World Map of Fault Lines

Fault lines are the lines that mark a boundary between two tectonic plates of the Earth's crust. Earthquakes usually take places along these fault lines, as two plates move in different directions and built up pressure is released as the plates slip suddenly.

Why do tectonic plates move like that? Well, the plates are part of the Earth's crust, which sits on top of the Earth's mantle which is semi-molten. Since the mantle is not as hard as the crust, the crust "floats" so to speak, or slips/slides.

One fault line well known to North Americans is the San Andreas fault, which runs up the westcoast of the USA and was the location of the San Francisco earthquake.

Below you can find a world map of fault lines that may help you with your reference.

Faults of Southern California

Faults of Southern California

Below is a map of southern California, with five regions highlighted:
1. Southern Coast Ranges and Central Valley area is orange.
2. Sierra Nevada and Basin and Range area is green.
3. Mojave region is yellow.
4. Extreme southern end of California is red.
5. Los Angeles area is blue-violet.

This map is clickable. Clicking on a region will take you to an enlarged relief map of the area, with local faults highlighted in a variety of colors, and linked to pages detailing information about these faults. In all of the maps, that segment of the San Andreas fault which is visible will be red, and scales for distances and elevations will be given. A few city and highway labels will also appear on the smaller maps.

Select a region
These maps were created using public-domain fault data which was modified by hand to more accurately reflect our current understanding of California's fault system. The 1994 Fault Activity Map of California and Adjacent Areas by Charles W. Jennings (available from the California Geological Survey ) was used as a guide. These maps should not be considered as zoning guides, nor be used for risk assessment. Because of the sheer number of faults in southern California, this is not an exhaustive collection. The faults featured within this section were chosen typically because they are larger and/or exhibit more recent offset than others. Most, if not all, of this material has a certain level of uncertainty to it. Our understanding of faults is constantly expanding, and new faults and better data may be added to these maps as our knowledge about these structures advances.
Note: Some browsers may distort the color palette of these images. If the elevation scale seems "odd" or "wrong", you may be experiencing this problem. Also, to use these imagemaps your browser must be compatible with client-side imagemaps. Comments are welcome.




RELATED INFORMATION

Alphabetical Fault Index
Historic Earthquakes in Southern California
Research Tools

U.S. Fault Lines GRAPHIC: Earthquake Hazard MAP

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Some may be curious of the U.S. fault lines with earthquakes in the news lately.
The fact is most states are at risk of major earthquakes, with 39 of the 50 states in moderate to high risk areas for seismic activity, ABC News reports.
This map courtesy of the U.S. Geological Survey (USGS) shows the major earthquake hazard areas within the United States based on fault lines:

Latest Earthquakes

USA (Magnitude 1+)

March 28, 2011 02:28:33 UTC
California Nevada Map U.S. Map Alaska Map Hawaii Map Puerto Rico Map

World (Magnitude 4.5+)

March 28, 2011 02:13:55 UTC
World Map

Regional Offices

Our scientists study earthquakes around the world. Learn about research conducted in our regional offices, regional seismic network monitoring, and local activities of interest.

Additional Resources

California-Nevada Fault Map centered

Earthquake map centered at 36°N,121°W Advanced National Seismic System Homepage M=1.8 2011/03/27 18:51:23 M=2.2 2011/03/27 14:45:33 M=1.3 2011/03/27 04:34:48 M=1.1 2011/03/26 23:51:36 M=2.5 2011/03/26 18:46:20 M=1.7 2011/03/26 08:24:17 M=1.4 2011/03/26 05:29:55 M=2.4 2011/03/26 04:25:39 M=1.7 2011/03/25 18:21:25 M=3.0 2011/03/25 18:08:17 M=2.7 2011/03/25 16:55:10 M=1.4 2011/03/24 18:11:35 M=2.5 2011/03/24 15:51:38 M=1.4 2011/03/24 06:44:25 M=2.2 2011/03/24 06:35:47 M=1.2 2011/03/24 01:07:29 M=2.8 2011/03/23 22:44:32 M=1.1 2011/03/23 07:40:25 M=1.4 2011/03/23 02:15:27 M=1.7 2011/03/23 02:13:54 M=1.1 2011/03/22 21:41:55 M=1.5 2011/03/22 21:04:31 M=1.8 2011/03/22 21:03:44 M=1.2 2011/03/22 19:02:55 M=2.1 2011/03/22 17:00:26 M=1.2 2011/03/22 15:29:55 M=1.3 2011/03/22 11:14:13 M=1.2 2011/03/22 07:45:31 M=1.2 2011/03/22 06:15:26 M=1.0 2011/03/21 21:22:38 M=2.0 2011/03/21 16:22:57 M=1.0 2011/03/21 12:06:33 M=1.3 2011/03/21 12:02:53 M=1.2 2011/03/21 06:46:07 M=1.4 2011/03/21 06:40:40 M=1.8 2011/03/20 22:46:48 Calaveras fault Cambria fault Hosgri fault zone Los Osos fault zone Monterey Bay fault zone O\'Neil fault system Oceanic-west Huasna fault zone Ortigalita fault Paicines fault Quien Sabe fault Reliz fault Rinconada fault San Andreas fault zone San Gregorio fault San Joaquin fault San Simeon fault Santa Maria River fault system Sargent fault Vergeles fault map centered at 37° N, 122° W map centered at 37° N, 121° W map centered at 37° N, 120° W map centered at 36° N, 122° W map centered at 36° N, 120° W map centered at 35° N, 122° W map centered at 35° N, 121° W map centered at 35° N, 120° W

Instructions

Tips

  • Red lines are known faults (orange lines are unnamed faults).
  • Thin grey lines are roads.
  • Magnitude = ? for new earthquakes until a magnitude is determined (takes 4-5 minutes).
  • Earthquakes can appear near a fault without having occurred on that fault. To associate an earthquake with a fault requires viewing both of them in three-dimensions.
  • Maps show events recorded in the past 7 days.
  • Maps are updated whenever a new earthquake has been located. Try to reload this page if you do not have the most current map.

Types of Faults



A fault is a fracture or zone of fractures between two blocks of rock. Faults allow the blocks to move relative to each other. This movement may occur rapidly, in the form of an earthquake - or may occur slowly, in the form of creep. Faults may range in length from a few millimeters to thousands of kilometers. Most faults produce repeated displacements over geologic time. During an earthquake, the rock on one side of the fault suddenly slips with respect to the other. The fault surface can be horizontal or vertical or some arbitrary angle in between.
Earth scientists use the angle of the fault with respect to the surface (known as the dip) and the direction of slip along the fault to classify faults. Faults that move along the direction of the dip plane are dip-slip faults and described as either normal or reverse, depending on their motion. Faults that move horizontally are known as strike-slip faults and are classified as either right-lateral or left-lateral. Faults that show both dip-slip and strike-slip motion are known as oblique-slip faults.  
A normal fault is a dip-slip fault in which the block above the fault has moved downward relative to the block below. This type of faulting occurs in response to extension and is often observed in the Western United States Basin and Range Province and along oceanic ridge systems.
 
A thrust fault is a dip-slip fault in which the upper block, above the fault plane, moves up and over the lower block. This type of faulting is common in areas of compression, such as regions where one plate is being subducted under another as in Japan and along the Washington coast. When the dip angle is shallow, a reverse fault is often described as a thrust fault.
A strike-slip fault is a fault on which the two blocks slide past one another. These faults are identified as either right-lateral or left lateral depending on whether the displacement of the far block is to the right or the left when viewed from either side. The San Andreas Fault in California is an example of a right lateral fault.
Go back to the "Earthquakes" page Go back to the previous page Move on to the "Magnitude" page



Earthquake Basics

Epicenter, hypocenter, aftershock, foreshock, fault, fault plane, seismograph, P-waves, magnitude, intensity, peak acceleration, amplification...
We hear them. After big earthquakes, we say them. But what do these terms mean? What do they mean for what we felt and what we will feel the next time? Do we really understand what seismologists are saying?
This section describes how earthquakes happen and how they are measured. It also explains why the same earthquake can shake one area differently than another area. It finishes with information we expect to learn after future earthquakes.

Earthquakes and Faults

What is an earthquake?

An earthquake is caused by a sudden slip on a fault, much like what happens when you snap your fingers. Before the snap, you push your fingers together and sideways. Because you are pushing them together, friction keeps them from moving to the side. When you push sideways hard enough to overcome this friction, your fingers move suddenly, releasing energy in the form of sound waves that set the air vibrating and travel from your hand to your ear, where you hear the snap.
The same process goes on in an earthquake. Stresses in the earth's outer layer push the side of the fault together. The friction across the surface of the fault holds the rocks together so they do not slip immediately when pushed sideways. Eventually enough stress builds up and the rocks slip suddenly, releasing energy in waves that travel through the rock to cause the shaking that we feel during an earthquake.
Just as you snap your fingers with the whole area of your fingertip and thumb, earthquakes happen over an area of the fault, called the rupture surface. However, unlike your fingers, the whole fault plane does not slip at once. The rupture begins at a point on the fault plane called the hypocenter, a point usually deep down on the fault. The epicenter is the point on the surface directly above the hypocenter. The rupture keeps spreading until something stops it (exactly how this happens is a hot research topic in seismology).

Aftershocks

Part of living with earthquakes is living with aftershocks. Earthquakes come in clusters. In any earthquake cluster, the largest one is called the mainshock; anything before it is a foreshock, and anything after it is an aftershock.
Aftershocks are earthquakes that usually occur near the mainshock. The stress on the mainshock's fault changes during the mainshock and most of the aftershocks occur on the same fault. Sometimes the change in stress is great enough to trigger aftershocks on nearby faults as well.
An earthquake large enough to cause damage will probably produce several felt aftershocks within the first hour. The rate of aftershocks dies off quickly. The day after the mainshock has about half the aftershocks of the first day. Ten days after the mainshock there are only a tenth the number of aftershocks. An earthquake will be called an aftershock as long as the rate of earthquakes is higher than it was before the mainshock. For big earthquakes this might go on for decades.
Bigger earthquakes have more and larger aftershocks. The bigger the mainshock, the bigger the largest aftershock, on average, though there are many more small aftershocks than large ones. Also, just as smaller earthquakes can continue to occur a year or more after a mainshock, there is still a chance for a large aftershock long after an earthquake.

Foreshocks

Sometimes what we think is a mainshock is followed by a larger earthquake. Then the original earthquake is considered a foreshock. The chance of this happening dies off quickly with time just like aftershocks. After three days the risk is almost gone.
Sometimes, the chance that an event is a foreshock seems higher than average — usually because of its proximity to a major fault. The Governor's Office of Emergency Services will then issue an advisory based on scientists' recommendations. These are the only officially recognized short-term "predictions."

What is a fault?

Earthquakes occur on faults. A fault is a thin zone of crushed rock separating blocks of the earth's crust. When an earthquake occurs on one of these faults, the rock on one side of the fault slips with respect to the other. Faults can be centimeters to thousands of kilometers (fractions of an inch to thousands of miles) long. The fault surface can be vertical, horizontal, or at some angle to the surface of the earth. Faults can extend deep into the earth and may or may not extend up to the earth's surface.

How do we know a fault exists?

  • Past fault movement has brought together rocks that used to be farther apart;
  • Earthquakes on the fault have left surface evidence, such as surface ruptures or fault scarps (cliffs made by earthquakes);
  • Earthquakes recorded by seismographic networks are mapped and indicate the location of a fault.
Some faults have not shown these signs and we will not know they are there until they produce a large earthquake. Several damaging earthquakes in California have occurred on faults that were previously unknown.
Carrizo Plain National Monument along the San Andreas fault

How do we study faults?

Surface features that have been broken and offset by the movement of faults are used to determine how fast the faults move and thus how often earthquakes are likely to occur. For example, a streambed that crosses the San Andreas fault near Los Angeles is now offset 83 meters (91 yards) from its original course. The sediments in the abandoned streambed are about 2,500 years old. If we assume movement on the San Andreas has cut off that streambed within the last 2,500 years, then the average slip rate on the fault is 33 millimeters (1.3 inches) per year. This does not mean the fault slips 33 millimeters each year. Rather, it stores up 33 millimeters of slip each year to be released in infrequent earthquakes. The last earthquake offset the streambed another 5 meters (16 feet). If we assume that all earthquakes have 5 meters (5000 millimeters) of slip, we will have earthquakes on average every 150 years: 5000 millimeters divided by 33 millimeters per year equals 150 years. This does not mean the earthquakes will be exactly 150 years apart. While the San Andreas fault has averaged 150 years between events, earthquakes have occurred as few as 45 years and as many as 300 years apart.
Types of Faults 

Earthquake Faults




Salt Lake County Earthquake Fault Map
Salt Lake County fault map

What is a fault? A fault is a break in the earth's crust along which movement can take place causing an earthquake. In Utah, movement along faults is mostly vertical; mountain blocks (for example, the Wasatch Range) move up relative to the downward movement of valley blocks (for example, the Salt Lake Valley).
Why are faults a concern? Faults with evidence of Holocene (about 10,000 years ago to present) movement are the main concern because they are most likely to generate future earthquakes. If the earthquake is large enough, surface fault rupture can occur.
What is surface fault rupture? With a large earthquake (about magnitude 6.5 and greater), the fault rupture can reach and displace the ground surface, forming a fault scarp (steep break in slope). The resulting fault scarp may be several inches to 20 feet in height, and up to about 40 miles in length, depending on the size of the earthquake.
What are the effects of surface fault rupture? An area hundreds of feet wide can be affected, called the zone of deformation, which occurs chiefly on the downthrown side of the main fault and encompasses multiple minor faults, cracks, local tilting, and grabens (downdropped blocks between faults). Buildings in the zone of deformation would be damaged, particularly those straddling the main fault.
Also, anything crossing the fault, such as transportation corridors, utilities, and other lifelines, both underground and above ground, can be damaged or broken. The ground can be dropped below the water table on the downdropped side, resulting in localized flooding.
Surface fault rupture can also cause tectonic subsidence, which is the broad, permanent tilting of the valley floor down toward the fault scarp. Tilting can cause flooding along lake and reservoir shorelines nearest the fault; along altered stream courses; and along canals, sewer lines, or other gravity-flow systems where slope gradients are lessened or reversed.
Where and when is surface fault rupture likely to occur? On the Holocene fault on which a magnitude 6.5 (approximate) or larger earthquake occurs. On average, these earthquakes may occur once every 120 years on various faults in the Wasatch Front region; once every 350 years somewhere along the central part of the Wasatch fault (between Brigham City and Nephi); once every 2,000 years at any specific locality along the central Wasatch fault; and once every 5,000 to 20,000 years or more on other Holocene faults in the state.
What can be done to protect homes? Faults can be avoided by setting homes back a safe distance. Special-study areas have been delineated along faults where geologic studies are recommended to assess the hazard, locate faults, and recommend setbacks. However, the use of special-study areas in land-use ordinances varies by county and city, as does the level of enforcement.
Therefore, buyers, particularly of older homes (pre-1985), should personally check available fault maps to see if the home is near a fault (within a few hundred feet) and, if so, may want a geological site investigation performed. For newer homes, buyers should check with the county or city to determine whether geologic studies were performed for the site or subdivision and, if so, look at a copy of the geologic report.
Where to get additional information. Detailed fault and special-study-area maps used in ordinances are available at county and city planning departments.
Quaternary Fault and Fold Database of the United States (outside link)
Located on USGS website. Contains information on faults and associated folds in the United States that are believed to be sources of M>6 earthquakes during the Quaternary (the past 1,600,000 years). Maps of these geologic structures are linked to detailed descriptions and references.
Other earthquake hazards: liquefaction and ground shaking.

Earthquake Fault Maps

Earthquake Fault Map of Salt Lake County; PI-3 pdf
Earthquake Fault Map of Utah County; PI-11 pdf
Earthquake Fault Map of Davis County; PI-2 pdf
Earthquake Fault Map of Weber County; PI-1 pdf
Earthquake fault map of a portion of Cache County, Utah, (pdf) PI-83
Earthquake fault map of a portion of Tooele County, Utah, (pdf) PI-84
Earthquake fault map of a portion of Washington County, Utah, (pdf) PI-85

Faults and Earthquakes

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
First-time Visitors: Please visit Site Map and Disclaimer. Use "Back" to return here.

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

What Causes Earthquakes?

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

Elastic Rebound Theory

elastic rebound 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 Left-Lateral Fault
    • Right-Lateral
    • (San Andreas - 21 Ft. in 1906) Right-Lateral Fault
  • Dip-Slip - Vertical Motion
    • Normal (Extension) Normal Fault
    • Reverse or Thrust (Compression)
    • (San Fernando, 1971
    • Alaska, 1964 - up to 150 Ft.) Thrust Fault
  • Nappe
  • Overthrust

Fault Structures - Normal Faults

Fault-Block Structures - 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.
 Overthrust Fault 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.

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

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

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.
How Seismographs work 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.
Seismic Waves

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

Isoseismals, 1906 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

San Francisco, 1906 San Francisco, 1906
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).
San Francisco, 1906 San Francisco, 1906
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

isoseismals, New Madrid 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

Seismic Magnitude Scale 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.
Seismic Magnitude Scale There are not too many familiar analogies for very large earthquakes, but very small events overlap the energies of many familiar phenomena.

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
eastern earthquakes

U.S. Earthquakes, 1973-2002

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

Seismic Risk Level Maps for the U.S.

Probable ground acceleration in 50 years. Blue = small, red = large
U.S. seismic hazard
Probability of damage in 100 years. Blue = negligible, green = low, red = high.
  U.S. seismic hazard

Seismic Gaps

seismic gaps seismic gaps
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.
Earthquake frequency
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.

Seismology and Earth's Interior

Successive Approximation in Action

seismic modeling
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.
seismic modeling
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
seismic modeling
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

seismic 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).
seismic refraction
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.
seismic waves
 The animation below shows the wave front of a P-wave in the earth.
seismic waves

Inner Structure of the Earth

seismic waves
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.
earth's interior 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.

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Created 15 Jan 1997; Last Update 02 March 2010
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