When death toll is reported more than the village population, then better sense prevails!
The Latur earthquake occurred at 3.56 am local time on September 30, 1993. The main area affected were the districts of Latur and Osmanabad, including the Ausa block of Latur and Omerga of Osmanabad in Maharashtra, Western India.
Mr PNV Nair, our Editor, asked us to rush to the Latur for a coverage. We left by car around 7.30 pm the day quake occurred carrying some water bottles, biscuits fruits.
We reached Latur town around midnight and there was deadly silence with little or no street lights. We found a hotel and myself and Sushil checked in with a typewriter.
We called up Deccan Chronicle and described the scenario. DC staff watched Doordarshan and compiled the stories along with PTI and UNI agency copies.
We came out in search of food, but there were no hotels open. Finally, a family spotted us and asked us what we are looking for. We told them we are journalists to cover the quake. They offered us food and we left for hotel.
Our driver Sanjeev Reddy refused to sleep inside the hotel saying the hotel may collapse if quake occurs again. He also advised us to keep doors and windows open for emergency exit. That was scary!
In the morning we left the hotel for reporting to Killari and Holi villages, which was worst hit. We spoke a large number of people. Infact, Latur more specifically Killari was famous for grapes.
We went by Doordarshan death figures which put death toll at about 10,000 on the first day. It was more a guess based on flattened villages.
The next day local Marathi and other newspapers reported 15,000 to 20,000 dead. We reported about 15,000 deaths. Officials refused to give guess figures saying they cannot give information unless the body is physically counted.
We decided to meet the Collector IAS officer Praveen Singh Pardeshi. We met him and asked about the varying death toll in newspapers.
“What can I tell you? When the village population is 8000, some have survived and are in relief camps, you people reported 10,000, 20,000 dead. Please confirm and report. We cannot give you figure unless the body is recovered and accounted for. It takes time to get correct casualty and injured figures.”
In the nights, outskirts of Latur were really scary with officials disposing off bodies in mass funerals. Series of pyres were lit along the roads.
We were sceptical about the death toll and decided focus more on human interest stories like who people escaped from jaws of death, how families lost kith and kin and ghostly villages etc.
There were after shocks and we were advised by Mr Nair to stay in safe place. We moved to Solapur nearby.
After days passed by the final figures was:
Death of 7,928 people
Injury to 16,000 individuals
15,854 livestock perished
Complete destruction of 52 villages
30,000 houses collapsed entirely
27,000 houses sustained severe damage to amenities and infrastructure
2,11,000 houses across 13 districts suffered varying degrees of damage.
The devastation was largely due to extremely vulnerable housing stock constructed of uncoursed random rubble stone masonry with heavy earthen roofs supported by timber planks and posts.
About 67 villages around Killari in Latur district and contiguous areas were completely destroyed. The total property loss was approximately $333 million (Rs. 11.8 billion).
In response to the scale of the destruction, Govt. of Maharashtra launched an equally massive reconstruction program, known as MEERP (Maharashtra Emergency Earthquake Rehabilitation Program).
This program involved the relocation of 52 totally destroyed villages, the reconstruction of 22 severely damaged villages, and the repair/strengthening in about 2400 villages. The initiative placed a strong emphasis on rebuilding with better housing and infrastructure.
Our fortnight stay in Latur, Solapur was a great experience. Spot reporting was quite challenging. We typed out reports sitting under a tree or hotel and faxed it to Deccan Chronicle from Solapur post office. Several journalists from Mumbai and other places too were present.
My wife Dr C G Bhagirathi, Susheel parents were worried as there were after shocks and death toll was rising. We called up our families daily and assured nothing to worry.
Mr T Venkatram Reddy, Deccan Chronicle Chairman, Mr PNV Nair, Editor, Mr M P Rabindranath Asst. Editor and other staff members congratulated us for the excellent coverage. We visited again after a year to report on rehabilitation and resettlement.
An investigation into the Latur earthquake of September 29, 1993 in southern India
Abstract
The devastating ( Mw 6.2) Latur earthquake of September 29, 1993 in South India has claimed an estimated 11.000 human lives.
With an Imax of VIII, the earthquake was felt to an average distance of 750 km. More than 125 shocks were reported to have been felt during August 1992-March 1993.
Out of these, during October-November 1992, several shocks of M ≥ 2.0 were recorded at the NGRI seismic station at Hyderabad which is the closest (220 km) to the epicentre. No such shocks occurred for at least 8 months before the Latur earthquake.
The aftershocks were monitored by a network of up to 21 stations between October 8, 1993 and January 31, 1994. A majority of the aftershocks occurred within a 10-km radius from the main shock.
On the basis of the location of aftershocks in the first few days, a plane dipping at an angle of 45° towards the southwest and striking at 135° is inferred to be the fault plane which extends to a depth of 4.5 km and on projection meets the surface in the vicinity of the observed surface rupture.
Assuming the aftershock zone to be the rupture zone, the stress drop is estimated to be 7 MPa with a maximum displacement of 1.7 m for the main earthquake. A unique discovery is the high concentration of helium in the soil in the immediate vicinity of the surface rupture indicating that the rupture extends to the surface from a depth of a few km.
A detailed broadband magnetotelluric (MT) investigation revealed the presence of an anomalously high conductive zone at a depth of 6-10 km. Observation of a PC phase, lagging behind the Pg phase by about 0.6 to 0.8 s in the seismograms of aftershocks, indicating a low-velocity zone at 7 to 10 km depth, is consistent with the MT results. This highly conductive low-velocity layer is inferred to be fluid-filled.
The main stress regime in Peninsular India is NE compressive stress due to plate tectonic movement. Erosion of the basalt cover in the Deccan Plateau may be adding additional compressive stress in the region.
The existence of a low-velocity highly conductive fluid-filled layer will enhance stress concentrations in the uppermost brittle part of the crust causing the earthquake.
NICEE Remembers the 1993 Latur Earthquake
on its 30th Anniversary
Slide show recalling the aftermath of the Latur Earthquake
Thirty years ago, in the morning hours, a moderate earthquake (~M6.4) shook the
Marathwada region of Latur and Osmanabad districts of Maharashtra.
The event proved to be one of the deadliest earthquakes to have struck India in recent times
which resulted in over 8,000 fatalities, 16,000 injuries, and over one million homeless people.
The region is geologically a part of the Deccan Plateau (Trap) which has been widely regarded
as a stable continental region (SCR) for its limited potential for seismic activity as opposed to
highly seismically active Himalayan plate boundary regions. The area that was in the lowest
seismic zone was moved to Zone III, the third highest in the 2002 revision of the seismic
code IS 1893.
The devastation was largely due to extremely vulnerable housing stock constructed of
uncoursed random rubble stone masonry with heavy earthen roofs supported by timber planks
and posts. About 67 villages around Killari in Latur district and contiguous areas were
completely destroyed. The total property loss was approximately $333 million (Rs. 11.8 billion).
In response to the scale of the destruction, Govt. of Maharastra launched an equally massive
reconstruction program, known as MEERP (Maharashtra Emergency Earthquake
Rehabilitation Program). This program involved the relocation of 52 totally destroyed
villages, the reconstruction of 22 severely damaged villages, and the repair/strengthening in
about 2400 villages. The initiative placed a strong emphasis on rebuilding with better
housing and infrastructure.
Effects of Killari earthquake on the paleo-channel of Tirna River Basin from Central India using anisotropy of magnetic susceptibility
Saumitra Misra
The Killari Earthquake (Moment magnitude 6.1) of September 30, 1993, occurred in the state of
Maharashtra, India, has an epicenter (18°03′ N, 76°33′ E) located at ~ 40 km SSW of Killari Town.
The ~ 125 km long basin of Tirna River, close to the Killari Town, currently occupies the area that has witnessed episodic intra-cratonic earthquakes, including the Killari Earthquake, during last 800 years. The anisotropy of magnetic susceptibility (AMS) study was performed on ~ 233 soft sedimentary core samples from six successions located in the upper to lower stream of the Tirna River basin in the present study in order to evaluate the effects of earthquake on the river flow dynamics and its future consequence.
Published: 25 November 2020
Effects of Killari earthquake on the paleo-channel of
Tirna River Basin from Central India using anisotropy of magnetic
susceptibility
Scientific Reports
volume 10, Article number: 20587 (2020) Cite this
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Abstract
The Killari Earthquake (Moment magnitude 6.1) of September
30, 1993, occurred in the state of Maharashtra, India, has an epicenter (18°03′
N, 76°33′ E) located at ~ 40 km SSW of Killari Town.
The ~ 125 km long basin of Tirna River, close to the
Killari Town, currently occupies the area that has witnessed episodic
intra-cratonic earthquakes, including the Killari Earthquake, during last
800 years.
The anisotropy of magnetic susceptibility (AMS) study was
performed on ~ 233 soft sedimentary core samples from six successions located
in the upper to lower stream of the Tirna River basin in the present study in
order to evaluate the effects of earthquake on the river flow dynamics and its
future consequence.
The AMS Kmax orientations of the samples from the
upper reach of the river section suggest that the sedimentation in this part of
the river was controlled by a N–S to NNW–SSE fluvial regime with a low or
medium flow velocity.
In the middle reaches of the basin, an abrupt shift in the
palaeo-flow direction occurred to W–E with low velocity flow. However, a NW–SE
higher palaeo-flow regime is identified in the following central part of the
basin in down-stream direction, followed by a low-velocity palaeo-flow regime
at the lower reach of the Tirna basin.
We attribute the sudden high flow velocity regime in the
central part of the river basin to an enhanced gradient of the river that
resulted from the reactivation of a NW–SE fault transecting the Tirna River
basin at the Killari Town.
As the NW–SE faulting in regional scale is attributed as the
main cause of Killari Earthquake, the reactivation of this fault, thus, could
enhance the further possibility of an earthquake in near future, and hence
leading to devastating flood in the almost flat-lying downstream part of the
Tirna River.
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Introduction
A devastating earthquake [Moment magnitude (Mw):
6.1] occurred close to Killari town (18°03′ N, 76°33′ E) in Maharashtra state,
India1,2
(Fig. 1),
on September 30, 1993. This earthquake was considered as an example of
intra-cratonic earthquake that occurred in a region of low background
seismicity that resulted a poorly developed geomorphologic expression, and
had long recurrence interval2,3.
Multiple hypotheses were proposed in explaining the origin of this earthquake
that include (a) rupture along a new fault plane2,3,4;
(b) subduction of Indian plate beneath Tibetan plate5,6,7,
and (c) flexural buckling of Indian plate to the north in contact with Tibetan
plate8.
The most recent hypothesis suggests that the Killari Earthquake resulted due to
a deep crustal/lithospheric dynamics9,10.
Some other opinion suggests that the existing faults were reactivated to
generate the earthquake11,12.
The Killari Earthquake produced a NW–SE trending deformation zone having a
length of ~ 3 km and width of 300 m, which was associated with
surface rupture2,3.
The geomorphological features that resulted from such deformation includes
relative subsidence and/or uplift, and small scale local upheavals1,2.
A ~ 40 m long and 2–8 m wide, narrow elevated linear ridge is also
observed close to the Killari Town13.
Also, a 17 km long, NW–SE reverse fault that extends from Talni to Killari
Town with a dip 45°SW was suggested to have produced from the focal mechanism
of the main shock (Fig. 1)5,14,
although an elaborate structural analyses of this fault is awaited. It was also
suggested that the Killari Earthquake had resulted from the reactivation of
pre-existing fault5.
Figure 1
Geological map of the Tirna River basin showing the
lineaments and location of studied outcrops (modified after Chetty and Rao13
and Babar et al19).
The major (red dashed lines) and minor (black dashed lines) lineaments
occurring along NE–SW, NW–SE, E–W and WNW–ESE directions. The sampling sites
are: Ter, Dhutta, Makani, Sastur, Killari and Sawari. Red Star indicates
epicenter of 1993 Killari Earthquake (Mw = 6.1). The dextral
displacements of the Tirna River faults are shown in F1 and F2.
The ~ 125 km long Tirna River, the subject of present
study, is currently flowing towards the east along a channel that occupies
~ 10 km long WSW–ENE trending fault zone in the vicinity of the earthquake
epicenter (Fig. 1). Several
subsurface faults trending mainly NW–SE, and a few in N–S, NE–SW and ENE–WSW
are also identified in satellite imageries crosscutting the Tirna River basin13.
The relative movement along these NW–SE and ENE–WSW lineaments results in
a well-developed mosaic of block structure13.
However, the WSW–ENE lineament, along which the Tirna River is currently
flowing close to the Killari Town, crosscuts all lineaments and exhibits
sinistral strike-slip movement13.
A re-investigation of the Tirna River basin has significance because this NW–SE
trending river basin is confined in an area that contains epicenters of most of
the earthquakes (M < 6.5) that occurred during the
last ~ 800 years (Fig. 2 inset
map).
Figure 2
Digital elevation model (DEM) of Tirna River basin (Source: http://srtm.csi.cgiar.org) and inset map
shows location of historic earthquakes around Killari (modified after Jain et
al2
and Rajendran et al5).
The main focus of scientific research in and around Killari
Town, till date, was mostly concentrated in evaluating the origin of 1993
earthquake2,3,4,5,6,7,9,10,15.
However, no scientific study has been undertaken till date to evaluate the
possible impact of earthquake on the regional environment in and around this
area. The local agricultural economy and habitation in and around Killari Town
mainly depends on the Tirna River16. Hence, in the
present paper, we have attempted to identify the possible changes in
palaeo-flow pattern of the Tirna River due to the earthquake using the
technique of Anisotropy of Magnetic Susceptibility (AMS) of soft sediment cores
collected from its floodplains, and tried to evaluate its future impact on the
surrounding environment. An attempt has also been made to re-evaluate the
future consequence of the Killari Earthquake.
Geological setting
The ~ 125 km long Tirna River is an important tributary
of the Manjara River that flows to the Godavari River basin in Maharashtra,
India (Fig. 1). The
Tirna Basin extends from 17°51′13′′ N to 18°28′33′′ N latitudes and 75°47′51′′
E to 76°57′14′′ E longitudes, and covers an area of ~ 8280 km2 with
a maximum and minimum elevation of ~ 746 m to NW and ~ 553 m to E,
respectively, with a general slope from ~ 10.3° towards NW to ~ 5.5°
towards the east (Fig. 2). The main
course of Tirna River is found to be controlled by a combination of NW–SE and
WSW–ENE fault systems (Fig. 1). From its
origin to the west upto the location Ter, the main course of the river changes
from NW–SE to WSW–ENE; from Ter to Makani, the main channel of the river flows
in NW–SE; finally, from Makani to Sawari, the main channel of the river orients
WSW–ENE. The NE–SW lineament has poor control in shaping the direction of the
river channel. The NW–SE fault, however, is found to be still active in
controlling the course of the Tirna River, which are indicated by the dextral
displacements of the Tirna River channel to the west of Ter along F1F1
fault (box 1 in Fig. 1) and along
F2F2 fault close to Makani (box 2 in Fig. 1), and
formation of a NW–SE tributary along F3F3 fault close to
Killari (box 3 in Fig. 1). This
basin has dendritic drainage pattern, where the lower reaches of the basin have
streams of low gradient and more sinuosity. The river also shows the evidences
of channel movement by avulsion largely controlled by the lineaments
(Fig. 1).
The older palaeo-levees exist in the form of ridges of around 4–5 m high
at Ter, Killari, Sastur and Makani villages along the Tirna River flood-plain.
The abnormally thick (~ 12–15 m) sediments are deposited near the Ter
Village at the bed level of the Tirna River forming mounds17,18.
In the exposure scale, these deposits are marked by curvilinear deposits over
the silty or sandy bank deposits preserved along the older course of the river.
The Quaternary deposits of the Tirna River basin have been divided into three
informal lithostratigraphic Formations; viz., (i) the oldest dark grey silt
Formation, (ii) the middle light grey silt Formation, and (iii) the youngest
dark grayish brown silt Formation19.
The early and late Formations of the Tirna River are thought to be
stratigraphically equivalent to the Ramnagar and Bauras Formations,
respectively, of the Narmada River alluvium20,21.
The Quaternary sediments present in the area can be tentatively classified as
pre-floodplain, floodplain, and pedi-plain deposits.
Materials and methods
Sampling and measurements
The sediment samples for the present study were collected in
8 cm3 cylindrical plastic containers (2.5 cm diameter and
2.2 cm height) using a portable soft sediment corer and were oriented
using a magnetic compass. Two hundred and thirty three (233) oriented samples
were collected from six sections along the Tirna River basin (Figs. 1, 2). The
sampled sections were named as TR [Ter, number of sample (n) = 17], DT (Dhutta,
n = 30), MK (Makani, n = 37), ST (Sastur, n = 26), KL (Killari, n = 57) and SW
(Sawari, n = 66), which are located from the upstream to downstream of the
Tirna River across the Killari Earthquake epicenter close to the Killari Town.
Sampling process for this work focused primarily on fine‐grained sediments such
as silt and clay samples and zones of pedogenically altered and disturbed
horizons were avoided.
The laboratory analyses for rock magnetic investigations
were carried out at the Indian Institute of Geomagnetism, Navi Mumbai, India.
The measurement of low-field (300 Am−1 at 920 Hz) AMS for
each specimen was carried out using a MFK1 kappabridge with measurements in 64
directions on three mutually orthogonal planes, using an automatic rotator
sample holder. The azimuths and magnitudes of principal susceptibility axes (Kmax,
Kint, and Kmin) were calculated using SUFAR software
supplied by AGICO, together with other magnetic anisotropy parameters such as
anisotropy ratios, expressed as corrected degree of anisotropy (P′) and shape
(T)22.
The analysis of the AMS data was performed using the Anisoft 5 software.
Isothermal Remanent Magnetization (IRM) was imparted with a
pulse magnetizer at a forward field of 20 and 1000 mT and at backward
fields of 300 mT. All remanences were measured using a Molspin spinner
magnetometer. The magnetization acquired at 1000 mT was defined as the
saturation isothermal remanent magnetization (SIRM). The remanent coercive
force (Hcr) characteristic was obtained using a MMPM9 pulse
magnetizer and a Molspin spinner magnetometer. Selected samples of each profile
were subjected to high-temperature magnetization and hysteresis loop
measurements in order to gain further insights into magnetic mineralogy and
grain size. For representative samples, thermomagnetic runs of magnetic
susceptibility (k–T curves) were performed using a Kappabridge
KLY-4S (AGICO) equipped with a furnace. The sample was heated from room
temperature to 700 °C and cooled back to room temperature in an argon
atmosphere. Hysteresis loops and First-order reversal curves (FORCs) were
obtained by an alternating gradient magnetometer Micromag 2900 with a maximum
field of ± 1 T. Values of saturation magnetization (Ms),
saturation remanent magnetization (Mrs), coercive force (Hc)
and coercivity of remanence (Hcr) were calculated from the
hysteresis loops. FORC diagrams were processed using the FORCinel software23.
Anisotropy of magnetic susceptibility (AMS) method
The AMS of a rock sample corresponds to a symmetric,
second-rank tensor that can be described by a triaxial ellipsoid with principal
eigenvectors Kmax > Kint > Kmin
representing the maximum, intermediate and minimum susceptibility axes
respectively of the tensor ellipsoid24.
The AMS technique has been widely used in geological science to evaluate the
orientation of mineral fabric in igneous rocks or structurally deformed rocks25,26,27,28,29,30,
naturally deposited soft-sediments31,32,33,34,35,36,37,38,39,40,41,42,43,44
and in laboratory deposited sediments45,46,47.
The experimental studies suggest that in fluvial
environment, minimum susceptibility axes (Kmin) of the AMS ellipsoid
always orient perpendicular (i.e., vertical) to the flow direction on a near
horizontal depositional surface irrespective of weak to moderate (less
than ~ 1 cm s−1) or strong (greater than or equal
to ~ 1 cm s−1) current velocity36,40,46,47,48,49,50,51,52.
The Kmax axes, on the other hand, will be either parallel or
perpendicular to the direction of flow when the current is either weak to
moderate or strong, respectively36,40,46,47,48,49,50,51,52.
Statistically, the long axis of Kmin enveloping ellipse is parallel
to the orientation of the main cluster of Kmax axes in stereographic
projection in low to moderate velocity environment and perpendicular in high
velocity environment36,
39,45,46,47,49.
The resultant shape of AMS ellipsoid of sediments is, therefore, prolate in
fluvial environment, and the shape is oblate for sediments deposited in still
water environment47.
However, superimposition of some anisotropy on this oblate shape is possible if
elongated grains roll on the shallowly sloping depositional surface and are
preferentially aligned perpendicular to the slope47.
Following the pioneering laboratory experiment45,
several authors36,37,38,39,40,43,44
applied these AMS techniques to study paleo-current directions in sedimentary
rocks, especially in sediments of appropriate grain size. As the orientation of
AMS ellipsoid in sedimentary environment is controlled by the direction and
velocity of water flow47,
this technique can be efficiently used to evaluate the palaeo-velocity of Tirna
River because the main flow direction of this river is known to be dominantly
eastward (Fig. 2).
Wide varieties of parameters have been used to describe the
axial magnitude relationships of the susceptibility ellipsoid53.
The corrected anisotropy degree expressed as P′ = exp [2{(ƞ1 − ƞm)2 + (ƞ2 − ƞm)2 + (ƞ3 − ƞm)2}]½
where ƞ1 = ln K1, ƞ2 = ln K2, ƞ3 = ln
K3, ƞm = (ƞ1 + ƞ2 + ƞ3)/3;
and K1, K2 and K3 are maximum, intermediate
and minimum susceptibility vectors respectively22.
P′ shows the degree of anisotropy and reflects the eccentricity of the
ellipsoid, i.e., the intensity of preferred orientation of minerals in a rock.
The shape parameter, expressed as T = (2 ƞ2 − ƞ1 − ƞ3)
/ (ƞ1 − ƞ3), reflects the symmetry of the ellipsoid. It
varies from − 1 (perfectly prolate shape) through 0 (triaxial shape, transition
between prolate and oblate shape) to + 1 (perfectly oblate shape).
Results
Magnetic carriers
Temperature (T) dependent of magnetic susceptibility (χ)
curves (χ–T) for selected samples from Ter, Dhutta, Makani, Sastur, Killari and
Sawari sections are shown in Fig. 3. There is
a decrease in χ values at ~ 300–400 °C and ~ 580 °C in heating curves
(Fig. 3a–e)
in almost all the measured samples. In sample Sawari 0.8, an increase in
susceptibility is seen to be present with increase in temperature, which before
dropping significantly at ∼ 350 °C. It is then seen to rise and is followed by a sharp
drop at ∼ 580 °C
(Fig. 3f). The
sharp drops in susceptibility value at ∼ 350 °C and ∼ 580 °C indicate that the magnetic carrier in the samples is
titanomagnetite and magnetite, respectively. The increase in χ between room temperature and the Curie temperature is typical
for Ti-rich titanomagnetite53,54
for Ter, Dhutta, Makani, Sastur and Killari samples (Fig. 3a–e). For
all the samples, susceptibility is seen to have recovered gradually with each
successive step during cooling (Fig. 3), and the
destruction of magnetic properties of Ti-rich titanomagnetite phases53,54
is the cause of the irreversibility of cooling curve.
Figure 3
(a–f) Typical magnetic susceptibility versus
temperature plots (χ-T curves) for representative samples from different depths
from six studied sections Ter, Dhutta, Makani, Sastur, Killari and Sawari.
Figure 4 shows IRM
acquisition curves for saturated isothermal remanent magnetization and remanent
coercivity spectra for all the six sedimentary sections. The IRM acquisition
curves for Ter, Dhutta, Makani, Sastur and Sawari samples are seen to increase
rapidly from 0 to 150 mT, where ~ 70 to 80% of the maximum magnetization
is acquired by 200 mT, and all samples achieve near complete saturation at
1000 mT (Fig. 4a–d,f).
These observations suggest the low-coercivity ferrimagnetic mineral as the main
magnetic carrier in our samples, with a possibility of having traces of
high-coercivity magnetic mineral(s). For the Killari samples, < 95% of
saturation is achieved at ~ 200 to 300 mT, indicating contribution is
predominantly from a low-coercivity mineral (Fig. 4e). The
values of remanent coercivity of representative samples (Fig. 4a–f) range
between 25 and 80 mT, suggesting predominance of low-coercivity
ferrimagnetic minerals.
Figure 4
(a–f) Acquisition of isothermal remanent
magnetization and backfield demagnetization for representative samples from the
selected sites.
FORC diagrams and hysteresis parameters
To identify domain state that help in determining grain size
and also to discriminate different components within a magnetic mineral
assemblage56,57,58,59,60,61,62,63,
FORC diagrams and hysteresis parameters are obtained for the samples
(Fig. 5).
The FORC diagrams of the samples show characteristics of vortex state magnetic
grains by a peak at BC ~ 10–15 mT with closed contours57,58,59,60,61.
The hysteresis properties of the analyzed samples, viz., Ms, Mrs,
Hc and Hcr are summarized in Fig. 5e. In Mrs/Ms
versus Hcr/Hc plot63,
the samples are seen to form a cluster within the pseudo-single domain (PSD)
grain size. The hysteresis ratios are consistent with a dominant low-coercivity
ferrimagnetic component magnetic grain size, most likely magnetite.
Figure 5
(a–d) Representative first-order reversal
curve (FORC) diagrams for the representative samples from different depths,
(FORCinel) (e) Results of Hysteresis measurements presented in a Day
plot64.
AMS
Stereographic projections of Kmax and Kmin
principal axes of AMS ellipsoids from all six sections are shown in Fig. 6. The Kmin
axes of Ter soft-sediment samples (N = 17) are mostly sub-vertical and dipping
towards the north, where as the sub-horizontal Kmax axes in this
section are oriented primarily in N–S direction. In P′ versus T plots, Ter
sedimentary samples plot only in the oblate quadrant with relatively low P′
values of < 1.015 (Fig. 7a).
Figure 6
Equal area projections (lower hemisphere) of the directions
of Kmax (solid squares) and Kmin axes (open circles) of
AMS ellipsoids. The spreads in orientations of Kmax and Kmin
axes are qualitatively indicated in the stereograms. The lineament pattern
along the Tirna River basin also shown (dashed lines).
Figure 7
Variation of anisotropy degree (P′) and shape parameter (T)
for the selected sites (a) Ter and Dhutta (b) Makani and Sastur (c)
Killari and (d) Sawari.
Dhutta sedimentary succession, ~ 20 km SE of Ter, shows
sub-vertical to inclined Kmin axes mostly towards NNW-SSE, and
horizontal to sub-horizontal Kmax axes that have orientations mostly
between NNW-SSE and NNE-SSW (Fig. 6). The long
axis of Kmin ellipse is statistically parallel or sub-parallel to
the orientation of Kmax axes. In P′ versus T plot, the majority of
data from Dhutta succession plot in the oblate field (73%) with P′
(< 1.015), although subordinate spread in prolate field (27%) is also
noticed (Fig. 7a). The
majority of data from the Dhutta section are highly overlapping with those from
the Ter succession in P′ versus T plot.
In the Makani section, in the middle reaches of the Tirna
River, Kmin axes are also vertical to incline that are oriented
mostly towards E–W, although a few Kmin axes show inclination
towards NNE (Fig. 6). The
corresponding Kmax axes are horizontal to sub-horizontal and the
majority of data are oriented in E–W. Like the Dhutta section, the long axis of
Kmin ellipse is also statistically parallel or sub-parallel to the
orientation of Kmax axes. In P′ versus T plot, data are plotted in
oblate field in majority of cases (73%) with significant distribution in
prolate quadrant (27%) as well. All the data have P′ < 1.015 (Fig. 7b).
In the Sastur succession, the majority of Kmin
axes are vertical to incline in E-W direction (Fig. 6), whereas
the Kmax axes are horizontal to sub-horizontal and are oriented in
ENE–WSW direction in average. Like the Dhutta and Makani sections, the long
axis of Kmin ellipse is also statistically parallel or sub-parallel
to the orientation of Kmax axes. The P′ versus T plot reveals that
the majority of data from Sastur succession lie in the oblate field (85%) with
a few in prolate field (15%), wherein and the P′ values for the majority of
data are ≤ 1.015 (Fig. 7b).
However, a few data in the oblate field has P′ values between ~ 1.015 and 1.04.
In the Killari sedimentary section, Kmin axes are
vertical to incline in NW–SE (Fig. 6). The
horizontal to sub-horizontal Kmax axes are oriented from ESE–WNW to
NE–SW. Unlike the previous three locations in upstream direction (Dhutta,
Makani, Sastur), the trends of Kmin axes in the Killari section are
perpendicular to highly oblique to those of Kmax axes. In P′ versus
T plot, the Killari data are mostly distributed in the oblate field (72%),
however, a spread in the prolate fields (28%) is also noticed. The P′ values
are ≤ 1.015, however, a few samples have P′ values between 1.03 and 1.04. The
Dhutta, Makani and Killari sections are distinct from the Ter section by having
a low to important prolate component (~ 27%) in P′-T plot (Fig. 7a–c).
In the Sawari section, the distribution of Kmin
axes are sub-vertical to inclined, the inclined Kmin axes are mostly
oriented in NW–SE, although spread of a few sub-horizontal Kmin axes
towards NE is also seen (Fig. 6). The Kmax
axes are horizontal to sub-horizontal, and trend mostly in NW–SE direction.
Unlike the Killari section, the long axis of the enveloping ellipse on the main
cluster of Kmin axes of the Sawari section is statistically parallel
to the orientation of the majority of Kmax axes, which is, however,
similar to those of the Dhutta, Makani and Sastur sections. In P′ versus T
plot, the Sawari data mostly plot in oblate field (85%) and a few in prolate
fields (15%). The P′ values for these samples show higher range of variation of
1.03 (Fig. 7d).
Discussion
The rock magnetic experiments on the present Tirna River
soft sedimentary core samples help to infer ferrimagnetic minerals and PSD
magnetic grain sizes predominantly (Figs. 3,4,5) in the
samples. The step-like decrease in magnetic susceptibility at temperature
300–400 °C and 580 °C ranges suggests the mineral to be
titanomagnetite with various Ti contents54,55
(Fig. 3a–f).
The presence of magnetite/titanomagnetite is also supported by inference from
the IRM and FORC study (Figs. 4 and 5). The
decrease in magnetic susceptibility at 580 °C shows that magnetite grains
were also present. Although, the river sediments, in general, contains more
oxidized form of iron oxides, i.e. hematite, the presence of
magnetite/titanomagnetite in the Tirna River sediments, therefore, suggests
that the river sediments were subjected to protected surface alteration.
The experimental studies suggest that the orientation of Kmax
axes of AMS ellipsoids of river deposits is the function of two factors viz.,
direction and velocity of river flow47.
As the flow direction of the Tirna River is constant mainly from NW to SE
followed by W to E (Figs. 1 and 2), the
orientations of Kmax AMS axes can, therefore, be interpreted as the
function of flow current of the river41.
The orientations of Kmax axes of AMS ellipsoid suggest that in the
upper reaches of the Tirna River, at the Ter and Dhutta sections, the
sedimentation was controlled mainly by the N–S and NNW–SSE flowing fluvial
regimes, respectively (Fig. 6). The
spread of main cluster of Kmax axes in Dhutta section parallel (or
sub-parallel) to the long-axis of Kmin ellipsoid suggests medium to
low palaeo-flow regime in this part of the Tirna River37.
The low velocity of the palaeo-river flow in the upper reach of the Tirna River
is also interpreted by mostly oblate shape of the AMS ellipsoids in P′ versus T
diagrams64,65,66
(Fig. 7a).
The sub-parallel relations of long-axes of Kmin ellipsoids with the
cluster distributions of the Kmax axes in Makani and Sastur sections
(Fig. 6)
are also suggestive of low-velocity palaeo-fluvial environment47,48.
However a subordinate spread in the prolate filed of Dhutta and Makani sections
in the P′ versus T diagrams perhaps indicate local seasonal variation in river
flow velocity.
In the central part of the river including the Makani,
Sastur and Killari sections, the flow direction, as indicated by distribution
of long-axes of Kmax ellipsoids, seems to have shifted, in general,
from E–W (in Makani and Sastur) to SW–NE (Killari) directions, although a wide
variation of flow directions is observed in Killari section (Fig. 6). The near
orthogonal relationship between the long-axis of Kmin ellipsoid and
respective clusters of Kmax axes in the Killari section, suggesting
high velocity with greater than ~ 1 cm s−1 in the central
part of the Tirna River36,47,48.
The high velocity of the palaeo-river flow in central part of the Tirna River
is also suggested by mostly oblate to prolate shape of the AMS ellipsoids
mainly from the Killari section in P′ versus T diagrams29,
65,66,67
(Fig. 7c).
However, in the Sawari section the long-axis of Kmin AMS ellipsoid
is oriented parallel (or sub-parallel) to the distribution of the Kmax
AMS cluster (Fig. 6),
indicating dominantly low-flow palaeo-regime in this section of the Tirna River36.
The mostly oblate shape of the AMS ellipsoid in P′ versus T (Fig. 7d) also
support low-velocity regime in this lowest reach of the Tirna River.
The WNW–ESE Tirna River basin presently occupies an area
that has been active seismologically for the last ~ 800 years2,
5
(Fig. 2
inset map), and this seismicity is proposed to have been propagated by movement
along a major NW–SE inter-cratonic reverse fault situated close to the Killari3,5,
although a detail structural information on this fault is awaiting (Fig. 1). The
moderate dextral displacements of the main Tirna River channel along the F1F1
fault to the west of Ter, and F2F2 fault to the west of
Makani (Fig. 1) indicate
that the NW–SE fault system is still active at present post-dating the
formation of Tirna River. Therefore, it can be concluded that a possible
reactivation along the NW–SE F3F3 fault, crosscutting the
Tirna River channel to the west of Killari (Figs. 1, 6), in the
recent geological past has enhanced the gradient of the Tirna River towards the
east of this lineament. This recent movement along this fault plane, in turn,
increases the slope of this part of the river channel resulting relatively high
velocity of river flow in the central section of the Tirna River basin close to
this fault zone at Killari during monsoon.
Hence, further movement along this presently active NW–SE
fault could lead to the possibility of a major earthquake in this area in
future. A fresh movement along this reverse fault could have further increased
the gradient of the Tirna River close to the Killari town. This could lead to
the possibility of sudden flash flooding in the Sawari section and to its
downstream direction, to the east, particularly during the monsoon season. So
no further habitation close to the Sawari section of the Tirna River is
recommended on this study. However, if the movement along major intra-craton
major fault system development in the Deccan Traps basalts was responsible for
repeated earthquake in this area2,3,4,5,6,7,11,
a detail regional analyses of the existing fault system as well as rock
mechanical data are important for further prediction of earthquake in this
area.
Conclusions
In this present study, we determine paleo-flow directions of
the Tirna River sediments using anisotropy of magnetic susceptibility with the
help of rock magnetic studies. The AMS study results are summarized below:
- (a)
AMS analyses indicate that the recent sedimentation in the
upper reaches of Tirna River (i.e., Ter and Dhutta sectors) was dominated by
N–S to NNW–SSE fluvial regime with low to medium flow velocity
(< 1 cm s−1).
- (b)
Low fluvial velocity (< 1 cm s−1)
with an abrupt shift of flow direction to E–W was observed in the middle
reaches of the river in Makani and Sastur sectors. In the lower reach of the
Tirna River, in the Sawari section, low fluvial velocity is also observed in
NNW–SSE direction.
- (c)
High flow velocity (> 1 cm s−1) with
a SW–NE flow direction was observed in central section of Tirna River in the
Killari sector. This sudden change to high fluvial velocity in the central
section was resulted due to higher slope of the river valley in this area due
to the NW–SE faulting transecting the river channel at Killari sector.
- (d)
As the Killari Earthquake is sensitive to regional
intra-cratonic faulting2,3,4,5,6,7,11,
an in depth study on the regional fault system along with a detail AMS study on
Tirna River soft sedimentary cores are important for better understanding the
possibility of further earthquake in future. This is because further movement
along these fault system could have affects the flow pattern of Tirna River
leading to the possibility of future flooding in this area.
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