Seismological Research Letters; November/December 2007; v. 78; no. 6;
p. 591-599; DOI: 10.1785/gssrl.78.6.591
© 2007 Seismological Society of America
Aftershock Investigation in the Andaman-Nicobar Islands: An Antidote to Public Panic?
O. P. Mishra1,
O. P. Singh1,
G. K. Chakrabortty2,
J. R. Kayal1, and
D. Ghosh2
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SUMMARY
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Public panic prevailed in every part of the Andaman and Nicobar Iislands of
India following the megathrust Sumatra earthquake (Mw 9.3)
on 26 December 2004. In this article, we present a very brief analysis of our
continuous three-month (January-March 2005) monitoring and recording of
aftershock data following the main earthquake to show how this endeavor
reduced public panic and constituted an important ingredient to a disaster
management program for the Andaman-Nicobar region. Monitoring was conducted
using six short-period three-component temporary digital seismograph stations
set up in different parts of the Andaman and Nicobar islands. Our findings
demonstrate that 1) there was no aftershock gap zone as recorded by the
far-distant seismographic network, hence negating the possibility of immediate
strong quakes in the Andaman and Nicobar islands; 2) there was no strong
shaking at full moon (26 January 2005) due to tidal stresses, although the
rate of aftershock activity increased by about 31% from the events of the
preceding day; 3) there is a north-south trending 850 x
350-km2 rupture area beneath the Andaman and Nicobar islands; and
4) eruptions of mud at the Baratang volcanic zone and lavas at the Barren
volcanic zone occurred because of strong shaking due to the mainshock and a
series of aftershock clusters within 100 km of the individual volcanic zones.
These eruptions may continue for a couple of years, until the aftershock
sequence ceases. We show that this new dataset from the local seismographic
network constituted an important factor in reassuring the coastal people and
islanders about imminent dangers from future earthquakes and tsunamis, and we
propose geochemical tests of the erupted mud samples from the Baratang
volcanic zone, with geophysical exploration followed by drilling, to ascertain
the presence of oil and gas reserves and the phenomenon of gas seepage under
the land and sea in the Middle Andaman region of India.
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INTRODUCTION
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"Science is an interface of society," Prabhas Pande, organizing
secretary and director of the Kangra Earthquake Centenary Seminar, said in his
welcome speech at the Kangra Earthquake centenary seminar on 4 April 4, 2005.
This concept, however, suffers a serious setback when we consider the
scientific achievements in predicting natural calamities such as earthquakes,
volcanic eruptions, and tsunamis. The Andaman and Nicobar Islands region was
one of the worst-affected states of India in the great Sumatra-Andaman
earthquake (hereafter referred to as MSE 9.3) (Mishra et al.
2007a,
b). There was nothing but
chaos, panic, fear, and despair among the people in this region after this
killer earthquake, the second-largest earthquake in the history of seismology
(only the 1960 Chilean earthquake is greater, with Mw 9.5)
(Stein and Okal 2005). In the
absence of a history of devastating tsunamis in the Indian Ocean, particularly
in the Andaman-Nicobar region, there was a lack of awareness about tsunamis
among the Indians compared to people who inhabit other tsunami-prone coastal
areas of the world. The coastal people and Andaman and Nicobar islanders were
in shock and were afraid of further imminent strong quakes and tsunamis in the
region. Questions were raised about whether the islands would submerge into
the Bay of Bengal, what was the possibility for future volcanic eruptions, and
what the inhabitants of the islands could expect in the near future. People
were fleeing the islands because of misinformation and misinterpretation of
earthquakes both before and after the arrival of the seismological aftershock
monitoring team from the Geological Survey of India (GSI).
As soon as the GSI team arrived, it rushed to the Andaman administration to
explain its objectives. The administration replied, "We are too busy to
help you because of our gigantic workload of earthquake rehabilitation in
different parts of the islands." Then the seismological team explained
that science offers us protection from blind belief systems and a better
quality of life when properly applied. We assured the administration that the
seismological team would neither disturb the ongoing rehabilitation work nor
expect to receive any services from the administration. Rather, the team was
there to help the public through truthful scientific experiments and to render
service to the islanders in whatever way it could. The Andaman-Nicobar
administration, however, provided all infrastructural supports to the
seismological team during the investigation. It was a great challenge to us,
and we diligently followed our scientific duty to unravel the hidden mystery
of stress release in the islands, which helped us to educate islanders about
earthquake genesis and magmatic eruptive processes. Most islanders were
terrified and confused by the irresponsible prediction of further earthquakes
and tsunamis by some scientists and astrologers. What was sorely needed after
MSE 9.3 was a candid and plausible explanation of earthquakes and tsunamis,
from scientists in common language. We were subsequently recognized for our
significant contribution toward meeting that goal (see appendix A).
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Figure 1. Tectonic map of the study area with the 26 December 2004 Sumatra-Andaman
mainshock (Mw 9.3) shown by a black asterisk (MS) and four
(1-4) big aftershocks (Mw  6.3) denoted by black
asterisks and their corresponding CMT solutions (from the U. S. Geological
Survey) marked with 1 to 4. The black-and-white shed in the CMT projections
denote compressive and tension zones, respectively. The open triangles denote
seismograph locations (from south to north: Car Nicobar; Hutbay; Port Blair;
Rangat; Diglipur; and Narcondum) installed by Geological Survey of India
following the mainshock. Volcanic zones (BV: Barren volcano; NV: Narcondum,
and BT: Baratang mud volcano) are denoted by black triangles. The study region
is demarcated by a rectangle in the insert map of India.
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The Andaman and Nicobar Islands region of India is poorly instrumented,
with only a solitary permanent analog station of the national network of the
India Meteorological Department (IMD) at Port Blair, which was upgraded to a
three-component digital broadband seismograph just after MSE 9.3 hit on 26
December 2004. Our aftershock investigation in this poorly instrumented region
provided us with useful information on the earthquake processes
vis-à-vis the status of the magma chamber beneath the volcanic
zones. Continuous monitoring of aftershocks in the ruptured source zone by a
close-spaced temporary network shed light on detailed tectonics, dimension of
the rupture zone, and the structural heterogeneity that play a major role in
causing earthquakes (Frohlich
1989; Kisslinger and Hasegawa
1991).
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FIELD SURVEY
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Six seismographs were installed by a seismological team of the Geological
Survey of India in different parts of the Andaman and Nicobar islands,
covering Car Nicobar, Hutbay, Port Blair, Rangat, Diglipur, and Narcondum
along a part of a delineated mainshock rupture zone about 1,300 km long that
extended from north Sumatra (Indonesia) to Diglipur (India) along the Andaman
trough, where four big aftershocks occurred within 12 hours of the MSE 9.3
earthquake (figure 1). Due to
obvious constraints on geographical locations because of the elongated nature
of the archipelago, it was not possible to obtain good azimuthal control in
laying the temporary network. However, we made the best possible network and
managed to monitor all installed seismographs continuously using air, sea, and
land conveyances with the help of the local administration. The station
spacing ranges from 90 to 130 km. For the first time in the history of Indian
seismology, a seismometer was installed in the volcanic zone of Narcondum for
better coverage in the North Andaman region
(figure 1). Efficient
instrument parameters were set up in event-triggered mode for recording a
large number of aftershocks (ML > 2.0) by seismographs
operated with rechargeable power-safe batteries of 100 amperes per hour. The
seismographs were equipped with global positioning systems (GPS) to obtain
precise coordinates of station locations and high-precision time.
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DATA ANALYSES AND DISCUSSION
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Analyses of about 18,000 aftershocks (ML
3.0) recorded during the period from 6 January to 16 March 2005
(figure 2A) showed that the
decay of the aftershock sequence did not strictly follow the Omori power law
t-p in the month of January 2005. There was a sudden burst
of aftershock activity (figures
2B-D), which consistently generated fear among the
inhabitants.
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Figure 2. (A) Graph showing aftershocks recorded until 16 March 2005 at different
seismograph stations as shown by their alphabetical codes (PBR: Port Blair;
CNB: Car Nicobar; HTB: Hutbay; RGT: Rangat; DGP: Diglipur; NCD: Narcondum).
(B-D) Graphs showing temporal variations of aftershocks at different
seismograph stations from 6 to 31 January 2005. The star denotes the
occurrence of the mainshock (Mw 9.3).
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Aftershock Activity and Public Panic
Prior to the second burst of aftershock activity at the end of January, the
p-value was estimated to be 0.9532 for aftershocks (M
4.5), near the normal value of 1.0. The aftershocks are, however, still
continuing due to multiscaled fractures and ruptures generated by the MSE 9.3
beneath the Andaman and Nicobar islands. Although the temporary six-station
network lacked good azimuthal coverage, the epicenter locations
(figure 3A) were determined
with a fair degree of precision by a multistation method of locating
aftershocks (Havskov and Ottemoller 2000), and the observed source area is
fairly consistent with that estimated by teleseismic stations
(Lay et al. 2005)
despite poor focal depth locations of those aftershocks that occurred outside
the temporary network (Mishra et
al. 2003). Our analysis revealed a prominent north-south
trending 850 x 350 km2 rupture area beneath the Andaman and
Nicobar islands (figure 3A),
which is larger than previous estimates
(Lay et al. 2005). Our
estimate of the rupture dimension from the distribution of aftershocks beneath
the Andaman and Nicobar islands is possibly a truer dimension of the rupture.
Three classes of aftershocks were observed: 1) initial aftershocks that
describe the mainshock rupture, 2) secondary aftershocks that represent growth
in the original aftershock zone due to distant events, and 3) aftershocks
caused by the reactivation of the pre-existing fault system that propagated
from the Nicobar Islands to the north of the Andaman Islands
(figure 3A). We studied the
entire rupture zone by dividing it into 10 different blocks depending upon
aftershock clusters and geotectonic features of the region
(figure 3B). It is interesting
to note that the teleseismic data (Mw 5.0)
(figure 3C) shows a prominent
gap near 10° N. There was a growing apprehension among the local people
that the anticipated gap zone could be a probable source of a large earthquake
in the coming days. We were able to reduce this apprehension by showing our
epicentral location of many aftershocks of magnitude less than 5.0 in the gap
(figure 3A), thereby
demonstrating some stress release in the gap zone. In this way, our aftershock
investigation functioned as an antidote to public panic. In order to
understand the nature of the heterogeneity in the rupture zone, an estimate of
b-value was made from the frequency magnitude relation of the
aftershocks in 10 different blocks with not less than 240 events in each block
(figure 3A,
table 1). The b-value
varies in the different blocks from 0.49 to 1.03, with an average value of
0.7723, which indicates local variation of the compressive stress as well as
heterogeneity in the region.
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Figure 3. (A) Epicenter map of aftershocks located by the multiple-station method in
this study. The variation in size of circles denotes the variation in
magnitude of aftershocks (see magnitude scale) and the variations in color of
the circles denote variations in depths of aftershocks (see depth scale). The
black star denotes the mainshock epicenter. The white triangle denotes the
location of the Baratang mud volcano. The black triangles denote Barren and
Narcondum volcanoes. The white boundary zones denote the aftershock clusters
or swarms around the volcanic zones. WAF represents the West Andaman fault.
(B) The entire study area is divided into 10 (1-10) different blocks for
studying b-values (table
1), and composite fault-plane solutions of aftershock clusters
(table 2 and
figure 4). (C) Epicenter map of
aftershocks recorded by teleseismic network ascribed to USGS/IRIS. The black
triangles denote Barren and Narcondum volcanoes while the black star denotes
the Sumatra-Andaman mainshock. The question marks denote anticipated
aftershock gap that does not exist in figure 3(A).
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Sudden stress changes cause large changes in seismicity; the seismicity
rates climb where the stress increases in terms of aftershocks and fall where
stress drops (Stein 1999).
Sudden bursts of aftershocks may be categorized as epidemic-type aftershocks
(ETAs) that deviate from the normal power law. Normally the larger aftershocks
behave as mainshocks and generate new sequences of aftershocks
(sub-aftershocks), and the deviation from Omori's law could be directly
related to the occurrence of some large aftershocks (M 6.0) in
the region. Dieterich's (1994)
time-dependent nucleation process does not explain temporal decay of
aftershocks because some other effects, such as viscoelastic or poro-elastic
processes, may be involved in the earthquake generating processes. A recent
study by Gavrilenko (2005)
demonstrated that hydromechanical coupling in response to an earthquake could
be the possible causes of ETAs. The subsurface of the Andaman and Nicobar
islands has been badly devastated and possibly associated with several minor
to major underwater faults by submarine ruptures
(Sieh 2005), through which the
permeation of a huge volume of sea-water in the aftershock zone cannot be
ruled out. This process of water permeation in the rupture zone might have
facilitated the occurrence of more aftershocks
(Rojstaczer et al.
1995) due to poro-elastic effects and hence, a burst of
aftershocks was observed. Our interpretation is supported by the earlier study
made on fault-zone heterogeneity and the rheology of the fault-zone materials,
and the presence or absence of water or pore fluids that have a tendency to
influence the aftershock activity (Zhao
et al. 2002; Mishra 2003).
The people of the Andaman and Nicobar islands were very eager to know the
role of earth tides in inducing earthquakes, especially during a full moon, as
foretold by some astrologers and fortune tellers. Some residents moved to
higher land and hills to save themselves from anticipated tsunamis at full
moon on 26 January 2005, because the 26 December 2004 tsunamigenic MSE 9.3
earthquake occurred on a day when the moon had been full! We field
seismologists had to provide a pragmatic and scientifically truthful
explanation to these good people who had suffered enough. The distortion of
the Earth caused by the pull of the sun and moon may modulate the seismicity
rates or the rate of aftershock, provided earthquake occurrence is influenced
by static stress changes (Stein
1999). This is so because unlike earthquakes, the tides produce no
strong motion (shaking), but they do alter the stress on faults
(Stein 1999). Though we found
that there was no strong shaking at full moon (26 January 2005) caused by
tidal stresses, the rate of aftershock activity was enhanced by about 31% from
the events that occurred on the preceding day
(figure 2). Previous work by
Vidale et al. (1998)
on seismicity and tidal stresses found that the rate of seismicity during the
peak tidal unclamping is 1.0% higher than average. The enhanced aftershock
activity on 26 January 2005 (figures
2B-D) supports the hypothesis that the tides perceptibly alter the
rate of seismicity, suggesting that the much larger off-fault stress changes
associated with earthquakes was indeed one cause of aftershock rate changes
(Das and Scholz 1981). The
nonoccurrence of large earthquakes in the region on the date of the full moon
(26 January 2005) significantly reduced public panic despite an increase of
microtremors on that day.
Mud Eruption and Public Panic
The Baratang area (located 100 km northeast of Port Blair) between Port
Blair (South Andaman) and Rangat (Middle Andaman)
(figure 1) showed mud eruptions
on 28 December 2004, just two days after the MSE 9.3
(figure 4A), which created
havoc in the region due to several valid questions such as: Why did mud erupt
after a big earthquake? Was there any chance for gas or lava eruption through
the mud crater? What could be its future implications in Middle Andaman? We
determined the composite fault-plane solutions of three aftershock clusters at
different depth ranges (figure
4B). The solutions (figure
4C) show that the area in the vicinity of Baratang is associated
with a thrust fault, indicating the dominant compressive forces at depths
ranging from 0-30 km. The accretionary subduction complex beneath the Andaman
Islands is associated with a number of imbricated thrust fault systems; one of
the most important faults is the north-south oriented Jarwa thrust fault,
which lies right beneath the Baratang volcano
(Roy 1992). The expulsion of
mud and slurry materials at the Baratang area (triangle in
figure 3A) might have been
initiated by the squeezing up of these materials through the thrust zone due
to compressive forces during the 9.3 MSE sequence. The origin of the mud
volcanoes is, however, debatable due to their unusual and enigmatic phenomena.
Mud volcanoes are often formed along fault lines in areas of weakness in the
Earth's crust and are associated geologically with younger sedimentary
deposits. Geologists describe mud volcanoes as "capricious" and
are still arguing about their origins
(Seach 2005). Some believe
that such volcanoes are created during the sedimentary process itself, while
others argue that different processes (such as seismic activity) also are
involved. We suggest that the strong shaking of the 26 December 2004 MSE 9.3
and its aftershock sequences has reactivated the Jarwa thrust zone along which
the Baratang mud volcano materials were expelled to the surface from the
accretionary prisms of the subduction complex
(Curray 2005). The main cause
of concern is the possibility of mud volcano eruptions in the form of toxic
gases and violent explosions (Seach
2005). Elsewhere in the world, mud volcanoes have exploded with
great force, shooting flames into the sky and depositing huge quantities of
mud on the surrounding area (e.g., Azeri mud volcano of Azerbaijan,
2001; Aso mud volcano of Japan, 1997) as reported by Seach
(2005). If this is the case,
then the occurrence of huge and violent magmatic eruptions in the Baratang
area may prove to be fatal. However, the likelihood of such an explosion from
the Baratang mud volcano cannot be ascertained at present because of its
unknown history of eruption and virgin geochemical status. On the other hand,
mud volcanoes are one of the visible signs of the presence of oil and gas
reserves under the land and sea in the Caspian region. Gas seeps are a related
phenomenon; they occur when a pocket of gas underground finds a passage to the
surface (Seach 2005).
Geochemical tests of the erupted mud samples from the Baratang volcanic zone
and geophysical exploration with drilling are highly warranted at this
juncture to ascertain the presence of oil and gas reserves and the phenomenon
of gas seepage under the land and sea in the Middle Andaman region. If
exploration proves that oil and gas reserves are present, then the tsunami
could possibly turn out to be a boon in disguise for islanders and the people
of coastal India.
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Figure 4. (A) Photo showing mud eruption at Baratang on 28 December 2004 in Middle
Andaman. Arrow indicates location of the mud crater. (B) Ten blocks same as
shown in figure 3(B) with
aftershock clusters at 0-15,16-30, > 31 km depths. (C) Composite
fault-plane solutions denoted by numeric values in parentheses
(table 2). Photo by O. P.
Mishra.
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Public Panic over Possible Lava Eruption
Fear among islanders persisted that the Barren Island volcano, the only
active volcano in India, would erupt due to the MSE 9.3 earthquake. The
volcano erupted after a long quiescence of around 200 years in April 1991 and
continued to be active up to July 1995
(Banerjee and Shaw 2001). Even
in 2001 Barren was active (Sengupta et
al. 2005). We examined the composite fault-plane solution
(figure 4C) of the aftershock
clusters in and around the Barren (active) and Narcondum (dormant) volcanic
zones (figure 3A). The
aftershock clusters resembled the "swarm" type of earthquakes,
which suggests the movement of magma. Under the volcanic zones, clusters of
aftershocks are useful indicators of an impending eruption
(Linde and Sacks 1998).
However, the composite fault-plane solutions of the aftershock clusters at a
depth of 0-15 km show thrust faulting, indicating the lack of tensional
stress, which may be due to concealed cold felsic materials as a structural
barrier at this depth range (Banerjee and
Shaw 2001; Sengupta et
al. 2005). At relatively deeper depths ( 16 km), though,
the fault solutions were found to be normal for the Narcondum volcanic zone
(table 2). This may be
attributed to rock failure in the weakened crust at this depth range by the
process of underheating (Zhao et
al. 2002). The possibility of Barren and Narcondum eruptions
in the near future was indicated by Mishra et al.
(2007a,
b) based on fault-plane
solutions and a series of repeated shaking by moderate to large aftershocks
adjacent to these volcanic zones. About 150 days after the MSE 9.3 the Barren
volcano erupted lava on 28 June 2005
(figure 5) and the Narcondum
volcano emitted smoke and sand. These eruptions support the interpretation and
the hypothesis that the volcanic system is disturbed by earthquakes
(Linde and Sacks 1998). There
have been many volcanic eruptions triggered by earthquakes (M
4.8) elsewhere in the world. A study by Linde et al.
(1994) showed that large
earthquakes (M 7.0) were able to trigger volcanic eruptions
within a radius of 750 km. The Nyiragongo volcano (Congo, Zaire) and Ulawun
volcano (New Britain, Papua New Guinea) erupted due to two strong regional
earthquakes, each at a distance of about 150 km. We found that an aftershock
(Mw 6.4) occurred to the northeast within a
distance of 40-45 km of the Baratang volcanic zone and another aftershock
(Mw 6.3) occurred about 100 km northwest of the
Narcondum volcanic zone within 12 hours of the MSE 9.3 earthquake
(figure 3). A series of
aftershocks (M 5.0) also occurred closer to the Barren and
Narcondum volcanic zones. The occurrence of the third
(Mw 6.4) and fourth
(Mw 6.3) big aftershocks and a series of small to
moderate aftershocks (M 4.0) within 100 km of the volcanic zones
promoted volcanic eruption in the Andaman Islands region. It has been
suggested that seismic waves from earthquakes have the potential to increase
the pressure in magma chambers even at large distances, and a premature
eruption may result if a seismic wave accelerates the ascent of magma close to
the critical pressure state (Linde et
al. 1994). An estimate of 3-D seismic tomography may allow us
to better understand this enigmatic problem of whether deeper structure
influences shallow seismicity and volcanism
(Zhao et al.
2004).
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TABLE 2 Composite Fault-Plane Solutions of Aftershock Clusters at Three
Different Depth Ranges in 10 Different Blocks
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Figure 5. Lava eruption (bright areas) from Barren Island volcano through several
craters. The photos were taken from different directions at a distance of
about 300 m from the volcanic source. Arrows indicate locations of lava
craters. Photos by O. P. Mishra.
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CONCLUDING REMARKS
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Our aftershock investigation in the Andaman-Nicobar region provided
scientific and moral support to the people in the region and an antidote to
the public panic that prevailed immediately after the 26 December 2004 MSE
9.3. Continuous aftershock monitoring may provide information about the
pattern of stress release in the affected region. Such monitoring, and a basic
understanding of the science of seismology, will instill confidence among
local residents that they can ignore irresponsible predictions of earthquake
and volcanic eruption based on non-scientific propaganda (e.g.,
astrology, fortune tellers etc.). The large amounts of aftershock data
obtained by our local investigation in the Andaman and Nicobar islands
provides us with a wealth of valuable tools and information for probing the
nature of the beast. Our aftershock investigation and field survey suggest
that geochemical tests of the erupted mud samples from the Baratang volcanic
zone, together with geophysical exploration followed by drilling, may
ascertain the presence of oil and gas reserves and the phenomenon of gas
seepage under the land and sea in Middle Andaman. If the integrated
investigation proves that oil and gas reserves are present beneath the Andaman
region, then the tsunami could have significant long-term positive economic
impact on the islanders and other people of India. We recommend that countries
without sufficient permanent seismic networks get seriously involved in
aftershock monitoring and investigation to generate a seismological databank,
which can be used to better understand seismogenesis and the volcanic eruptive
processes of a region. Such an effort can help to develop a comprehensive
hazard mitigation model for any region, and it can also constitute an
important ingredient for disaster management programs for earthquake prone
regions elsewhere in the world.
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APPENDIX A
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ACKNOWLEDGMENTS
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We thank Shri P. M. Tejale, Director General, Geological Survey of India
(GSI), for the support and motivation to conduct field surveys in very arduous
conditions. Prof. Dapeng Zhao is gratefully acknowledged for his significant
comments on the interpretation of our observations during my stay at the
Geodynamics Research Center, Ehime University, Japan. Dr. L. K. Das is
sincerely acknowledged for his thoughtful comments on the original manuscript.
Logistic support by the Indian Air Force and Andaman-Nicobar administration
are gratefully acknowledged. Constructive and thoughtful comments by Prof.
Shamita Das, Dr. Susan Hough and Prof. Luciana Astiz were most helpful in
shaping the final presentation of this research. Authors are thankful to the
SRL production staff for their meticulous and favorable support
during production of the accepted manuscript.
1 Central Geophysics Division, Geological Survey of India, Kolkata 700016, India 
2 Geophysics Division, Eastern Region Office, Geological Survey of India, Kolkata 70009, India
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niom_mishra2005{at}yahoo.co.in
(O.P. M.)
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