Seismology: Fundementals 2

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Ali [email protected]

Department of Earth SciencesKFUPM

Introduction to Seismology

Strong Motion Seismology

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Previous Lecture

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Richter's Local Magnitude Earthquake Magnitudes Magnitude SaturationWhat causes saturation?Are Mb and Ms still useful? What is the better estimate of M? Moment magnitude Strength of Earthquake Magnitude-Energy Correlation

How to compute the energy released by an earthquake?

This figure was produced in cooperation with the US Geological Survey, and the University of Memphis private foundations

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log E = 1.5 Mw + 11.8

Where Es is seismic energy in ergs. Then,

From: Kanamori, H., 1977, The Energy Release in Great Earthquakes, Journal of Geophysical Research, v82, p. 2981- 2987.

E = Mo/(2 x 104)

log Mo - log(2 x 104) = 1.5 Mw + 11.8 Mw = (log Mo - 16.1) / 1.5

1 erg= 10-7 Joule

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Example: Energy for Mw=4.0?

The moment, Mo(4), would be: Mo(4) = E x (2 x 104)

= 1.26 x 1015 Joules It has been found that a 1 kton explosion will generate seismic waves approximately equivalent to a magnitude 4 earthquake.

The total seismic energy radiated from the source, E(4), would be:

E(4) = 10**(1.5*4 + 11.8)= 10**17.8 ergs = 6.3 x 1011 Joules

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1 erg= 10-7 Joule

Energy per ton of TNT = 4.18 x 109 Joules

The energy released by TNT (trinitrotoluene) and the TNT equivalent of the Hiroshima nuclear bomb (McGraw-Hill Encyclopedia of Science and Technology, 1992):

EarthquakeDamage

A rescue worker stands by a crack in Parks Highway caused by a 7.9 earthquake that rocked a sparsely populated area of Alaska. By Jimmy Tohill, AP

Collapse of Fourth Avenue near C Street, Anchorage, due to earthquake caused landslide. Before the earthquake, the sidewalk at left, which is in the graben, was at street level on the right. The graben subsides 11 feet in response to 14 feet ohorizontal movement. Anchorage district, Cook Inlet region, Alaska. 1964

Close-up view of tsunami damage along the waterfront at Kodiak.

Aerial view of collapsed sections of the Cypress viaduct of Interstate Highway 880. [H.G. Wilshire, U.S. Geological Survey]

A crack system destroys drivewayadjacent to summit road 0.8 km (1/2 mi) southeast of Highway 17. [J.K. Nakata, U.S. Geological Survey]

Only one magnitude. A certain amount of energy was released.Different intensity. Depends on your location. Parks and Plates

©2005 Robert J. Lillie

A Given Earthquake has:

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Intensity is a qualitative (non-numerical) rating of the ground shaking at a given place. It is inferred from the effects of the earthquake on the land and any buildings in the area. First-hand accounts from people are also used. Some scales of intensity are the Modified Mercalli, JMA, and MSK Intensity Scales. Quantitative (numerical) measures of ground motion from accelerograms are descriptions of shaking “severity” and should not use the word “intensity”.

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HARRY O. WOOD and FRANK NEUMANN, Modified Mercalli intensity scale of 1931, pp.277-283, BSSA

Felt

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Felt Report Form for a New Event

From: http://earthquakescanada.nrcan.gc.ca/dyfi/unknown_form_e.phpIntr

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DID YOU FEEL IT?REPORT IT

HERE!

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From: http://www.data.scec.org/Module/Pics/s3a1answ.gif

M=5.6, July 8, 1986, Palm Spring Earthquake

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EarthquakeIntensity Generally

Increases with Distance from

Epicenter

Faultline

Parks and Plates©2005 Robert J. LillieIn

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EarthquakeIntensity Generally

Increases with Distance from

Epicenter

Parks and Plates©2005 Robert J. Lillie

INTENSITY

Magnitude of the earthquake.Distance from the earthquake focus.Local soil conditions.

Factors that determine how intense the shaking was at a given location:

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Earthquake Intensity Database Search 1638-1985

From: http://www.ngdc.noaa.gov/seg/hazard/int_srch.shtml

U.S. Geological Survey map

New Madrid Earthquakes (1811-12)

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The dynamic rupture process in this hypothetical earthquake would occur in about 8 seconds.

This sequence of intensity maps shows how the intensity grows as a fault rupture increases in length.

This sequence, shown for the northern Hayward fault, starts from a hypothetical magnitude 5 and grows to a magnitude 7.1 earthquake.

Animated Example from Hayward Fault

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Damage Scenario

A damage scenario is a representation of the possible damage caused by an earthquake to the built environment in an area. It is in terms of parameters useful for economical and engineering assessment or postearthquake emergency management.

A probabilistic earthquake scenario is a representation of earthquake effects. The earthquake has a specified probability of exceedance during a prescribed period in an area. Adapted from the International Handbook of Earthquakeand Engineering Seismology, Aki and Lee[1]

From the International Handbook of Earthquake and Engineering Seismology, Aki and Lee[1]In

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Used at Los Angles Co. Emergency Operations Center, By LA County Office of Emergency Management for Training

Scenario, November 9, 2000.

LA County Emergency Operations Center

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Earthquake Damage ScenarioShake Map: Example for Verdugo Fault

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EXPLANATION of the online "Intensity Maps" activity

Source : http://www.data.scec.org/Module/s3act01.htmlIntr

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Assignment: Measuring Earthquakes

http://www.data.scec.org/Module/s3act01.html

I. Finding Isoseisms II. Locating an Epicenter

by Creating an Intensity Map

Due to April 15, Group Based Work is suggested.

Exercises

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Previous Lecture

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MagnitudeHow to compute the energy released due to an

earthquakeEarthquake Damage Magnitude-Intensity Mercalli Scale of arthquake IntensityData Acquisition for Earthquake

Example: Palm Spring Earthquake, M5.6, July 8, 1986Relationship between Intensity and Distance Animated Example of Intensity: Hayward Fault

Damage ScenarioEarthquake Planning Scenario: Example for Verdugo Fault

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Faults

When people stand across the fault, their heads are next to the hanging wall, their feet next to the foot wall.

a) Normal fault: A normal fault is a dip-slip fault where the rock above the fault moves down compared to the rock below the fault.

b) Reverse fault: A reverse fault is a type of dip-slip fault. The hanging-wall fault block of a reverse fault moves upward relative to the footwall fault block. (A thrust fault is a low-angle reverse fault.)

c) Strike-slip fault: A strike-slip fault is a fault where two plates slide horizontally past each other. It is called a strike-slip fault because the “slip” of the fault is parallel to the “strike” of the fault.

Parks and Plates©2005 Robert J. Lillie

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A fault is a fractureacross which there has been sliding parallel to the fracture surface.

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Faults

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StrikeStrike--slip faulting slip faulting animationanimation

Source: Photo taken from Martin Miller's website, University of Oregon. [email protected]

Strike-Slip Fault

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Normal Faulting Normal Faulting animationanimation

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Thrust Thrust Faulting Faulting

animationanimation

Source: University of Victoria.Intr

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Focal Mechanisms

There are special graphics, called focal mechanisms or “beach balls” that we use as shorthand to describethe style of faulting.

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Seismographs

A seismograph is an instrument which writes a permanent continuous record of earth motion.[1] Measuring the three orthogonal components of ground motion at a seismic station.

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Simplified motions of seismic waves

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Seismic waves are waves of energy that travel through Earth and along its surface. The waves are produced by earthquakes, explosions, or some other disturbance. Seismic waves are studied to locate and understand earthquakes and to determine the structure of Earth's interior.

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a)Body and surface wave paths from an earthquake located SSE of a station.

b) Seismograms from each of the three seismometers, responding to arrivals of the body (P= compressional, S= shear) and surface (L=Love, R= Rayleigh) waves

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Initial arrival as a compression pushes the groundup; Z-component shows an upward first motion.

Initial Ground Motions and Z-component seismograms for P-waves

Pushes the ground up

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Initial Ground Motions and Z-component seismograms for P-waves

Initial arrival as a dilatation pulls the grounddown; Z-component shows an downward first motion

Pulls the ground down

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Dilatation

Compression

Initial P-wave Radiation Pattern:from an Earthquake occurring on a reverse fault

a)Waves radiate outward in quadrants of compression and dilatation

b)The Z-component seismograms for the three stations highlighted in (a).In

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a) Map view of radiation pattern for right-lateral, strike-slip fault occurring along the San Andreas transform plate boundary

b) First-motion information for arrival at stations indicated in (a), plotted as a focal mechanism solution.In

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c) Auxiliary fault interpretation of the first-motion in (a), showing that the same radiation pattern could have resulted from a left-lateral fault.d) Focal mechanism solution for (c) is exactly the same as that resulting from right-lateral fault in (a).In

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Case Work: Fault interpretation

May 1, 2003- Bingol Earthquake (Mw=6.4)

May 1, 2003Mw=6.4

What is the strike of fault?

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Case Work: Fault interpretation

TUBITAK Earthquake Stations

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Case Work: Fault interpretationEarthquake Aftershocks

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Case Work: Fault interpretationEarthquake Aftershocks

Depth Distribution

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Observed surfaced deformation due to Bingol Earthquake

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Homework: VIRTUAL SEISMOLOGY

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You have learned how to pinpoint the location of an earthquake by measuring the speed of seismic waves radiating away from the focus of the earthquake. Now, we can determine an earthquake's magnitude by measuring the strength of ground shaking as you did for global earthquakes. Learn how to do both these things by visiting the virtual earthquake web page. http://www.sciencecourseware.org/VirtualEarthquake/VQuakeExecute.htmland completing the exercise. It should take you about 30 minutes. Turn in your certificate of completion at the beginning of class on Monday, 26 March.