Truyen, Nguyen Ta Nam, 2018. Assessment of Earthquake-induced liquefaction hazard in Urban areas of Hanoi city using LPI-Based method. Vietnam Journal of Earth Sciences, 40(1), 78-96, DOI: 10.15625/0866-7187/40/1/10972 3. Nguyen Hong Phuong, Nguyen Ta Nam, Pham The Truyen, 2018. Development of a Web-GIS based Decision Support System for earthquake warning service in Vietnam. Vietnam Journal of Earth Sciences, 40(3), 193-206, DOI: 10.15625/0866-7187/40/3/12638 4. Nguyen Hong Phuong, Pham The Truyen, Nguyen Ta Nam, 2016. Probabilistic seismic hazard assessment for the Tranh River hydropower plant No2 site, Quang Nam province. Vietnam Journal of Earth Sciences, 38(2), 188- 201, DOI: 10.15625/0866-7187/38/2/8601 5. Nguyễn Hồng Phương, Phạm Thế Truyền, 2015. Tập bản đồ xác suất nguy hiểm động đất Việt Nam và Biển Đông. Tạp chí Khoa học và Công nghệ biển, tập 15 số 1; 2015: 77-90, DOI: 10.15625/1859-3097/15/1/6083.
INTRODUCTION The necessity of the Thesis Hanoi, the capital of Vietnam, is the political, cultural, educational and scientific center of the country. According to the census results of 2019, Hanoi has an area of 3,324.92 km², located in 17 of the world's largest capitals, with more than 8 million people. The average population density of Hanoi is 2,398 people/km2, which is 8.2 times higher than the national average population density. Besides the high population density, the process of urbanization of the capital has also taken place rapidly in recent years. Hanoi seems to become a great construction with a series of infrastructure development projects and urban areas. In terms of natural conditions, Hanoi is a vulnerable area to earthquake hazards. According to previous studies, Hanoi is located on the fault zone of the Red river, Chay river, Lo river, and in the intensity zone level VIII (the MSK scale) on the national seismic zoning map. With the dense population in urban areas, Hanoi might suffer significant losses if an earthquake occurs. Over the past decade, great progress has been made in the methodologies of seismic hazard and seismic risk assessments in the world. This can be mentioned as follows: after a series of earthquake disasters occurred recently, the ground motion prediction equations have also been updated and newly built in accordance with the seismotectonic provinces as the NGA-WEST1, NGA-WEST2 and SHARE; building damages and casualties models have also been upgraded. With the aforementioned issues, PhD student selected the topic "Urban seismic risk assessment for the Hanoi city", in order to enhance research capacity in depth on the topic of urban seismic risk assessment. Based on inheriting the results of previous studies, combined with updated data, and applying the state-of-the-art methodology for seismic risk assessment and cutting edge computing technology is performed on the GIS environment to assess seismic risk for the Hanoi urban area. The purposes of the Thesis: - Upgradation and enhancement of the urban seismic risk assessment tool in Vietnam based on the inherent results of the previous studies and the cutting-edge methodology of seismic risk assessment in the world. - Development of a realistic portrayal of seismic risk for the Hanoi urban area based on the state-of-the-art methodologies for seismic risk assessment and updated dataset. The main contents of the Thesis: - Assessment of seismicity for Hanoi city and adjacent areas. - Seismic hazard assessment for Hanoi city. 1
- The correction of ground motion amplification for Hanoi urban area - Seismic risk assessment for Hanoi urban area. New contributions of the Thesis: - Upgradation and enhancement of the urban seismic risk assessment tool called ArcRisk, with the function of a decision support system, operating in the GIS environment. ArcRisk was built based on World’s advanced methodologies such as Hazus-MH, Openquake, with modifications to suit the application conditions in Vietnam. - Establishment of a set of spectral acceleration maps for Hanoi city at 0.3s and 1.0s, corresponding to return periods of 475, 975, 2475, and 9975 years. - Application of direct correction technique for ground motion amplification based on Vs30 data. - The estimation of the building damage and casualties due to earthquake in the five most populated districts of Hanoi City using both probabilistic and deterministic approaches of seismic risk assessments show a realistic portrayal of the earthquake threat to urban communities. The Thesis structure: The thesis is divided into 5 Chapters as follows: Chapter 1. Literature review Chapter 2. Methodology, data and urban seismic risk assessment tool. Chapter 3. Seismotectonic characteristic of the Hanoi and adjacent areas Chapter 4. Seismic hazard assessment for Hanoi city Chapter 5. Seismic risk assessment for Hanoi urban area CHAPTER 1: LITERATURE REVIEW 1.1. A review of seismic hazard and risk assessments in the world 1.1.1. A review of seismic hazard assessment in the world Up to now, there are two main approaches in seismic hazard assessment: (1) probabilistic approach (PSHA) and (2) deterministic approach (DHSA). In which, the classical PSHA methodology first proposed by Cornell and Esteva in 1968 is used (Cornell, 1968; Esteva, 1968). In the original Cornell-Esteva approach, then McGuire (1976, 2004, 2008) inherited and developed. Over the past decades, the PSHA has made a great progress by incorporating more detailed and extended components such as seismic hazard desegregation, nonlinear site response analysis, vector value PSHA, near-fault effects, aftershock hazard... At the same time, PHSA is a comprehensive accounting of uncertainties related to occurrence, source, wave propagation and site effects. 2
The deterministic seismic hazard assessment (DSHA) method has been applied around the world in the early 20th century. The basis of the DSHA is the development of scenario earthquakes with a definition of magnitude and distance to affect the considered sites. The ground motion results were obtained from the scenario earthquake and ground motion prediction equations (GMPE). The DSHA results are often presented in the form of seismic zoning maps, established as a national or regional level, representing the boundaries of the regions with ground motion values in the study area. 1.1.2. Seismic risk assessment studies Seismic risk assessment is considered here as the likelihood of human and property losses resulting from the exposure and the vulnerability to seismic hazard. Up to now, seismic risk assessment has been carried out in many countries and regions of the world, regardless of whether the seismicity of the study area is high or low. The scope of the seismic risk assessment projects is also very different, from city level (RADIUS, 1999; Risk-UE, 2003), national (HAZUS, 1999, 2003, 2017), regional (ESRM, 2020) or global (Open Quake, 2020). 1.2. A review of seismic hazard and risk assessments in Vietnam 1.2.1. Seismic hazard assessment in Vietnam In Vietnam, assessment of seismic hazard has been started since 1960s. However, the national seismic hazard map was first released in 1985 by Pham Van Thuc and Nguyen Dinh Xuyen. In 1989, the map was upgraded in the joint project between Vietnam and Soviet. In the continuous years, the national seismic hazard has been carried and published by various authors such as Nguyen Hong Phuong (1993), Nguyen Dinh Xuyen (1996), Nguyen Hong Phuong (1997), Tran Thi My Thanh (2002), Nguyen Dinh Xuyen (2004), Cao Dinh Trieu (2008), Nguyen Hong Phuong (2010), Nguyen Hong Phuong and Pham The Truyen (2015). The results of seismic hazard assessment in Vietnam always reflect the world’s advanced methodologies. However, most of the results have been carried out by independent research groups, the latest national seismic hazard map was established in 2004 and still used in the national standard TCVN 9386: 2012. Meanwhile, in the world, the national seismic hazard map has been updated periodically or after major earthquakes to serve for updated their national seismic building design code. 1.2.2. Seismic risk assessment in Vietnam In 2001, the first urban seismic risk assessment methodology for Vietnam was developed by Nguyen Hong Phuong based on the HAZUS99 methodology, with modifications to suit the application conditions in Vietnam and carried out the seismic risk assessment for the Hoan Kiem district. 3
With the successes achieved from the above research project, Nguyen Hong Phuong et al. conducted the seismic risk assessment for several urban districts of Hanoi and Ho Chi Minh City in 2006, 2008, 2010 and 2012. Seismic risk assessment in Vietnam has been carried out since early 21th century with modern methodology and technology at that time. However, in order to approach the evolution of seismic risk research in the world in recent years, following limitations of previous studies need to be improved: Firstly, the appropriate ground motion prediction equations (GMPEs) were applied in the methodology at that time. However, with the new GMPEs have been developed recently, which will minimize the uncertainties in seismic hazard assessment of study areas. Secondly, throughout the seismic risk assessment methodology in Vietnam, ground motion acceleration (PGA) values were calculated based on earthquake scenarios corresponding to the site class B according to the American site classification standard (NEHRP). Then, the PGA values were corrected based on the ground types and spoil amplification factors defined by NEHRP. However, Vietnam National Standards TCVN 9386-2012 was released the ground types and soil amplification factors. Thirdly, previous seismic risk assessment studies were based on the earthquake scenarios and without considering the probabilistic seismic risk assessment. Whereas in Vietnam, seismic design code was built on the PGA maps corresponding site class B with return period 475 years. Fourthly, the casualty estimation results were based on the HAZUS99 methodology, so the calculation of casualties only considered the indoor casualty. CHAPTER 2: METHODOLOGY, DATA AND SEISMIC RISK ASSESSMENT TOOL Urban seismic risk assessment is essentially an estimation of the earthquake damage to communities in the study area. Figure 2.1 shows the procedure of urban seismic risk assessment. This is a general procedure; it has only applied to Hanoi city, but not also been able to carry out for any cities in the earthquake's impact. 4
Figure 2. 1. Urban seismic risk analysis and loss estimation procedure. 2.1. Seismic hazard assessment 2.1.1. Probabilistic seismic hazard assessment Based on an algorithm of Cornell, 1976, R.K. McGuire built EQRISK program as a tool to calculate and map seismic hazard, where the seismic hazard is calculated by the formula. P[ A]    P[ A M , r ] f M (m) f R (r )dMdr (2. 1) r M 2.1.2. Deterministic seismic hazard assessment. In the seismic hazard assessment, the traditional deterministic method is also called the seismic hazard assessment based on scenarios. The procedure of DHSA includes four main steps, illustrated in Figure 2.2. 5
Figure 2. 2. The procedure of determinisctic seismic hazard assessment 2.2. Site effect evaluation In Vietnam, the ground motion values were assessed by PSHA or DSHA for the rock site, which corresponds to the site of ground type A according to the Vietnam National Standards TCVN 9386:2012 on Design of structures for earthquake resistances. The amplification of ground shaking to account for local site conditions is based on the ground types and soil amplification factors defined by the TCVN 9386- 2012. This is called an indirect correction technique for ground motion amplification (Figure 2.3). In the world, the direct correction technique for ground motion amplification via Vs30 maps has been widely accepted (Figure 2.3). Figure 2. 3. The procedure of the correction of ground motion amplification 6
2.2. Ground motion amplification The PGA maps were calculated for the rock site, which correspond to the site of ground type A according to the Vietnam National Standards TCVN 9386-2012 on Design of structures for earthquake resistances. To obtain the reliable ground shaking values, a site condition need to be evaluated, considering the local site effects caused by geological setting, ground water level and some other site conditions. This procedure is called the indirect correction of amplification ground motion. In recent years, in the world, the amplification ground motion is also corrected directly by using Vs30 maps. This approach is called the direct correction of amplification ground motion. 2.3. Seismic risk assessment 2.3.1. Assessment building damage method Results of ground shaking assessment obtained from both probabilistic and deterministic approaches were used as input for building damage assessment. A two steps procedure of building damage assessment was developed based on HAZUS method as described below. At the first stage, the Capacity Spectrum method was applied to estimate the peak response of a building to ground shaking caused by an earthquake (Mahaney et. al., 1993; FEMA, 1996; SSC, 1996; Freeman et al., 1998, Kircher et al., 2006). The method compares a graphical representation of the force-displacement capacity curve of the structure with the response spectrum representation of earthquake demands. Building response is characterized by building capacity curves, which describe the pushover displacement of each building type and seismic design level as a function of laterally-applied earthquake load. To assess damage, capacity curve of each building is converted to a set of coordinates defined by Sd-Sa format, where Sa is spectral acceleration, and Sd is spectral displacement. The peak building response can be estimated as the intersection of the building capacity curve and the response spectrum of ground shaking demand at the building’s location (demand spectrum). In the following stage, damage state of each building structure type caused by seismic shaking in the study area was estimated. Structural damage of a building under seismic load can be expressed in the form of lognormal fragility curves that relate the probability of being in, or exceeding, a building damage state to a given demand parameter (the response spectrum displacement in our case). The spectral displacement, Sd, that defines the threshold of a particular damage state (ds) is assumed to be distributed by: Sd = S d ,ds .εds (3) 7
where S d ,ds is the median value of spectral displacement of damage state, ds, and εds is a lognormal random variable with unit median value and logarithmic standard deviation, βds. The conditional probability of being in, or exceeding, a particular damage state, ds, given the spectral displacement, Sd, is defined by the function: 1 𝑆 𝑃[(𝑑𝑠|𝑆𝑑 )] = Ф [ 𝑙𝑛 ( ̅ 𝑑 )] (4) 𝛽𝑑𝑠 𝑆𝑑,𝑑𝑠 where S d ,ds is the median value of spectral displacement at which the building reaches the threshold of damage state, ds, βds is the standard deviation of the natural logarithm of spectral displacement for damage state, ds, and Ф is the standard normal cumulative distribution function. The whole procedure of building damage assessment was carried out by the ArcRisk tool. The capacity curves of all building types of study area were constructed with different anti-seismic design levels and resided in ArcRisk. In addition, the S d ,ds and βds values for each building type were also pre-defined for all damage states. 2.3.2. Estimation casualty method In this study, the vulnerability and exposure models proposed by the HAZUS- MH methodology were adopted to estimate casualties in Hanoi City, assuming that these casualties are directly caused by structural damage of buildings due to earthquakes. The casualty model uses the structural damage states computed by the previous direct physical damage module (D1- Slight, D2 - Moderate, D3 - Extensive, D4 - Complete, and D5 – Complete with collapse) as input. The exposure model is defined by the distribution of population in the study area. The casualties are estimated for three cases: inside the buildings, outside the buildings, and commuting. The expected number of casualties associated with different building damage states is obtained by multiplying predefined casualty rates by the number of occupants presumably present in structures at the time of the earthquake, according to local census data. The casualty model yields the estimates of four injury severity levels (C1- Light injury non necessitating hospitalization; C2-Injury requiring hospital treatment; C3-Severely injured and C4-Death), at three times of day (2:00 a.m., 2:00 p.m., and 5:00 p.m). 2.4. Database - Seismotectonic database is inherent from previous research projects and studies. The catalog includes all historical and macroseismic events (collected from 1311 to 1903) and instrumental events (recorded from 1903 to 2018) within a region bounded by φ=17.5-23.5°N; λ=102.0-108.5°E. All events of magnitude lower 4.0 were removed from the catalog. - Geology engineering map of Hanoi region at a scale of 1: 25000. 8
- VS30 values obtained from 191 seismic survey points and 181 microtremor survey points. 2.4.4. GIS database of building inventory and population The procedure of seismic risk assessment has many factors involved in the calculation process, some of which are simplified and assumed. Building inventory and population data are considered as constant values over time. - Figure 2.2 shows the map structural building inventories in the study area, which was inherited from previous studies. - Figure 2.3 illustrates the map of population density of Hanoi city at ward level, based on the 2019 Viet Nam Population and Housing Census. Figure 2.2. Distribution of structural building inventories in the study area. Figure 2.3. Population distribution by ward in Hanoi city updated until 2019. 9
2.5. Seismic risk assessment tool Figure 2.1 illustrated the procedure of urban seismic risk assessment conducted in the ArcRisk. The procedure is carried out through many stages, in which the stages have the relationship "cause and effect” together, or in other words, the performance of each stage used as the input for the next period. ArcRisk is developed in a GIS environment, including three main modules: Module 1 - Define the study area; Module 2 - Evaluation of ground motion; Module 3 - Estimation of losses. CHAPTER 3: SEISMOTECTONIC CHARACTERISTICS IN HANOI CITY AND ADJACENT REGIONS To incorporate all possible impacts from seismic sources for Hanoi city, the study area was enlarged to the whole Northern Vietnam territory. Fig. 3.1 shows the seismotectonic map of Hanoi city and surrounding areas, developed based on up-to- date knowledge on seismically active faults and an earthquake catalog updated until 2019. Fig. 3. 1. Map of seismic active faults and an earthquake catalog in North Vietnam and adjacent areas. 3.1. Active faults The results of detail geomorphologic investigation show that the Red River fault consists of two branches stretching along the two banks of the Red River. According to the geophysical data, the Red River fault is a deepseated fault that crosses through the Moho, with the average depth of more than 30 km (Bui Cong Que, 1983). Geomorphology and topographical offsets suggest that these strike-slip movements are combined with normal slip (Phan Trong Trinh et al., 2012). 10
The Chay River fault is also identified as a deep-seated fault, stretching along the NE boundary of the Elephant Range metamorphic massive from Lao Cai to Viet Tri, with a length of hundreds of kilometers. Located in the NE and almost parallel to the RRFZ is the Lo river fault. According to the geological data, the fault appeared in Early Paleozoic. The fault is mainly identified as a right strikeslip, but along the SW side of Tam Dao mountain, it appears as a normal fault, dipping 70–80° to the SW direction. 3.2. Seismicity of Hanoi region While the large earthquakes were not recorded in the Vietnamese part of the RRFZ, the events with medium magnitude occurred quite frequently (Fig. 3.1). During less than a century, from 1910 to 2005, 33 earthquakes with magnitude exceeding 4.0 have been instrumentally recorded within the zone. In addition, it is worth to mention the historical events, which might have occurred during the years 1277, 1278, 1285 and can be traced in the ancient annals. As described in literature, the first event “had caused a crack of 7 zhangs length (~24 meters) in the surface", while the second event was “a swam of three strong shakings during a day, and the third event “had made the gravestone in Bao Thien temple broken in two, and caused landslide in the Cao Son mountain” (Nguyen Dinh Xuyen, 2004). As evaluated by seismologists, the shakings of these historical earthquakes are comparable with intensity VII or VII-VIII on the MSK-64 intensity scale. 3.3. Investigation of the unified scaling law for earthquakes in the North of Vietnam. The results of the analysis of waiting time between two inter-earthquakes and the unified scaling law of earthquake in the North of Vietnam were performed for both long-term and short-term seismicity cases. The analysis results show that in the case of long-term seismicity, the good data collapse is observed when all three curves with cutoffs of M= 4.0, M= 4.5 and M = 5.0 respectively fall neatly onto a single curve. As for the short-term seismicity case, the same good collapse of data is observed with the cutoffs of M =3.0 and M =4.0. It should be noted that the kink at x=1 in this case corresponds to the cutoff value equaling to M =4.0, which is suitable for seismic hazard calculation. CHAPTER 4: SEISMIC HAZARD ASSESSMENT FOR HANOI CITY In this Chapter, the seismic hazard assessment for the Hanoi city is presented in two probabilistic (PSHA) and deterministic (DSHA) approaches. The procedure of seismic hazard assessment consists of the following steps: 1) Define the seismic sources in Hanoi and adjacent areas; 2) Estimation of seismic hazard parameters for the seismic source zones; 3) Selection GPEMS for the study area; 11
4) Calculation and establishment of seismic hazard maps. 4.1 Seismic source models The seismic source models of the Hanoi region were developed based on a digitized database of the seismically active faults defined throughout Vietnam. For this study, a seismic source model is defined along seismically active faults by summing all the possible rupture zones caused by maximum earthquakes, which might occur within the given zone. In another word, this is the projection of tectonic fault plans counting from the lowest active layer to the Earth's surface. However, while delineating a seismic source zone boundary; this rule can be extended, depending on certain observed earthquake epicenter distribution, a set of faults in cases of scattered earthquake data. The acceptable boundary for a seismic source zone has to maintain all seismotectonic characteristics of the zone as a whole, namely the azimuthal location, direction of main geologic structures and a cluster of earthquake epicenters. In total, 18 seismic source zones were delineated (Fig. 4.1). Fig. 4. 1. Map of the seismic source zones used in this study. 4.2. Estimation of seismic hazard parameters for the seismic source zones In this thesis, value M0 = 4.0 has been selected according to the investigation of the earthquake unified scaling laws. In order to calculate the seismic hazard of Hanoi region, the following earthquake hazard parameters, characterizing the level of seismicity, were estimated for each seismic source zone: 12
- Constants a, b in the Gutenberg-Richter magnitude-frequency relation and their deductive values , ; - Expected maximum magnitude Mmax; - Mean return period T(M) of the strong earthquakes with magnitude M. 4.3. Ground motion prediction models The establishment of an attenuation equation to be applied to a study region is important and usually considered as a separate stage in PSHA procedure. To select suitable GMPEs for Vietnam, a test has been carried out to compare the calculation results of 25 published GMPEs with seismograms of 39 earthquakes recorded in the territory of Vietnam and to find the GMPEs, best fit to Vietnamese data. As Hanoi city is located on the boundary of Northeastern and Northwestern seismotectonic provinces, an attempt has been made to select the GMPEs for these two provinces. In results of the best fit test, two GMPEs that can be applied to Hanoi region, which are the Campbell & Bozorgnia (2008) and Akkar et al., (2014). These GMPEs then were used for seismic hazard assessment of Hanoi city. 4.4. Seismic hazard maps of Hanoi city 4.4.1 Probabilistic seismic hazard maps of Hanoi city Results of seismic hazard assessment for Hanoi city are presented in terms of probabilistic seismic hazard maps. Program CRISIS2015 was used to compute hazard. Figs. 4.2 - 4.3 illustrate the probabilistic seismic hazard maps of Hanoi city, representing the spatial distribution of the median values of SA 0.2 sec and SA 1.0 sec (in unit of % g) with 10% and 2% probability of exceedance in 50 years and VS30 site class A. Analyzing the hazard maps of Hanoi city, the following can be concluded: Spatial distribution of seismic shakings in Hanoi city has a prolonged shape in NW-SE direction, where the highest hazard coincides with location of three active faults named Red River, Chay River and Lo River crossing the Hanoi city. For the whole territory of Hanoi city, the SA 0.3 sec. Values are in the range of 0.09-0.14 g and 0.16-0.32 g, corresponding to a return period of 475 and 2,475 years, respectively. The maximum shaking intensity of VIII level according to the MSK-64 scale is predicted for all downtown districts at the return periods of 475 years. For return periods of 2,475 years, the maximum shaking of IX intensity level according to the MSK-64 scale is predicted for such downtown districts as Cau Giay, Thanh Xuan and Ha Dong and a part of the Ba Dinh, Dong Da and Hoang Mai. For the whole territory of Hanoi city, the 1.0 sec. SA values are in the range of 0.03-0.05 g and 0.05-0.08 g, corresponding to a return period of 475, and 2,475 13
and 9,975 years, respectively. Thus, maximum shaking intensity in within the city can only reach to the VI and VII levels according to the MSK-64 scale. In the downtown area, the highest shaking values are predicted in such districts as Tay Ho, Hoan Kiem, Ba Dinh, Cau Giay, Dong Da, Hai Ba Trung, Thanh Xuan and Ha Dong. Maximum shaking in these districts can reach to the VI level according to the MSK-64 scale at the return periods of 475, and to the VII level according to the MSK-64 scale at the return periods of 2,475. Figure 4.2. Maps showing spectral acceleration at 0.3s (SA 0.3s) in Hanoi for 475 and 2475 years Figure 4.3. Maps showing spectral acceleration at 1.0s (SA 1.0s) in Hanoi for 475 and 2475 years 4.4.2 Deterministic ground motion assessment In parallel with the usage of PSHA, in this thesis the DSHA approach is also applied to the Hanoi city area, which is detailed below: 4.4.2.1 Defining scenario earthquake 14
A scenario earthquake is the event, most likely to have to occur in the future, and with predefined parameters. The scenario earthquakes were created for Hanoi city with the following assumptions: 1) Earthquake originated on one of the active tectonic faults which crosses through or nearby the city. 2) For fault ruptures, the closest point on the fault to the site is taken as the distance. For this study, the scenario earthquake originated on the Red river was created. 4.4.2.2 Ground motion assessment Figure 4.4 shows PGA map calculated from the Red river fault scenario earthquake using attenuation equation proposed by Campbell and Bozorgnia (2008). As can be seen from the map, spatial distribution of PGA values clearly reflects shaking attenuation from a single source, with higher values of PGA observed near the epicenters areas. The PGA value ranged from 0.04 to 0.2g corresponding intensity of VI-VIII on the MSK-64 scale can be expected in the Hanoi city region. Figure 4. 4. The PGA map calculated from the Red river fault scenario earthquake 4.5. Ground motion amplification Ground motion amplification to account for local site conditions is an important stage in the procedure of seismic hazard and seismic risk assessments. In this framework of the thesis, the direct correction of ground motion amplification is applied to PSHA approach, the indirect correction with DSHA. 15
4.5.1 The direct correction of ground motion amplification In order to serve the direct correction of ground motion amplification to account for the local site conditions, Vs30 map of the study area was established with 191 seismic exploration points and 181 microtremor points. In the study area, the SA 0.3s value, taking into account the local site conditions, corresponding to a 475 years return period, ranges from 0.156-0.178 g, which is equivalent to VIII on the MSK-64 scale. Regarding spatial distribution, SA 0.3s value tends to decrease from West to East of the study area, specifically in Thanh Xuan district, SA 0.3s value was the largest 0.178g and smallest in Hoan Kiem district 0.156g. This is quite consistent since the Red River - Song Chay seismic source zone is located in the northwestern side of the five districts, and Thanh Xuan district is closer to the source than the rest. Besides, the SA value also reflected in the distribution of the Vs30 value, which has shown quite clearly in the area of Thanh Xuan district. Meanwhile, the SA 1.0 value, taking into account the local site conditions, corresponding to a return period of 475 years, ranges from 0.081-0.091g, which is equivalent to VII on the MSK-64 scale. In terms of spatial distribution, like the case of SA 0.3s, the largest value of SA 0.1s in Thanh Xuan district is 0.091g. However, the smallest value located in the Northwest area of Ba Dinh and Dong Da districts is 0.081g, which is completely consistent with the distribution Vs30 map, in this area, the value Vs30 is from 180 to 360 (m/s). 4.5.2 The indirect correction of ground motion amplification The scenario earthquake was used for estimation of building damage in the study area (See Fig. 4.4). ArcRisk tool was used to generate the PGA map with the chosen scenario earthquake. To develop elastic response spectra for ground types A to E of Hanoi city, spectral acceleration for the short periods, SAS, maps are developed from PGA, and spectral acceleration for the long period, SAL, is inferred from short period spectral acceleration, SAS, based on the factors given for rock (Ground type A) locations. To obtain the reliable shaking values, the amplification of ground motion to account for local site conditions is based on the ground types and soil amplification factors defined by the TCVN 9386-2012. For site effect evaluation, the engineering geological map of Hanoi region at a scale of 1: 25000 was used as initial basis for the classification of ground types. Improvement of site classification was made by using the VS30 values obtained from 191 seismic survey points and 181 microtremor survey points. 16
Regarding spatial distribution, SA 0.3s and SA 1.0s tend to decrease from West to East with the maximum SA value is in the Western area of Thanh Xuan district and the minimum value is in the East area of Hoan Kiem district. The highest SA 0.3s in Thanh Xuan district is 0.29g, corresponding to the intensity level IX and the smallest in Hoan Kiem district area is 0.18g, corresponding to the intensity level VIII on the MSK-64 scale. Similar to the case of SA 0.3s, the largest value of SA 1.0s in the Western area of Thanh Xuan district is 0.107g and the smallest value in Hoan Kiem district is 0.067g, which corresponds to the intensity level VII on the MSK-64 scale. The results obtained are quite consistent, with the Red river earthquake scenario was built on the west side of the five districts and the Thanh Xuan district being closer to the source than the rest. In addition, the SA 0.3s and SA 1.0s values also reflect the local site amplification quite clearly in the area of Ba Dinh district. The results SA 0.3s and SA 1.0s will be the input data for calculating building damage and casualties. CHAPTER 5: SEISMIC RISK ASSESSMENT FOR HANOI URBAN AREA In this Chapter, the results of seismic risk assessment in the Hanoi urban area were assessed using both PSHA and DSHA methodologies by the ArcRisk program. The methodologies were applied to assess the building damage and casualties for five downtown districts of Hanoi city. 5.1. Estimation of building damage in Hanoi urban area 5.1.1. Probabilistic estimation of building damage The results of building damage estimation obtained from the probabilistic method are illustrated here. Figure 5.1 shows the probability of slight, moderate, extensive and complete damage to building stock. Spatial distribution of probability all states damage to building stocks in the area is quite similar: slight state ranges from 13.96 to 14.65%, moderate state is from 6.22 to 8.10%, extensive state is from 1.28 to 1.9% and complete state is from 0.07- 0.15%. The probability of states damage to building stocks illustrated here can also be considered as the number of building damages caused by the earthquake out of the total number of buildings located in the study area. 17