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What are earthquakes ? Why do they occur ? Why can't we predict them ?Although we still can't predict when an earthquake will happen, we have learned much about earthquakes as well as the Earth itself from studying them. We have learned how to pinpoint the locations of earthquakes, how to accurately measure their sizes, and how to build flexible structures that can withstand the strong shaking produced by earthquakes and protect our loved ones.
Just the Facts An earthquake is a sudden shaking of the ground. They generate seismic waves which can be recorded on a sensitive instrument called a seismograph. The record of ground shaking recorded by the seismograph is called a seismogram.
What is a Seizomgraph Earthquakes generate seismic waves which can be detected with a sensitive instrument called a seismograph. Advances in seismograph technology have increased our understanding of both earthquakes and the Earth itself. Perhaps the earliest seismograph was invented in China A.D. 136 by a man named Choko.
A Long Time Ago This early eastern seismoscope consisted of a copper vessel with eight dragon heads attached to it, positioned above eight frogs. Each dragon head held a ball in its mouth, which, when dropped due to the strong shaking of an earthquake, would fall into the open mouth of the frog directly below it. By noting which frogs contained balls after a strong earthquake, it was possible to determine how the Earth had moved in response to the earthquake.
Floating Plates The Earth's outermost surface is broken into 12 rigid plates which are 60-200 km thick and float on top of a more fluid zone, much in the way that icebergs float on top of the ocean.
Shifting and Sliding Along the boundary separating any two plates, the relative motion between the plates can be classified into one of 3 categories: Divergent Convergent Transformatinal
Divergent Motion: Movin' Apart This is the most common kind of motion along the mid-ocean ridges. This is a system of undersea mountain ranges that extends beneath the world's oceans and connects together like the seams on a baseball. You can simulate divergent plate motion by placing your palms down with your index fingers touch. Now pull your hands apart.
Convergent Motion: Comin' Together This is the most common kind of motion at subduction zones. This motion happens where dense oceanic plates collide and slide beneath continental plates. You can simulate convergent plate motion by placing your hands together in front of you with your palms down. Now gently slide your bottom hand downward and your top hand upward, while keeping them in contact.
Transformational Motion: A good example of this type of motion is the San Andreas Fault which runs through California. You can simulate transformational plate motion by placing your hands together in front of you with your palms down. Now slide your left hand away from you and your right hand toward you.
The movement between plates and along faults is not smooth. They move in jerks, giving rise to earthquakes. The locations of earthquakes throughout the world delineate the major tectonic boundaries. What a Jerk!
Faults are narrow zones in the Earth, usually extending no more than about 10 miles deep, which separate rigid crustal blocks. A well known fault is the San Andreas Fault which separates the Pacific plate from the North American plate. The Pacific plate has San Francisco and Los Angeles on it, while the North American plate contains the rest of California and the U.S. The Pacific plate is moving to the northwest at a rate of about 4 inches per year. It's Not Your Fault
Under Stress Earthquakes occur on faults. Because faults have friction, they resist the forces trying to move the pieces apart. As the forces build, the fault remains locked and the blocks get deformed because of the increasing stress. Eventually the stresses get so high that the fault breaks. This releases the built up stress and allows the sides of the fault to slide past one another. This is what we call an earthquake. The blocks return to their locked state until the stresses build up enough to cause another earthquake.
How to Release Stress You can simulate the earthquake by placing your hands together with your thumbs up. By pushing your hands together you create pressure between your hands. As you try to slide your hands apart, you create friction which stops you from sliding your hands easily. Stress builds up in your hands and arm.
Spreading the Motion Tectonic plates are somewhat flexible. The motion between them is not confined entirely to their own boundaries. The motion extends into their interiors and is spread out among a system of faults all around the plate's boundary.
A Drop of Water Earthquakes create seismic waves which shake the ground as they pass. They sometimes cause buildings to topple. Earthquakes create waves just like waves of water moving across the ocean and waves of air moving across a field of wheat. Consider what happens when a drop of rain hits a pond of water. The drop disturbs the flat surface of the water and creates waves that travel outward in all directions from the disturbance. These waves travel on the surface of the pond, along the interface between the water and the air.
Around the World Earthquakes generate seismic waves which travel all around the world and can be detected with a sensitive instrument called a seismograph. . This was first discovered in 1889 by E. von Rebleur Paschwitz who noted that waves recorded on a horizontal pendulum in Potsdam, Germany were generated by an earthquake far away in Tokyo, Japan.
The Slinky and The Rope Earthquakes generate several kinds of seismic waves including P, for "Primary" and S, for "Secondary" waves. P Waves The P waves move in a compressional motion similar to the motion of a slinky, while the S waves move in a shear motion perpendicular to the direction the wave is travelling. S Waves
A Shaking Record A seismogram is a record of the ground shaking recorded by a seismograph. The P waves travel fastest through the Earth so they arrive at a seismograph first, followed by the S waves and lastly by the surface waves. The figure above is the seismogram of the 1989 Loma Prieta earthquake recorded at a seismograph in Kongsberg, Norway, 8400 km (about 5,200 miles) away.
Locatin' the Shakin' Seismologists locate earthquakes by measuring the time between the P and S waves in a seismogram.After a seismogram "feels" an earthquake, scientists compare the time difference of these waves to figure out how far away the earthquake is. It takes at least three seismograms to locate exactly where the earthquake is. One seismograph can only tell how far away it is from that seismograph. The earthquake could be located anywhere on a circle of radius equal to this distance and centered on the observation station. By measuring the S-P times at 3 or more stations these circles can be drawn around each station and where they meet indicates the earthquake location
The Bigger The Better For very large earthquakes (Magnitude greater than 7), the Richter scale saturates, so that the S waves from an M 7 earthquake are not appreciably different from those of an M 7.8 earthquake. For these larger events, seismologists use a slightly different measure of the earthquake size called the moment magnitude. The moment magnitude is directly related to the size of the earthquake rupture area or fault plane, and does not saturate for large events. For smaller events the Richter and moment magnitudes are similar.
The Great San Francisco earthquake The Great San Francisco earthquake occurred on Apr. 18, 1906 at 5:12 a.m. and had a magnitude of 8.25 on the Richter scale. It is the largest earthquake to occur along the San Andreas fault in the last 200 years. 700 deaths were attributed to the earthquake, though current estimates place the death toll 3-4 times higher. Though the ground shaking only lasted some 45-60 seconds, the earthquake triggered fires throughout the city that lasted over 12 hours and destroyed much of the downtown.
The Loma Prieta earthquake The Loma Prieta earthquake of 1989 was centered in the southern Santa Cruz mountains. It was our nation's most costly natural disaster, causing over $6 billion in property damage, 62 deaths and 3,757 injuries. 41 of the deaths occurred when the double-decked section of the 880 freeway in Oakland collapsed
The convergent motion generated by the fault raised the Santa Cruz mountains by approximately 3 feet, leading scientists to propose that the Santa Cruz mountains have grown from repeated fault ruptures over millions of years. Photos by Deanne Fitzmaurice.
