development of geodynamic model of bangladesh - 2012

Earthquake Risk and Damage Assessment and Subsequent Development of Scenario-based Contingency Planning for Rangpur, Dinajpur, Mymensing, Tangail, Bogra and Rajshahi Munici-palities / City Corporations and Detailed Building Inventory of the Said Towns Including Dhaka and Chittagong City Corporation Areas

July, 2012

Submitted by

Asian Disaster Preparedness Center SM Tower, 24 th Floor, 979/69 Paholyothin Road, Samsen Nai Phayathai, Bangkok 10400, Thailand

Comprehensive Disaster Management Programme (CDMP-II) Ministry of Food and Disaster Management (MoFDM) Disaster Management and Relief Division (DMRD) Government of the People's Republic of Bangladesh

Task I: Development of Geodynamic Model of Bangladesh

Development of Geodynamic Model of Bangladesh

Table of Contents

Abstract .................................................................................................................................................................. 2

Introduction .......................................................................................................................................................... 1

Active tectonic framework of Bangladesh .................................................................................................. 1

Historical earthquakes ...................................................................................................................................... 4

1548 earthquake ................................................................................................................................................. 6

1676 earthquake ................................................................................................................................................. 7

The great Arakan earthquake of 1762 ......................................................................................................... 8

1822 Bengal earthquake ................................................................................................................................... 9

1842 earthquake ............................................................................................................................................... 10

1845 earthquakes ............................................................................................................................................. 11

1865 earthquake ............................................................................................................................................... 12

1869 earthquake ............................................................................................................................................... 13

The great Indian earthquake of 1897 ......................................................................................................... 14

The Srimongal earthquake of 1918 ............................................................................................................. 15

Active structures ............................................................................................................................................... 17

Major plate-boundary structures ................................................................................................................ 17

Active structures in the CTFB ........................................................................................................................ 26

Earthquake potential of active structures ................................................................................................ 38

Reference ............................................................................................................................................................ 41


Abstract

Geomorphic investigation and published studies indicate that two major active

tectonic belts threaten Bangladesh with large and potentially destructive

earthquakes. These two elements are the Himalayan system in the north and the

Arakan subduction-collision system in the east. The Himalayan Frontal Thrust fault

and the Dauki fault are the principal components of the former and the

Chittagong-Tripura Fold Belt of eastern Bangladesh is a manifestation of the latter.

Judging from the length of individual active folds within the CTFB and the nature of

historical earthquakes, these anticlines and their associated faults are able to

produce earthquakes up to about M 7.5 individually.


1

Introduction

The seismic potential of Bangladesh is high, because the country is on and near two

major tectonic elements of the on-going Indian-Eurasian plate collision. However,

investigation of the seismic potential of these two systems is in its infancy. Steckler

et al. [2008] are, for example, among the first to begin to address the seismic hazard

of the region with scientific rigor.

In the past two decades, the improved availability and resolution of terrestrial

imagery from satellites and aerial surveys has facilitated evaluation of active

structures. For example, mapping by Sieh and Natawidjaja [2000] delineated the

2000-km long Sumatran fault in unprecedented detail, and mapping by Shyu et al.

[2005] yielded the first comprehensive, reliable modern map of Taiwans active faults

and estimates of their seismic potential. Data from space-borne surveying systems

include the nearly global digital elevation model from NASAs Shuttle Radar

Topography Mission (SRTM), as well as a variety of high-resolution (~1 m) imagery.

The availability of such datasets greatly facilitates investigation of large regions with a

consistency of detail and standards.

We have applied such techniques to mapping the active tectonic features of

Bangladesh and surrounding regions. We have used primarily the 90-meter digital

elevation model from the Shuttle Radar Topographic Mission (SRTM) and

high-resolution satellite imagery. However, because sedimentation and erosion rates

in Bangladesh are high, landforms may be eroded faster than the rate at which

tectonism forms them. Our use of other geological information has helped us

identify and analyze such features. These data include published subsurface

stratigraphic and structural data, such as seismic reflection profiles, and a recently

refined historical earthquake catalog. Taken together these and the satellite data

allow us to identify most major seismically active structures and allow us to estimate

their seismic potential.

Active tectonic framework of Bangladesh

The Bangladesh region is strongly affected by the on-going Indian-Eurasian plate

collision process. To the north, the collision of the Indian plate with Asia has created

the spectacular Himalayan mountains, bounded along their southern flank by the

Himalayan Frontal Thrust (HFT) [Figure 1]. This great north-dipping thrust fault runs


2

more than 1500 km from Pakistan to Assam and has produced many large

continental earthquakes, some greater than M 8. South of the HFT in the

Bangladesh region, is another large north-dipping reverse fault, which lies along the

southern flank of the Shillong Plateau. This Dauki fault is bringing ancient

continental rocks of the Shillong plateau over thick sediments of of the

Ganges-Bramhaputra delta [Figure 2].

Another major active tectonic belt appears along the eastern side of Bangladesh.

Oblique subduction and collision between the Indian and the Burma plate has

produced the N-S trending Indo-Burma range along the western edge of the Bay of

Bengal [Curray, 2005]. The width of the range gradually increases from south to north.

North of the latitude of southernmost Bangladesh, it is ~300 km wide. The eastern

half of the range is the Chittagong-Tripura fold belt [Figure 2]. It comprises many

young anticlinal folds in young sediment of the thick Ganges Delta. The higher,

eastern half of the Indo-Burma range comprises older rocks and reflects the longer

convergence history of the Indian and the Burma plate.

The Arakan megathrust changes its orientation from nearly N-S to NE-SW at the

latitude of the Shillong Plateau. Northeast of the Shillong Plateau, this fault system

becomes the Naga thrust fault, which forms the northern flank of the Naga hills

(northern Indo-Burma range). Further northeast, the southeast-dipping Naga-Arakan

fault system approaches the north-dipping Himalayan frontal thrust and terminates

in a complex, poorly understood fashion at the Eastern Himalayan Syntax, near the

Indian-China boarder [Figure 2].

Recent geodetic analyses suggest that each of these four major structures

accommodate more than 1 cm/yr of shortening [e.g., Socquet et al., 2006; Banerjee

et al., 2008]. Analysis of GPS data across Nepal indicates that the Himalayan frontal

thrust accommodates 18 to 21 mm/yr of convergence, a range of values that is

consistent with the much longer-term fault slip rate established from geomorphic

analysis [Bettinelli et al., 2006]. Recent analysis of GPS data implies 4 to 7 mm/yr of

shortening across the the Dauki fault. This translates into ~11 mm/yr fault slip rate

on the Dauki fault along the northern border of Bangladesh [Banerjee et al., 2008].

To the south, analysis of GPS data implies shortening across the Arakan megathrust

of ~ 23 mm/yr [Socquet et al., 2006]. Across the Naga thrust, however, GPS yields a

puzzling result -- insignificant rates of convergence [Jade et al., 2007]. Instead, it

shows right-lateral motion across the Naga thrust fault.


3

Figure 1. The tectonic setting of the Bangladesh region is dominated by the structural elements of the Indian-Asian collision. The Himalayan Frontal Thrust (HFT) and Dauki Fault (DF) thrust Asian lithosphere over Indian lithosphere. Blue arrow indicates the motion of India relative to Asia. After Tapponnier et al. [1982].

Figure 2. Major active tectonic elements of the region also include the southeast-dipping Naga thrust and the Arakan megathrust. Estimated current convergence rates range from 11 to 23 mm/yr.


4

Historical earthquakes

Historical damaging earthquakes in the region can be plausibly ascribed to

rupture of faults associated with the four major active tectonic systems just

described. Although there are historical reports of earthquakes as far back as the

mid-16th century, the written record is far more reliable and detailed since the

mid-18th century.

Table 1 lists significant historical earthquakes in and around Bangladesh from

several published earthquake catalogues. Szeliga et al. [2010] and Martin and Szeliga

[2010] provide the most up-to-date earthquake catalog for this region since the great

earthquake of 1762.

Figure 3. Large (M>6) historical earthquakes in the catalogue of Szeliga et al. [2010].


5

Table 1 Major historical earthquakes in and around the Bangladesh area

No Name Date M Source area Reference

1 -- 1548 -- Dauki fault or

Arakan megathrust

Iyengar et al., 1999

Steckler et al., 2008

Morino et al., 2011

2 -- 1676 -- CTFB or

Arakan megathrust

Iyengar et al., 1999

Martin & Szeliga 2010

3 Arakan

earthquake 1762 8.5 Arakan megathrust

Halstead, 1841

Oldham, 1883


4 Bengal

earthquake 1822 7.1 CTFB ? Martin & Szeliga 2010

5 -- 1842 7.3 Western Bengal Ambraseys and Dauglas, 2004


6 -- 1845 7.1 Shillong plateau Martin & Szeliga 2010

7 -- 1865 CTFB or

Arakan megatrust Martin & Szeliga 2010

8 Cachan

earthquake 1869

7.4 /

8.3

Indoburman range or

Arakan subduction zone

Ambraseys and Dauglas, 2004


9 Great Indian

earthquake 1897 8.0 Dauki fault

Oldham, 1899

Richter, 1958

Abe, 1994


10 Srimongal

earthquake 1918 7.4

CTFB or

Arakan megathrust

Stuart, 1920



6

1548 earthquake

The 1548 earthquake is one of the earliest damaging earthquakes recorded in

the region. Detailed information about ground shacking and damage is lacking, but

the earthquake is known to have damaged both Sylhet and Chittagong and to have

caused significant liquefaction [Figure 4] ["Banglapedia Earthquake"; Bilham and

Hough, 2006; Steckler et al., 2008]. Iyengar et al [1999] suggest that the intensities

might have been as high as IX in the southern Assam Valley. Steckler et al. [2008]

suggest the earthquake resulted from slip on the megathrust beneath the

Chittagong-Tripura folding belt, because of the severity of shaking and damage in the

area. Morino et al. [2011] suggest that rupture of the Dauki fault produced the

earthquake, based on paleoseismologic investigation and geochronological analysis

of the Dauki fault.

Figure 4. The documented felt locations of the 1548 earthquake in eastern India.


7

1676 earthquake

Records of the 1676 earthquake are sparse and ambiguous. Iyengar et al [1999]

indicates that both Chittagong and the Balasore area may have felt this earthquake,

Chittagong being severely damaged by both (either?) a cyclone and (or?) the

earthquake [Figure 5]. However, the felt records from Chittagong and the Balasore

region may represent separate events, since the records give no exact date.

If the same earthquake were felt in both Chittagong and the Balasore area, and

severe damage occurred at Chittagong, the likely source of the earthquake would be

t the Arakan megathrust or an upper plate fault within in the Chittagong-Tripura fold

belt.

Figure 5. The felt location of 1676 earthquake. The two records are from different

sources, and no exact date is mentioned in either record.


8

The great Arakan earthquake of 1762

The Aarakan earthquake in 1762 is best-known historical earthquake along the

eastern side of Bay of Bengal. The earthquake was strongly felt from Cheduba and

Ramree Islands of western Myanmar to the area near Dhaka and caused heavy

damage in the Chittagong region [Figure 6]. A British officer who surveyed the

Myanmar coast about 80 years later documented several meters of uplift associated

with the earthquake [Halstead, 1841]. Our recent measurements and radiometric

dtates of marine terraces implies that the 1762 earthquake was generated by failure

of the Arakan megathrust and an accompanying upper-plate fault [Wang et al., in

preparation].

The extent to which the 1762 rupture extended northward toward Bangladesh

is unclear, because all we have to interpret are historical reports of coseismic land

subsidence in the Chittagong area. Cummins [2007] has interpreted this to be

associated with megathrust rupture seaward of the coastline. If this interpretation is

correct, the total length of the fault rupture during the 1762 earthquake could well

be ~ 350 km or even longer, and the earthquake magnitude could be greater than or

equal to M 8.5.

Figure 6. The rupture of the Arakan megathrust in 1762 could have been as great as shown by the dashed rectangle, if reported

coseismic land subsidence in the Chittagong area (northernmost large dot) was the result of offshore rupture of the megathrust.

The four dots farther south along the coast are localities where we have confirmed uplift in 1762.


9

1822 Bengal earthquake

This earthquake was felt in many parts of Bangladesh [Figure 7]. People found

it difficult to stand in the Comilla area, but no material damage occurred there. At

Mymensingh, south of the Shillong plateau, houses were demolished or badly

fractured [Martin and Szeliga, 2010]. Although information about this earthquake is

very sparse, Szeliga et al. [2010] suggest the source of the earthquake was close to

the western edge of the Chittagong-Tripura Folding belt, and they estimate a

magnitude of 7.1.

Figure 7 Location of 1822 earthquake from Szeliga et al. [2010]


10

1842 earthquake

This earthquake was felt throughout most of Bangladesh and in parts of Assam

and Bihar. Records collected and analyzed by Martin and Szeliga [2010] report

damage of several buildings in western Bangladesh. Szeliga et al. [2010] estimates

the earthquake magnitude at M 7.3, with the center of the quake close to the

India-Bangladesh border [Figure 8]. Ambraseys and Dauglas [2004] suggest a lower

earthquake magnitude (M 6.8) and a center inside Bangladesh.

Figure 8 Location of 1842 earthquake from Szeliga et al. [2010]


11

1845 earthquakes

Three distinct earthquakes rocked northern Bangladesh from July 1845 to

August 1845. The strongest, on 6 August, damaged several buildings at Sylhet and

Guwahati. The tremor was felt strongly at Cherrapunji and other places around

Bangladesh. Szeliga et al. [2010] estimate a magnitude of 7.1 for this earthquake and

a center near the northern flank of the Shillong plateau [Figure 9]. Their analysis also

suggest the two earlier earthquakes were centered very close to the Shillong plateau.

Therefore, it is plausible that all three events resulted from rupture of a structure

associated with the Dauki thrust beneath the Shillong plateau.

Figure 9 Locations of 1845 earthquakes from Szeliga et al. [2010]

(After Szeliga et al., 2010)


12

1865 earthquake

A large earthquake shook the Sandwip Island area in 1865. Analysis of

intensity records led [Szeliga et al., 2010] to estimate a magnitude of M 6.8. This

earthquake was felt along the Arakan coast and in the Bengal area. Liquefaction and

ground cracks occurred northeast of Chittagong city, where the intensity was

strongest [Martin and Szeliga, 2010]. The location of the stongest shaking suggests a

source within the Chittagong-Tripura fold belt or on the Arakan megathrust fault.

Figure 10 Locations of 1865 earthquakes from Szeliga et al. [2010]

(After Szeliga et al., 2010)


13

1869 earthquake

The Cachan earthquake of 1869 occurred in the Silchar area, in the IndoBurman

range east of Bangladesh. Damage extended from Silchar to the Manipur area and

included extensive liquefaction. The quake was felt in both northeast and eastern

India as well as the in adjacent parts of Myanmar. On the Brahmaputra River,

people observed energetic seiches [Martin and Szeliga, 2010]. Based upon intensity

records, Szeliga et al. [2010] estimate the center of this earthquake to have been

near the India-Myanmar boarder and that its magnitude was M 8.3. Ambraseys

and Dauglas [2004] suggest a far lesser magnitude of M 7.4 but a similar source

location. It is likely that this earthquake resulted from rupture within the

Wadati-Benioff zone of the downgoing Indian plate lithosphere beneath the

Indoburman range.

Figure 11 Location of 1869 earthquakes from Szeliga et al. [2010]


14

The great Indian earthquake of 1897

The 1897 earthquake is the first Indian earthquake for which levels of shaking

were documented in a contemporary earthquake report [Oldham, 1899]. The

region of very strong shaking (EMS intensity IX) includes the Attrabari, Rambrai and

Shillong areas [Martin and Szeliga, 2010]. The Sylhet and Mymensingh regions

experience severe damage. The tremor was felt less strongly over much of South

Asia, including lower Myanmar, the Assam valley and much of the Indian

subcontinent.

Estimates of the magnitude of the earthquake varies, but all are greater than M

8.0. Richter [1958] calculates an Ms 8.7. Abe [1994] recalculated the magnitude as

Ms 8.0 from instrumental records. Szeliga et al. [2010] suggests a magnitude of M

8.4 based on seismic intensity records. All of these magnitude estimates suggest

that surface rupture would have been at least 100 to 200 km long, based on

magnitude-rupture empirical relationships [Wells and Coppersmith, 1994].

No evidence of surface rupture has been found, however. Thus the source of

the earthquake currently is still debated. Bilham and England [2001] postulate the

Oldham fault, on the northern edge of the Shillong Plateau, ruptured, based upon

triangulation measurements. However, no one has found the surface trace of the

Oldham fault, and the geomorphological expression of the plateau suggests that

there is no fault break along the northern flank of the Shillong plateau.

The Dauki fault is a more likely source for the 1897 earthquake. It outcrops

along the southern flank of the Shillong Plateau and dips under the region of highest

intensities, on the plateau [Figure 12]. Preliminary paleoseismological work at one

locality along the Dauki fault revealed no evidence of surface rupture in 1897

[Morino et al. 2011]. A more comprehensive paleoseismological study would be

needed firmly establish or refute the Dauki fault as the source of the great

earthquake.


15

Figure 12. The fact that the high-intensity area of 1897 earthquake coincides with the

Shillong Plateau suggests that the source of the earthquake was the Dauki fault.

Intensity map is from Rajendran et al. [2004]

The Srimongal earthquake of 1918

The Sirmongal earthquake of 1918 caused extensive damage along the eastern

Bangladesh border, particularly in the Balisera Valley near Srimongal, where seismic

intensity reached EMS 8 to 9 [Martin and Szeliga, 2010]. Buildings were also

damaged in Dhaka and Sylhet, as well as some adjacent parts of in India. The tremor

was throughout Bangladesh and adjacent parts of India and Myanmar [Stuart, 1920].

Pacheco and Sykes [1992] estimate a magnitude Ms 7.4 centered close to the

western front of the Chittagong-Tripura fold belt. The highest intensities of this

earthquake occurred in the northernmost part of the Chittagong-Tripura fold belt.

This suggests that the earthquake was generated by a structure within the

Chittagong-Tripura fold belt or the Arakan megathrust beneath it.


16

Figure 13. The high-intensity area of 1918 earthquake and likely center.


17

Active structures

To categorize the active structures with the seismic potentials in the near future,

we first define these structures into two basic groups based on their scales. The first

group, major plate-boundary structures includes the major collision and subduction

interface that are capable to produce M ~ 8 earthquake solely. The second group

includes the structure that mostly within the deformation belt, but shows the

evidence of active at least during the late-Quaternary period.

We include different aspects of data to address on the activity of both the

plate-boundary scale structures and the second-order active structures. These data

include the structural analyses from published dataset, the geodetic data, the

existing paleoseismology studies and the geomorphic investigations from the remote

sensing dataset.

Overall, we define five different patches of plate-boundary faults as the primary

seismic sources in this area. Among the active structures within the deformation belt,

we suggest 13 structures within the Bangladesh-India region are the candidate of

potential seismic source in the region. The high-resolution satellite imagery shows

some of these structures may accompany with the young faulting on their limbs.

However, future detail study is needed to verify their history of active.

Major plate-boundary structures

1. Main frontal thrust

One of the most prominent seismic sources in the region is the Himalayan

Frontal Thrust (HFT). The fault runs along the southern flank of the Himalayan

Mountains and has produced several great earthquakes in the past several hundred

years [Figure 14] [Kumar et al., 2010]. Two of these occurred near Bangladesh in the

20th

century. The 1934 Bihar-Nepal earthquake (Mw~ 8.1) severely shook a large

region northwest of Bangladesh. Judging from felt reports, rupture of a section of

the megathrust about 300 km long produced the earthquake. The 1950 Assam

earthquake (Mw 8.5) involved rupture of at least 400 km of the megathrust along the

northern edge of the Assam Valley, northeast of the Bangladesh [Figure 14] [Kumar

et al., 2010]. Between the 1934 and 1950 rupture patches is a 500-km-long patch

that has not produced a great earthquake in recorded history. This seismically

quiescent gap is only ~60 km north of the Bangladesh border, so it must be

considered to be a significant potential source of strong ground shaking over at least

the northern sectors of Bangladesh.


18

This section of the Himalayan megathrust has not been well studied, but studies

of its neighbor to the west provides some suggestions about its structural geometry

and kinematics. There, in eastern Nepal, the HFT (the Main Himalayan Thrust (MHT))

dips ~10 northward and has a long-term slip rate of ~ 18 mm/yr plate convergence at the eastern Nepal area [Ader et al., 2012]. Geodetic analysis suggests the fault is

locked from the surface to the depth of 15 to 20 km. This locking depth

corresponds to an ~ 100-km-wide locked patch. The analysis of geodetic data by

Banerjee et al. [2008] suggests a slip rate beneath the Bhutan Himalaya is about 21

mm/yr.

Figure 14 Historical earthquakes along the Himalayan Frontal Thrust fault

Paleoseismological studies at both ends of this patch suggest that the most

recent large rupture of the fault was several hundred years ago. This result is

consistent with the lack of a historical record of earthquakes along this reach of the

megathrust. Kumar et al. [2010] suggest the last rupture took place no later than the

15th

century and likely occurred in the 12th

century. If the most recent event occurred

in the 12th

to 15th

century and if the fault is fully locked and accumulating potential

slip at 2 cm/yr, we might expect the fault is now capable of slipping 12 to 18 meters.

2. Dauki fault system

Slip on the famous Dauki fault has formed the steep, 270-km-long southern

flank of the Shillong plateau, where the elevation rises northward from near sea level


19

to more than 1500 m. This impressive fault scarp extends from the Brahmaputra

River Delta to the western flank of the Indoburman range, where the Shillong Plateau

and Indoburman range meet near the northeastern corner of Bangladesh [Figure 15].

The Dauki fault dips northward at an angle of between 40 and 50 beneath the Shillong plateau [e.g. Bilham and England, 2001; Biswas et al., 2007]. An analysis of

geophysical data suggests the fault extends to a depth of 35 km and cuts completely

through the Indian continental crust to the Moho [Nayak et al., 2008]. If the Dauki

fault is locked from the surface to 35 km deep, the fault width in the down-dip

direction would be ~50 km.

An elastic deformation model based upon geodetic data yields a slip rate of ~11

mm/yr [Banerjee et al., 2008]. This would be a third of the total plate convergence

rate between the Tibetan Plateau and the Indian plate. If the fault is totally locked

and accumulating potential slip at this rate, then potential slip would be

accumulating at about one meter per century. For example, a 5-meter rupture

might occur every about every 500 years and a 10-meter rupture might occur every

1000 years.

Although it is still debated, the last major seismic event on the Dauki fault is

likely to have been the great (M~8.0) Indian earthquake of 1897. Its bell-shaped

patch of high intensity covers most of the Shillong plateau, above the north-dipping

Dauki fault and the southern limit of strongest intensities lies close to and parallel to

the trace of the surface trace of the Dauki fault. Even so, a plaeoseismological

study at one site along the Dauki fault failed to yield clear evidence of surface

rupture during the 1897 earthquake. Instead, fissures caused by liquefaction is a

very shallow part of the trench indicate strong shaking during the 1897 earthquake

[Morino et al. 2011]. The same trench also revealed an earlier rupture of the fault in

the 16th

century.

Our geomorphic investigations based upon satellite data show that most of the

geomorphic features at the base of the Shillong plateau are related to fluvial

processes associated with river flow parallel to the mountain front. The high

erosional and deposition rates associated with this active fluvial system obscure

younger tectonic landforms of the Dauki fault. Nevertheless, we have found several

features that we suspect to be young fault scarps. Figure 16 shows one such

south-facing scarp near the Bichanakandi area. The lateral continuation of this scarp

is unclear, due to fluvial erosion both to the east and west.


20

Figure 15 Simplified Dauki fault patch (black rectangle) along its mapped surface

trace (red line).

Figure 16 Suspected Dauki fault scarp near the Bichanakandi area. Image is from

Google Map.


21

3. Arakan megathrust (Ramree section)

Our studies of the structure, tectonic geomorphology, paleoseismology and

historical seismicity of the Arakan megathrust have led us to separate the fault into

four distinctive sections. Only the three northernmost sections are relevant to this

report on the seismic hazard of Bangladesh. The farthest south of these three is the

500-km-long Ramree section, which extends from near Myanmars Fouls Island in the

south about to Bangladeshs Chittagong in the north. The megathrust dips gently

northeastward toward the coastline at an angle of about 16. This is the dip of a

plane intersecting the offshore trench and lay atop the Wadati-Benioff zone.

The 4-decade-long instrumental seismic record contains very few earthquakes

on the megathrust. This implies that it is wholly locked and the blocks above and

below are accumulating strain at the rate of convergence of the Indian and Burma

plates [Figure 17]. Recent analysis of GPS data suggests that the convergence rate

across the megathrust is ~23 mm/yr at a latitude just south of the

Bangladesh-Myanmar border [Socquet et al., 2006]. Most of this strain is likely to be

accommodated by the megathrust. However, youthful deformation of the

over-riding plate indicates that part of the convergence occurs across upper-plate

structures within the Indoburman range [Nelson et al., 2004].

The great Arakan earthquake of 1762 is the latest major rupture of the Ramree

section of the Arakan megathrust. The rupture was at least 350 km long, if one

assumes that, in addition to uplift reported in the south, reported subsidence in the

Chittagong region was associated with slip on the megathrust [Halstead, 1842; Mallet,

1878; Oldham, 1833]. Shishikura et al. [2009] suggests the recurrence interval of

1762-type earthquakes is about 900 years, based on the ages of uplift marine

terraces they measured and dated along the Myanmar coast. Our unpublished data

demonstrates that uplift events are more frequent than this, which implies that large

earthquakes are also more frequent.


22

4. Arakan megathrust (Chittagong section)

We define the megathrust north of the Chittagong area as a separate

seismogenic patch, because of a significant difference in the megathrust geometry

[Figure 18]. Maurin and Rangin (2009) suggest the dip of this Chittagong section of

the megathrust is nearly flat. Slip on this shallow-dipping section of the megathrust

has resulted in the construction of the Chittagong-Tripura fold belt, a broad swath of

anticlinal ridges and synclinal troughs formed within the accreted Bengal Fan

sedimentary sequence. These large but secondary structures arise from bends in

the megathrust and/or secondary faults associated with it.

Steckler et al. [2008] suggest the megathrust dips 5-7 to the east. They deduce a slip rate of 1 to 2 cm/yr, based upon a structural analysis of previously

published data. One important result of the very shallow dip of the megathrust is

that the width of the shallow part of the fault must be much greater than it is along

the Ramree section. If the locking depth of the fault is about 35 km, then the width

of the seismic patch would be about 300 km.

Although the earthquake history of the Chittagong section extends back to the

16th

century, the historical reports are so scant prior to the 20th

century that

associations of large historical earthquakes with specific parts of the megathrust or

specific secondary faults of the Chittagong-Tripura fold belt remain spectulative.

Steckler et al. [2008] suggest that 1548 earthquake may have resulted from rupture

Figure 17. Ramree section of the Arakan megathrust (red rectangle). Colored rectangles are earthquake epicenters for the period 1973 to 2008, from the NEIC catalogue. Warmer colors indicate earthquakes with shallow hypocenters and cooler colors indicate deeper hypocenters. The pattern northeast of the Ramree section shows the descent of the Wadati-Benioff zone beneath the Indoburman range. The 60-km on the top of the Wadati-Benioff zone is approximately at the westernmost light blue hypocenters.


23

of the megathrust event, since severe damage extended across most of the fold belt,

from Syhelt to Chittagong. To the contrary, the Morino et al. [2011] paleoseismic

study of the Dauki fault suggested to them that the 1548 earthquake resulted from

slip on the Dauki fault. Additional paleoseismological studies would be needed to

resolve the matter definitively.

The length of the Chittagong section, its exceptional width, the high rate of

convergence across it, and its very long period of quiescence imply that it is capable

of generating exceptionally large earthquakes. For example, if it has indeed been

quiet since at least 1548 and is accumulating potential slip at a rate of 1 to 2 cm/yr,

then one can reasonably surmise that at least 5.5 to 11 meters of slip could occur

during the next great rupture of the megathrust. The magnitude of such an event

could be Mw 8.8, given the width and length of the Chittagong section.

Figure 18 Along the Chittagong section of the Arakan megathrust the shallow,

seismogenic part of the great fault is very flat, exceptionally wide, and blind. That is,


24

its westernmost limit does not crop out at the surface. Our estimate of the blind

megathrusts geometry is illustrated in the cross-section. The solid red line suggests

the width of its shallow, seismogenic portion is over 200 km.

5. Naga thrust fault

The Naga thrust fault is the northeastern extension of the Arakan megathrust

and the third of the three megathrust sections that could affect Bangladesh. This

350-km-long fault system separates the Naga hills from the Assam valley, northeast

of the Shillong Plateau. In great contrast to the Chittagong section, this system of

fault-propagation folds and southeast-dipping thrust faults outcrops at the surface

within a narrow band. Shallow seismic and well data indicate that the dip of the Naga

thrust is about 23 [Figure 19]. Young tectonic landforms are very prominent in high-resolution satellite

imagery and SRTM digital elevation model along the entire Naga fault system. These

features include uplifted and anticlinally deformed fluvial terraces and small tectonic

scarps along the mountain front. The existence of these features implies that the

Naga thrust fault system has been active at least during the late Quaternary period.

A recent geodetic study, however, shows insignificant modern shortening across

the Naga fault system [Jade et al., 2007]. These two results suggest that the fault is

active but accumulating strains at a rate lower than 5 mm/yr, about the uncertainty

of the GPS measurements.


25

Figure 19. Our mapping of tectonic landforms shows that the active Naga thrust

fault traverses the northern flank of the Naga hills from the Shillong plateau to the

northeastern end of the Assam valley. The cross-section shows the fault

accommodates the rocks of the Naga hills over-riding the continental sedimentary

rocks of the Assam valley. After Kent [2002].


26

Active structures in the CTFB

The five major plate-boundary fault systems that we have just described are not

the only potential sources for destructive earthquakes in Bangladesh. Most

prominent among secondary structures in the region are those associated with the

anticlines of the Chittagong-Tripura fold belt (CTFB). Geomorphological and

structural/stratigraphic analyses demonstrate that at least thirteen of these

anticlines are active (Table 2 and Figure 20). Because of high erosional and

depositional rates, not all of these structures show clear, youthful geomorphic

features. Hence, we rely also on published seismic reflection profiles to assist in our

evaluation of whether or not they are active.

Most of the anticlines that display evidence of youthful activity are within 100

km of the western deformation front of the Chittagong-Tripura fold belt. We

suspect that this reflects the westward propagation of the megathrust and overlying

fold belt into the Ganges delta sedimentary section. This suggests that other folds

within 100 km of the deformation front, including those not marked by us as active,

may also belie activity of the underlying megathrust and related secondary

structures.

Table 2. A preliminary list of active anticlines in Bangladesh

Code Anticline name

Evidence

Reference Growth strata

Deformed

surface

SM St. Martins Island This study

Da Dakshin Nila This study

M Maheshkhali Maurin and Rangin, 2009

Khan et al., 2005

J Jaldi Maurin and Rangin, 2009

Khan et al., 2005

P Patiya Maurin and Rangin, 2009

SW Sandwip This study

L Lalmai This study

H Habiganj John and Nur Alam, 1991

R Rashidpur Curiale et al., 2002

S Sylhet John and Nur Alam, 1991

Steckler et al.,2008

F Fenchunganj This study

Ha Hararganj This study

Pa Patharia Sikder and Alam, 2003


27

Figure 20. Thirteen anticlines of the Chittagong-Tripura fold belt with geomorphic

and/or structural/stratigraphic evidence for recent activity. Table 2 lists more

information about these anticlines, which are marked with white code letters in black

boxes. Other folds and related faults within 100 km of the deformation front may

also be active.


28

St. Martins Island

The anticline beneath the St Martins Island has long been known from offshore

structural maps. Uplifted marine terraces on the island are clear evidence of recent

activity. High-resolution satellite imagery shows two distinct terraces ringing the

island [Figure 21]. Young uplifted corals also exist at several locales along the

modern coastline. The uplifted corals suggest very young tectonic uplift of the island

[Environmental profile of St Martins Island, 2010]. The lengths of the anticline and

the related fault are unclear from published research..

Figure 21 Marine terraces on the southern part of the St Martins Island imply that it

is rising on the crest of an active anticline. Image is from Google Map.

Dakshin Nila anticline

As on nearby St Martins Island, marine terraces on the southern tip of Dakshin

Nila imply that it is an actively rising anticline. Its alignment with and proximity to the

anticline of St. Martins Island suggests that they are kinematically related. The

geomorphology of the Dakshin Nila anticline suggests that its western limb is steeper

than its eastern limb. This asymmetry would be consistent with the existence of an

underlying east-dipping thrust fault.

The steepness of the western flank may be evidence that the fault cuts through

its western limb and crops out along or near the west coast of the island.

High-resolution satellite imagery shows a steep west-facing scarp between coastal


29

plain and foothills; this may be the fault scarp of the suspect thrust fault [Figure 22].

Field investigation would be needed to test this hypothesis.

Figure 22. The steep western flank of the foothills may be the scarp of a

northeast-dipping thrust fault beneath the Dakshin Nila anticline. Image is from

Google Map.

Maheshkhali anticline and fault

The activity of the Maheskhali anticline was documented in the report of the

CDMP-1 project. The SRTM digital elevation model and high-resolution satellite

imagery both show clear evidence of recent uplift along the southern part of the

anticline [Figure 23]. The base of its western limb exhibits a continuous linear scarp

on the SRTM topography. As in the case of the Dakshin Nila anticline, this suggests

that the fault associated with the anticline may crop out the surface.

SRTM topography indicates that the Maheshkhali structure may further extend

to the north, beneath Kutubdia Island. Profiles from the 90-m SRTM show the

islands surface is warped about 4 meter [Figure 24]. We have no evidence that the

fault underlying the Maheshkhali anticline breaks to the surface; we suspect

therefore that it is blind, at least along its northern part.


30

Figure 23. The fault scarp on the southwestern side of the Maheshkhali anticline.

Image is from Google Map

Figure 24. The surface profile across the northern part of Maheshkhali anticline

shows a 4-meter bowing of the surface of nearby Kutubdia Island, which we interpret

as anticlinal deformation. Source of topography is the 90-meter SRTM dataset.


31

Jaldi anticline

The seismic-reflection profile across the Jaldi anticline shows clear growth strata

in the shallow part of the profile. The age of the growth strata indicate that the

anticline has been active since the Pleistocene [Maurin and Rangin, 2009]. Khan et

al. [2005] use soil morphology to infer an average uplift rate of nearly 3 mm/yr. The

shape of the Jaldi anticline is well exhibited in satellite imagery. This also supports

the contention that the anticline is an active structure.

A clear linear scarp is apparent in high-resolution satellite imagery along the

southwestern limb of the anticline [Figure 25]. This may reflect the existence of an

underlying fault near or breaking the surface. The scarp does not extend to the

southern part of the anticline, so this speculative southwest-bounding fault would

become a blind fault along the southern part of the fold.

Although a seismic-reflection profile suggests another thrust fault along the

eastern limb of the anticline, we do not see clear evidence of it in satellite imagery.

The eastern-bonding fault may also be a blind fault beneath the young and

undeformed sediment, or is slipping at a lesser rate than the southwest-bounding

fault of the Jaldi anticline.

Figure 25. A sharp demarcation between coastal plain and foothills may represent a

fault scarp along the southwestern side of the Jaldi anticline. The image is from

Google Map.


32

Patiya anticline

A seismic-reflection profile shows clear growth strata against the Patiya anticline

in the shallow subsurface; this imdicates that the anticline has been active in the

Pleistocene [Maurin and Rangin, 2009]. It also shows the eastern limb of the

anticline is cut by a west-dipping thrust fault. The trace of this eastern-bounding fault

may be apparent in a high-resolution satellite image along the central part of the

anticline [Figure 26]. However, we caution that there is no clear evidence for

deformation of young fluvial sediments along this lineament, so we cannot be sure

that it represents a fault scarp. Additional inspection using stereoscopic aerial

photography and in the field would be needed to resolve the true nature of the

lineament.

Figure 26. A tectonic scarp along the eastern flank of the Patiya anticline. Image

from Google Map.

Sandwip Island

SRTM digital topography shows that the surface of Sandwip Island has been

anticlinally warped about 4 meters. This slight warping is also reflected in the

drainage pattern of the island [Figure 27].


33

Figure 27. Image and surface profile of the Sandwip Island. The white dashed

line shows the watershed of the island and the yellow line shows the location of the

profile. Elevation profile is from 90-meter SRTM topography. Image is from Google

Map.

Lalmai anticline

The Lalmai anticline lies close to the western front of the Chittagong-Tripura

folding belt, east of Dhaka. The seismic reflection profile published by Sikder and

Alam [2003] clearly shows the gentle folding of this young anticline. Moreover, the

gentle folding of the anticline is clear in the warping of the young surface of the

Comilla terrace. The western side of Comilla terrace is bounded by a linear scarp,

which may well be a fault scarp. The CDMP-1 project report also mentions this scarp,

but no clear feature was found during their field investigations.

Habiganj anticline

The seismic-reflection profile published by John and Nur Alam [1991] shows

clear growth-strata on the shallow part of the Habiganj anticline. The age of the

growth strata indicate that the anticline has been growing throughout the late

Quaternary period. The profile also suggests an east-dipping reverse fault along the

western limb of the anticline. A lack of clear topographic scarps in high-resolution

imagery suggests that active faulting beneath the fold is blind.


34

Rashidpur anticline

Geomorphic observation from satellite imagery suggests the Rashidpur anticline,

near the northwestern front of the Chittagong-Tripura fold belt is active. A

seismic-reflection profile shows clear growth strata across the northern extension of

the anticline [Curiale et al., 2002]. The same profile also shows a secondary fault

developing beneath its eastern limb. That structure is not clearly in evidence in

high-resolution satellite imagery, although the eastern topographic slope of the

anticline is sharp [Figure 28]. Field investigations aided by examination of

stereoscopic aerial photographs could lead to a better resolution of the nature of this

landform and its relation to the activity of the anticline.

The Srimongal earthquake of 1918 was centered close to the Rashidpur anticline.

The earthquake report shows that structures on the eastern flank of the anticline and

in the adjacent valley were badly damaged and that the earthquakes strongest

intensities were experienced there.

Figure 28. High-resolution satellite imagery of the Rashidpur anticline. A green

dashed line shows the northern limit of high intensity during the 1918 earthquake.

Image is from Google Map.

Sylhet anticline

Published seismic-reflection profiles [e.g. Steckler et al., 2008] clearly show

young growth strata on the flanks of the Sylhet anticline. High-resolution satellite

imagery shows the top of the anticline is highly eroded, and no uplifted terrace


35

surfaces appear to have been preserved. Faults associated with the Sylhet anticline

do not appear to outcrop at the surface, so they are likely blind. The existence of a

tectonic feature in the middle of this rapidly depositing and subsiding floodplain

suggests that the Sylhet anticline is active. Details regarding its rate of folding and

seismic history await field investigations of affected Pleistocene and Holocene

deposits and landforms.

Figure 29. A false-color map of SRTM topography clearly shows the Sylhet anticline

rising out of a part of the delta that is rapidly sedimenting and subsiding. The

underlying faults that have created the anticline are not apparent at the surface, so

we speculate that they are blind.

Fenchunganj anticline

The Fenchungani anticline is topographically manifest as gentle low hills on the

active floodplain of the northern part of the Chittagong-Tripura folding belt.

High-resolution satellite imagery shows that the anticline deforms young fluvial

sediment across the southern part of the fold, where deformed fluvial surfaces are

well preserved and largely uneroded. The northeastern limb of this fold is

well-expressed [Figure 30] and may be either a fault or a fold scarp.


36

Figure 30. An east-facing scarp is well-expressed along the eastern flank of the

Fenchunganj anticline.

Hararganj anticline

The northernmost part of the Haraaganj anticline may deform a young fluvial

surface east of the Fenchunganj anticline. SRTM topography shows the surface at

the northern Hararganj anticline is bowed about 15 meters [Figure 31]. A careful

inspection of stereoscopic aerial photographs and a field survey of late Quaternary

landforms and sediments here would be necessary to ascertain the degree of recent

activity of this anticline.


37

Figure 31. An SRTM digital elevation map and the surface profile of the Haraganj

anticline suggest that young deformation may extend northward from the clear

topographic expression of the anticline.

Patharia anticline

The Patharia anticline is the easternmost anticline in the Chittagong-Tripura fold

belt for which we have evidence of young activity. A seismic-reflection profile

published by Sikder and Alam [2003] shows a fanning of dip angles and an angular

unconformity at shallow levels that implies activity during the late Quarternary

period. A lack of clear fault scarps on the flank of the fold implies that the underlying

faults are blind.


38

Earthquake potential of active structures

The lengths and widths of active faults or fault segments can be used to

calculate the potential size of earthquakes generated by them via the formula for

moment magnitude, Mw. Since we are interested in estimating the size of

earthquakes generated by the five large thrust faults associated with the Arakan and

Himalayan megathrusts, we use the Mw equation proposed by Strasser et al. [2010]

for megathrusts and other subduction-zone structures:

Mw = 4.441 + 0.846 * log10(A)

where A is the area of the fault patch that produces the anticipated earthquake.

Mw is related to seismic moment (Mo) by this equation:

log10(Mo) = 1.5 * Mw + 16.1

and Mo is related to the area of seismic rupture as follows:

Mo = AD

where D is the average slip of the fault plane and is the shear modulus.

Later we will use the slip-rate estimates to estimate recurrence intervals via this

equation:

D = r * T (r = slip rate; T = recurrence interval)

Earthquake scenarios for major structures

Table 3 lists the five major seismic structures of Bangladesh and their estimated

parameters. Plugging these parameters into the Mw equation suggests that each of

them is capable of producing earthquakes greater than Mw 8. Their estimated

recurrence times range from 150 to 540 years, based on their average slip rate and

average co-seismic fault slip. The estimated recurrence intervals should be

considered minimum values, because during most known megathrust events peak

fault slip on the megathrust is much greater than the average fault slip (e.g., 2005

Nias, 2010 Chilean, 2011 Japan). Therefore, a recurrence interval based on an

average value of co-seismic fault slip is probably an overestimate of the actual

frequency of earthquake. Future paleoseismological studies of these structures could

greatly improve the quality of seismic recurrence estimation. Also such work will


39

provide better constraints on estimations of possible earthquake magnitudes.

Table 3 The potential earthquake magnitude of major structures near Bangladesh

Fault name

Length

(km)A

Dip

angle

Locking

depth (km)

Fault width

(km)

Slip rate

(mm/yr)

Mmax

Average

slip (m)

Recurrence

interval (yr)

Date of

last event

Main Frontal Thrust 440 10 20 115 21 8.4 3.5 175 1100(?)

Dauki fault 260 45 35 50 11 7.9 2.5 250 1897

Arakan megathrust

(Rahkine section)

440 16 30 108 23 8.4 3.4 150 1762

Arakan Megathrust

(Chittagong section)

360 5 30 340 20 8.75 4.5 200 1548?

Naga thrust 360 23 20 50 5 8.0 2.7 540 ?

Earthquake scenarios of active structures within the CTFB

We use a different Mw equation to calculate earthquake scenarios for secondary

active structures within the Chittagong-Tripura fold belt. Because most of these

anticlines are related to underlying thrust faults and their fault widths are mostly

unconstrained, we use magnitude-length relationship from Wells and Coppersmith

[1994] to calculate plausible earthquake magnitudes:

M = 4.49 + 1.49 * log10(RLD) (RLD = subsurface rupture length)

We later use the magnitude-fault area relationship of Wells and Coppersmith

[1994], and the moment-magnitude equation to calculate average fault slip and the

likely recurrence interval by assuming a constant slip rate for all of these structures.

Log (A) = -3.99 + 0.98 * M

where A is the rupture area.

log10(Mo) = 1.5 * Mw + 16.1

where Mo is the seismic moment. And

Mo = AD

where D is the average slip.


40

D = r * T

r is the assumed slip rate and T is the recurrence interval.

The slip rate we assumed for each of the structure is 3 mm/yr, about 30 % of the

shortening rate across the entire Chittagong-Tripura fold belt from analysis of GPS

data by Jade et al. [2004]. We chose this slip rate because it appears that two to

three anticlines are currently active at any particular latitude across the CTFB.

Table 4 lists a scenario for each of the 13 anticlines that we have described

above, assuming that they act independently. Our calculations suggest these

structures are capable of producing earthquakes ranging in magnitude from M 6.3 to

M 7.5, with recurrence intervals ranging from 250 to 1100 years. Although these

earthquake magnitudes are smaller than the magnitudes generated by the five larger

active faults, they are still dangerous, because most them are close to populated

areas.

Table 4 The potential earthquake magnitude of structures within Chittagong-Tripura folding belt.

Name

Length

(km)

Slip rate

(mm/yr)

Mmax

Average

slip (m)

Recurrence

interval (yr)

Date of last

event

Reference

St. Martins Island > 16 3 6.3 0.75 250 ?

Dakshin Nila 40 3 6.8 1.5 515

Maheshkhali 50 3 7.0 1.8 610

Jaldi 40 3 6.8 1.5 515

Patiya 40 3 6.8 1.5 515

Sandwip 50 3 7.0 1.8 610

Lalmai 50 3 7.0 1.8 610

Habiganj 105 3 7.5 3.3 1100

Rashidpur 62 3 7.2 2.2 720 1918 (M 7.4)

Sylhet 22 3 6.5 1 340

Fenchunganj 45 3 6.9 1.7 570

Hararganj 50 3 7 1.8 610

Patharia 46 3 7 1.7 570


41

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