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What are earthquakes ? Why do they occur ? and 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. What's Shakin'?
Just the Facts The record of ground shaking recorded by the seismograph is called a seismogram. An earthquake is a sudden shaking of the ground. They generate seismic waves which can be recorded on a sensitive instrument called a seismograph.
What is a Seismograph? • 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 m an named Choko.
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.
It's not your Fault • 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 Fransicso 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.
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.
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.
Layers of the Earth • By analyzing the seismograms from many earthquakes, scientists have discovered that three main levels or shells exist within the Earth: • CRUST • MANTLE • CORE
Don't Eat the Earth's CRUST • CRUST • MANTLE • CORE • The Earth's outermost surface is called the crust. The crust is typically about 25 miles thick beneath continents, and about 6.5 miles thick beneath oceans. The crust is relatively light and brittle. Most earthquakes occur within the crust. The crust in this image is much smaller in the real earth!
MANTLE in the Middle • CRUST • MANTLE • CORE • The region just below the crust and extending all the way down to the Earth's core is called the mantle. The mantle is relatively flexible so it flows instead of fracturing.
The CORE of the Matter • The earth has three main levels. • CRUST • MANTLE • CORE • Beneath the mantle is the Earth's core. The Earth's core consists of a fluid outer core and a solid inner core. Because the outer core contains iron, when it flows it generates a magnetic field. This is the source of the Earth's magnetic field.
The State of Quakes • Now that we've learned a little about what causes earthquakes, we can take a closer look at some historical quakes in California. • California has many active faults including the San Andreas fault which sometimes produce large earthquakes. • For this reason there is a very long history of active earthquake research in California to understand how and why earthquakes occur.
History and Technology • The history of earthquake monitoring technology and research is closely linked to the history of California earthquakes. California earthquakes are closely monitored with state-of-the-art equipment searching for any early warning signs that might someday be used to forecast earthquakes.
The Great San Francisco Quake • 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.
The Great Quake part II • 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 Great Quake part III • Geologists of the day noted that the strongest shaking within the city occurred on land that had recently been reclaimed from the San Francisco Bay. The fact that soft landfill greatly amplifies the shaking caused by earthquakes was "rediscovered" in 1989 when the Loma Prieta earthquake wreaked havoc on the North Beach and Marina districts.
The Long Beach Quake • The Long Beach Earthquake was on Mar. 10, 1933. It had a magnitude of 6.3 on the Richter scale. This earthquake occurred in the Newport-Inglewood fault, killing 115 people in Long Beach and the Los Angeles Basin. The earthquake toppled several unreinforced school buildings (fortunately the schools were not in session at the time) and led to the enactment of the Field Act mandating construction standards for California schools.
Loma Prieta part I • 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
Loma Prieta part II • In San Francisco the damage was especially severe in the Marina district because that area is built on landfill debris from the 1906 quake that was dumped into the bay. The image below shows how a building in the Marina fell and crushed a car
Loma Prieta part III • 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. Photo by Deanne Fitzmaurice.
The Landers Quake • The Landers earthquake occured on June 28, 1992 at 4:52 a.M. And had a magnitude of 7.3 on the Richter scale. • The largest earthquake to hit southern California in 40 years, the Landers earthquake was centered 120 miles east of Los Angeles in the Mojave desert. • The earthquake ruptured 5 separate faults for a total length of 85 km, creating very large surface breaks. (Like the one in the road!) • The Landers earthquake suprised many seismologists because it triggered swarms of small earthquakes in areas very far away as well.
The Earthquake At 4:31 A.M. (local time) on Monday, January 17, 1994, a magnitude 6.8 earthquake woke nearly everyone in southern California. The earthquake epicenter was beneath the San Fernando Valley, 20 miles (32 km) west-northwest of downtown Los Angeles, near the community of Northridge (34° 13' N, 118°32'W). The main shock occurred on a shallowly-dipping, previously unknown thrust fault. The rupture started at a depth of about 12 miles (19 km) and, during the course of the main shock, traveled upward and northward, spreading both eastward and westward. The focal mechanism of the main shock from both first motions and teleseisms shows a N 60° W striking and 35° to 45° south dipping plane. Rock on the south side of the fault surged upward and over the rock on the north side. As a result of the quake, the Earth's crust south of the San Fernando Valley moved slightly closer to the Earth's crust north of the valley, and the mountains just north of the valley are slightly higher.
The Northridge Quake 1994 Northridge Fashion Mall: The Northridge Fashion Mall was extensively damaged by the earthquakeOther department stores and parking structures were also damaged. Windows were shattered throughout the mall. Severe damage and partial collapses were seen in other, smaller shopping centers near the epicenter. . Bullocks Department Store in the mall collapsed. One parking structure collapsed into a pile about twenty feet high.
Magnitude 6.8NORTHERNALGERIA 2003May 21 18:44:19 UTC • Tectonic Setting • The earthquake occurred in the boundary region between the Eurasian plate and the African plate. Along this section of the plate boundary, the African plate is moving northwestward against the Eurasian plate with a velocity of about 6 mm per year. The relative plate motions create a compressional tectonic environment, in which earthquakes occur by thrust-faulting and strike-slip faulting. Analysis of seismic waves generated by this earthquake shows that it occurred as the result of thrust-faulting.
Remarks At least 1,785 people killed, 7,600 injured and extensive damage in the Algiers-Bourmerdes-Thenia area. A tsunami generated with an estimated wave height of 2 m caused damage to boats and underwater telephone cables off the Balearic islands, Spain. Also felt in Monaco and southeastern Spain.
2003 May 26 09:24:32 UTCMagnitude 7.0 NEAR THE EAST COAST OF HONSHU, JAPAN • This thrust earthquake occurred near the north-east shore of the island of Honshu, Japan. In this region, the convergence of the Pacific plate and the Eurasian plate generates numerous earthquakes. The Pacific plate is moving west at a rate of about 9 cm per year relative to the Eurasian plate. The surface expression of the boundary between these plates is the north-south trending Japan trench which is about 220 km east of the epicenter. Given the earthquake's fault mechanism and depth, it likely resulted from the release of compressional stresses within the Pacific plate that continuously develop as it descends into the mantle.
