Faults and Earthquakes complete notes

Some Important Earthquakes

• 1755 - Lisbon, Portugal
  • Killed 70,000, Raised Waves in Lakes all over Europe
  • First Scientifically Studied Earthquake
• 1811-1812 - New Madrid, Missouri
  • Felt over 2/3 of the U.S.
  • Few Casualties
• 1886 - Charleston, South Carolina
  • Felt All over East Coast, Killed Several Hundred.
  • First Widely-known U.S. Earthquake
• 1906 - San Francisco
  • Killed 500 (later studies, possibly 2,500)
  • First Revealed Importance of Faults
• 1923 - Tokyo
  • Killed 140,000
• 1964 - Alaska
  • Killed about 200
  • Wrecked Anchorage.
  • Tsunamis on West Coast.
• 1976 - Tangshan, China
  • Hit an Urban Area of Ten Million People
  • Killed 650,000

Greatest Earthquakes and Volcanic Eruptions

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What Causes Earthquakes?

Most Quakes Occur along Faults (Fractures in Earth's Crust)

Elastic Rebound Theory

Here we have a landscape with a road, a fence, and a line of trees crossing a fault. As the crust moves, the rocks adjacent to the fault are deformed out of shape (in reality the deformation is spread across many kilometers - if it were this obvious, earthquake prediction would be easy). Eventually the rocks are so stretched out of shape that they cannot bear the stress any longer. The fault slips, and the stage is set for the next cycle of strain buildup and release.

Epicenter and Focus

Focus 

Location within the earth where fault rupture actually occurs 

Epicenter 

Location on the surface above the focus

Types of Faults

Faults Are Classified on the Basis of the Kind of Motion That Occurs on Them

• Joints - No Movement
• Strike-Slip - Horizontal Motion (Wrench Faults)
  • Left-Lateral 
  • Right-Lateral
  • (San Andreas - 21 Ft. in 1906) 
• Dip-Slip - Vertical Motion
  • Normal (Extension) 
  • Reverse or Thrust (Compression)
  • (San Fernando, 1971
  • Alaska, 1964 - up to 150 Ft.) 
• Nappe
• Overthrust

Fault Structures - Normal Faults

- Horst (uplifted block) - Graben (or rift valley - downdropped block) - Tilted Fault Block

Fault Structures - Reverse Faults

Nappes or Overthrusts are shallowly-dipping thrust faults found in almost all mountain ranges. Because they are nearly horizontal, they often have very complex outcrop patterns.

A Window (W) is an opening where erosion cuts through a shallowly-dipping thrust fault to expose the rocks below. A klippe (K) is an isolated remnant of a thrust fault block.

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Major Hazards of Earthquakes

• Building Collapse
• Landslides
• Fire
• Tsunamis (Not Tidal Waves!)

Safest & Most Dangerous Buildings

• Small, Wood-frame House - Safest
• Steel-Frame
• Reinforced Concrete
• Unreinforced Masonry
• Adobe - Most Dangerous

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Tsunamis

• Caused Probably by Submarine Landslides
• Travel about 400 M.p.h.
• Pass Unnoticed at Sea Cause Damage on Shore
• Warning Network Around Pacific Can Forecast Arrival
• Whether or Not Damage Occurs Depends on
  • Direction of Travel
  • Harbor Shape
  • Bottom
  • Tide & Weather

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Seismology

Ideally, we'd like to be able to hover above the earth during and earthquake and watch the earth move beneath us. Since my anti-gravity belt is in the shop for repairs, the closest we can come is with a pendulum.

Contrary to intuition, an earthquake does not make the pendulum swing. Instead, the pendulum remains fixed as the ground moves beneath it. A pendulum with a short period (left) moves along with the support and registers no motion. A pendulum with a long period (right) tends to remain in place while the support moves. The boundary between the two types of behavior is the natural period of the pendulum. Only motions faster than the natural period will be detected; any motion slower will not.

Since earthquake vibrations can have periods of many seconds, we need a pendulum with a very long period. We can construct a pendulum with a very long arm, or we can build a compact instrument by building a horizontal pendulum. If the pendulum is built like a swinging gate, the restoring force (force pulling it back toward the center of its swing) can be made very weak, and the pendulum can have a period as long as we like.

Seismic Waves

Seismic waves come in several types as shown below:

P-Waves

Primary (they arrive first), Pressure, or Push-Pull. Material expands and contracts in volume and particles move back and forth in the path of the wave. P-waves are essentially sound waves and travel through solids, liquids or gases. Ships at sea off the California coast in 1906 felt the earthquake when the P-wave traveled through the water and struck the ship (generally the crews thought they had struck a sandbar).

S-Waves

Secondary (arrive later), Shear, or Side-to-side. Material does not change volume but shears out of shape and snaps back. Particle motion is at right angles to the path of the wave. Since the material has to be able to "remember" its shape, S-waves travel only through solids.

Surface Waves

Several types, travel along the earth's surface or on layer boundaries in the earth. The slowest waves but the ones that do the damage in large earthquakes.

Magnitude and Intensity

Intensity

How Strong Earthquake Feels to Observer

• Depends On:
  • Distance to Quake
  • Geology
  • Type of Building
  • Observer!
• Varies from Place to Place
• Mercalli Scale- 1 to 12

We can plot earthquake intensity by gathering reports from observers. Although the reports will be subjective, and vary somewhat, most observers will agree on the intensity criteria, for example, feeling the quake while driving. For very strong quakes, damage provides fairly objective measures of intensity.

Isoseismals from the 1906 San Francisco Earthquake

Overall, the pattern is pretty simple: high intensity close to the San Andreas Fault, dropping off with distance. But why is there a disconnected island of high intensity in central California? The band of low (IV) intensity parallel to the coast coincides with the Coast Ranges. Soils here are very shallow - usually less than a meter to bedrock. Observers here felt mostly a sharp jolt. In contrast, the high intensity in central California coincides with the Central Valley, where young and unconsolidated sediments are kilometers deep. Unconsolidated material shakes like jelly in an earthquake. Note how intensity VI follows the shoreline of San Francisco Bay, where there are also thick unconsolidated sediments.

Intensity and Geology in San Francisco

At left is an isoseismal map for San Francisco itself . Everything was shaken hard, and of course intensities were extremely high close to the fault. But note how in the city intensity can vary by two levels within a couple of hundred meters. At right is a geologic map. Note that low intensity correlates closely with bedrock at or near the surface (Franciscan metamorphic rocks and serpentine).

When we examine intensity compared to depth to bedrock (right) the pattern becomes even clearer. Candlestick Park, where game 3 of the 1989 World Series was about to begin, owes its reputation for being a windy ball park to being near a steep hill. Its location on bedrock meant that fans felt a sharp jolt, there were a few cracks in the concrete, and little else. (The First Amendment gives San Francisco the right to call it 3-Com Park if they like - it also gives me the right to ignore them.) The Marina District was shaken badly because it's on artificial fill, in fact, much of it is rubble from the 1906 earthquake. The deep filled valley in northeastern San Francisco is occupied by the commercial center of the city but the modern construction is steel-frame and was undamaged in the 1989 earthquake.

San Francisco and New Madrid Compared

The map at left compares the isoseismals from the 1906 San Francisco earthquake and the 1811-1812 New Madrid quakes. There is a lot less intensity data for the New Madrid events so local details are missing. Intensity estimates are based on reports from places shown as blue dots.

Although the New Madrid events were big, they owe their vast felt areas to the layer-cake geology of the Midwest. The flat strata and relative lack of geologic complexity (especially compared to California) mean that seismic waves travel very efficiently for long distances with little loss of energy.

Magnitude - Determined from Seismic Records

Richter Scale:

• Related to Energy Release
• Exponential
• Magnitude-Energy Relation
  • 4 - 1
  • 5 - 30
  • 6 - 900
  • 1 Megaton = about 7.0
  • 7 - 27,000
  • 8 - 810,000
• No Upper or Lower Bounds
• Largest Quakes about Mag. 8.7

A Seismograph Measures Ground Motion at One Instant
But --

• A Really Great Earthquake Lasts Minutes
• Releases Energy over Hundreds of Kilometers
• Need to Sum Energy of Entire Record

Seismic - Moment Magnitude

• Modifies Richter Scale, doesn't replace it
• Adds about 1 Mag. To 8+ Quakes

Magnitude and Energy

	Magnitude and energy for large earthquakes. Near-surface earthquakes are measured in terms of their surface waves, but deep earthquakes don't produce much surface waves. Deep earthquakes are measured in terms of their P- and S- waves. The two scales are defined to coincide as well as possible for normal deep earthquakes.
	There are not too many familiar analogies for very large earthquakes, but very small events overlap the energies of many familiar phenomena.

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Strategies of Earthquake Prediction

• Lengthen Historical Data Base
  • Historical Records
  • Paleoseismology
• Short-term Prediction
  • Precursors
• Long-term Prediction
  • Seismic Gaps
  • Risk Levels

Eastern North America Earthquakes 1534-1994

Source: USGS Data

U.S. Earthquakes, 1973-2002

Source, USGS. 28,332 events. Purple dots are earthquakes below 50 km, the green dot is below 100.

Seismic Risk Level Maps for the U.S.

Probable ground acceleration in 50 years. Blue = small, red = large

Probability of damage in 100 years. Blue = negligible, green = low, red = high.

  

Seismic Gaps

• Modelling
  • Dilatancy - Diffusion: cracks open as rocks deform. and fluids moving into the cracks weaken the rock, hastening its failure.
  • Stick - Slip: studies of how and why materials slip.
  • Asperities: sticking points on faults, typically bends. Many fault ruptures seem to be bounded by bends or kinks in the faults. Allows estimates of the likely magnitude of earthquakes along fault segments.
  • Crack Propagation: studies of how cracks form, expand, and join.

Are Earthquakes Getting More Frequent?

It was only in 1885 that a seismograph in Europe detected an earthquake in Japan, and we have global coverage, even for very large events, only since 1900 or so. Below is a graph, based on USGS data, for the annual number of M=7.5 and M=8 earthquakes from 1900 to 2001.

The high levels between 1900 and 1918 were real. The instruments might have overrated some events, but also it is still possible that some events were missed in those years.

There was a steady decline between 1968 and 1984. Curiously, not a single person during those years asked me whether earthquakes were becoming less frequent.

The graph above shows earthquake fatalities since 1800 from the U.S. Geological Survey list of significant earthquakes. The totals are not exact for any year but give an idea of trends. For example, the database for 1892 lists only two fatalities. Does anyone really believe there were only two earthquake fatalities worldwide in 1892, let alone the gaps where there are no reported fatalities?

Note that the scale is logarithmic. The dozen or so events with more than 100,000 fatalities account for a large fraction of the total. Even in recent decades there have been quiet years with only a few hundred fatalities. There have been about 4.5 million earthquake fatalities since 1900, 6 million since 1800, and 10.5 million since 1500.

There is an overall increasing trend, partly due to better reporting, partly due to larger populations in at-risk areas, and population pressures forcing people into ever more dangerous ground. However, some seismologists believe we have not seen the worst. World population has tripled since 1950 and that is too short a time for us to conclude we have seen the worst case scenarios. A repeat of the 1923 Tokyo earthquake at the worst possible time, or a tsunami like 2004 but directed north toward Bangladesh, could conceivably produce disasters with million-plus fatalities.

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Seismology and Earth's Interior

Successive Approximation in Action

Assume the Earth is uniform. We know it isn't, but it's a useful place to start. It's a simple matter to predict when a seismic signal will travel any given distance.

Actual seismic signals don't match the predictions

• If we match the arrival times of nearby signals, distant signals arrive too soon
• If we match the arrival times of distant signals, nearby signals arrive too late.
• Signals are interrupted beyond about 109 degrees

We conclude:

• Distant signals travel through deeper parts of the Earth, therefore ..
• Seismic waves travel faster through deeper parts of the Earth, and ..
• They travel curving paths (refract)
• Also, there is an obstacle in the center (the core).

Wave Refraction

When marchers in a parade turn a corner, the inner marchers slow down and the outer ones speed up. When waves of any kind change speed, they also change direction (refract).

Refracted waves always travel the shortest possible path in terms of time. Path B is the fastest one possible.

Path A covers a shorter distance, but the slower velocity more than cancels out the savings in distance.

However, if a little is good, a lot is not necessarily better. Path C dips down into a region of even higher velocity than B, but the velocity is not fast enough to make up for the longer path length.

Seismic Waves in Earth's Interior

There are two ways to look at waves. One is to track ray paths, the path of any particular impulse. The other way is to track wave fronts, the boundary of the wave as it travels outward. A surfer riding a wave travels a ray path. The crest of the wave is the wave front. The animation below shows ray paths of a P-wave in the earth. 

 The animation below shows the wave front of a P-wave in the earth. 

Inner Structure of the Earth

Seismic signals can bounce off boundaries in the Earth. Each leg of a signal describes its history up to that point. A P-wave travelling through the outer core is labelled K, a bounce off the core is denoted by lower-case c. We don't see any S-waves passing through the core, the principal line of evidence that the outer core is fluid.

A P-wave in the inner core is I and an S-wave in the inner core (remember, it's solid!) is J. There are so many variables to match, that by the time we successfully account for all the observed seismic signals, we can be pretty confident it is the correct solution.

The overall structure of the Earth.

Seismic Tomography

Seismic tomography is a method of using seismic signals to map the earth's interior in three dimensions.

Crustal Deformation complete notes

Economic Consequences of Geologic Structures

• Tracing Coal Seams, Aquifers, etc.
• Ore Deposits are often localized along faults and folds
• Petroleum Traps

Types of Folds

Small Structures

• Anticline
• Syncline
• Monocline
• Homocline

Large Structures

• Dome
• Basin
• Arch, Swell, Upwarp

Foliation

Foliation is a sheetlike structure that forms when rocks are deformed. As the figures show, it forms in a variety of ways, but in every case, the foliation is at right angles to the direction of greatest compression.

a. Foliation can simply form when objects in the rocks are flattened. b. Foliation can form when flattening causes platy mineral grains to align, much the way toothpicks would be aligned when swept up by a broom. c. Solutions often remove large amounts of material from rocks as they are being deformed. The solutions move in the direction of least resistance. d. Elongate crystals grow in the direction of least resistance. e. Pockets of molten material may form during high-temperature metamorphism, and these may be flattened. This is one possible way banding in gneiss forms. f. Shear, like along a fault, also produces foliation. As the shear deformation becomes greater, the foliation becomes stronger and more closely aligned with the fault plane.

Banding in Gneiss

a. Often the banding in gneiss is a relic of original bedding, especially if the original rocks had alternating beds of dissimilar composition. b. As ferromagnesian minerals form, they accumulate iron and magnesium from their surroundings, which become depleted. The result is a mass of ferromagnesian minerals surrounded by quartz and feldspar. c. As rocks begin to melt, the granitic components melt first, following Bowen's Reaction Series in reverse. As the melt collects, the remaining rock becomes enriched in ferromagnesian minerals. This is another way to create alternating bands of light and dark minerals. d. As folding progresses, the sides (or limbs) of folds become progressively more thinned out. Alternating bands of segregated and finely-intermingled minerals result.

In reality, all of these mechanisms may be at work simultaneously in gneiss formation.

Folds and Foliation

On a small scale (microscopic to centimeters), foliation forms by a variety of mechanisms, but always at right angles to the direction of greatest compression

On a large scale (centimeters to kilometers), rocks fold. The axial plane of the fold is also at right angles to the direction of greatest compression

 

Therefore, foliation in deformed rocks is generally parallel to the axial plane of the fold. This relationship makes it easy for a geologist working in deformed rocks to tell something about the orientations of large folds.

The Importance of Minor Folds

When rocks fold, the layers slip over each other, and thin beds are sometimes crinkled into so-called minor folds. Note that the folds are Z-shaped on one side of the fold and S-shaped on the other. A geologist can tell that she's crossed the axial plane of a fold by observing that the minor folds change from S- to Z-shaped (or vice versa).

How Geologists Use These Clues

	Here's an outcrop that might be seen in the field. The beds are dipping steeply to the left, hinting perhaps that the beds are on the flanks of an anticline somewhere off to the right. However, the foliation also dips to the left, and the minor folds are inconsistent with an anticline off to the right.
	Mentally, we picture the axial plane of the fold as parallel to the foliation. The other side of the fold is roughly a mirror image of the side we can see. We can guess it's a tight anticline with its right side overturned. There must be a syncline to the right of the outcrop. Note that we have no idea how big the fold is. The axial plane could be a meter or a kilometer away. What we are interested in is figuring out what kind of fold it is and how it is oriented.
	We can mentally fill out the sketch to get an idea of the shape of the fold. Note that we still have no idea how big the fold is, but we know it's an anticline and have some idea of its shape and orientation. Note that we can conclude the beds in the outcrop are overturned. If we just naively concluded there was an anticline to the right and a syncline to the left we'd be exactly wrong.
	How do we know the axial plane is on this side of the outcrop? It doesn't matter. Here we place the axial plane to the right and construct the mirror image.
	If we put the axial plane on the other side of the outcrop we would conclude the fold to the right was a syncline, and there is an anticline to the left. We end up with the same results either way.

Oh, by the way, why can't we have the axial plane running through the outcrop itself? Answer below.

Diapirs

Salt domes are the best known and most common diapirs. In the cross-section above, light blue represents salt and other shades represent other sedimentary rock layers. Salt is both very ductile and much less dense than most other rocks. If it finds a weak spot in the overlying rocks (left) it begins to flow upward. The pressure of adjacent rocks on the salt layer, plus the low density of the salt, cause the salt to continue rising (center). In advanced stages, the salt often takes on a mushroom shape (right) and can even become entirely disconnected from its roots. The upturning of layers adjacent to the salt creates numerous traps for petroleum. In the North Sea and Gulf of Mexico, the search for petroleum basically amounts to a search for salt domes.

The salt can and often does reach the surface. On the Gulf Coast, the salt is very vulnerable to solution, and collapse due to solution is common. In extremely arid regions, notably Iran and Morocco, salt that reaches the surface can flow downhill as a salt glacier.

Intrusions like granite also commonly rise as diapirs. Some even have a teardrop outline.

Isostasy

Isostatic Rebound Since the Pleistocene

Isostatic Rebound in Canada

Isostatic Rebound in Scandinavia

 

It's probably no accident that two of the largest epicontinental seas (seas extending deep into the continent) on Earth are Hudson Bay and the Baltic, both dead center in areas of active isostatic uplift. In all likelihood, the crust in these regions is still depressed and has not finished rising, and when uplift is complete both seas will mostly or entirely disappear. Gravity measurements suggest that the crust in the Hudson Bay region has another 100 meters still to rise.

Epeirogeny

Epeirogeny is gentle uplift or subsidence of the crust, sometimes by kilometers, but with little igneous activity, faulting, metamorphism, or intense deformation. The mechanisms of epeirogeny are not well understood. Epeirogenic regions are characterized by domes, arches, and basins. The Great Lakes region is one of the best illustrations of epeirogeny anywhere.

Basins and Uplifts in the Midwest

Legend Cretaceous (Gray) Jurassic (Blue-Gray, Michigan) Permian (Dark Blue) Pennsylvanian (Violet) Mississippian (Blue) Devonian (Light Blue) Silurian (Light Green) Ordovician (Brown) Cambrian (Yellow) Precambrian (Red) Units within the Great Lakes are horizontally ruled.

The map above shows the geology of the Great Lakes region. Note that the pattern around Michigan is like a bulls-eye, with younger rocks in the center. Thus, we find Silurian rocks in eastern Wisconsin and Ontario, but they are buried beneath younger rocks in Michigan and are below the surface. Thus the rocks sag downward under Michigan. The structure is like a syncline, but much more broad and gentle, and it's more or less equidimensional. We call such a structure a basin. In Wisconsin, we find old rocks in the center becoming younger as we move away. We'd have to drill downward to find Precambrian rocks in LaCrosse or Green Bay, but they are on the surface in Wausau. Hence the rocks in Wisconsin arch upward, like an anticline but again much more broad and gentle. If this structure were closed on all sides we would call it adome, but since it's open at each end we refer to it as an arch

Note also how the Great Lakes correlate with the rock units. Lake Michigan, the main portion of Lake Huron, and Lake Erie are mostly underlain by Devonian rocks, mostly shales, which were easily excavated by the glaciers. The Green Bay lowland and Green Bay itself were scoured out of soft Ordovician shales (the Maquoketa Formation). So were the channels between the islands in Lake Huron and the mainland, Georgian Bay, the lowland across southern Ontario, and Lake Ontario. Four of the Great Lakes have their shapes and locations determined by structures in the Paleozoic rocks (and Lake Superior is excavated out of relatively soft Precambrian sedimentary rocks as well).

The outer edge of the Silurian rocks forms the Niagara Escarpment, which wraps around the north side of the Michigan Basin. It runs along Lake Winnebago and Green Bay, into Michigan, across the Lake Huron islands, down the Bruce Peninsula, across Ontario, and into New York. The Bruce Peninsula, the peninsula in Ontario that separates Lake Huron from Georgian Bay, is a geological and ecological twin of the Door Peninsula, but less developed.

Legend Cretaceous (Gray) Jurassic (Blue-Gray, Michigan) Permian (Dark Blue) Pennsylvanian (Violet) Mississippian (Blue) Devonian (Light Blue) Silurian (Light Green) Ordovician (Brown) Cambrian (Yellow) Precambrian (Red) Units within the Great Lakes are horizontally ruled.

The diagram above gives a three-dimensional view of the structure of the rocks. The vertical scale is very much exaggerated. Precambrian rocks are exposed in Michigan and Ontario, and are 5 km beneath the surface in the center of the Michigan Basin. We know that because a consortium of oil companies and universities drilled a well there in the 1970's. Thus the Michigan Basin is 500 km across but only 5 km deep. At the scale of this drawing, the true depth of the Michigan Basin would be about one screen pixel deep. No attempt has been made to show the complex folding in the Appalachians.

Note that all units are not present everywhere. Cambrian rocks are absent north and east of the Michigan Basin, and Silurian rocks are absent in the southwestern part of the map area. Layers do not extend forever; they may be absent because they were eroded away or because they were never deposited in certain areas.

Basins are not just empty holes filled by sediments. Everywhere the rocks in this region show evidence of being deposited in shallow water (100 meters deep at most). The sediments accumulate because the crust is subsiding. Note how in Michigan sediments accumulated from the Cambrian through the Pennsylvanian, then stopped. Later on, in the Jurassic, there was a little more subsidence, allowing a little more deposition to take place. It's fairly common for sedimentary basins to resume activity after long periods of inactivity, and it seems to correlate with major periods of mountain-building, but the exact process is still not well understood.

This diagram shows the principal basins and arches of the upper Midwest

Orogeny

Types of Mountains

• Volcanic
• Erosional
• Fault Block
• Fold

A cross-section through a typical mountain range looks something like this. As we proceed from the stable interior of the continent into the mountain range, we encounter a belt of shallow-water marine sedimentary rocks. These rocks become thicker and more intensely deformed the farther we go into the mountain range. There are frequently uplifted blocks of deep continental crust, then a belt of quite different rocks. In the most intensely-deformed and metamorphosed part of the mountain range, the rocks consist of thick, deep-water marine sedimentary rocks with abundant volcanic rocks. These rocks are often intruded by batholiths.

Mountain belts are thought to begin as ordinary continental shelves and slopes as shown above. The continental shelf rocks are sometimes termed the miogeocline and the deep-water continental slope rocks are often termed the eugeocline.

Deformation, metamorphism and igneous activity may begin when plate subduction begins along the continental margin.

Sometimes plate movements may cause the leading edge of a continent to collide with another fragment of crust, often a volcanic island arc. This is another way to juxtapose shallow-water rocks with deep-water and volcanic rocks.

Answer

If we had the axial plane running through the outcrop, we would actually see the curve of the fold. Since we don't, the axial plane has to be off to one side somewhere.