unit 17

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Earthquakes

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Earthquakes are not random events. Earthquakes occur when stresses build up inside the earth to point where they exceed the strength of the surrounding rock, or exceed the force of friction holding two sides of a fault in place. The most common place we see this is at plate boundaries.

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Different colored dots represent earthquakes that originated at different depths. Note that the deepest ones (red) are associated with subduction zones. You can even tell which plate is being subducted by which side the red dots are on. (Red dots on left side means the plate on the right side is being subducted.) We’ll take a closer look at a cross-section of the earth here.

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Same color dots. In this example, red dots (deeper earthquakes) are on the right, which means the plate on the left is being subducted. Volcanic mountain ranges and ocean trenches share a similar relationship. The subducting plate will always be on the same side as the trench.

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Earthquake hazard map for the U.S. We expect earthquakes near active plate boundaries such as the West Coast and the Aleutian Islands, but what about the Mississippi River and Hawaii? Aleutian Islands (volcanic island arc)

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Hawaii is over a hot spot. Expanding magma exerts great force on surrounding rocks. The area along the Mississippi is known as the New Madrid fault zone. This is thought to be a failed rift zone that is still somewhat active. The largest earthquake in U.S. history occurred here in 1812. Aleutian Islands (volcanic island arc)

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Cause of ground shaking Ground waves are called seismic waves . Seismic waves are generated when rock is flexed, and snaps back into place after the rock breaks or slides along a fault.

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Forces place stress in opposite directions on either side of a fault. The rock on either side begins to move, but friction along irregular surfaces of the fault prevents slippage, so the rock flexes. The stress eventually exceeds the friction and the fault gives way. As the rock snaps back into place, seismic waves are generated.

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This is known as elastic rebound – flexed rock snaps back into place after a rupture. The amount of displacement is called the slip . slip

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Seismic Waves 1. Types of seismic waves P waves (primary waves) These are compressional waves. They do not form the traditional “S” shape of an ocean wave, but compress and expand rock as they travel. (Think of a slinky held from above and jiggled up and down.) These are the fastest waves, traveling 6-8 km/s. P waves travel through solids and liquids (meaning they travel through the earth’s liquid outer core.) compression and expansion as wave moves to right

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S waves (secondary waves) S waves have the characteristic “S” shape of an ocean wave. These are slower than P waves, traveling 4-5 km/s. S waves travel through solids, but not through liquids. Surface waves Surface waves also have the characteristic “S” shape, but travel only along the surface. These are the slowest waves, but cause the most damage.

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2. Measuring seismic waves Seismic waves are measured using a seismometer (also called a seismograph ). Two different configurations are needed to measure P waves (back and forth motion) and S or Surface waves (up and down motion). In both configurations, a sensor is suspended so that it remains stationary while the instrument around the sensor is shaken. The original versions used springs or cantilevers with a pen suspended over paper on a slowly turning drum. When seismic waves passed, the pen remained stationary while the paper and drum were shaken. Modern instruments employ the same concepts but with more sophisticated equipment and electronic sensors.

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3. Finding where an earthquake originated focus – location where the slip occurred beneath the surface epicenter – point on the surface directly above the focus When the origin of an earthquake is mentioned, we usually refer to the epicenter. We can find the epicenter using S and P waves because they travel at different speeds. The farther away the epicenter, the greater the lag time between the arrival of the first P waves, and the arrival of the first S waves.

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5600 km 8600 km With one station, we may know an earthquake occurred 1500 km away, but we don’t know in what direction. 1500 km At a second station, the earthquake originated 5600 km away. A third station allows us to pinpoint the location of the epicenter. This narrows the origin down to two possible locations.

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Earthquake Magnitude There are three different scales used to report the intensity of an earthquake: Richter Scale Moment Magnitude Scale Modified Mercali Scale The Moment Magnitude scale is preferred by many geophysicists, but is rarely used among the general population. Only the Richter Scale and the Modified Mercali Scale will be covered in this section.

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1. Richter Scale When a news reporter comes on TV and announces that a magnitude 6.2 earthquake hit San Bernardino, California, they are using the Richter Scale. So what does this number mean? The Richter Scale provides a measure of the energy of an earthquake at the epicenter (not wherever felt – only at the epicenter). Each number is 10 times more powerful than the previous number (logarithmic scale). For example, 6 is 10 times stronger than 5, which means 7 is 100 times stronger than 5. We can feel an earthquake starting at about magnitude 3. Earthquakes are destructive starting at about magnitude 5.

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Frequency of different magnitude earthquakes Magnitude Frequency < 3 (unfelt without detectors) ~800,000 per year (over 2,000/day) 6 ~100 per year 8+ once every 3-10 years Magnitude 6 and 7 are potentially destructive earthquakes, but we don’t hear about 100 of them each year because most are out in the ocean.

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This figure provides more detail along with examples of specific earthquakes. It also compares the energy released by various earthquakes with other familiar energetic events. The tsunami generating earthquake near Sumatra in December, 2004, was a 9.3, making it one of the strongest ever recorded.

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2. Modified Mercali Scale The Modified Mercali Scale is a measure of the destructiveness of an earthquake where felt. The Richter scale has no theoretical limits (though there is a practical limit to how much the earth can shake) and reported intensities can include decimal values (for example, 6.4 or 5.7). The Mercali scale is divided into 12 discrete categories based on reported damage. These categories are designated 1 through 12, often as Roman Numerals (I through XII). (Make sure you know your Roman numerals up through XII for the exam.)

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The next two slides provide details of the scale. You do not need to memorize the whole table, but you should remember of few facts that are underlined.

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Influence of local geology Earthquakes of equal magnitude in two different places will not necessarily feel the same at equal distances. The geology between the epicenter and the location where felt plays an important role. Seismic waves attenuate (grow weaker) more as they travel through rock than through unconsolidated sediments. This is illustrated in the next slide.

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Two earthquakes nearly identical in intensity at their epicenters are shown below. The colored zones represent areas of equivalent shaking or damage. The New Madrid earthquake was felt over a much wider area than the California earthquake because southern California is mostly rock, while the mid-South is mostly unconsolidated sediments.

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The last really big earthquake from the New Madrid region was an estimated 8.0 on the Richter Scale in 1811 and 1812. This earthquake was felt as far away as Maine! Note Oxford is near zone VIII on this Mercali scale – moderate to heavy damage. (No damage reported because Oxford was not established until 1837.) No lines are drawn west of the Mississippi River because the west was still largely unsettled and no first hand accounts are available. Modified Mercali Scale

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Is the New Madrid still active? Could another big one occur? The answer is yes to both. This map shows the epicenters of minor earthquakes measured since 1974. Another big one will almost certainly happen, though perhaps not in our life time.

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Earthquake Hazards Earthquakes rarely kill anyone directly – just a bouncy, unnerving ride Deaths are typically caused by (in order of significance) collapsing buildings fire (broken gas lines, burning buildings) landslides tsunamis (created by normal or reverse faulting under large body of water)

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Ironically, it is not the tallest buildings that are necessarily at greatest risk. Just as slight but well timed shaking can create large waves in a rope, so seismic waves can be magnified in buildings of a specific height or design. Buildings

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Mexico City, 1985 The smaller and taller buildings received much less damage than the center building of intermediate size.

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Mexico City, 1985 This hospital was destroyed while the much taller building behind it remained standing.

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In this earthquake in Japan in 1964, buildings did not collapse, but tipped and sank into sand liquefied by the seismic vibrations.

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Fire San Francisco, 1906 Fires, started when the earthquake ruptured gas lines, burned down 42,000 buildings. Earthquake Hazards

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The 1970 Yungay landslide in Peru that killed 17,000 people was used as an example in the Mass Wasting unit. The landslide was triggered by an earthquake. Avalanche deposits that buried several towns. Landslides

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Earthquakes can generate tsunamis in two ways. 1. If a submarine normal or reverse fault ruyptures, water will be displaced and generate a wave. It is the slippage that causes the tsunami, however, not the seismic waves. 2. An earthquake can shake loose sediment causing a submarine landslide. The ocean surface suddenly drops above the source of the slide, and is suddenly elevated above where the slide travels, both creating a large wave. Tsunamis

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December 26, 2004, a thrust (reverse) fault associated with a convergent boundary off the coast of Sumatra suddenly ruptured, releasing an earthquake registering 9.3 on the Richter scale. Ironically, the earthquake itself caused little damage. Immense damage was inflicted by a tsunami. A large section of the ocean floor was abruptly elevated as the thrust fault slipped, which suddenly raised the ocean above the fault. Immense waves traveled across the Indian Ocean destroying coastal cities and villages as far away as Africa. Epicenter of earthquake at star.

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Banda Aceh, Indonesia Before tsunami After tsunami

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Tectonic Setting The Indian plate is subducting under the Burma microplate. Slippage along this fault zone caused the earthquake and tsunami.

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Top 5 earthquakes in recorded history 1. Chile 1960 9.5 2. Prince William Sound, Alaska 1964 9.2 3. Andreanof Islands, Alaska 1957 9.1 4. Kamchatka 1952 9.0 When this map was made, the Sumatra earthquake was thought to be a 9.0, making it the 5 th largest. It has since been upgraded to a 9.3, making it the 2 nd largest.

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March 11, 2011 9.0 earthquake (epicenter at star) Convergent boundary Japan is a volcanic island arc formed by the subduction of the Pacific plate

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Tsunami forms by sudden uplift or sudden drop in water level. Wave radiates outward from the displacement.

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Waves from the Japan tsunami reached Hawaii and the west coast of the US. They weaken as they radiate, so waves hitting the US were small.

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North of Sendai, Japan 2011 Before tsunami After tsunami

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Predicting Earthquakes 1. Early warning tremors Small earthquakes often precede big earthquakes In China in 1975, warning of a big earthquake was issued due to a series of small ones (and strange animal behavior). The big one hit five hours later. The early warning saved thousands of lives. However, most earthquakes are not as cooperative. In 1976, a massive earthquake hit Tangshan, China, with little warning – over 250,000 people were killed.

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2. Seismic gap method Big earthquakes sometimes occur at approximately regular intervals. The longer the time since the last major earthquake, the sooner the big one is coming California - big earthquakes seem to occur about once in 100 to 150 years. The last really big one was in 1906, which leveled much of San Francisco. Another big one is probably coming somewhere between tomorrow and 30 years from now. (The 6.9 near San Francisco in 1989 wasn’t it!)

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When a segment of a fault slips, stress increases on the adjacent locked segments. If segments slip in sequence, the next slip point may be predicted. Turkey provides a good example of how this can work. 3. Monitoring movement and stress along faults

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Turkey is circled in these maps. The African plate is moving north, slowly closing the Mediterranean Sea. The plate boundary cuts through Turkey. Note all the earthquake activity in the map on the right.

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Sections of the Anatolian Fault have ruptured in a semi-regular sequence. This observed sequence allowed the 1999 earthquake (9) to be predicted a few years before it occurred. 1 2 3 5 4 6 7 9 8

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Unique uses of seismic waves 1. Detection of underground nuclear weapons testing The red countries are nations we know have tested nuclear weapons, but that deny having them (somewhat outdated now). Our knowledge is not from spies, but from geophysicists studying seismic waves.

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Natural earthquakes yield prolonged shaking. Nuclear blasts yield a high energy initial P-wave that dies out rapidly, with very little S waves. (There is no slippage with an explosion to generate the S waves.)

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2. Monitoring volcanoes Set off blasts or strike the ground on one side of the volcano, and measure the seismic waves on the other side. P waves will pass through liquid magma, S waves will not. By analyzing the “shadow” created by the missing S waves, the size of the magma chamber and any changes over time can be monitored. S P P & S P only magma

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3. Exploring the earth’s interior Wonder how we can tell the earth’s outer core is liquid when our deepest drill holes can’t even reach the mantle? Seismic waves! The same principle used to monitor volcanoes is used to analyze the earth’s core. P only P only P & S P & S Earthquake epicenter no S waves, core must be liquid

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Parting thought - Statue of Louis Agassiz, a famous geologist, that stood atop the Geology Building at Stanford until the earthquake of 1906. We can shape and control many natural processes to our liking, but when nature unleashes its full force, man always gets stood on his head!

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Good luck on the final exam and whatever comes next for you! End of Earthquakes