seismology: fundementals 1
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Ali [email protected] of Earth Sciences
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Introduction to Seismology
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The What, Where and How of Earthquakes
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Parks and Plates©2005 Robert J. Lillie
Types of Plate Boundaries
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Mid-Atlantic ridge
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Internal heat
engine
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Measuring plate motions
From: McClusky et al., 2003, GJI, 155, 126–138
Velocities of the Arabian Platewith respect to the Eurasian Plate
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BSSA, Vol 70, 70, No. 6, pp. 2291-2293, December 1980Intr
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IN THE TIHAMAT-ASIR REGION
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From: AMBRASEYS andMELVILLE, 1989, BSSA, Vol. 79
Crustal Earthquakes in NW Arabia
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Mw=5.0March 11
2002
From: Rodgers et al., 2006, Tectonophysics, 415 (2006) 57–64Intr
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Lillie, Figure 7.1Intr
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Elastic Rebound (H.F.Reid, 1906)
See pp. 398, Bullen and BoldIntr
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LOCATION OF EARTHQUAKE
FocusActual point of rupture within the Earth.EpicenterPoint on Earth’s surface directly above the focus.An asperityis the roughness on the fault surface. It is a region of high shear strength.
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Parks and Plates©2005 Robert J. LillieIn
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Parks and Plates©2005 Robert J. LillieIn
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Release of Accumulateenergy
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Lillie, Fig. 7.4Intr
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How to determine Distances ?
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http://www.geol.binghamton.edu/faculty/jones/eqlocate.html
Ali [email protected] of Earth Sciences
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Introduction to Seismology
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•Measure log (A) in mm
•Measure S-P in second
•Measure Distance in km
•Estimate Magnitude
Richter’s Local Magnitude
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Earthquake Magnitude
ML - Local (Richter) magnitude
MW - Seismic Moment magnitude
MS - Surface wave magnitude
MB- Body wave magnitude
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Magnitude Saturation
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What Causes Saturation?The rupture process.
Small earthquakes rupture small areas.Large earthquakes rupture large areas.
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Are mb and Ms still useful?
YES!
Many (most) earthquakes are small enoughthat saturation does not occurEmpirical relations between energy releaseand mb and Ms exist The ratio of mb to Ms can indicate whethera given seismogram is from an earthquakeor a nuclear explosion (verificationseismology)
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What is the better estimate of M?
The seismic moment – Mo
Invented in the 1960s to circumvent magnitude limitationsHas physical units of energy (Nm, cal, J)Is the product of three factors thatindicate the size of the earthquake:Mo = (shear modulus) x (rupture area) x (slip offset)
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Moment Magnitude
Moment-Magnitude ScaleSeismic Moment = Strength of Rock x Fault Area x Total amount of Slip along Rupture
M0 = µ A D
Moment Magnitude Mw = 2/3 x [log10M0(dyne-cm) –16]
Measurement Analysis requires Time
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STRENGTH OF EARTHQUAKEMagnitude
Relates to the amount of energy released by the earthquake.
IntensityRelates to the effects (damage) observed at the surface.
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Logarithmic (not linear) scale.
Increase 1 order of magnitude means:The seismic waves are 10 times as large.
Parks and Plates©2005 Robert J. Lillie
Question:Compare M=6 EQ to M=7 EQ?
Logarithmic (not linear) scale.Increase 1 order of magnitude
means:The seismic waves are 10 times as
large.30 times as much seismic energy
was released by the earthquake.
Respond:Seismic waves 10 times as big. 30 times more energy.
Parks and Plates©2005 Robert J. LillieIn
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Next Question:An M=8 EQ is equivalent how many M=6 EQ going off at the same time?
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Start with a magnitude 6 earthquake.
A magnitude 7 earthquake releases 30 times the amount of seismic energy as a magnitude 6 earthquake.
A magnitude 8 earthquake releases 30 times the amount of energy as a magnitude 7 earthquake.
30 x 30 = 900
It would take about 900 magnitude 6 earthquakes going off at the same time to pack the same wallop as a magnitude 8 earthquake!
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Seismic Energy900 times as big.
Seismic waves100 times as big.
This figure was produced in cooperation with the US Geological Survey, and the University of Memphis private foundations
Magnitude -Energy Correlation
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http://www.geol.binghamton.edu/faculty/jones/eqlocate.html
web.ics.purdue.edu/~braile/edumod/eqlocate/tutorial.pdf
For Tutorial and Download
Assignment: Estimate Location of One Earthquake Due to Next Wednesday
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I played zooming and keys of arrow and arranged the location of above map. Good luck for rest of exercise.In
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Seismogram
Seismogram in one window
Step 3