seismic hazard assessment
TRANSCRIPT
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July, 2011
PROBABILISTIC SEISMICHAZARD ASSESSMENT FOR
YANGON REGION,MYANMAR
Myo Thant, Soe Thura Tun, Maung Thein, Win Swe, Than Myint
Myanmar Earthquake Committee
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MYANMR EARTHQUAKE COMMITTEE July, 2011
REPORT
ON
PROBABILISTIC SEISMIC HAZARD
ASSESSMENT FOR YANGON REGION,
MYANMAR
Prepared
By
Myo Thant, Soe Thura Tun, Maung Thein,
Win Swe, Than Myint
July, 2011
MYANMAR EARTHQUAKE COMMITTEE
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ContentsAbstract ...................................................................................................................................... 1
Introduction ................................................................................................................................ 2Seismotectonics.......................................................................................................................... 2Analyzed Data ............................................................................................................................ 6Methodology .............................................................................................................................. 7Earthquake Sources Identification ............................................................................................. 7Earthquake Sources Characterization ...................................................................................... 10
Estimation of seismicity parameters (a- and b- values) ....................................................... 10 Estimation of maximum earthquake potentials .................................................................... 12 Estimation of earthquake recurrence rate............................................................................ 13
Attenuation Relationship ......................................................................................................... 14Site Characterization ................................................................................................................ 15Calculation of the Seismic Hazard ........................................................................................... 16
Discussion and Conclusions .................................................................................................... 23Acknowledgements .................................................................................................................. 23References: ............................................................................................................................... 24
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Abstract
According to the seismicity and the records of the previous considerable high
magnitude earthquakes, Yangon Region can be regarded as the low to medium seismicity
region. Moreover, tectonically the present region is bounded by the subduction zone of the
Indian Plate and Burma Plate in the west and the right lateral Sagaing fault in the east. The
most significant earthquake happened around this region is the Bago earthquake of 5th May,
1930 with the magnitude of 7.3. This earthquake caused 500 casualties and great destruction
in Bago. However, considerable damage and 50 deaths were recorded in Yangon. It was
originated from the Sagaing fault. The seismic hazard analysis is performed for Yangon
Region by applying the probabilistic way. In conducting seismic hazard analysis, firstly the
most possible seismic sources are identified and the seismic source parameters are then
determined for each sources. Based on the seismicity, focal mechanism study of the previous
events, and the geological data, the main seismic sources to cause the earthquake potentials
for this region are subduction zone of Indian and Burma Plates, Sagaing fault, and
Kyaukkyan fault. Including those sources, fifteen areal seismic sources and two fault sources
are identified. After that the seismic source parameters such as the seismicity parameters of
a- and b- values, maximum magnitude of earthquake potentials and earthquake recurrence
parameters for certain magnitude of each seismic source are estimated. The seismic hazard
analysis are carried out for 10% probability of 10 years, 20 years, 50 years and 100 years and
The seismic hazard maps are represented in terms of peak ground acceleration for those
recurrence intervals.
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Introduction
Yangon region is the major Region of Myanmar and the ancient capital is also located
in this region. Based on the seismicity and the records of the previous considerable high
magnitude earthquakes, this region can be assumed as low to medium seismicity region
(Figure 1). Some of the large earthquakes can be observed in the records and those events
caused the considerable damages to some buildings and some casualties in and around
Yangon region (Figure 2), e.g. the magnitude 7.3, earthquake which struck on May 25, 1930
and December 3, 1930. The former earthquake, well-known Bago earthquake, caused 50
deaths and great damages in Yangon while 500 casualties were resulted in Bago. The other
significant earthquakes are Yangon earthquakes of September, 1927 and December, 1927.
These events also resulted in a certain amount of damage in Yangon. Therefore the seismic
hazard analysis was performed for this region by applying the probabilistic way.
Seismotectonics
The major tectonics of Myanmar comprises of the subduction zone of Indian Plate
and Burma Plate in the west, and the collision zone of Indian Plate and Eurasia Plate in the
north. The rate of subduction is 35 – 50 mm/yr and the direction of subduction is NE to NNE
(Kayal, 2008). The other major structures present within Myanmar are the major fault system
of well-known Sagaing fault, Kyaukkyan fault, Gwegyo thrust, Kabaw fault system and WestBago Yoma fault. Most of the earthquakes happened in the central, relating with Sagaing
fault, and eastern part of Myanmar is not greater than 40 km in focal depth while the
earthquakes happened along the western portion represent the shallow, intermediate and deep
focus earthquakes. In the later case, the shallow focus earthquakes along the western margin
belongs to the subduction zone earthquakes and the focal depth of the earthquakes which
epitomize the subduction zone earthquakes gradually increase to the eastward. In the eastern
margin of the western portion, the shallow focus events indicate the correspondence of the
Gabaw fault system.Most of the earthquakes happened in the central part of Myanmar belongs to the
major structure, right lateral Sagaing fault which run through the central part of the country
with the length of more than 1,000km. It runs from the gulf of Mataban in the south through
Bago, Pyinmana, Yamethin, Tharzi, and Sagaing till Putao in the north. The records of the
previous significant earthquakes showed some destructive earthquakes originated from this
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fault are 7 and greater in magnitude. The focal mechanisms of the previous earthquake
happened along the Sagaing fault represents the strike-slip mechanisms (Figure 3),
confirming the compression force in NE-SW direction and extensional force in NW-SE
direction. However, the events which are located in the northern most part of Sagaing fault,
i.e. northern segments of that fault show strike-slip mechanism with the dominant trustmechanism. This character corresponds to the gradual changes or influence of the collision of
Eurasia and Burma Plates on the Sagaing fault system. The second-most significant fault
system is the Kyakkyan fault which strikes in nearly N-S and it extends southward from Pyin
Oo Lwin – Naungcho area through Taunggyi – Innle Lake with the length of above 450 km.
The largest earthquake happened from this fault with the magnitude of Richter magnitude 8.1
on 23rd May, 1912 (Chibber, 1913, Win Swe and Win Naing, 2008 and Mg Thein and Tint
Lwin Swe, 2006). However, very few (about 5 small events) have been recorded around this
fault.
Rather than the above mentioned faults, Kabaw fault is another important thrust fault
system. In the records, the maximum magnitude of the earthquake happened in the area of
this fault system is 7.7M, which struck on 12, March 1952. The second most high magnitude
earthquake, 7.2M 16, Aug 1938 event, and the third one is Magnitude 7 earthquake also
happened near Hommalin. Both of the last earthquakes happened at the deep foci with the
depth of 100km and 180km and the events happened in and around the area of this fault
systems are mostly greater than 60km in focal depth. It doubts that these events were
originated from the Kabaw Fault System and these seem to be subduction zone earthquakes.
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Figure (1). The seismicity of Myanmar (Data Source – ANSS earthquake catalog, 1963 –2009)
The present region, Yangon Region is tectonically bounded by the Indian-Burma plates subduction in the west, Sagaing fault in the east, West Bago Yoma fault in the north,
Kyaykkyan fault in the north-east, and the Andaman rift zone in the south. The earthquakes
observed in the Andaman sea region are shallow focus earthquakes that show not only the
normal fault mechanisms but also the strike-slip fault mechanisms.
In and around Yangon region, most of the earthquakes happened are in shallow focus
earthquakes, especially within about 250km in radius. Most are related with Sagaing fault,
some blind faults and subduction zone of Indian and Burma Plate (Part of Eurasian Plate),
and the Andaman Rift Zone. Moreover, some other faults whose geometry and other parameters are not well-known in and around this region also generated some earthquakes.
Small numbers of intermediate and deep focus earthquakes can be seen in this region and
those are caused by the subduction zone of Indian-Burma Plates.
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Figure (2). The Events of the considerable high magnitude earthquakes records.
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Figure (3). The focal mechanisms of the previous earthquakes (Data Source – CMT catalogof Harvard University, 1963 – 2009).
Analyzed
Data
The earthquake catalog of ANSS (1963-2009) is utilized with the complement of
USGS (1973-2009) and declustering of the dependent earthquakes was also carried out for
the present research by removing the foreshocks and aftershocks. For focal mechanisms
study, the CMT catalog of Havard University (1963-2009) is used in this research.
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Methodology
The seismic hazard analysis for Yangon Region was carried out by probabilistic way,
i.e. Probabilistic Seismic Hazard Analysis (PSHA) which was developed by Cornell (1968).
It includes four steps: (1) Identification and characterization of the possible seismic sources,
(2) Characterization of the spatial and temporal distribution of the earthquake recurrence, (3)
Determination of the ground motion using the ground motion predictive relationship, and (4)
Estimation of the probability that the ground motion parameter will be exceeded during the
particular time period (Kramer, 1996 and Reiter, 1990). The EQRISK program (McGuire,
1976) was used to calculate the seismic hazard, the ground motion parameter, peak ground
acceleration (pga).
Earthquake
Sources
Identification
Seismic sources are identified as areal seismic sources and fault sources according to
the geology (tectonics), seismicity and focal mechanisms study of the previous earthquakes.
Thirteen areal seismic sources are identified as eight sources (MAS_02A, MAS_02B,
MAS_03, MAS_17, MAS_18, MAS_19, MAS_20, and MAS_21) in the west of Yangon
Region, representing the subduction zone earthquakes, two (MAS_15 and MAS_16) in the
south, Andaman Rift Zone, one (MAS_22) representing Gwegyo thrust and West Bago Yoma
fault in the north, and two (MAS_10 and MAS_11) in the east, including some strike-slip
faults (Figure 4 and Table 2). In this case, some faults are identified as the areal seismicsources due to the lack of adequate information.
In the surroundings of this region, two significant fault systems are observed and
those are well-known Sagaing fault and Kyaukkyan fault. Two fault sources are therefore
identified and the fault segmentation of those faults was done and three major segments for
Sagaing fault as SGSMN in the northern portion of that fault, SGSMM in the middle portion
and SGSMS in the southern portion. The Kyaukkyan fault was also segmented as three
segments from north to south as KK01, KK02 and KK03. For this region, the most influence
segments from those two fault sources are SGSMM02_02, SGSMS01, SGSMS02, SGSMS03and SGSMS04 which are of Sagaing fault and KK03, the southernmost segment of
Kyaukkyan fault (Figure 5,6 and Table 2).
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(a)
(b)
Figure (4). The map of areal seismic sources, displaying (a) the epicentral distribution of the previous earthquakes happened 1963 – 2009, and (b) the epicentral distributionof the most significant events.
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(a)
(b)Figure (5) The map showing the most prominent faults around Yangon Region, displaying (a)
the epicentral distribution of the previous earthquakes happened during 1963 –2009, and (b) the epicentral distribution of the most significant events.
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Figure (6). The map of the fault seismic sources used in this seismic hazard analysis andsome other faults around Yangon Region.
Earthquake
Sources
Characterization
The characterization of the seismic sources include the determination of the seismic
source parameters of each seismic source such as seismicity parameters; a- and b- values, the
maximum magnitude of the earthquake potentials ( M max), and the recurrence rate of a certain
magnitude (lower bound earthquake) earthquake.
Estimation of seismicity parameters (a‐ and b‐ values)
The seismicity parameters of a- and b- values are estimated by applying the classical
Gutenberg-Richter recurrence law, with the complementary of the maximum likelihood
method of Aki (1965). The Gutenberg-Richter recurrence law can be expressed as the
following equation.
Log Nm = a – bm Eq.1
Where Nm is the number of earthquakes having magnitude ≥ m, a is the measure of seismic
activity and b is a parameter of the ratio of larger to smaller earthquakes (Gutenberg and
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Richter, 1944). The seismicity parameters obtained for seismic sources are shown in Figures
(7 and 8).
(a)
(b)
(c) (d)
(e) (f)
Figure (7). Gutenberg-Richter relations for the areal seismic sources: (a) MAS_02&03, (b)
MAS_10&11, (c) MAS_15&16, (d) MAS_17&18, (e) MAS_19, 20&21, and (e)MAS_22.
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(a) (b)
(c)
Figure (8). Gutenberg-Richter relations for the fault sources (a) SGSMM_02, (b) SGSMS01-04, and (c) KK03.
Estimation of
maximum
earthquake
potentials
The magnitude of the earthquake potential is another important seismic source
parameter in seismic hazard analysis. This parameter was estimated for areal seismic sources
and fault sources separately. For areal seismic sources, maximum likelihood method of Kijko
(2004) was used and the relationship is as follow:
)exp()}]exp(/{)}()([{ min22211maxmax nmnn E n E mm obs−+−−+= β Eq. 2
where, E 1( z ) = {( z 2 + a1 z + a2)/ [ z ( z
2 + b1 z + b2)]} exp (-z ) Eq. 3
n1 = n / {1 - exp [- β (mmax - mmin)]} Eq. 4n2 = n1 exp [- β (mmax - mmin)] Eq. 5
in which n is the number of earthquakes ≥ mmin, β = 10 Logb, a1 = 2.334733, a2 = 0.250621,
b1 = 3.330657, and b2 = 1.681534.
It must be noted that Eq. 2 does not constitute a direct estimator for mmax since
expressions n1 and n2, which appear on the right-hand side of the equation, also contain mmax
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and the maximum earthquake potential, mmax is obtained by the iterative solution of Eq. 2.
However, mmax can be obtained without interaction when mmax - mmin ≤ 2, and n ≥ 100, the
parameter mmax in n1 and n2 can be replaced by mmax(obs), (Kijko, 2004). The resulted
maximum earthquake potentials for each areal seismic source are represented in Table (2).
In estimating the maximum earthquake magnitudes for fault sources, the empiricalrelationships of the fault (rupture) length and the magnitude shown in Table (1) are used. The
results are listed in Table (2).
Table (1) The empirical relation of the fault (rupture) length and the earthquake magnitude
Relationship Authors
Ms = 2 log L + 1.33 log ∆σ + 1.66 (Mohammadioun & Serva, 2001)
M = (LogL+6.4)/1.13 (Ambraseys and Zatopek, 1968)
M = 2.0 logLmax + 3.5 (Iida, 1965)
M = 1.7 LogL + 4.8 (Matsuda, 1977)
a1* mmax= (logL+1.86)/0.5 (Papazachos et al., 2004)
a2* mmax= (logL+2.3)/0.59
M= (LogL+1.9)/0.5 Inoue et al. (1993)
L = 2W (Cheng and Cheng, 1989)
∆σ N = 10.6 W0.5 (Mohammadioun & Serva, 2001)∆σSS = 10.6 W0.8 (Mohammadioun & Serva, 2001)
ΔσR = 10.6 W1.6 (Mohammadioun & Serva, 2001)a1* For oblique faults (σ=0.13, 6.0≤M≤7.5)
a2* For strike-slip faults(σ=0.14, 6.0≤M≤8.0)
Estimation of earthquake recurrence rate
The recurrence rate of the lower bound magnitude earthquake for each seismic source,
which is used in calculating the seismic hazard, ground motion parameter is determined by
the Gutenberg-Richter recurrence relation and the results are displayed in Table (2). The focal
depth of the earthquake potential which is the other seismic source parameter is also
estimated based on the depth of the previous earthquakes.
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Table (2) Seismic source parameters of each seismic sources; fault specific sources and arealseismic sources (M0 =5.5 for all fault sources and 5 for all areal seismic sources).
No Sources Mmax β λ F
1 SGSMM02_SG02 7.8 1.2692 0.0763 152 SGSMS01 7.9 0.6977 0.0486 153 SGSMS02 7.8 1.2388 0.1706 154 SGSMS03 7.8 1.2388 0.1706 155 SGSMS04 7.8 1.2388 0.1706 156 KK03 8.1 1.2319 0.0117 157 MAS_02A 7.6 1.6194 0.7874 808 MAS_02B 7.6 1.6194 0.7874 459 MAS_03 7.6 1.6194 0.7874 15
10 MAS_10 8.2 1.6229 0.2780 1511 MAS-11 8.2 1.6229 0.2780 15
12 MAS_15 7.2 1.3548 0.3336 1513 MAS_16 7.2 1.3548 0.3336 1514 MAS_17 9.1 1.6636 0.4855 1515 MAS_18 9.1 1.6636 0.4855 1516 MAS_19 6.6 2.2227 0.1321 1517 MAS_20 6.6 2.2227 0.1321 1518 MAS_21 6.6 2.2227 0.1321 1519 MAS_22 7.2 1.4649 0.1208 15
Attenuation Relationship
The seismic hazard which is represented by the ground motion is calculated by
utilizing the predictive attenuation equation. The attenuation relation used in conducting
PSHA for this area is the relationship developed by Takahashi et al. (2000) and can be
expressed by the equation described below and represented by the graph in Figure (9).
k h
dM
w S hec X bX aM Y w+−+⋅+−−= δ )20()10(log)(log 1010 Eq. 6
in which Y is peak ground acceleration in gal, M w is moment magnitude, X is source distance
(km), h is focal depth (km), δh = 0 (h<20) or 1 (h≥20), S k is constant for the site condition,
and a, b, c, d and e are the constants.
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Figure (9). The graph of peak ground acceleration attenuation (Takahashi et al., 2000).
Site
Characterization
Since the ground motion parameters are greatly depends on the surface geology, the
surface condition in Yangon Region is characterized based on the existing bore-hole data in
Yangon, the geological map of Myanmar, that of Yangon and soil map of Myanmar. The
bore-hole data are obtained at around 40 sites in Yangon, from that the SPT values are
transformed to shear wave velocity profile by using the empirical relation of Sadish et al.
(1997). The soil types are then determined by using UBC (uniform building code) shown in
Table (3). Some of the bore-hole data are obtained till 30m, although some are just around
15m in depth. Therefore the average shear wave velocity in 30m are attained from some
bore-hole, it is still required to carry out farther more for site characterization. Based on the
available data and geological information, the subsurface soils are classified as rock and
medium soil type in Yangon Region (Figure 10).
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Table (3) Subsurface soil classification according to UBC (uniform building code)
Ground Description Sv30 (m/s)
Hard Rock >1500
Rock 760 – 1500
Very dense soil and soft rock 360 – 760
Stiff soil (Medium soil) 180 – 360
Soft soil <180
Figure (10). The site condition map of Yangon Region
Calculation
of
the
Seismic
Hazard
The seismic hazard is calculated for Yangon Region in grid interval of 0.1 ̊ x 0.1 ̊ for
both rock sites and by implementing the subsurface conditions (Figure 11). The seismic
hazards are calculated for 10% probability of exceedance in 10, 20, 50, and 100 years and
represented in term of peak ground acceleration.
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Figure (11) Map of the sites locations in 0.1 ̊ x 0.1 ̊ grid interval, where the seismic hazards arecalculated.
The seismic hazard maps (peak ground acceleration in rock condition) for Yangon
Region are depicted in Figures 12 – 15. The PSHA map of peak ground acceleration (pga)
expected in 10% probability of 10 years on rock condition is represented in Figure (12). The
maximum pga value is obtained as 0.150 g and the minimum one is 0.018g. Therefore, forthis recurrence interval, the maximum seismic hazard area corresponds to the eastern part of
Yangon Region while the lowest hazard areas belong to the eastern margin of the Region and
the western portion and northern portion. The pga map predictable in 10% probability of
exceedance in 20 years is illustrated in Figure (13). In this recurrence interval, the maximum
seismic hazard zone comprise the eastern portion of Yangon Region with value of 0.29g,
while the minimum hazard area are in south-western portion and the eastern margin with the
value of 0.031g. The pga map anticipated for 10% probability of exceedance in 50 years can
be observed in Figure (14). While it is acquired as the maximum pga value of 0.25g,representing the area of eastern portion of the Region, the minimum pga value is attained as
0.041g, comprising the central and southern portion of the Region. Moreover, the northern
portion also involves in the moderate seismic hazard zone with the pga value of 0.125g.
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Figure (12) Probabilistic seismic hazard (pga) map of Yangon Region expected at 10% in 10years on rock.
Figure (13) Probabilistic seismic hazard (pga) map of Yangon Region expected at 10% in 20years on rock.
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Figure (14) Probabilistic seismic hazard (pga) map of Yangon Region expected at 10% in 50years on rock.
Figure (15) Probabilistic seismic hazard (pga) map of Yangon Region expected at 10% in 100years on rock.
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Figure (15) demonstrates the pga map expected for 10% probability of 100 years. In
this hazard map, it can be seen that the maximum hazard zone also embraces the eastern
portion with value of 0.275g. The seismic hazard zone distribution pattern is similar with that
of pga map in 10% probability of exceedance in 50 years. The moderate pga value is 0.2g in
the northern portion and the minimum one is 0.047g which is in the southern portion of theRegion.
The seismic hazards are also calculated for Yangon Region by involving the site
conditions (surface geology). Those hazard maps are paraded in Figures 16-19. The pga map
with 10% probability of exceedance in 10 years is displayed in Figure (16). The maximum
pga value is obtained as 0.30 which belongs to eastern portion of the Region and the
minimum one is 0.02g. Figure (17) exposes the pga map of 10% probability of exceedance in
20 years, in which the maximum pga consequence is 0.35g and the minimum one is 0.03g,
while Figure (18) depict the pga map with 10% probability of exceedance in 50 years, in
which the maximum pga is 0.45g and the minimum 0.05g. The pga map with 10% probability
of exceedance in 100 years is shown in Figure (19) and the highest pga value is procured as
0.55g and the lowest one is 0.06g.
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Figure (16) Probabilistic seismic hazard (pga) map of Yangon Region with 10% in 10 years(The site conditions are embraced).
Figure (17) Probabilistic seismic hazard (pga) map of Yangon Region with 10% in 20 years(The site conditions are embraced).
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Figure (18) Probabilistic seismic hazard (pga) map of Yangon Region with 10% in 50 years(The site conditions are embraced).
Figure (19) Probabilistic seismic hazard (pga) map of Yangon Region with 10% in 100 years(The site conditions are embraced)
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Discussion
and
Conclusion
The most significant seismic sources for Yangon Region are the subduction zone of
Indo-Australia – Burma plates and the Sagaing fault. Hazard is the highest in the area along
the Sagaing fault. The most contribution of the seismic hazard to Yangon Region is resulted
from the Sagaing fault since the source – to – site distance is closer than those from other
source and the recurrence rate is also high for the large earthquake. The second – most
influence seismic sources are the areal seismic sources of MAS_21 from where the intraplate
earthquakes can be originated and MAS_22 which occupies the West Bago Yoma fault, and
Gwegyo Thrust because the source – to – site distances from these sources are also close
although the recurrence rate is low for the high magnitude earthquakes. Although the
Kyaukkyan fault caused the largest earthquake in Myanmar, the 8.1 Magnitude, May 23,
1912 event, because of its low recurrence rate of large earthquake and distant source – to –
site distance from that fault, the small influence of this source cause this region negligible
seismic hazard. Even though tectonically the second – most significant seismic source rather
than Sagaing fault for this region is the subduction zone of the Indo – Australia Plate and
Burma Plate, the seismic sources (MAS_17, 18, 19 and 20) from this also result the low
seismic hazard because they occur far from the site. However, the recurrence rate of large
earthquakes from the sources MAS_17 and 18 are high and those from sources MAS_19 and
20 are low. The other one is the Andaman Rift zone (seismic sources: MAS_15 and 16) also
contribute the low seismic hazard to this region due to the distal source from the site.For the present region, there are some issues that are still needed to carry out,
concerned with fault parameters such as slip rate and recurrence rate of certain large
earthquake by performing the paleoseismic analysis. For the site characterization in this
seismic hazard analysis, the bore-hole data used are very diminutive in amount (around 30
bore-holes), some are in shallow depth. Therefore it is still required to conduct the site
characterization process.
Acknowledgements
The authors are grateful Myanmar Engineering Society (MES), Myanmar Geoscience
Society (MGS) and Myanmar Earthquake Committee (MEC) for the support to carry out this
research.
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