Tóm tắt Luận án Urban on seismic risk assessment for Hanoi city

rrected 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 = dsdS , .εds (3) 8 where dsdS , 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: 𝑃[(𝑑𝑠|𝑆𝑑)] = Ф [ 1 𝛽𝑑𝑠 𝑙𝑛 ( 𝑆𝑑 𝑆̅𝑑,𝑑𝑠 )] (4) where dsdS , 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 dsdS , 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. 9 - 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. 10 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). 11 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; 12 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: 13 - 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 14 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 15 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. 16 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. 17 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. 18 a) b) c) d) Figure 5. 1. Probabilistic case: maps of structural building damage probabilities in four damage states: a) Slight; b) Moderate; c) extensive and d) Complete. 5.1.2. Deterministic estimation of buildings damage The results of building damage estimation caused by the Red river scenario earthquake (M=6.3; h=17 km) are illustrated in figure 5.2. Spatial distribution of structural building damage shows heavier damages in several wards, which are located in Western part of Ba Dinh, Dong Da, and Thanh Xuan districts. This results are quite reasonable since the epicenter of scenario earthquake is located in the Western study area. Similar to the probabilistic estimation, the result of building damage estimation in each state is quite similar in five urban districts of Hanoi city. Specifically, the slight damage ranges from 17.6 to 20.9%; moderate state is from 10.7 to 12.1%, extensive state is from 2.7 to 3.5% and complete state is from 0.2 to 0.3%. The structural building damage values are illustrated here can also be considered as the number of building damage caused by the earthquake out of the total number of buildings located in the study area. 19 a) b) c) d) Figure 5.2. Deterministic case: maps of structural building damage probabilities caused by the Red river scenario earthquake in study area in four damage states: a) Slight; b) Moderate; c) Extensive and d) Complete. 5.1.3. Comparison between the results from deterministic and probabilistic analyses The results from both deterministic and probabilistic methods show a general rule that building damages in the study area are mostly impacted by the ground shakings. Figure 5.3 shows the building damage results from deterministic and probabilistic analyses. In which, the deterministics based on the Red river earthquake scenario, the probabilistics was based on the SA at 0.3s and 1.0s corresponding to return period of 475 years. Comparing the areas of damaged buildings by structure shows that the most damage belongs to the concrete buildings. This can be explained by the fact that in the study area the area of concrete buildings (group C) is 5.4 times larger than the area of masonry buildings (group M). In group C, for the light damage state the highest probability of damage belongs to C3H and C3M building types, accounting for 27.97% and 31.74% in the deterministic case and 23.63% and 26.40% in the probabilistic case. In group M, for the light damage state the highest probability of damage belongs to 20 URMM and URML building types, accounting for 24.83% and 21.39% in the deterministic case and 26.55% and 22.66% in the probabilistic case. Figure 5. 2. Proportion of damaged building area to total usable building area (in %) in five downtown districts of Hanoi city in four damage states. a) results of deterministic case (the Red river scenario); and b) results of probabilistic case (the Sa 1.0s and Sa 0.3s maps with 10% probabilities of exceedance in 50 years). 5.2. Casualty estimation for Hanoi urban area. The Arcrisk was ugraded according to HAZUS-MH methodology and modifications based on the population structure national census survey data 2019. The Arcrisk was performed to evaluate the casualty for five urban districts of Hanoi city. According to the results obt