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Early Tectonic IdeasEarly Tectonic Ideas

Herodotus of Herodotus of Halicarnassos(484 BC-ca. Halicarnassos(484 BC-ca. 425 BC), now part of Turkey, 425 BC), now part of Turkey, observed seashells in the observed seashells in the hills of Egypt and concluded hills of Egypt and concluded that they had lain beneath that they had lain beneath the sea.the sea.

Predecessors Anaximander Predecessors Anaximander and Xenophanes are quoted and Xenophanes are quoted as making similar as making similar statements.statements.

Thus the first aspect of Thus the first aspect of tectonics to be noted was tectonics to be noted was vertical motion.vertical motion.

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Wegener, in Wegener, in 1924, 1924, proposed a proposed a reconstructioreconstruction of n of Pangaea, Pangaea, based based primarily on primarily on geometric geometric fit.fit.

From USGS “This Dynamic Earth, Kious and Tilling,1996

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Smith and Hallam, 1970, generated fits which were consistent with structural and paleontological data.

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Sequence of key ideasSequence of key ideas

1924 Wegener: Continents drift1924 Wegener: Continents drift 1962-1963 Harry Hess (Princeton) and 1962-1963 Harry Hess (Princeton) and

Bob Dietz (San Diego): Seafloor Bob Dietz (San Diego): Seafloor spreads and carries continents with it.spreads and carries continents with it.

1965 J. Tuzo Wilson: Transform fault 1965 J. Tuzo Wilson: Transform fault idea solves mystery of ridge offsets and idea solves mystery of ridge offsets and fracture zones. Uses name “plates”.fracture zones. Uses name “plates”.

1967 Lynn Sykes: Fault mechanisms 1967 Lynn Sykes: Fault mechanisms support transform fault idea.support transform fault idea.

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Current Plate GeographyCurrent Plate Geography

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Velocities from the Global Velocities from the Global Positioning System confirm that Positioning System confirm that

movement continuesmovement continues

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The signs left on The signs left on continental cratons by continental cratons by plate tectonic activity plate tectonic activity are not as clear as we are not as clear as we see when we have see when we have todays maps of the todays maps of the ocean basins before ocean basins before us.us.

The remains of The remains of unsubducted unsubducted fragments of oceanic fragments of oceanic lithosphere are stuck lithosphere are stuck onto the edges of onto the edges of continents and continents and represent many cycles represent many cycles of basin openings and of basin openings and closing (Wilson closing (Wilson cycles). These were cycles). These were not so easy to not so easy to interpret.interpret.

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Key Theoretical Advance:Key Theoretical Advance:Transform faultsTransform faults

1965- J. Tuzo Wilson, A new class of 1965- J. Tuzo Wilson, A new class of faults and their bearing on Continental faults and their bearing on Continental

DriftDrift

This explained: Segmentation of Mid-Atlantic Ridge,This explained: Segmentation of Mid-Atlantic Ridge, Long linear features (Fracture Zones) in Pacific described by MenardLong linear features (Fracture Zones) in Pacific described by Menard

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Seismic Seismic Proof:Proof:1967 Lynn 1967 Lynn

Sykes, Sykes, Mechanism of Mechanism of Earthquakes Earthquakes and Nature of and Nature of

Faulting on the Faulting on the Mid-Oceanic Mid-Oceanic

RidgesRidges Provided real-Provided real-time evidence for time evidence for Plate Tectonics Plate Tectonics in addition to in addition to historic evidence historic evidence from continent from continent shapes and shapes and magnetic magnetic anomaly paterns.anomaly paterns.

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Earthquakes Earthquakes give us give us

information information about their about their

sources sources through the through the waves they waves they

radiate.radiate.

Surface waves Surface waves (L,R) are bound (L,R) are bound to the surface, to the surface,

while body while body waves travel waves travel through the through the

interior.interior.

Body waves can Body waves can be polarized.be polarized.

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Elastic rebound Elastic rebound mechanismmechanism

(a) Initial state(a) Initial state

(b) During (b) During strainstrain

(c)After (c)After earthquakeearthquake

Figure from Stein and Figure from Stein and Wysession, 2003Wysession, 2003

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Many faults are not as simple as the strike-slip Many faults are not as simple as the strike-slip example in the previous slide. The geometric example in the previous slide. The geometric variables which define fault plane geometry and variables which define fault plane geometry and relative motion are defined below.relative motion are defined below.

Figure from Stein and Wysession, 2003Figure from Stein and Wysession, 2003

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Other simple geometries and slip angles.Other simple geometries and slip angles.

Figure from Stein and Wysession, 2003Figure from Stein and Wysession, 2003

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Up motion occurs in the compression quadrantUp motion occurs in the compression quadrantDown motion occurs in the dilation quadrantDown motion occurs in the dilation quadrant

The little doughnuts show where the rays The little doughnuts show where the rays whose seismograms are shown exit from the whose seismograms are shown exit from the

box.box.

The following several slides are from Stein and Wysession, 2003

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For a small fault, the fault plane and the For a small fault, the fault plane and the auxiliary plane are indistinguishable auxiliary plane are indistinguishable

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The amplitude The amplitude depends on the angle depends on the angle of the vector to the of the vector to the receiver as seen from receiver as seen from the fault.the fault.

For P waves this For P waves this radiation pattern can radiation pattern can be represented as a be represented as a beach ball, which beach ball, which represents a FOCAL represents a FOCAL SPHERE around the SPHERE around the source.source.

For S waves it is more For S waves it is more complicated because complicated because of the polarization.of the polarization.

The lower panel shows The lower panel shows the direction of first the direction of first motion for S waves on motion for S waves on a sphere.a sphere.

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This slide shows the This slide shows the radiation pattern from radiation pattern from a source which is a source which is called a double called a double couple, since it is couple, since it is composed of two composed of two vector pairs instead of vector pairs instead of one.one.

It conserves angular It conserves angular momentum, which is momentum, which is usually a good thing.usually a good thing.

Here the right panels Here the right panels show the direction of show the direction of first motion for both P first motion for both P and S waves. and S waves.

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In the real earth, we make observations on the In the real earth, we make observations on the surface, which we can map back to the focal surface, which we can map back to the focal sphere by ray tracing, since we know the sphere by ray tracing, since we know the earth's velocity structure (of which more later).earth's velocity structure (of which more later).This means that the fault and auxiliary planes This means that the fault and auxiliary planes will map onto the earth's surface in a will map onto the earth's surface in a diagnostic pattern, as these rays are mapped.diagnostic pattern, as these rays are mapped.

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One of Sykes plots from One of Sykes plots from MARMAR

transform faultstransform faults Black dots Black dots

indicate indicate compressional compressional first motionfirst motion

Open circles Open circles indicate indicate dilatational dilatational motionmotion

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Projections are ways of displaying all or part of Projections are ways of displaying all or part of the surface of the spherical (or more the surface of the spherical (or more complicated) earth on a flat surface, such as a complicated) earth on a flat surface, such as a piece of paper.piece of paper.

Useful properties of projections are that they Useful properties of projections are that they bebe(1) conformal, that is, that for small areas, (1) conformal, that is, that for small areas, angles are preserved. So a right angle on the angles are preserved. So a right angle on the earth's surface is represented as a right angle earth's surface is represented as a right angle on the map.on the map.(2) equal-area. So areas are conserved.(2) equal-area. So areas are conserved.Unfortunately, we cannot have both Unfortunately, we cannot have both simultaneously.simultaneously.

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A more fanciful A more fanciful descriptiondescription

From From

Six Books of Six Books of Optics, useful for Optics, useful for philosophers and philosophers and mathematicians mathematicians alikealike

François François d'Aiguillon, 1613d'Aiguillon, 1613

Figure by Peter Figure by Peter Paul RubensPaul Rubens

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To visualize To visualize the fault the fault and and auxiliary auxiliary planes onto planes onto a flat piece a flat piece of paper, of paper, we use a we use a stereograpstereographic net.hic net.

The The azimuth azimuth coordinate coordinate is used to is used to plot strike plot strike and, and, naturally, naturally, dip is for dip is for the dip.the dip.

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Here are Here are three fault three fault planes planes mapped onto mapped onto the lower the lower hemisphere.hemisphere.

All NS All NS striking striking planes will planes will be be meridians.meridians.

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To draw planes with To draw planes with different strikes, just rotate different strikes, just rotate the line.the line.

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To plot a plane To plot a plane normal normal (perpendicular) (perpendicular) to plane A, to plane A, rotate the net rotate the net until A is on a until A is on a meridian.meridian.

The pole for The pole for plane A will be plane A will be 90 degrees 90 degrees from its from its “equator”. All “equator”. All planes through planes through this pole will be this pole will be normal to plane normal to plane A.A.

Then any Then any meridian in a meridian in a rotated net, if it rotated net, if it passes through passes through the pole of A, the pole of A, will be normal will be normal to A.to A.

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Plotting PointsPlotting Points

The direction a ray leaves the The direction a ray leaves the focus is defined by its azimuth focus is defined by its azimuth from north, and its dip from north, and its dip (measured from horizontal) or (measured from horizontal) or its takeoff angle, measured from its takeoff angle, measured from vertical.vertical.

To plot a ray on the focal sphere, To plot a ray on the focal sphere, use its dip to plot it on the use its dip to plot it on the equator of the net. This would be equator of the net. This would be correct if the azimuth were east correct if the azimuth were east (090), then rotate the point as (090), then rotate the point as shown here. This not the same as shown here. This not the same as following the meridian.following the meridian.

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The compressional area The compressional area of the P radiation of the P radiation pattern is, by pattern is, by convention, shown as convention, shown as black.black.

The direction of motion The direction of motion on the fault depends on on the fault depends on which plane is the fault which plane is the fault plane.plane.

Choose a fault plane, Choose a fault plane, then imagine standing in then imagine standing in the white area. Then the white area. Then follow the directions of follow the directions of Darth Vader and “come Darth Vader and “come to the dark side”to the dark side”**. You . You will then be following will then be following the motion of the earth the motion of the earth on that side of the fault.on that side of the fault.

* Noted by Vince Cronin

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Here are beachball Here are beachball representations of some representations of some faults with combinations of faults with combinations of dip-slip and strike-slip dip-slip and strike-slip behavior.behavior.

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The fault geometry also The fault geometry also controls the radiation of controls the radiation of surface waves, since they surface waves, since they are composed of are composed of interference patterns of interference patterns of body waves.body waves.

Here are radiation Here are radiation patterns of Love waves patterns of Love waves (made of SH body waves) (made of SH body waves) and Rayleigh waves (made and Rayleigh waves (made of P and SV waves).of P and SV waves).

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The classical method for The classical method for finding focal mechanisms is to finding focal mechanisms is to assemble seismograms which assemble seismograms which observe a quake and trace observe a quake and trace their rays back to the focal their rays back to the focal sphere, plotting the points as sphere, plotting the points as filled circles if the first motion filled circles if the first motion is dilational (down) and solid is dilational (down) and solid if up (compressional). if up (compressional).

By examination, two planes By examination, two planes are fit dividing the are fit dividing the compressional and dilational compressional and dilational quadrants. Needless to say, quadrants. Needless to say, the results of this process are the results of this process are do not always define planes do not always define planes uniquely.uniquely.

Now mechanisms are usually Now mechanisms are usually derived using moment tensor derived using moment tensor inversion, about which more inversion, about which more later.later.

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We have looked at focal We have looked at focal mechanisms from the mechanisms from the point of view of earth point of view of earth motions. These motions motions. These motions have clear interpretations have clear interpretations in the case of the strike-in the case of the strike-slip fault.slip fault.

These motions are, These motions are, however, not immediately however, not immediately indicative of the forces indicative of the forces which produce them.which produce them.

So we introduce the So we introduce the pressure and tension axes.pressure and tension axes.