Earthquakes Vibration of the Earth produced by the rapid release of energy. ….. Massive energy! Earthquakes occur along plate boundaries at points called faults. Energy is stored in the rocks which produces stress and strain… until the rock breaks! Releasing stored energy in the form of seismic waves.

Focus and Epicenter The focus is the earthquake's underground point of origin or hypocenter. The epicenter is the point on the Earth’s surface that is directly above the point where an earthquake originates or focus.

Stress and Strain: Rock Behavior Strain - the result of stress or deformation .

Stress and Strain: the forces of the earthquake Tectonic forces apply stress to rock in three basic forms 1. Compression: pushing together or compression 2. Tension : Stress that acts to lengthen an object or pull it apart. 3. Shear/Transform: Stress that acts parallel to a surface. It can cause one object to slide over another. The most general definition is that shear acts to change the angles in an object.

Elastic Rebound To view this animation, click “View” and then “Slide Show” on the top navigation bar.

Stress and Strain along Faults

Stress and Strain along faults

Fault Types There are three basic fault types Normal faults form when the hanging wall drops down. The forces that create normal faults are pulling the sides apart, or extensional.

Fault Types There are three basic fault types 2. Reverse faults form when the hanging wall moves up. The forces creating reverse faults are compressional, pushing the sides together.

Fault Type 3. Strike-slip faults have walls that move sideways, not up or down. That is, the slip occurs along the strike, not up or down the dip. In these faults the fault plane is usually vertical, so there is no hanging wall or footwall. The f orces creating these faults are lateral or horizontal, carrying the sides past each other.

FAULTS

Faults To view this animation, click “View” and then “Slide Show” on the top navigation bar.

Normal Fault                                                     

Reverse Fault

Strike-Slip Fault

Strike Slip Fault

Strike Slip Fault

P-Waves or Primary Waves S-Waves or Secondary Waves Seismic Waves: There are two types of body waves P-Waves or Primary Waves S-Waves or Secondary Waves

P-Waves P waves arrive first. Primary, pressure waves. Particle motion is along the direction of travel of the wave. P waves can travel through solids, liquids or gases.

Earthquake Waves To view this animation, click “View” and then “Slide Show” on the top navigation bar.

P-Wave Motion Push-Pull Motion

P-Wave Motion P waves are compression waves - the wave pulse or pulses travels through the rock in a series of compression pulses. On either side of the compression the rock is stretched.

S-Wave Motion S-shake or shear wave

S-Wave Motion S waves are characterized by a sideways movement. The rock materials are moved from side to side as the wave passes. Travel are like water waves. Rocks are more resistant to sideways motion so the S wave travels more slowly.

Surface Waves The surface waves are the slowest of the three earthquake wave types.

1. L-waves or long waves. Complex motion. Up-and-down and side-to-side 1. L-waves or long waves.Complex motion. Up-and-down and side-to-side. Slowest. Causes damage to structures during an earthquake.

Seismic Wave Motion To view this animation, click “View” and then “Slide Show” on the top navigation bar.

Using Seismic Waves to Study Earth's Interior Seismic Waves travel through the entire Earth Both S and P waves travel throughout the body of the earth, and can be picked up by seismometers - machines that record earthquakes - anywhere in the world.

Seismic waves as “x-rays” to look inside the earth P-Waves travel through solid and liquid

Seismic waves as “x-rays” to look inside the earth However, it turns out that S waves cannot travel through the core, and only P waves are recorded in some places: S-Waves travel only through solids

A seismometer records the vibrations from earthquakes A seismometer records the vibrations from earthquakes. Mechanical versions work by way of a large mass, freely suspended. In the example on the left, a rotating drum records a red line on a sheet of paper. If the earth moves (in this case from left to right) the whole machine will vibrate too. However, the large mass tends to stay still, so the drum shakes beneath the pen, recording a squiggle! Seismometers

Seismograph: the record of the Earthquake The record of an earthquake, a seismograph, as recorded by a seismometer, will be a plot of vibrations versus time. On the seismograph, time is marked at regular intervals, so that we can determine the time of arrival of the first P-wave and the time of arrival of the first S-wave.  

Seismograph

Triangulation If three arrival times are available at three different seismic stations then triangulation can be used to find the location of the focus or epicenter and the time of occurrence of the earthquake. The distance between the beginning of the first P wave and the first S wave tells you how many seconds the waves are apart.

Triangulation P waves move about 5.5 kilometers per second (k/s) through granite, whereas the slower S waves move only about 3 k/s through granite. Imagine that at station A a P wave is detected and the S wave follows 42.8 seconds later. Since the S wave is 2.5 k/s slower than the P wave, difference in speed multiplied by the time difference will give the distance to the source. Thus, the earthquake epicenter is 107 km away from station A (42.8 s times 2.5 k/s= 107 km). Although we can determine the distance, we still don't know the direction, which is why we need data from the other stations.

Triangulation Since the P (or “primary”) waves travel faster than the S (or “secondary”) waves, P waves will arrive at a given seismograph station sooner than S waves. In other words, the S waves lag behind the P waves. In fact, the time difference between when the P waves arrive at a seismograph station and when the S waves arrive at the same station is called Time Lag. Knowing the time lag for a number of seismograph stations is essential in pinpointing the location of the epicenter of an earthquake.

Collecting data from the recording stations: Station A: San Francisco, California P-Wave arrival 3:02:20 S-Wave arrival 3:06:30 What is the time difference between P and S wave arrivals? 4:10

Collecting data from the recording stations: Station B: Denver, Colorado P-Wave arrival 3:01:40 S-Wave arrival 3:05:00 What is the time difference between P and S wave arrivals? 3:20

Collecting data from the recording stations: Station C: Missoula, Montana P-Wave arrival 3:01:00 S-Wave arrival 3:03:00 What is the time difference between P and S wave arrivals? 2:00

Difference in arrival times: San Francisco: 4:10 minutes/sec Denver, Colorado: 3:20 minutes/sec Missoula, Montana: 2:00 minutes/sec

Locating the Epicenter Finally we plot the P and S wave travel-time curves to find the distance from each station to the earthquake epicenter. We do this by finding the unique epicenter distance where the difference in the P and S wave travel times is exactly equal to the difference you calculated from the seismogram. (we use a time/distance curve plot)

WE TAKE A PIECE OF PAPER, AND MARK OFF THE DIFFERENCE IN ARRIVAL TIME 4:10 2800Km

WE MOVE THE PAPER UNTIL THE TWO TICK MARKS LINE UP WITH THE P AND S CURVES WHEN TICK MARKS LINE UP, GO STRAIGHT DOWN AND READ THE EPICENTER DISTANCE EPICENTER DISTANCE OF 2800 KM

EPICENTER DISTANCES San Francisco: 4:10 Denver, Colorado: 3:20 2,800km Denver, Colorado: 3:20 2,000km Missoula, Montana 2:00 1100km

Epicenter Distances Using the map scale, and a drafting compass we set it to the appropriate length for the distance from the first location to the epicenter. Place the compass point at this location and draw an arc using the distance as the radius. Repeat for the other two locations. The intersection of the three arcs identifies the epicenter of the earthquake.

Recording Board Difference in arrival times: San Francisco: 41:0 2,800km Open your compass to the EXACT distance on the scale. 1,000 2,000 3,000 4,000 5,000

. . . Use your carefully set compass to draw a circle around each seismic station. . . . You've found a possible epicenter! Does this seem like a likely location for an earthquake to occur?

Seismographs To view this animation, click “View” and then “Slide Show” on the top navigation bar.

Measuring Earthquakes Mercalli scale Richter scale Magnitude is a measurement of earthquake strength based on seismic waves and movement along faults

Earthquake Strength The intensity or strength of an earthquake is measured by seismologist in two main ways: 1.The Richter Scale measures the amount of energy that an earthquake releases Each number of magnitude is 10x stronger than the number below it.

The Richter Scale The Richter scale is a rating of the size of seismic waves as measured by a particular type of mechanical seismograph Developed in the 1930’s All over the world, geologists used this for about 50 years Electric seismographs eventually replaced the mechanical ones used in this scale Provides accurate measurements for small, nearby earthquakes Does not work for big, far ones

Earthquake Strength 2. The Mercalli Scale Measures the amount of damage from an earthquake Ranges from I to XII Based on common earthquake occurrences such as "noticeable by people" "damage to buildings" chimneys collapse" "fissures open in the ground”.

The Mercalli Scale Developed in the twentieth century to rate earthquakes according to their intensity The intensity of an earthquake is the strength of ground motion in a given place Is not a precise measurement But, the 12 steps explain the damage given to people, land surface, and buildings The same earthquake could have different Mercalli ratings because of the different amount of damage in different spots The Mercalli scale uses Roman numerals to rank earthquakes by how much damage they cause

How Earthquakes Cause Damage The severe shaking provided by seismic waves can damage or destroy buildings and bridges, topple utility poles, and damage gas and water mains With their side to side, up and down movement, S waves can damage or destroy buildings, bridges, and fracture gas mains.

Earthquake damage in Anchorage Alaska on March 27, 1964

San Francisco is built on sandy soil or fill San Francisco is built on sandy soil or fill. Many structures built on this type of “soft” soil were badly damaged during the 1989 Loma Prieta earthquake.

Tsunami Damage, Gleebruk, Indonesia 2005

Tsunami To view this animation, click “View” and then “Slide Show” on the top navigation bar.

Thanks to Mr. Welch for the use of his materials.