**recent advances in Seismic INSTRUMENTATION**

**and data interpretation in india**

S.N. Bhattacharya and R.S. Dattatrayam

India Meteorological Department, Lodi Road, New Delhi 110003.

*Included with permission from Current Science.*

**Abstract**

*The seismic instrumentation during later half of the twentieth century
had seen rapid growth, enabling generation of very useful data sets not
only for unraveling the very important features of Earth's interior but
also in understanding the complex geodynamic processes. By the middle of
twentieth century, the optomechanical seismographs in operation at
seismological observatories in India were upgraded with electromagnetic
seismographs. During 1970s, these observatories were upgraded with
visible seismographs. Since early 1990s, a trend has emerged to upgrade
and replace the existing analogue seismographs with digital broadband
seismographs to increase the dynamic range and frequency band of
recording by using the digital recorders and force balance seismometers.
Time keeping has also been improved by GPS synchronization. The data
acquisition systems of some of the observatories are connected to
Central Receiving Station, India Meteorological Department (IMD), New
Delhi, by telephone modem for near real time data reception. At present,
there are 212 seismological observatories being operated in India by
various R&D institutions, river valley authorities and Universities. The
Department of Earthquake Engineering (DEQ), University of Roorkee is
operating an Indian National Strong Motion Instrumentation Network
(INSMIN) and few Strong motion arrays in Himalayan region and Northeast
India. IMD is engaged in the manufacture of analogue seismographs since
1960s. Portable analogue recorders are being manufactured at CSIO,
Chandigarh, where design of digital recorders is also in progress. The
digital data processing and analysis in time and frequency domains, has
greatly improved the hypocentral location capability and ability to
evaluate detailed source parameters.*

**INTRODUCTION**

Seismic instrumentation in India started in 1898 with the installation
of the first seismograph of the country at Alipore (Calcutta) by India
Meteorological Department (IMD). Till 1930s, there were only four
seismological observatories in operation at Calcutta, Bombay, Kodaikanal
and Agra. However, these were supplemented by the observatories at Dehra
Dun, Colombo and later at Hyderabad with the co-operation of Surveyor
General of India, the Superintendent of Colombo Observatory and the
Director of Nizamia Observatory respectively.

By the year 1970, IMD seismological network had grown to comprise 18
permanent observatories under national network and 12 observatories in
North India for studying seismicity around river valley projects and
Delhi region. During 1963-64, the observatories at Shillong, New Delhi,
Poona and Kodaikanal were upgraded to conform to World-Wide Standardized
Seismological Network (WWSSN) standards under USGS collaboration. During
1965, Bhabha Atomic Research Centre (BARC) established a seismic array
station at Gauribidanur near Bangalore. In India, at present there are
212 seismological observatories run by various Central and State
Government organizations, research institutions and Universities. The
analogue seismographs are being steadily replaced by digital
seismographs at many of these observatories.

The conventional approach of analyzing the seismograms, produced by
analog seismographs was mostly confined to identification of various
phases, in an attempt to locate the hypocentral parameters. With the
advent of computer-aided digital recording systems, the seismic data
analysis became possible not only in time domain but also in frequency
domain so that more detailed information about earthquake source and
Earth's interior can be inferred. Seismic instrumentation in India was
earlier reviewed by Srivastava^1^. In the present article we shall
concentrate on a brief review of conventional seismographs and recent
advances in seismic instrumentation as well as some approaches to
digital data analysis and interpretation.

**SEISMIC INSTRUMENTATION**

The main objective of the seismic instrumentation is to record the
ground motion arising due to natural and man-made disturbances and, in
particular, to monitor the seismicity of a given region. A network of
observatories, equipped with seismographs is operated to record the
ground displacement / velocity. Depending on the nature of application,
the inter-station spacing of the network may vary from few tens or
hundreds of metres in mines, through kilometres to tens of kilometres
for monitoring of seismicity towards mapping of active faults,
investigations of crustal structure, study of induced seismicity,
aftershock studies etc. The regional/national networks cover a few
hundreds of kilometres depending on the geographical extent of the
region. It may be added that the data collected from such seismological
observatories during the 20th century had greatly contributed in
understanding the structure and physics of the Earth's interior and
geodynamic processes.

# Seismographs

A seismograph consists of a seismometer, which senses the ground motion,
and a recorder to record the motion. In a vertical component
seismograph, a heavy mass is made to hang through a spring from a rod
fixed to the ground. With the movement of the ground, the rod as well as
the recorder move, but the mass does not move initially due to its
inertia and therefore the spring extends. Thus, relative to the
recorder, the mass moves up or down in a vertical direction and this
relative motion is recorded on the recorder. The mass continues to move
up and down for some time, like a free pendulum, even after the ground
ceases to move. To avoid this, a damping arrangement is made, so that it
responds only to the ground movement^2,3^. Using the same principle, the
horizontal component of the ground motion can also be recorded with a
suitable arrangement by making the pendulum to move like a two-way swing
door. In order to get a three-dimensional representation of the ground
motion, it is necessary to record it in three orthogonal (perpendicular)
directions, generally, in vertical (Z), north-south (N) and east-west
(E) directions.

### Inertial pendulum seismometer

As stated above, the ground motion causes the mass of the seismometer to
undergo forced motion of a damped pendulum. The damping of a seismometer
is defined by a parameter h. A pendulum is called critically damped (h
= 1) if, (i) after moving in one direction, it comes back to the
equilibrium position without crossing over to the other direction and
(ii) the time T^'^ taken to come back from extreme position to
equilibrium position is half the free period of the pendulum (i.e.
period without damping). If T^'^ is more than half the free period, the
pendulum is overdamped (h \>1) and if the pendulum does not satisfy the
condition (i), it is underdamped (h\< 1).

The ground motion can be represented by Σj Aj exp(i ωj t) or, in a
simpler way, Σj Ajsin (ωj t), where angular frequency, ωj = 2 π / Tj and
Tj is the period. Considering one representative term of the summation,
i.e. A sin (ω t), the motion of the mass of the seismometer is given by,
B sin (ω t + b), where

B/A = 1 / √\[ { (T/TS)2 -1}2 + 4 h2 (T/TS)2 \] (1)

b = arctan {2 h T TS /(TS2 - T2) } (2)

Here TS is the free period of the seismometer mass, T is the period of
the ground motion under consideration and h is the damping
coefficient^2,4^. B/A = I(T), is known as the dynamic magnification
(response) of the seismometer and b is the phase change. I(T) is plotted
against T/TS in Figure 1, for different values of h. It may be seen that
I(T) = 1 when T/TS \<\< 1. However, I(T) is nearly equal to 1 up to T/TS
=1, if h is between 0.60 and 0.71. Thus, to get a flat response nearly
up to the free period of the seismometer, it is kept somewhat
underdamped with a value of h between 0.60 and 0.71, with most favored
value as 0.707 (= 1/√2).

### Optomechanical seismograph: Wood-Anderson seismograph

In order to record small ground motions, seismographs are designed to
magnify the ground motion before it is actually recorded.
*Magnification* of a seismograph gives the factor by which the ground
motion is magnified. We have seen above that with h = 0.7 the maximum
magnification is only 1. However, we can achieve higher magnification by
employing optical methods. In an optomechanical seismograph, a ray of
light is made to fall on a mirror fixed to the mass of the seismometer
and the reflected light is recorded at a distance on a photographic
paper. An example of one of the earliest seismographs of this type is
Milne-Shaw seismograph, which records the horizontal component of ground
motion. In this seismograph, static magnification (Vs) = 250 to 300, h
=1 and Ts =10 to 12 sec. Actual magnification of a seismograph is the
product of dynamic magnification (I) and the static magnification (Vs).
Another important example of this type of seismograph, which is widely
in use, is *Wood-Anderson seismograph*. This seismograph also records
horizontal component of the ground motion and was developed in 1920s by
two scientists, Wood and Anderson. Here the pendulum consists of a
cylindrical mass (generally made of copper), which is attached to a
vertical suspension wire^5,6^. During ground motion, the cylinder
rotates around the axis of the wire and a mirror attached to the
cylinder reflects a light beam on the photographic paper wrapped on a
recorder drum. A horseshoe magnet surrounding the copper cylinder acts
as a damping (eddy current) device. For a standard Wood-Anderson
seismograph, TS = 0.8 sec, h = 0.8 and Vs = 2800. However, Wood Anderson
seismographs with Vs = 1000 are also in use.

### Electromagnetic seismometer

The initial part of this seismometer is the same as an inertial pendulum
seismometer. In addition, here a coil is attached to the mass (M), which
is kept in a magnetic field as shown in Figure 2. As the ground moves,
the coil attached to the mass also moves in the magnetic field and
generates a voltage across the coil terminals, which is proportional to
the velocity of the mass of the seismometer. The constant of
proportionality is referred to as the electrodynamic constant, G
\[mv/(mm/sec)\] of the seismometer. The output voltage of the coil
terminals gives the measure of the ground motion. An external resistance
provides the required electromagnetic damping of the seismometer.

### Electromagnetic seismograph

Here the output of an electromagnetic seismometer is connected to a
sensitive mirror galvanometer. The mirror reflects a light beam towards
a photographic paper on a recorder. In an electromagnetic seismograph,
the seismometer and galvanometer are connected by a combination of
resistances to give proper damping to the seismometer and the
galvanometer, as well as to have suitable magnification^7,8,4^.
Electromagnetic seismographs offer much higher magnifications in
comparison to optomechanical seismographs.

### Broadband seismometer

During an earthquake, the ground vibrates mostly in the period range of
0.02 sec to 1000 sec depending on how far the earthquake is from the
recording station and how big it is. To increase the frequency range of
recording, it is necessary to increase the free period of the
seismometer. But increase of free period of a pendulum seismometer
causes instability and nonlinearity. Today, *broadband seismometers*
with electronic feedback mechanism can provide increased period without
compromising stability and linearity*.* In a pendulum type seismometer,
relative motion between the frame and the mass produces the signal. The
force on the mass due to ground motion is equal to the product of the
mass of the seismometer with the sum of acceleration of the frame and
the relative acceleration between the frame and mass. However, if a
force is applied by the frame to the swinging mass in such a way that
the relative displacement (and consequently, the relative velocity as
well as acceleration) between frame and mass is reduced to zero, the
applied force would then be a direct measure of the acceleration of the
mass. This can be done in principle by detecting the relative
displacement between the frame and the mass, generating a current signal
corresponding to it, and feeding the current back to the coil moving
with the mass and the magnet fixed on the frame. In such a system, the
mass-spring arrangement becomes much less critical so far as the
response characteristic is concerned. However, the feedback amplifier
has to be of wide bandwidth and high dc gain. By making the gain of the
feedback loop dependent on frequency, a very wide variety of response
characteristics can be obtained. For example, the STS2 seismometer from
Streckeisen SG, has only a 300 gm mass. It uses a capacitive
displacement transducer, and a feedback coil with a force constant of 50
Newton per Ampere, which is fed from displacement transducer via a
network. The response of the seismometer is same as that of a pendulum
seismometer with a free period of 120 sec and electrodynamic constant
(G) of 1500 mv/(mm/sec).

### Visible recording seismograph or modern analogue seismograph

The conventional photographic recording is getting obsolete because of
high recurring costs involved in photographic charts. Further, it is
sometimes necessary to observe the recording continuously. In a visible
seismograph, the output of electromagnetic seismometer is fed to an
amplifier; the amplified voltage is fed to a galvanometer attached to a
pen for recording on a paper. Visible recording is usually made through
(a) ink recording on a plain paper, or (b) scratching on a smoked paper/
heat sensitive paper. In the heat sensitive recording mode, the pen
remains hot and it removes scratching the white chemical cover of the
recording paper to bring out the black background.

Figure 3 explains the steps in the measurement of the ground velocity on
a recorder. From Figure 3, it may be seen that the ratio of trace
amplitude to the ground velocity, may be expressed as,

\[B G S K sin(ωt+b)\] / \[A sin(ωt)\]

= I G S K \[sin(ωt+b) / sin(ωt)\] (3)

where, I = B/A (equation 1). Here S and K are the gain of amplifier and
recorder sensitivity respectively. Thus,

*velocity magnification* = I G S K (4)

where I and S are functions of angular frequency ω. It may be noted
that, ω = 2πf = 2π / T, where f is linear frequency in Hz and T is
period in sec. S(ω) is normally constant from a very low frequency to
about 50 Hz. Since G and K are constants, G S K = C, is nearly a
constant. Thus from (4),

Velocity magnification = C I(ω)

which shows that velocity magnification curve is nearly parallel to
magnification curve I(ω) of inertial pendulum seismometer over a wide
band; the expression for I(ω) can be obtained from (1) by changing T =
2π/ω and TS = 2π/ωS. However, in seismology we use period more often
than frequency and we may also write

Velocity magnification = C I(T) (5)

We can obtain displacement magnification by multiplying the velocity
magnification with ω or 2π/T. We note that at low periods, displacement
magnification rapidly increases as the period decreases. However, at low
periods, background noise, mostly arising from cultural sources also
increases rapidly as period decreases, due to which we need to decrease
magnification at low periods. For this reason, we use a low pass filter
in amplifier, which decreases the gain below a desired period.

The response of a seismograph is described by its dynamic range and
bandwidth. *Dynamic range* is expressed as,

Dynamic range (db) = 20 log10 (maximum amplitude / resolution)

For example, the MEQ800 visible recorder with smallest readable signal
amplitude of 0.2 mm and maximum pen deflection of 25 mm has a dynamic
range of 42 db.

*Corner period* (or frequency) of a seismograph is defined as the
period, at which the magnification drops by 3 db of peak value (i.e.
0.707 of peak magnification). One corner period *Tlc,* is at the lower
side of the period range and the other corner period, *Thc,* on the
higher side of it. The range (*Tlc - Thc*) is defined as the
*bandwidth.* This is the period range within which the seismograph is
most sensitive to ground motion. For an inertial pendulum seismometer,
the magnification drops exactly by 3 db at the free period of the
seismometer, when we set h = 0.707.

### Digital recording seismograph

In a digital recording seismograph, the amplifier output is fed to a
digital recorder, which records the ground motion in terms of digital
counts. Thus, a ground velocity of A sin(ωt) gives an output of B G S
sin(ωt+b) mV from the amplifier (Figure 3). The output of amplifier then
goes to a digital recorder, which converts the voltage to counts, say, L
counts / mV. Thus, it records B G S L sin(ωt+b) counts. The velocity
magnification is I G S L = C.I (T) counts/ (mm/sec). With h = 0.707,
maximum value of I is 1 and hence maximum velocity magnification is C =
GSL.

In a digital recorder, the analogue-to-digital converter (ADC), also
called as digitizer, samples the input signal at regular intervals,
defined by N samples per second (SPS). For a unique representation, any
harmonic must have at least three samples per wavelength. Thus, the
frequency defined by f~N~ =0.5N represents the harmonic with lowest
frequency and is called, the Nyquist frequency. Contamination of
computed spectra at frequencies higher than f~N~ is termed as
*aliasing*. To remove the effect of aliasing, a sharp (high order)
low-pass filter, called *anti-alias filter*, is generally set at 0.4N.

The minimum digitization step is called a *dig*it or *count*. The
smallest unit of a digital value or word is called Byte (= 8 bits). A
16-bit ADC can count values from -32768 (-215) to 32768 (215), giving a
dynamic range of 90 db. On the other hand, a 24-bit ADC, which can count
from -8388608 (-223) to 8388608 (223) gives a dynamic range of 138 db.
Higher dynamic range in digital recording, as compared to analogue
recording, gives the advantage to record the ground motions from very
small magnitude earthquakes as well as from large magnitude earthquakes
without saturation. The three parameters crucial in the design of
digital seismograph are bandwidth, dynamic range and the bit resolution
of the digitizer.

A 3-component digital seismograph recording continuously at a sampling
rate of 20 SPS per channel with an ADC resolution of 24 bit (3 bytes)
would require a storage capacity of 3 (comp.) x 20 (SPS) x 3 (byte) x 24
(hrs.) x 3600 (sec) ≈ 15.5 MB per day. Several types of large storage
devices (of the order of few gigabytes) and data compression techniques
are available to store the data so generated. The present day systems
offer recording in different streams, for example, one stream shall
record at 20 SPS in continuous mode and other stream shall record 80 or
100 SPS in trigger mode i.e. when signal amplitude exceeds a predefined
threshold value. In trigger mode, normally recording is based on the
ratio of short-term average (STA) to long-term average (LTA) of the
recorded signal. The time at which this ratio exceeds a predefined
threshold value is called *trigger time*. The recording starts a few
seconds (set by *pre-event time*) before the trigger time. The recording
continues till the signal amplitude restores to the background value,
or, till such time, set by *post-event duration,* after the STA/LTA
ratio falls below the threshold value. Some of the commonly used
combinations of seismometer - Data Acquisition systems in India are
discussed below:

***(i) STS2 -- Q680**:* This is a combination of a force balance
seismometer (STS2) and data acquisition system (Q680LVG, in short, Q680)
of Quanterra make. The seismometer has a free period of 120 sec and
electrodynamic constant G = 1500 mV/(mm/sec). The gain of Q680 (which
includes amplifier) is SL = 0.42735 x 103 counts/mV. With h = 0.707, the
seismograph has a bandwidth up to 120 sec and the peak velocity
magnification is GSL = 641.0 x 103 counts/(mm/sec).

***(ii) CMG40T -- 72A***: The broadband sensor CMG40T of GURALP make is
also a force balance seismometer and is equivalent to an inertial
seismometer with free period of 30 sec; the electrodynamic constant is
800 mV/(mm/sec). The data acquisition model RT 72A of Reftek make has a
gain of 0.52466 x 103 counts/mV. With a damping of h = 0.707, the
seismograph has a bandwidth up to 30 sec and the peak velocity
magnification is 419.0 x 103 counts/(mm/sec).

***(iii) L4C -- 72A***: In this case the data acquisition system
referred in (ii) above is connected to a short period electromagnetic
seismometer L4C or L4C3D with 1 sec free period and G = 240 mV/(mm/sec).
With damping h = 0.707, the seismograph has a bandwidth up to 1 sec and
peak velocity magnification is 125.9 x 103 counts/(mm/sec).

The Data Acquisition systems described above have 24 bit resolution and
employ a timing device that includes a crystal clock synchronized by
Global Positioning System (GPS) receiver. The magnification curves for
the above combinations are shown in Figure 4. These broadband
seismograph systems have astounding capabilities over the conventional
analogue systems and thus have revolutionized the seismic
instrumentation in India. They are capable of simultaneously recording
very long period surface waves and high frequency body waves ranging
from minimum Earth noise levels up to the strong accelerations expected
from large nearby earthquakes. They offer excellent digital data
processing options, viz., filtering, rotation of seismograms, conversion
of seismogram traces through differentiation / integration, etc.
Figure-5 depicts vertical, radial and transverse component broadband
velocity and displacement seismograms of a Koyna event recorded at
Karad.

**Strong Motion Accelerographs**

During violent ground vibrations due to large earthquakes, the
seismographs meant for recording weak motions of the ground either go
off the scale or stop functioning after recording the onset. To
effectively record such strong ground motions in near epicentral zone,
another type of instrument, called strong motion accelerograph (SMA), is
used. In the design of SMAs, we use electromagnetic sensors or force
balance sensors of very low period so as to make the output voltage
proportional to ground acceleration instead of ground velocity as in
weak motion sensors described above. In analogue SMA, recording is taken
on a 70 mm photographic film, normally without absolute time marks. In
digital SMA, recording is done in digital mode with absolute time using
GPS synchronization. Since such instruments are meant to activate only
during a strong motion, a triggering device is used for recording.
Modern digital accelerographs have provided excellent near-field data
sets for several significant earthquakes that are useful in
understanding the effects of ground shaking on structures and also to
assess the attenuation characteristics of the media.

**Seismological observatories**

IMD operates and maintains the national seismological network consisting
of 45 observatories spread over the length and breadth of the country.
Till 1970, these observatories were equipped with photographic recording
seismographs such as Milne-Shaw, Wood-Anderson and electromagnetic
seismographs. However, in 1970s many of these observatories were
upgraded with visible seismographs. A Seismic Research Observatory (SRO)
system with a broadband borehole seismometer and a digital data
recording system was installed at Central Seismological Observatory
(CSO), Shillong, in 1978. The India Meteorological Department started
operation of five stand-alone digital seismographs during early
nineties. The seismological observatory at National Geophysical Research
Institute, Hyderabad equipped with WWSSN type analogue seismograph
system was upgraded during early nineties with a very broadband
seismograph in collaboration with GEOSCOPE.

During 1996-97, ten seismological observatories under the national
network in the Peninsular India had been upgraded to the standards of
Global Seismic Network (GSN) by deploying STS2-Q680 seismograph system
at each of these observatories. The data acquisition system Q680,
records 80 SPS data in trigger mode and 20 SPS data in continuous mode.
It has also got provision for recording data of any two streams on an
analogue drum recorder for continuous display. Presently one
short-period vertical component and one long period vertical component
simulated streams are being recorded on analogue drum recorders. A
digital recorder (Atlus K2 of Kinematrics make) with an internal
triaxial force balance accelerometer (FBA) serves as a secondary data
acquisition system at each of these observatories. The Q680 also records
the output of FBA. In these observatories, the Q680 is connected locally
to a computer where the data can be downloaded and analyzed. The Q680 at
each of these 10 observatories are also accessed from the Central
Receiving Station at New Delhi, where the data is downloaded in
near-real time using telephone modem. During 1999-2000, fourteen more
existing observatories of the national network have been upgraded with
another type of broadband digital seismograph (CMG40T-72A). Although
recording at these observatories is possible in different data streams
viz., continuous, ratio and amplitude trigger, etc. presently recording
is being done in continuous mode at 20 SPS. Trigger times are also
stored in a separate stream as an event log file.

In addition to IMD, a number of R & D institutions, Universities and
state Governments are also operating seismological observatories in
different parts of the country (Table 1 and Figure 6). These
observatories, adding up to 212, may be classified according to the
affiliation of organizations maintaining them. The three broad
categories of organizations operating these stations are R&D
institutions (106 stations), River valley project authorities (79
stations) and Universities (27 stations). These include the radio
telemetered seismic networks operated in Tezpur and Nainital by NGRI and
Kumaun University respectively. Another landmark development in the new
millennium in seismic instrumentation is a 16-element VSAT based seismic
telemetry system being deployed in and around Delhi by IMD.

**Microearthquake surveys**

Monitoring of microearthquake activity in a small area has many
applications, such as, evaluation of seismotectonics, study of
aftershocks and swarm type activities etc. Till mid-seventies, such
surveys were conducted by employing bulky electromagnetic seismographs
requiring photographic recording with dark room facility. However, since
late seventies these have been replaced by portable visible seismographs
with smoke paper recording. More recently, digital portable seismographs
such as L4C3D-72A with GPS time synchronization are being used in such
surveys to generate very high-resolution data. Such digital portable
seismographs were operated for monitoring (a) swarm type activity in
Pandhana Tehsil of Khandwa in Madhya Pradesh during 1998-99 and (b)
aftershock activity of Chamoli earthquake of 1999.

**Strong motion networks/arrays**

The spatial variability of earthquake intensities near the source play a
key role in the prediction of strong ground motion and response of
engineered structures. To achieve this objective, specially designed
strong motion arrays/ networks are operated in highly seismic areas.
Under a project sponsored by Department of Science and Technology on
Himalayan seismicity, the University of Roorkee has been operating three
Strong Motion arrays, one each in Northeast India (45 stations),
Pithoragarh region (50 stations) and Garhwal Himalaya (45 stations).
These instruments record 3-component ground accelerations in digital
form along with actual time. These strong motion arrays have yielded
very valuable data sets for several important earthquakes including the
Chamoli earthquake of March 29, 1999. The data has enabled estimation of
attenuation characteristics of the region under consideration and
response spectra of structures, which have significant application in
engineering design of critical structures. An Indian National Strong
Motion Instrumentation Network (INSMIN) with a set of strong motion
accelerographs and Structural Response Recorders (SRR) is also being
maintained by the Department of Earthquake Engineering, University of
Roorkee. The SRRs are meant for directly recording the response of
structures (buildings etc.) in the event of an earthquake.

**Design and development of seismic instruments**

IMD has been engaged in the manufacture of Wood-Anderson seismometers
and Electromagnetic seismometers together with their recording systems
since 1960's. The galvanometers required for the electromagnetic
seismographs were, however, imported. These systems are still in
production and use at various observatories of the national network of
IMD. Conventional type time marking devices viz., pendulum and crystal
clocks were also developed by IMD during fifties and seventies
respectively. During the last two decades, Central Scientific
Instruments Organization (CSIO), Chandigarh made serious efforts towards
indigenous development and manufacture of visible analogue recorders,
digital recorders and time marking systems using crystal clock. Analogue
visible recorders and time marking systems of CSIO are being used at
some of the IMD observatories. A 16-bit digital data acquisition system
has also been developed by CSIO. An analogue radio telemetered seismic
network developed by BARC in 1980s was in operation in and around Bhatsa
in Maharashtra till mid 1990s. Structural Response Recorders and
analogue SMAs are also being manufactured at the Department of
Earthquake Engineering, University of Roorkee since mid 1970s**.**

**seismic data interpretation: RECENT ADVANCES**

With the availability of digital waveform data, several standard
software packages have been developed in recent years for routine
analysis and processing of the data. The International Association for
Seismology and Physics of the Earth's Interior (IASPEI) has developed a
library of standard programs for various kinds of seismological data
processing and analysis, which is used extensively by the seismological
community; the most important of these are PITSA and SEISGRAM. The
scientists at University of Bergen, Norway have also developed a SEISmic
ANalysis software, called 'SEISAN', suitable for operation in single-
and multi-user environments. A seismic network automation software,
called SEISNET, was also developed by this group for remote access of
waveform data. The software effectively combines various types of
seismic data sources into one virtual seismic network to bring all the
data to one central site. SEISNET can only be used in combination with
SEISAN, since SEISAN programs and the database structure are used by
SEISNET. A general-purpose interactive program, called Seismic Analysis
Code (SAC), designed for the study of time series data by University of
California, is also available. The software works in UNIX environment
and has extensive graphics capabilities. Since it would not be within
the scope of this paper, to discuss all the applications of seismic data
analysis and interpretation, an attempt has been made to give a brief
description of various techniques /software widely in use for estimation
of earthquake source parameters and the crust and upper mantle
structure.

**Analysis of seismograms**

Identification of various phases in a seismogram is an essential and
foremost task of any seismic data interpretation. The actual arrival
times and amplitudes of various phases help respectively to estimate the
hypocentral location and magnitude of a seismic event. Identification of
various phases in a seismogram used to be a very tedious task in the
pre-digital era. It is now possible to simulate the arrival times of
various phases employing any travel-time model and display along with
the waveform. The SEISGRAM package of IASPEI software has the facility
to display Jeffreys-Bullen predicted phase arrival times for a given
origin time, epicentral distance and focal depth. Similarly, the SEISAN
software package has also got the facility to simulate the IASPEI91
predicted phase arrival times for any located event. The operator can
also interactively select various phases on the seismogram, which can be
combined and stored as a single event file (S-file), to locate the event
subsequently. These S-files contain the phase data and the names of the
corresponding waveform files. The data base part of SEISAN consists of
two directories REA and WAV. The REA directory and its sub-directories
contain phase readings and derived source information of all located
events while all the waveform data is normally stored in WAV directory.

**Determination of hypocentral parameters**

The conventional method of determining the hypocentral parameters viz.,
origin time, epicentre and focal depth, utilizes the times of arrival of
various phases recorded at different stations. Over the years, this
method of locating hypocentres, has been replaced by computer based
analytical methods. A number of techniques and algorithms have been
developed for quick and accurate estimation of hypocentral parameters.
The procedure essentially, involves assuming a trial solution and then
computing the theoretical travel times with respect to the assumed
solution. By applying an appropriate method, for instance, least-squares
method, the arrival-time residuals are minimized to get a new set of
location parameters. The procedure is repeated through an iterative
process till an acceptable error criterion is met. The final adjusted
parameters are then accepted as the best possible estimate of the source
location. These algorithms also take care of the possible errors in the
data by assigning suitable weights and judging the quality of results in
the form of statistical estimates of errors. Although a number of source
location algorithms are available in literature, the HYPO71 program
developed by Lee and Lahr^9^ is the most commonly used program. Few
other programs, such as HYPOLAYERS, HYPOINVERSE and HYPOELLIPSE are also
used by some researcher workers. However, these programs are useful for
locating earthquakes recorded by near observatories.

The HYPOCENTRE (Lienert and Havskov^10^) location package of SEISAN
software has capabilities to locate earthquakes locally, regionally and
globally. It uses the travel-time tables of IASPEI91. However, if
specified, it can adopt local crustal structure to calculate travel
times for those stations which are within a given hypocentral distance
range. Depending upon the epicentral distance, it is possible to
estimate various kinds of magnitudes like mb, Ms, M~L~ and Mc of an
earthquake making use of the wave amplitudes and periods and coda
duration. Given the instrumental response, it is also possible to
simulate seismograms of Wood-Anderson and few other types, which can in
turn be used to estimate corresponding magnitudes like M~L~, mb and Ms.
The SEISAN software package has got the facility to plot the initial
motions (polarity of first onset) on an equal area projection map to
work out plausible fault plane solutions.

Accurate estimation of focal depth very often poses problems for want of
depth phases from nearby stations. The uncertainty in focal depth
estimation increases as the epicentral distance from the nearest
seismograph station increases. However, with the help of digital
broadband records, it is possible to decipher the depth phases, normally
through filtering, which are otherwise not very clear in the narrow band
seismograms. The focal depth of the Jabalpur earthquake of 1997 could be
accurately determined, using the sPn phase recorded in the broadband
seismograms with a low pass filter at 2 Hz and waveform inversion
techniques^11,12^. The GPS time synchronization available with the
modern digital systems facilitate improvement in the time resolution
necessary for accurate estimation of hypocentral parameters.

To monitor earthquake activity in operational mode, the Central
Receiving Station (CRS), New Delhi, of IMD, pools every half an hour
through dial-up modem using SEISNET, the information pertaining to the
detected triggers from each of the ten broadband stations equipped with
STS2-Q680 seismographs. On identification of a trigger attributable to
an event, the operator at CRS decides to download the waveform data of
desired stations using dialup modem, and then processes the data for
hypocentral location. The hypocentral parameters thus evaluated in
operational mode are improved later by incorporating data from other
observatories and a monthly seismological bulletin is prepared in the
standard Nordic format using the SEISAN software package. Efforts are
being made to connect all the broadband stations of IMD to CRS through
VSAT communication facilities.

**Estimation of crust and upper mantle structure**

As is well known, most of the present knowledge about the otherwise
inaccessible deep interior of the Earth regarding its composition and
structure is based on seismic observations. There is an intense effort
to determine the internal structure with very high precision, so that
the composition and dynamic processes of the Earth's interior can be
better understood. It is possible to delineate, to a first
approximation, most of the one- or two- dimensional features of the
Earth's crust using the regional broadband network data. However, a much
more dense and closely spaced seismic network would be required to model
the three-dimensional complexities in the curst.

Dube et al. (ref. 13) used body wave travel-time data to derive crust
and upper mantle structure On the basis of body wave travel time data of
shallow earthquakes in India, Krishna et al (ref.14) found that the
P-wave travel time data reveal significant variations, while S-wave data
show comparatively better agreement with respect to the Jeffreys-Bullen
tables. Both P and S wave velocities in the sub-crustal region of the
Indian sub-continent are relatively high as compared to other regions of
the Earth.

Seismic observations through Deep Seismic Sounding (DSS) by controlled
explosive source constitute the most reliable approach for exploring the
structure of the crust and upper mantle. The source location and the
time information can be controlled accurately so that the derived
structure would be very accurate. For a review of the work done using
explosion data, the reader may refer Kaila and Krishna^15^. These
studies have helped not only in deriving the crustal structure but also
in addressing specific geological problems like delineation of
subterranean sedimentary structure and basement configuration, formation
and evolution of sedimentary basins and tectonic framework depicting
fault controlled crustal blocks. Seismic body waves recorded by
conventional short period seismographs on fast-run recorders (1 cm = 1
sec.) in various DSS field surveys were also extensively used by IMD to
obtain average crustal structure^16,17,18^. These results have helped in
improving the hypocentral locations of local earthquakes.

The dispersion of seismic surface waves has also been made use of in
deriving the crust and upper mantle structure by various investigators.
Bhattacharya^19^ gave a comprehensive review of the crust and upper
mantle models derived through inversion of the observed dispersion data
for the Himalaya, Indo-Gangetic plain and Peninsular shield regions.
Singh^20^ estimated the crustal structure of Bay of Bengal Fan and
Indian Ocean, making use of the surface wave dispersion of fundamental
and higher modes. Singh et.al. (ref. 12) derived the crust and upper
mantle structure of Peninsular shield region making use of the broadband
data of Jabalpur earthquake of 1997.

Another important development employing high-speed computation is the
three-dimensional imaging of the Earth's interior, called seismic
tomography. The technique essentially involves reconstruction of an
image of internal structure in terms of, say, time residuals of P- and
S- waves. Rai et al. (ref. 21) evaluated the 3-D velocity structure of
the south Indian shield. Rai et al. (ref. 22) made use of the data
generated by a digital array operated in Koyna region to map the 3-D
velocity structure and study the seismicity in detail. On the basis of
preliminary analysis of the data, they have inferred a high velocity
zone beneath the region of observed seismicity. Using the
well-constrained locations of the local events, the seismicity patterns
were also accurately delineated. Gupta et al. (ref. 23) also used this
data for making coda Q~C~ estimates for the region.

**Estimation of detailed source parameters**

The physical model for the tectonic earthquake source is usually
conceived as release of strain energy due to rupture along a fault plane
in the rocks. The actual faulting in an earthquake is a very complex
phenomenon. Nevertheless, the character of faulting in an earthquake can
be inferred from observed distributions of the directions of the first
onsets in waves arriving at the recording stations supplemented by the
S-wave polarization angles. From a well-determined polarity pattern of
first P-wave motions, it is possible to locate two nodal planes, one of
which represents the fault plane. Fault identification is normally made
from field evidence or distribution of aftershocks. Computer algorithms
are available to generate a set of fault plane solutions within a given
error criterion in terms of inconsistent polarities. The fault plane
solution so obtained describes the faulting pattern in terms of strike
and dip of the fault plane, slip direction, etc. For significant
earthquakes it is now a routine practice to work out the fault plane
solutions.

Based on the equivalence theorem, the displacement field produced by the
dislocation on a plane element in an elastic body equals that produced
by a double couple applied at the center of the source. Thus, the
strength of an earthquake source can be represented by a seismic moment.
For a simple fault source, it can be expressed as the product of average
slip, the fault area and the modulus of rigidity of the medium. Using
digital data, it is also possible to estimate seismic moment from the
far field displacement spectra. The spectra allow estimation of other
important parameters viz., corner frequency, source radius, stress drop
and the moment magnitude. Using the SEISAN and IASPEI software, these
source parameters are being obtained on routine basis.

In case of significant earthquakes, synthetic seismograms of body and
surface waves are computed in an attempt to invert for the source
characteristics. The seismic source is treated as a moving dislocation
along a fault and seismograms are calculated from a Green's functions
representation of the displacements. The process involves obtaining a
best fit between the synthetics and observed seismograms by varying the
parameters of source. Singh et al (ref. 24) have computed synthetic
seismograms to perform a moment-tensor inversion of band-pass filtered
(0.05-0.02 Hz) displacement seismograms of the 1997 Jabalpur earthquake.
The inversion yields reliable focal mechanism and seismic moment,
although the depth resolution is poor. Mandal et al. (ref. 25) and
Rastogi et al. (ref. 26) studied in detail the Koyna seismicity making
use of the digital data generated by a local network. The fault plane
solutions and focal depths were determined for several Koyna earthquakes
using CMT inversions.

Estimation of attenuation relations to predict ground motions during
future earthquakes forms an important element in the seismic hazard
assessment of any region. Similarly, the near source recordings play a
very important role not only in detailed estimation of source parameters
but also in the evaluation of attenuation characteristics. The excellent
recordings produced by the modern digital accelerographs (velocity and
displacement traces can be obtained by integrating the accelerogram)
offer a unique opportunity in this direction. Singh et al (ref. 12) made
spectral analysis of Jabalpur earthquake of 1997 to estimate a frequency
dependent Q of L~g~ waves. Making use of these estimates and assuming a
ω^2^-source model, they predicted peak acceleration and velocity as a
function of distance, magnitude and stress parameters. These predictions
are preliminary and need revalidation with more strong motion data in
future.

**CONCLUSIONS**

Keeping pace with the technological developments, the seismic
instrumentation and data interpretation in India have grown by leaps and
bounds over the years. Under a World Bank assisted project for seismic
instrumentation upgradation in the Peninsular shield region, 24
seismological observatories under the national network of IMD have been
upgraded with state-of-the-art digital seismograph systems having broad
frequency response, large dynamic range and accurate time keeping using
GPS synchronization. The data generated at some of these observatories
of IMD is presently available in near real time at CRS, New Delhi of
IMD. Also, a number of new observatories have been set up with similar
systems by various State and Central Government agencies. These digital
broadband systems have generated very useful, high-resolution data sets
for several significant earthquakes including Jabalpur \[1997\] and
Chamoli \[1999\]. This has greatly improved not only the hypocentral
location capabilities in operational mode but also the estimation of
detailed source parameters, crust and upper mantle structure, etc. in
research mode. A similar upgradation program for the extra-Peninsular
shield region together with matching communication facilities will be
useful for a near real time seismic monitoring covering the whole
country.

**Acknowledgments**:

The authors express their sincere gratitude to the Director General of
Meteorology for permission to publish this paper. The authors also
express their gratitude to the anonymous reviewers for their very useful
and constructive suggestions.

**References**

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2.  Richter, C.F., Elementary Seismology, W.H. Freeman & Co., San
    Francisco,1958.

3.  Bath, M. (1979). Introduction to Seismology, Birkhauser

4.  Aki, K. and Richards, P.G (1980). Quantitative Seismology: Theory
    and Methods, Vol.I, W.H. Freeman & Co., San Francisco.

5.  Tandon, A.N., Ind. J. Met. Geophys., 1951, **2**, 203-212.

6.  Agrawal, P.N (1991). Engineering Seismology, Oxford & IBH Publ. Co.,
    New Delhi.

7.  Chakraborty, S.K., Bull. Seism. Soc. Am., 1949, **39**, 205-218.

8.  Hagiwara, T.,Earthq. Res. Inst. Bull., Tokyo Univ., 1958, **36**,
    139-164.

9.  Lee,W.H.K., and Lahr, J.C.,. HYPO-71: A computer program for
    determining hypocenter, U.S.Geol. Surv. Open-File-rep., 1975,
    revised edition.

10. Lienert, B.R and Havskov, J., Seismo. Res. Letters, 1995, **66**,
    No.5, 26-36.

11. Bhattacharya, S.N., Ghose, A.K, Suresh, G, Baidya, P.R and Saxena,
    R.C., Current Science, 1977, **73**, 855-863.

12. Singh, S.K, Dattatrayam, R.S, Shapiro, N.M, Mandal, P, Pacheco, J.F
    and Midha, R.K., Bull. Seism. Soc. Amer., 1999, **89**, 1631-1641.

13. Dube, R.K., Bhayana, J.C and Chaudhury, H.M., Crustal structure of
    the Peninsular India, Pure Appl. Geophys., 1973, **109**, 1718-1727.

14. Krishna, V.G., Kaila, K.L and Reddy, P.R., Body wave travel time
    studies and inferences, Current Science, 1992, **62**, 163-176.

15. Kaila, K.L and Krishna, V.G., Current Science, 1992, **62**,
    117-154.

16. Srivastava, H.N., Verma, R.K and Verma, G.S., Mausam, 1983, **34**,
    267-274.

17. Srivastava, H.N., Verma, R.K, Verma, G.S and Chaudhury, H.M.,
    Tectonophysics, **110**, 1984, 61-72.

18. Mittal, V.K., Bhattacharya, S.N and Srivastava, H.N., Mausam, 1990,
    **41**, 59-64.

19. Bhattacharya, S.N., Current Science, 1992, **62**, 94-100.

20. Singh, D.D., Current Science, 1992, **62**, 155-162.

21. Rai, S.S, Ramesh, D.S, Srinagesh, D, Suryaprakasam, K, Mohan, G,
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Table 1

## Distribution of seismological observatories in India under different organisations

Organisation Abbreviation No. of stations

India Meteorological Department IMD 57

Wadia Institute of Himalayan Geology WIHG 11

National Geophysical Research Institute NGRI 20

Regional Research Laboratory, Jorhat RRLJ 10

Bhabha Atomic Research Centre BARC 2

Indian Institute of Geomagnetism IIG 2

Geological Survey of India GSI 1

National Institute of Rock Mechanics NIRM 1

Central Scientific Instruments Organisation CSIO 1

Centre for Earth Science Studies CESS 1

**Subtotal 106**

Maharashtra Engineering Research Institute MERI 31

Gujarat Engineering Research Institute GERI 17

Sardar Sarovar Narmada Nigam Ltd. SSNN 9

Narmada Valley Development Authority NVDA 10

Kerala State Electricity Board KSEB 12

**Subtotal 79**

Guru Nanak Deb University, Amritsar GNB Univ. 3

Delhi University D. Univ. 3

University of Roorkee UOR 9

Osmania University O. Univ. 1

Manipur University M. Univ. 4

Indian School of Mines, Dhanbad ISM 1

Kurukshetra University Kur. Univ. 1

Kumaun University Kum.Univ. 5

**Subtotal 27**

**Grand Total 212**

**Captions to Figures:**

> Figure 1. Dynamic Magnification I(T)=B/A and phase change b as a
> function of T/TS for a given damping constant h; here TS is the
> natural period of the seismometer and T is the period of the ground
> motion.

Figure 2. Principle of an Electromagnetic seismometer.

> Figure 3. Flow chart for a visible seismograph: Actual ground motion
> through recorded signal trace.

Figure 4. Period response of ground velocity for digital seismographs:

\(i\) STS2-Q680, (ii) CMG40T-72A and (iii) L4C-72A.

> Figure 5. Broadband seismograms of an earthquake in Koyna-Warna region
> of Maharashtra recorded at seismological observatory, Karad on April
> 6, 2000. Top three traces are vertical, radial and transverse
> component waveforms of ground velocity and the bottom three traces are
> vertical, radial and transverse components of displacement waveforms
> of the same earthquake obtained through integration of respective
> velocity waveforms given at the top. Length of each waveform is 30
> sec.
>
> Figure 6. Map showing seismological observatories in India operated by
> different organizations (See also Table-1).