tectonics_lecture-5

Seismology Group IIT Kharagpur

Seismology Measuring the interior

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)

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

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)

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

Mechanical Layering of the Earth

D’’ Layer

Continental crust

Oceanic crust

DEEP STRUCTURE

Combined View(A comparison)

Distribution of seismic velocities and density within the Earth

No S-wave

SHALLOW STRUCTURE

Seismic Reflection profiling

• Detail crustal images that reveal information about location of economic resources (oil and minerals)

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

Earthquake Distribution and Tectonic PlateMost Earthquakes result from motion of plates and are distributed along the plate margins.

Jigsaw of Plates

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)

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.

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

Seismograph Response

u(t) = U(ω)eiωt

z(t) = Z(ω)eiωt

Frequency Response Function H(ω)= Z(ω)/U(ω)

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)

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

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

Seismology Group IIT Kharagpur

From Incorporated Research Institutions in Seismology (IRIS)

Indian Network

Types of Installations and Seismometers

Seismology Group IIT Kharagpur

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

Seismology Group IIT Kharagpur

GPS

Sensor

DigitizerData Storage

Transmit via Telemetric Link

Basic Station Setup

Setup

Data Download and Servicing

Seismometer

Timing & Location

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

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

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.

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

Teleseismic Observation

Broad-band seismogram of the North Ridge earthquake recorded at HRV in Harvard, Massachusetts, about 5000 km epicentral distance.

Global Record Section: 1994 North Ridge, California Earthquake and corresponding propagation paths

Whole Earth Phase Direct Arrivals and Shadow Zones

Seismic Phases

Seismic Phases

Seismic Phases (a) Seismograms and

(b) Ray path for a deep focus earthquake beneath Tonga recorded on an array of seismographs in central Australia

Earthquake location determined from the arrival times of variousseismic phases

Measurements made from a seismic body-wave arrival