tectonics_lecture-5
DESCRIPTION
Seismology Group IIT Kharagpur Basic Concepts: Generates Seismic Waves Propagate away from source and samples the Earth structure Earthquakes (Passive Source) Recorded ground motion is SEISMOGRAM Speed of waves in the medium Travel Time Origin Time Arrival Time Location & Nature of Source Seismology Group IIT Kharagpur &TRANSCRIPT
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Seismology Group IIT Kharagpur
Seismology Measuring the interior
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IntroductionBasic Concepts:
Generates Seismic Waves
Propagate away from source and samples the Earth structure
Recorded ground motion is SEISMOGRAM
Earthquakes (Passive Source)
Free Surface ground motions caused by these propagating waves recorded at surface detectors(SEISMOMETERS)
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SEISMOGRAM = Source * medium of propagation * Receiver elastic waves (Earth Filter) response(Observation)
Location & Nature of SourceOrigin Time
Arrival Time&
Travel Time
Speed of waves in the medium
Phase &
Amplitude
Seismology Group IIT Kharagpur
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SEISMOGRAM = Source * medium of propagation * Receiver elastic waves (Earth Filter) response
Our goal is to understand the way in which the features of the observed seismogram are related to the properties of the source and structure of the Earth
• Little of the Earth is accessible by direct observations Deepest Well ~ 13Kms deep (Russia)
• Seismology is a primary (powerful) tool for exploring the Earth’s interior.
• Provides information on Variations of Velocity
Density
Attenuation with depth in the Earth.
• Existence of the Crust, Mantle, Liquid Outer Core and Solid Inner Core are inferred from the variations in velocity with depth,
• The idea of variations in chemical composition within the Earth is also based on Seismological Data.
(Observation)
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Compositional Structure of the Earth
Crust (rich in silica)
Mantle
John Milne Lord Rayleigh
Lord Rutherford
Average 35 km (continent) 7-8 km (ocean)
Inner Core
Derived from the mantle over the aeons by series of melting & reworking
Outer Core
Andrya Mohorovicic (Moho) 1909
Mantel or ‘coat’ by Emil Wiechert(in German)
-Discovered byR.D. Oldham 1906. - Correctly delineated by Beno Gutenberg in 1912from earthquake data.
-Fluid Outer Core (1926) From works on tides bySir Harold Jeffreys.
1929 Buller earthquake south Island of New Zealand.Inge Lehmann 1936 – solid Inner Core within the liquid Outer Core.
DEEP STRUCTURE
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Mechanical Layering of the Earth
D’’ Layer
Continental crust
Oceanic crust
DEEP STRUCTURE
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Combined View(A comparison)
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Distribution of seismic velocities and density within the Earth
No S-wave
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SHALLOW STRUCTURE
Seismic Reflection profiling
• Detail crustal images that reveal information about location of economic resources (oil and minerals)
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Fault Plane solutions giving fault plane geometry using P wave polarity
Radiation Pattern
Seismology is the primary method for studies of Earthquakes
e.g. nature of Faulting determined from resulting seismograms
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Earthquake Distribution and Tectonic PlateMost Earthquakes result from motion of plates and are distributed along the plate margins.
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Jigsaw of Plates
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Seismometers
Generates Seismic Waves
Propagate away from source and samples the Earth structure
Recorded ground motion is SEISMOGRAM
Earthquakes (Passive Source)
Free Surface ground motions caused by these propagating waves recorded at surface detectors(SEISMOMETERS)
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Seismometers
Pendulum Seismograph
Range of ground motion (in dB) and the period of ground motion spanned by the broad-band seismic system of IRIS-GDSN compared to the WWSSN instruments.
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Inertial-Pendulum Vertical and Horizontal Seismographs
Seismology Group IIT Kharagpur
Damping dash pot
Inertial Mass
* Damped Harmonic Oscillators
* Natural Frequency of vibration is given by : fo=1/2π(k/m)1/2
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Seismograph Response
u(t) = U(ω)eiωt
z(t) = Z(ω)eiωt
Frequency Response Function H(ω)= Z(ω)/U(ω)
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Response Function Curves
The Frequency response function can be written as:
H(ω) = ω2/(ω02 – 2εiω - ω2)
Where, ω = 2πf ε is the Damping parameter.
The strength of the damping relative to the stiffness of the spring is described by: h = ε /ω0 Damping Constant
Frequency of signal
Natural Frequency
Theoretically the absence of damping (h=0) results in an infinite response at resonance (ω0)
Critical damping: h=1 (No Oscillatory character of the Response)
Optimum damping is applied at h=0.707 (Flat Frequency Response)
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Modern Seismographs: Force Balance Feedback Principle
Ground Vibration
Motion in the Coil/Magnet
Induces a voltage proportional to the motion
Strength of current Proportional to the Ground Velocity
From EM
Generates a Feedback Current to restore the mass
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Three-Component Broadband Seismometers
Back Azimuth OH
tan φ = dE / dN
Incidence Angle i
tan i = OH / OV
= [(dE2 + dN
2)1/2 / dZ]
Z
E
N
i
ddZ
dN
dE
i
φO
H
V
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Seismology Group IIT Kharagpur
From Incorporated Research Institutions in Seismology (IRIS)
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Indian Network
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Types of Installations and Seismometers
Seismology Group IIT Kharagpur
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Seismology Group IIT Kharagpur
Weak motion
CMG-3TCMG-3TBCMG-3ESPCMG-3ESP Compact
Medium motion
CMG-6TCMG-6TDCMG-40T
Strong motion
CMG-5TCMG-5TBCMG-5TDCMG-5U
Digitizers and data modules
CMG-DM16R8CMG-DM24S6CMG-DM24S3CMG-DCMCMG-AM
Seismographs and Accelerographs
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Seismology Group IIT Kharagpur
GPS
Sensor
DigitizerData Storage
Transmit via Telemetric Link
Basic Station Setup
Setup
Data Download and Servicing
Seismometer
Timing & Location
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Generates Seismic Waves
Propagate away from source and samples the Earth structure
Recorded ground motion is SEISMOGRAM
Earthquakes (Passive Source)
Free Surface ground motions caused by these propagating waves recorded at surface detectors (SEISMOMETERS)
Seismic Waves
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Seismic Body Waves
P - waves - are Primary waves. They travel with a velocity that depends on the elastic properties of the rock through which they travel. V = Ö [(K + 4/3m )/r ]
S-Waves - Secondary waves, also called shear waves. They travel with a velocity that depends only on the rigidity and density of the material through which they travel: V = Ö [( m )/r ]
Vp=[(Κ+4/3μ)/ρ]1/2
Vs=[μ/ρ]1/2
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Seismic Surface WavesRayleigh Waves Named after Lord Rayleigh. Rolls along the ground just like a wave rolls across a lake or an ocean. Because it rolls, it moves the ground up and down, and side-to-side in the same direction that the wave is moving. Most of the shaking felt from an earthquake is due to the Rayleigh wave, which can be much larger than the other waves.
Love Waves Named after A.E.H. Love, a British mathematician who worked out the mathematical model for this kind of wave in 1911. It's the fastest surface wave and moves the ground from side-to-side.
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Local and Regional Observations
Pg
Sg
PnPg
Sg
SVD – 133 km
NEE – 368 km
Broad-band seismogram of the North Ridge earthquake recorded in Southern California
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Teleseismic Observation
Broad-band seismogram of the North Ridge earthquake recorded at HRV in Harvard, Massachusetts, about 5000 km epicentral distance.
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Global Record Section: 1994 North Ridge, California Earthquake and corresponding propagation paths
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Whole Earth Phase Direct Arrivals and Shadow Zones
Seismic Phases
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Seismic Phases
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Seismic Phases (a) Seismograms and
(b) Ray path for a deep focus earthquake beneath Tonga recorded on an array of seismographs in central Australia
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Earthquake location determined from the arrival times of variousseismic phases
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Measurements made from a seismic body-wave arrival