Learning log Describe what you think is happening during an earthquake. Essential Question: What causes earthquakes, how do we know where they are, and how are they described?

Earthquakes

Forces Within Earth Earthquakes are natural vibrations of the ground caused by plate movement in Earth’s crust or by volcanic eruptions In some instances a single earthquake has killed more than 100,000 people and destroyed entire cities. Anyone living in an area prone to earthquakes should be aware of the potential danger and how to minimize the damage that they cause.

Stress and Strain Most earthquakes occur when rocks fracture, or break deep within Earth Fractures form when stress (forces acting on a material) exceed the strength of the rock 3 kinds of stress: – Compression – decreases the volume of a material – Tension – pulls a material apart – Shear – causes a material to twist

Strain is the deformation of materials in response to stress. Rocks fail when stress is applied too quickly, or when stress is too great.

What is a fault? the fracture or system of fractures along which movement occurs. The surface along which the movement takes places is called the fault plane. 3 basic types of faults: – Reverse fault – form as a result of horizontal compression – Normal fault – caused by horizontal tension – Strike-slip fault – caused by horizontal shear Clip 0-13 on Engineering Earthquakes Reverse Fault Normal Fault Strike-slip Fault

Earthquake Waves The vibration of the ground during an earthquake are called seismic waves. Every earthquake generates 3 types of seismic waves: – Primary waves (P-waves) – causes rock particles to move back and forth as it passes

Earthquake Waves Secondary waves (S-waves) causes rock particles to move at right angles to the direction of the wave

Earthquake Waves Surface waves – causes rock particles to move both up and down and side to side. Surface waves travel along Earth’s surface and produce the most serious damage. P-waves and S-waves (body waves) pass through Earth’s interior

Earthquake Waves The first waves generated by an earthquake spread out from the point of failure of rocks Focus – Point where an earthquake originates, usually several km below the surface Epicenter – Point on Earth’s surface directly above the focus is the earthquake’s.

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Liquifaction – when earthquake vibrations make the loose soil behave like a liquid.

Measuring Energy Released and Locating Earthquakes More than one million earthquakes occur each year! More than 90% are not felt and cause little to no damage The ones that make the news are major seismic events that cause much damage.

Locating the Epicenter of an Earthquake The Triangulation Method

Locating the Epicenter In order to determine the location of an earthquake, the earthquake needs to be recorded on three different seismographs that are at significantly different locations. The other piece of information needed is the time it takes for P-waves and S-waves to travel through the Earth and arrive at a seismographic station.

The Triangulation Method Triangulation A mathematical method for locating the epicenter of an earthquake using three or more data sets from seismic stations. Seismograph - Earthquake monitoring instrument that records the seismic waves of the earthquake.

A seismograph records earthquake activity by plotting vibrations on a sheet of paper to create a seismogram. Above are some sample seismograms:

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, while 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 the 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 before S waves. In other words, the S waves lag behind the P waves. 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 3:02:20S-Wave arrival 3:06:30 What is the time difference between P and S wave arrivals?

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

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

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

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 Missoula, Montana 2:00 2,800km 1,100km 2,000km

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 1,000 2,000 3,000 4,0005,000 Open your compass to the EXACT distance on the map scale.

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Earthquake Magnitude & Intensity The amount of energy released during an earthquake is measured by its magnitude. Measured using the Richter Scale – based on the size of the largest seismic waves made by the quake 10-Fold (logarithmic) Scale: – meaning seismic waves of a magnitude-8 earthquake on the Richter scale are 10 times larger than a magnitude-7 and 100 times larger than a magnitude-6.

Earthquake Magnitude & Intensity Most seismologists use the moment magnitude scale – takes into account the size of the fault rupture, the amount of movement along the fault, and the rock’s stiffness – Moment magnitude values are estimated from the size of several types of seismic waves produced by an earthquake.

Earthquake Magnitude & Intensity Another way to assess earthquakes is to measure the amount of damage. Modified Mercalli Scale - measures the amount of damage done to the structures involved and is used to determine the intensity of an earthquake. uses the Roman numerals I to XII to designate the degree of intensity. Specific effects or damage correspond to specific numerals; the higher the numeral, the worse the damage.

Depth of Focus Another factor that determines a quake’s intensity is the depth of the quake’s focus – Can be classified as shallow, intermediate, or deep – Deep-focus = smaller vibrations at epicenter – Shallow-focus = larger vibrations at epicenter A shallow-focus, magnitude-6 quake will have greater intensity than a deep-focus, magnitude-8 quake Catastrophic quakes with high intensities are almost always shallow-focus quakes

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Seismic Belts Seismologists have collected and plotted locations of epicenters. – The global distribution of these epicenters reveals an interesting pattern. They are not randomly distributed Most earthquakes are associated with tectonic plate boundaries 80% occur along the Circum-Pacific Belt 15% occur along the Mediterranean-Asian Belt Most of the remaining occur at ocean ridges

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Locating an Earthquake Initially, the exact location of an earthquake’s epicenter and time of quake’s occurrence is not known But, it can be easily determined using seismograms and travel-time curves

Time of an Earthquake The exact arrival times of the P-waves and S- waves at a seismic station can be read from the seismogram. Using a travel-time graph, the time of occurrence can be determined by subtracting the travel time from the known arrival time of the wave.

Distance to an Earthquake To determine the location of the epicenter: – The locations of 3 seismic stations are plotted on a map – A circle whose radius is equal to the epicentral distance is plotted around each station – The point of intersection of these circles is the epicenter