Seismometry
• Seismic networks
• Instruments
Mainly based on:
Shearer, P.M., Introduction to seismology, Cambridge University Press, 1999.
Lay, T., and T.C. Wallace, Modern global seismology, Academic Press, 1995.
Seismic networks: Objectives
The three main purposes are:
• Seismic alarm (immediate response by civil defense, seismic risk
assessments, hazard maps and building codes)
• Seismic monitoring (incl. monitoring in volcanic regions, nuclear
explosions – international nuclear test ban treaty)
• Research – Earth interior and earthquake source
Networks are described in terms of:
• Scope of investigations
• Spatial resolution
• Quality of data in terms of frequency content and dynamic
range
Seismic networks: Frequency content and dynamic range
The noise amplitude is of the order of nanometers, whereas the signal amplitude is up to a
few meters. Thus, the full dynamic range spans
10 orders of magnitudes.
Seismic instruments: The sensors
The 2 main sensor types are:
• Seismometers – weak
motion
• Accelerometers – strong
motion
Seismometers are very
sensitive to small and distant
events and are thus too
sensitive for strong-motion
signals.
Waveforms of the April 4,
2010, Mw 7.2 El MayorCucapah earthquake
recorded at P494.
Co-located seismometer, accelerometer and GPS
Seismic instruments: The sensors
Seismic instruments: The sensors
Today's weak-motion sensors are roughly divided into three categories:
• The short-period (SP) seismometers measure signals from approximately 0.1 to 100
Hz, with a corner frequency at 1 Hz. They have a flat response to ground velocity for
frequencies greater than this corner frequency.
• The broadband sensors (BB) have a flat response to ground velocity from
approximately 0.01 to 50 Hz.
• The very broadband seismometers (VBB) measure frequencies from below 0.001 Hz
to approximately 10 Hz. They are able to resolve Earth's tides.
Seismic instruments: The standard inertial seismometer
Since the measurements are done in a
moving reference frame (the earth’s
surface), almost all seismic sensors are
based on the inertia of a suspended
mass.
The swinging system will have a
resonance frequency:
1
f0 =
2p
k
m
Seismic instruments: The standard inertial seismometer
It is convenient to define a
resonant angular frequency and a
dumping parameter as follows:
These substitutions give:
This equation shows that the Earth acceleration may be recovered by
measuring the displacement of the mass and its time derivatives.
Seismic instruments: The standard inertial seismometer
The stress balance equation of the
inertial seismometer may be
expressed in the frequency domain
by considering harmonic Earth
displacement function of the form:
where,
frequency.
, is the angular
Similarly, the displacement
response of the seismometer mass
can be expressed as:
And we have:
Seismic instruments: The standard inertial seismometer
Substituting these into the stress
balance equation gives:
Or:
Were Z(w) is the response function
of the sensor.
The response function is complex,
and may be expressed in polar form
as:
where the amplitude the phase lag
are real numbers.
Seismic instruments: The standard inertial seismometer
We obtained:
and:
• When h=1, the system is said
to be critically damped.
Seismometers generally
perform optimally at values
of damping close to critical.
• A polarity reversal at high
frequencies.
Seismic instruments: The standard inertial seismometer
We obtained:
and:
• The amplitude response falls
off at frequencies below the
resonant frequency and the
1-Hz sensor has little
sensitivity at periods longer
than 5 s.
• For small damping, a
resonant peak occurs in the
response spectrum near the
seismometer natural
resonance.
Seismic instruments: The standard inertial seismometer
We obtained:
and:
• When the damping increases
above 1, the sensitivity
decreases.
Seismic instruments: Extending the filtering response
Seismic instruments: Extending the filtering response
The frequency response function
(11.12) relates the Earth displacement,
u, to the sensor mass displacement, z.
In the case of a seismometer that
measures mass velocity, z ̇, such as that
shown in Figure 11.1, the response
function describes the sensor response
to ground velocity, u ̇. In general,
seismometers may measure the
displacement, velocity, or acceleration
of the sensor mass, and we may be
interested in recovering the
displacement, velocity, or acceleration
of the ground. It is important to be
aware of which combination is involved.
Each time derivative introduces a factor
of −iω in the frequency domain. Thus,
all other things being equal, velocity
and (especially) acceleration will be
enriched in high frequencies relative to
displacement.
Seismic instruments: Force Balance Principle (FBA)
Today, purely mechanical sensors are only constructed to have resonance frequencies
down to about 1.0 Hz (short period sensors), while sensors that can measure lower
frequencies are based on the Force Balance (FBA) principle of measuring acceleration
directly.
• A displacement transducer sends a
current through a feedback coil
through a resistor R in a negative
feedback loop.
• Feedback coil, which can exert a force
equal and opposite to the inertia force
due to the largest acceleration we
want to measure.
• The polarity of the current is such that
it opposes any motion of the mass,
and it will try to prevent the mass from
moving at all with respect to the
frame.
Seismic instruments: Velocity response functions
•
•
•
The original IDA (International Deployment of
Accelerometers) network was the first digital global
seismic net- work, it uses gravimeters designed to record
Earth’s normal modes at very long periods and recorded
one sample every 10 s.
Data from the Global Digital Seismo- graph Network
(GDSN) began to become available in the late 1970s. The
GDSN long-period channel recorded at one sample per
second; the GDSN short-period channel recorded at 20
samples per second. The GDSN response functions were
designed to avoid the microseism noise peak at 5 to 8 s
period (see Section 11.2).
Broadband instruments began to be widely deployed in
the late 1980s and early 1990s; the broadband stations in
the IRIS and GEOSCOPE networks have very wide
frequency responses.
Seismic instruments: Impulse response
• Instrument response can also be
described by the impulse response
function, which shows the seismograph
output in the time domain from a deltafunction input.
• In general, the impulse response function
will more closely approximate a delta
function as the instrument becomes
more broadband.
Noise matters: Noisy and quiet conditions
Noise matters: Surface versus boreholes
Noise matters: Long period noise
Noise matters: Distance from coast
Examples for coastline geometries that provide suitable interference conditions
for the generation of secondary microseisms (reproduced from Journal of
Seismology, 2, 1, 1998, “Ocean-generated microseismic noise located with the
Gräfenberg array”; Friedrich, Krüger & Klinge, p. 63, Fig. 13; Ó Kluwer Academic
Publishers, with permission of Kluwer Academic
Publishers).
Japan’s seismic network (as of 2003)
Hi-net: 696 high-sensitivity
seismograph network
F-net: 71 broadband seismograph
K-net: 1034 strong motion
seismographs installed at ground
surface
KiK-net: 659 stations with an
uphole/downhole pair of strongmotion seismographs is called KiKnet
Hi sensitivity stations in Japan
Strong motion stations in Japan
US Network
MEMS Seismology