It has been known that Bengkulu City (Indonesia) is vulnerable to undergo seismic damage. This study is initiated by measuring horizontal to vertical spectral ratio (H/V) to sites in Bengkulu City using microtremor. The inversion analysis is performed to generate shear wave velocity profile. A total of 218 sites is investigated in this study. The results show that observed H/V is consistent with the theoretical H/V. National Earthquake Hazard Reduction Program (NEHRP) code is adopted to classify the site class. The results also exhibit that Bengkulu City is dominated by Site Classes C and D. In general, this study could lead local government to reconsider seismic hazard mitigation for spatial plan.
Research
Seismic Hazard Microzonation of Bengkulu City, Indonesia
https://doi.org/10.21203/rs.3.rs-42761/v1
This work is licensed under a CC BY 4.0 License
Journal Publication
published 25 Feb, 2021
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Bengkulu City is a capital city of Bengkulu Province, Indonesia. This city is located in the western part of Sumatera Island, which is bordered with Indian Ocean. Recently, the city has been promoted as one of tourist destinations in Indonesia. However, earthquakes threats have identified as the main issue in Bengkulu City since last two decades. Mase (2017) reported that at least two major earthquakes had occurred within last two decades. The first strong earthquake happened on June 4, 2019, with Mw 7.9 (the Bengkulu-Enggano Earthquake), while the second strong earthquake happened on September 12, 2007 with Mw 8.6 (the Bengkulu-Mentawai Earthquake) (Fig. 1). Misliniyati et al. (2018) noted that those devastating earthquakes had triggered huge damage to structural buildings and lifelines facilities. Those earthquakes also triggered liquefactions along coastal area of Bengkulu City (Mase, 2017). Learning from earthquake events in the past, the effort to escalate seismic hazard mitigation in Bengkulu City should be prioritised.
Studies of earthquake hazard in Bengkulu City had been performed by several local researchers. Mase (2017) performed analysis of liquefaction potential along coastal area of Bengkulu Province based on site investigation data and geophysical measurement. Mase (2017) highlighted that liquefaction could happen along coastal area of Bengkulu City, especially during the Bengkulu-Mentawai Earthquake. Mase (2018) performed a reliability study of designed spectral acceleration for structural buildings in Bengkulu city and found that spectral acceleration resulted during seismic wave propagation had exceeded the designed spectral acceleration. Mase (2018) suggested that it is important to propose the updated design. Mase (2018) also recommended that seismic amplification effect should be considered in composing the updated designed spectral acceleration. Farid and Hadi (2018) investigated ground shear strain (γ) in Bengkulu City based on microtremor measurements. Farid and Hadi (2018) found that geological condition of Bengkulu City is dominated by alluvium materials. Those materials are composed of sandy soils, which are very sensitive to undergo liquefaction during earthquakes. In general, those previous studies had presented preliminary investigation on soil damage during earthquakes in Bengkulu City. However, seismic microzonation study related to local site condition has not performed yet.
Subsurface geophysical measurements included two major methods, which are widely known as passive and active methods. The implementation of passive method using microtremor is more preferable. Workability, low cost, and accuracy aspects are some reasons why this method is more chosen. Several researchers, such as Gosar (2010), El-Hady et al. (2012), and Mase et al. (2018a) had performed microtremor measurements for site investigation. Generally, those previous studies emphasised on depicting site characteristic based on time-averaged shear wave velocity for first 30 m depth (Vs30). Mase et al. (2018a) suggested that Vs30 is an important parameter to describe seismic vulnerability in an area. It is therefore very useful to be implemented in the area with high seismic intensity, such as Bengkulu City, Indonesia (Mase, 2019).
This paper presents a seismic microzonation study. The microtremor measurements were performed at hundreds of sites in Bengkulu City. The horizontal to vertical spectral ratio (H/V) is obtained from measurement. Furthermore, the inversion analysis is performed to generate shear wave velocity (Vs) profile. Vs profile is then analysed to determine Vs30, which is used to classify site condition. In general, the results could provide information about seismic vulnerability in Bengkulu City.
Bengkulu Province has been known as an area, which has a high seismic intensity (Mase, 2017). In Fig. 1, seismotectonic setting of Bengkulu Province is presented. There are three active tectonic settings which frequently triggered earthquakes in Bengkulu Province and its surrounding areas. The first earthquake source is Sumatra Subduction. This zone is the plate boundary between the Eurasian Plate and the Indo-Australia Plate. This subduction zone had triggered two major earthquakes in Bengkulu Province (the Mw 7.9 Bengkulu-Enggano Earthquake in 2000 and the Mw 8.6 Bengkulu-Mentawai Earthquake in 2007).
In Sumatra plain, a large fault system crossing Sumatra Island exists. This zone is known as Sumatra Fault System (Mase, 2020). This fault system is categorised as slip strike fault. Sieh and Natawidjaja (2000) mentioned that Sumatra Fault is situated between subduction zone in Southern Java and oblique subduction system in Sumatra. McCaffrey (2009) mentioned that this fault is associated with the oblique convergence between Indo-Australian Plate and Eurasian Plate. The earthquake history recorded that this fault had triggered both the Mw 6.8 Liwa Earthquake and the Mw 7.9 Alahan Panjang Earthquake in 1994 (Widiwijayanti et al., 1996 and Natawidjaja and Triyoso, 2007). Another active fault, which is known as Mentawai Fault System is located between Sumatra Subduction and west coast of Sumatra (Sieh and Natawidjaja, 2000). The Mentawai Fault system is also categorised as strike-slip fault zone. This fault had triggered a major earthquake, which was called the Mw 7.6 Padang Earthquake in 2009 (McCloskey et al. 2010).
Figure 2 presents geological formations of Bengkulu City. Generally, formation of alluvium terrace (Qat) is dominant, especially along coastal area up to middle part of Bengkulu City. This formation is composed of sediment materials, such as sands, silts, clays, and gravels. Mase (2017) mentioned that during the Mw 8.6 Bengkulu-Mentawai Earthquake, several liquefaction evidences were found on sites dominated by Qat. The reef limestone formation (Ql) is also found on coastline parts. Shallow soil deposits composed of sandy soils were underlain by reef limestone in this formation. At the borders between Central Bengkulu Regency and Seluma Regency and at several parts in the Northern Part of Bengkulu City, Bintunan formation (QTb) is found. This formation is composed of rock materials, such as polymictic conglomerate, breccia, reef limestone, tuffaceous claystone, and wood fossil. In the eastern part of Bengkulu City, which is bordered with Central Bengkulu Regency and Seluma Regency, Andesite formation (Tpan) is found. This area is high terrain area in Bengkulu City. Swamp deposit (Qs) is found in northern and western parts. This formation consisted of sand, silt mud, clay and plant fossils. Alluvium formation (Qa) is dominant in the middle part of Bengkulu City. Several materials, such as boulder, gravel, sand, silt, mud, and clay, are identified as the composing materials (Natural Disaster Agency of Bengkulu Province or BPBD 2018).
Mase et al. (2018b) had presented typical geological condition of Bengkulu City (Fig. 3). Generally, typical subsoils in Bengkulu City are composed of three main materials, i.e. clayey soil, sandy soil, and rock. Clayey soils, such as organic clay (OH), high plasticity clay (CH), and silty clay (CM) are found at first 3 m depth. These layers have cone resistance (qc) of 2 to 30 kg/cm2 and Vs of 94 to 353 m/s. Furthermore, granular soils composed of silty sands (SM), poor-graded sands (SP), clayey sands (SC), and clayey gravels (GC) are found at depth of 3 to 40 m depth, with qc of 30 to 250 kg/cm2 and Vs of 200 to 614 m/s. The soft-rocks composed of sandstones are generally found at depth of more than 40 m, which has qc more than 250 kg/cm2 and Vs of 476 to 760 m/s.
4.1 H/V Method
Several researchers, such as Mase et al. (2018), El-Hady et al. (2012), Bard (2004), and Lachet et al. (1996) had implemented microtremor measurement to observe local site condition. In its applicability, horizontal motion of microtremor, which is composed of shear waves is used as measurement assumption. Predominant period (T0) and Horizontal to Vertical Spectral Ratio (H/V) are estimated based on horizontal motions spectra which reflect the transfer function at investigated site.
The use of H/V method was firstly introduced by Kanai and Tanaka (1954) (popularised by Nakamura (1989)). This method is derived from spectral ratio of horizontal motion, which is transferred from microtremor measurement. Mase et al. (2018a) mentioned that H/V method is reliable to expect shear waves velocity at a site. Atakan (2009) explained that H/V from earthquake shaking recorded on sediment surface to bedrock surface is generally consistent with H/V from microtremor measurement. Lachet and Bard (1994) suggested that H/V method is very important in predicting sediment deposits. H/V is calculated by comparing Fourier Spectra between horizontal components and vertical component, as expressed in the following equation,
where, H(EW) and H(NS) are the Fourier amplitude spectra of horizontal in east-west and north-south directions, respectively, and V is vertical spectra value.
Mase (2019), Mase et al. (2018a), Kockar and Akgun (2012), Lachet et al. (1996) highlighted that H/V Method is very useful in predicting local site condition. Spectral ratios obtained from H/V method is relatively more stable than other raw noise spectra. Kockar and Akgun (2012) mentioned that f0is well correlated to clear peak of H/V or amplitude (A0). However, noises caused by human, environmental setting, and other active vibration sources at surface could affect measurement quality. Therefore, measurement should be carefully performed to ensure the appropriate result. However, Bonnefoy-Claudet et al. (2006) stated that H/V method is effective to predict predominant frequency (f0) (simply predicted by 1/T0). Raptakis et al. (2005) explained that even though the limitation of H/V has been recognised, the method is still widely used. SESAME (2004) compiled the criteria to determine the clear peak of H/V, which could help the engineers to determine site condition for engineering practice (the detail can be found in SESAME (2004)).
In this study, a triaxial geophone-broad band seismometer called PASI Gemini is used to measure microtremor on each investigated site. The seismometer consisted of three components, i.e. east-west (EW), north-south (NS), and up-down (UD). The equipment is also applicable to measure strong and weak motions. The digitisers are let to warm up for 5 minutes before measurement. This treatment is expected to avoid problem of low frequency range and to obtain reliable data. Each measurement is performed with the duration of 30 minutes. Once the measurement is completed, noises from recorded data are removed and processed based on SESAME (2004) criteria. Results of processed are then analysed to generate H/V curve. Next, H/V curve is analysed to generate Vs profile. In this study, the inversion technique introduced by García-Jerez et al. (2016) is used. The inversion of H/V is computed based on Monte Carlo sampling simulated annealing (García-Jerez et al., 2016).
The adopted model from García-Jerez et al. (2016) required several parameters for each layer. Those parameters are soil thickness, pressure wave velocity (Vp), shear wave velocity (Vs), soil density (r), and Poisson’s ratio (n). The assumption of elastic half-space is adopted for the bottom layer. Furthermore, each parameter is ranged from minimum to maximum value, which is defined from the a-priori knowledge of soil properties (Wathelet, 2008) in Monte Carlo Simulation. In this study, values of required parameters are taken from the information of typical soil layer in Bengkulu City. The shear wave velocity is derived from the correlation provided by Imai and Tonouchi (1982). Vs may be also predicted from the elastic theory which is related to Poisson’s ratio and elastic modulus (Salencon, 2001). To predict Vp, ratios of Vp/Vs are also firstly assumed in the model (Tatham, 1982). Several researchers, such as Wathelet (2008), García-Jerez et al. (2016), Mase et al. (2018a), and Mase (2019) stated that several parameters including Vp, Vs, and Poisson’s ratio could be categorised as the interdependent parameters. The Monte Carlo simulation is initiated with the starting guess whose parameters are randomly selected within the ranges listed in Table 1. The velocity profiles and other parameters are calculated repeatedly until measured H/V curve and calculated H/V curve are consistent with each other.
4.2 Site Classification
National Earthquake Hazard Reduction Provision or NEHRP (1998) introduced the criteria to classify local site condition. The criteria were derived based on time-averaged shear wave velocity for first 30 m depth (Vs30), which is expressed in this following equation,
In Equation 2, diis thickness of each layer, Vsi is shear wave velocity of each layer, and n is the number of layers for first 30 m depth. The criteria to determine site classification based on Vs30 are summarised in Table 2. Generally, the identification of site classification is very important in earthquake engineering practice. Likitlersuang et al. (2020) mentioned that the information of shear wave velocity for first 30 m depth is necessary to observe soil response during seismic wave propagation. In this study, the interpretations of site classification and shear wave velocity are elaborated.
4.3 Methodology
This study is systematically performed based on flow chart presented in Figure 4. This study is initiated by capturing geo-hazard issue in Bengkulu City. Several literatures related to earthquake aspect and geological condition of Bengkulu City were reviewed. Furthermore, data collection (cone penetration tests or CPT and microtremor records) was performed. CPT tests were addressed to obtain geological condition. CPTs were also used as a consideration in determining starting guess model. Microtremor measurement on each site was recorded by using PASI seismometer. Secondary data used in this study were geological map of Bengkulu City, typical geological condition, and topography map. Geological formation and typical geological condition would help to understand subsoils in Bengkulu City. For access information to sites, topography map would be useful in this study. There are 183 microtremor points and 35 cone penetration test (CPT) points measured in this study.
After the data collection, the data analysis was conducted. At this stage, microtremor record on each site is processed to generate H/V curve. To obtain the reliable H/V curve, the results were examined by SESAME (2004) criteria. If H/V curve was not fulfilled the criteria, measurement would be performed again. In this study, several H/V curves representing each geological formation in Bengkulu City (Figure 2) were also elaborated.
Afterwards, inversion analysis was performed. In this study, the model adopted from García-Jerez et al. (2016) was used. The initial model referred to geological condition of Bengkulu City (Figure 3 and Table 1). The model would randomly seek the best model based on the ranges.
From inversion analysis and measurement, several issues were discussed in this study. They are H/V curve, peak H/V (A0), f0 and the comparison of Vsprofile on each geological formation. Seismic microzonation map based Vs30 range was also elaborated in this study. In general, this study could describe seismic hazard vulnerability in Bengkulu City.
5.1 Measurement Results
To obtain the description of geophysical characteristic in Bengkulu City, several H/V curves (P-1 to P-6) which represent geological formations in Bengkulu City (Figure 2), are presented in Figure 5. All H/V curves have been also fulfilled the criteria of SESAME (2004).
In Figure 5, several H/V curves with sharp clear peak have been demonstrated by Figures 5b, 5c, and 5e. Those sites are located at QTb, Qa, and QI, respectively. A0 values of those sites are ranging from 2.2962 to 6.8101 and f0 are ranging from 6.0154 to 7.5231 Hz. A large A0indicates that there is a large impedance contrast between sediment and bedrock in a site. Meanwhile, a large f0 means there is a thin sediment thickness in a site. It seems to be realistic, since P-3 and P-5 tends to have thin sediment thickness underlain by the bedrock. Materials such as sand, silt, mud and boulder are dominant at Qa. Those materials have certainly different characteristic which are also possible to influence amplification. For P-2, the impedance contrast between sediment and bedrock is relatively smaller than P-3 and P-5 since A0is relatively smaller. The sediment thickness of P-2 is relatively thinner than P-3 and P-5 sites because of larger f0.
The flattered H/V curves are presented by P-1 and P-4, which represent Qs and Tpan formations. P-1 is located at swamp deposit dominated by sand, silt, mud, clay with plant remains. Those materials seem to be underlain by shallow bedrock. P-4 site is located at andesite formation which has a thin sediment thickness underlain by bedrock. A0 and f0 values for those sites are about 2.0918 to 2.7859 and 4.0152 to 4.501 Hz, respectively. Both parameters indicate that there is a medium contrast between rock and sediment in P-1 and P-4. The H/V curves also show that f0 values are smaller than P-2, P-3, and P-5. P-6 which represents Qa formation is composed of sand, silt, clay, and gravel. Both A0 and f0 values are 2.1518 and 4.0963 Hz, respectively. It indicates that impedance contrast between rock and sediment is relatively medium. Similar to other sites, thin sediment may be also found in this site since f0 value are quite large i.e. 4.0963 Hz.
5.2 Predominant frequency
In Figure 6, it can be seen that predominant frequency ranges are varied. There are five ranges representing distribution of predominant frequency. Those ranges can be grouped into high predominant frequency, medium predominant frequency, and low predominant frequency. High predominant frequency (f0> 5 Hz) can be found on several areas in Bengkulu City, especially in southern part, south-eastern part and western part. Those areas are generally dominated by Qs, Qa, Qat, QTb, and Tpan. Gosar (2010) mentioned that a high f0 indicates that a thin sediment thickness.
Low predominant frequency (f0 < 3 Hz) is found along southern part (some parts of coastal area) and northern part. Those areas are mostly dominated by Qat and QTb. The materials, such as sand, silt, clay, and sandstone are found in this region. Several researchers, such as Misliniyati et al. (2018) and Mase (2017) predicted that sand deposit along coastal area of Bengkulu City could be very vulnerable to undergo liquefaction. In terms of sediment thickness, a low predominant frequency indicates a thick sediment thickness. Medium predominant frequency (3 Hz £ f0£ 5 Hz) are concentrated in mid-plain of Bengkulu City. This area is generally dominated by Qa. Several sediment materials, such as sand and clay are found in this formation. Based on predominant frequency distribution, it can be predicted that sediment thickness at sites with medium predominant frequency is not much thicker than sediment thickness at sites with low predominant frequency. However, it may be thicker than sediment thickness at sites with high predominant frequency.
5.3 Amplitude (A0) or Peak H/V
A0 distribution is also divided into 5 categories, which can be grouped into low A0, medium A0, and high A0. In general, medium A0(3 £ A0 £ 5) is dominant. Based on geological condition, medium A0areas are dominated by Qa, Qat, and Tpan. Gosar (2010) mentioned that a larger A0 means a high impedance contrast between sediment and rock surface. The impedance contrast is the difference ability between the sediment and rock surface to propagate the seismic wave. Several areas, such as southern and northern parts of Bengkulu City have low A0 (A0 < 3). Those areas are also dominated by Qa, Qat, Qtb, and Qs formations. In terms of impedance contrast, a low A0 indicates a low impedance contrast between sediment and rock surface. Small parts on the coastline of Bengkulu City indicate high A0 (A0> 5). This area is dominated by Qat. A0is also related to the ability of sediment layer to amplify seismic wave propagation. Several studies (Mase, 2017; Misliniyati et al., 2018; Mase, 2020) had confirmed that for sites along coastal area of Bengkulu City could amplify during the Bengkulu-Mentawai Earthquake in 2007. Generally, the results are consistent with previous studies.
5.4 H/V comparison and Vs profile
The comparison between measured H/V and inversion H/V is presented in Figure 8. Generally, for all representative sites, H/V curves resulted from measurement and inversion are relatively consistent with each other. However, measurement curves slightly underestimated inversion curves. This is due to the fact that the contrast of velocity in predicted models is rather stronger than real ground condition (Souriau et al., 2011).
Furthermore, Vs profile is generated from the best model resulted from inversion analysis (Figure 9). In general, Vs profiles increase with depth up to 30 m depth on each investigated point. For first 30 m depth, Vs profiles consist of 4 to 5 layers. For P-2, P-3, P-4, P-5, thin layers with small difference of shear wave velocity profile are found. It indicates that the layer could have similar soil resistance. In addition, those thin layers are probably dominated by OH and CH, especially for first 2 m depth. Figure 9 also shows that Vs30 values at representative sites are ranging from 307 to 409m/s. Therefore, those sites can be categorised as Site Classes C and D.
5.5 Seismic Hazard Microzonation
The description of seismic hazard microzonation is presented in Figure 10. It can be seen that Bengkulu City is dominated by Site Class D. The map presents that most coastal areas are identified to have low Vs30 values. Vs30 values in those areas are generally within the range of 180 m/s to 360 m/s, which fall into Site Class D. In terms of geological condition, Site Class D are dominated by sediment materials, such as sandy soils and soft clayey soils which have low soil resistances. Mase (2017) and Mase (2018) also explained that sediment materials with low soil resistances are very vulnerable to undergo seismic impact, such as liquefaction and ground amplification.
Site Class C (Vs within the range of 360 to 760 m/s) is found in the eastern part of Bengkulu City. Generally, the eastern part of Bengkulu City is dominated by Tpan. This formation is composed of stiff materials. During the Bengkulu-Mentawai Earthquake, Mase (2017) and Misliniyati et al. (2018) reported that there was no significant damage found in the eastern part of Bengkulu City. It might be due to larger soil resistance existing in this area. Meanwhile, liquefaction, ground failure and structural damage were found massively along coastal area of Bengkulu City. Based on the finding, it can be concluded that areas of site class C are relatively safer from seismic impacts.
Overall, parameters of f0, A0and Vs30 are well correlated with geological condition in the study area. Misliniyati et al. (2018), Farid and Hadi (2018), and Farid and Mase (2020) mentioned that the updated spatial plan of Bengkulu City should consider seismic hazard microzonation. It would be effective to minimise the damage during earthquakes in Bengkulu City.
This paper presents the seismic hazard microzonation of Bengkulu City, Indonesia using the Inversion H/V of microtremor observations. A total of 218 sites are observed in this study. The inversion analysis using Monte Carlo simulation simulated annealing is performed to determine the best model of site condition. A0, f0 and Vs30 are presented to characterise local site condition.
The geophysical parameters of site, such as f0 and A0 are relatively consistent with general geological condition of Bengkulu City. f0 values can be used to estimate sediment thickness in the study area. Generally, Bengkulu City is dominated by Qs and Qat. The sediment thickness in the study area is relatively thin. From the information, it can be roughly estimated that bedrock surface in Bengkulu City may be located at shallow depth.
In Bengkulu City, most sites tend to have medium (3 ≤ A0 ≤ 5). It indicates that there is a medium impedance contrast between rock surface and sediment. For Bengkulu City, these dominant zones are dominated by Qa and Qat. Those formations are composed of sediment materials, such as clay and sand. During the Bengkulu-Mentawai earthquake, the magnification of peak ground acceleration in these dominant zones are relatively large. Generally, field observations are consistent with several evidences reported in previous studies (Mase, 2017 and Misliniyati et al., 2018).
This study also focused on seismic hazard microzonation in Bengkulu City. Site Class D is found on middle part to western part, whereas Site Class C is found on eastern part. Site Class D tends to have a lower soil resistance. It indicates that seismic damage could be more serious. In general, Site Class D is located along coastal area of Bengkulu City, which is prospectively developed as tourist area in near future. Therefore, spatial plan on the basis of seismic hazard assessment should be carefully considered in Bengkulu City.
Overall, the implementation of geophysical survey to determine site characteristic is successfully performed in this study. The inversion technique to determine Vs is reasonably performed. The results could provide a better understanding of seismic hazard mitigation in Bengkulu City. Finally, the results could give a recommendation to local government in updating the spatial plan in Bengkulu City. The framework implemented in this study can be used to investigate site characteristics in other areas.
- Available Data and Materials
All the datasets that have been used and analysed during the current study is available from the corresponding author on reasonable request.
- Competing Interest
I have declare that there is no any competing interests
- Funding
It is not applicable in this case
- Authors’ Contribution
- Lindung Zalbuin Mase: Conceptualisation, Software, Validation, Formal analysis, Data Analysis, Visualization, Writing-Original draft- review-editing, Mapping, Project Administration.
- Nanang Sugianto: Conceptualization, Methodology, Writing - original draft.
- Refrizon: Conceptualization, Methodology, Writing - original draft.
- Acknowledgement
This research was supported by Competitive Research Scheme from University of Bengkulu No. 2357/UN30.15/LT/2018. Authors would like to thank Soil Mechanics Laboratory and Geophysics Laboratory, University of Bengkulu for experiments and site investigations performed in this study.
H/V
horizontal to vertical spectral ratio
Vs
shear wave velocity
Vs30
time-averaged shear wave velocity for first 30 m depth
Qat
alluvium terrace
QI
reef limestone
QTb
bintunan formation
Tpan
andesite formation
Qs
swamp deposit
Qa
alluvium formation
OH
organic clay
CH
high plasticity clay
CM
silty clay
qc
cone resistance
SM
silty sand
SP
poor-graded sand
SC
clayey sand
GC
clayey gravel
T0
predominant period
H(EW)
Fourier amplitude spectra of horizontal in east-west direction
H(NS)
Fourier amplitude spectra of horizontal in north-south directions,
V
Fourier amplitude spectra of vertical
f0
predominant frequency
A0
peak of H/V or amplitude
EW
east-west
NS
north-south
UD
up-down
Vp
pressure wave velocity
Vs
shear wave velocity
ρ
soil density
ν
Poisson’s ratio
Vsi
shear wave velocity of each layer
di
thickness of each layer
n
number of layers for first 30 m depth
CPT
cone penetration test
NEHRP
National Earthquake Hazard Reduction Provisions
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Table 1. Range values of initial parameters for H/V inversion
|
Layer |
Soil Type |
Thickness |
Vs |
Vp |
Density |
Poisson Ratio |
|
USCS |
[m] |
[m/s] |
[m/s] |
[r] [kg/cm3] |
[n] |
|
|
1 |
OH/CH |
1-20 |
90-270 |
125-375 |
1800-2000 |
0.400-0.495 |
|
2 |
CM |
1-20 |
160-350 |
240-525 |
1800-2000 |
0.400-0.495 |
|
3 |
SM, SP, SC |
1-20 |
200-500 |
300-750 |
1800-2000 |
0.200-0.400 |
|
4 |
SW, SC, GC |
1-20 |
400-600 |
600-900 |
1800-2000 |
0.200-0.400 |
|
Half-space |
Sandstone |
~ |
500-760 |
750-1140 |
2000-2200 |
0.100-0.300 |
|
Remarks: OH = organic clay, CH = high plasticity clay, CM= silty clay, SM = silty sand, SP = poor-graded sand, SC = clayey sand, SW = well-graded sand, GC = clayey gravel |
||||||
Table 2. Site classification based on Vs30 (NEHRP, 1998)
|
NEHRP Site Class |
General Description |
Range of Vs30 [m/s] |
|
A |
Hard Rock |
Vs30 > 1500 |
|
B |
Rock |
760 < Vs30 < 1500 |
|
C |
Very Dense Soil and Soft Rock |
360 < Vs30 < 760 |
|
D |
Stiff Soil [15 < N < 50 or 50 kPa < N < 100 kPa] |
180 < Vs30 < 360 |
|
E |
Soil or any profile with more than 3 soft clay defiled as soil with PI > 20, w > 40%, and su < 25 kPa |
Vs30 < 180 |
|
F |
Soils requiring site-specific evaluation |
- |
|
Remarks: N = SPT value [blows/ft], su = undrained shear strength, PI = plasticity index, w = water content |
||