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EARTHQUAKES • The power of an earthquake is measured using a seismometer. A seismometer is a sensitive machine, which picks up the vibrations caused by the earthquake. It plots these vibrations on a seismograph.The strength, or magnitude, of an earthquake is measured using a scale called the Richter Scale. The Richter Scale is numbered 0-10: • An earthquake measuring one or two on the scale is very common and can happen everyday in places like San Francisco. These earthquakes are so small that they can only be picked up by a seismometer: people cannot feel them. • An earthquake measuring around seven or eight on the Richter Scale can be devastating, e.g. the earthquake in Turkey of 1999 which measured 7.5 on the Richter scale.
San Andreas Fault The San Andreas Fault is a geological fault that runs a length of roughly 800 miles (1300 kilometres) through western and southernCalifornia in the United States. The fault, marks a boundary between the Pacific Plate and the North American Plate. The fault was first identified in Northern California by U.C. Berkeley geology professor Andrew Lawson in 1895, and named by him after a small lake which lies in a linear valley formed by the fault just south of San Francisco, the Laguna de San Andreas. Following the 1906 San Francisco Earthquake, it was Lawson who also discovered that the San Andreas Fault stretched well southward into Southern California.
THE CALIFORNIA EARTHQUAKE OF APRIL 18, 1906 • On April 18th1906, shortly after 5:00 a.m., a great earthquake struck San Francisco and a long narrow band of towns, villages, and countryside to the north-northwest and south-southeast. Many buildings were wrecked; hundreds of people were killed; electric power lines and gas mains were broken. Fires broke out and burned wildly for days, utterly out of control because of severed water mains.
Loma Prieta earthquakeOctober 171989 The Loma Prieta was a major earthquake, and caused severe damage as far as 70 miles (110 km) away from its epicenter; most notably in San Francisco, Oakland, the San Francisco Peninsula, and in areas closer to the epicenter in the communities of Santa Cruz, the Monterey Bay, Watsonville, and Los Gatos. Most of the major property damage in the more distant areas resulted from liquefaction of soil used over the years to fill in the waterfront and then built upon.
There were at least 63 deaths (some say 66) and 3,757 injuries as a result of this earthquake. The highest concentration of fatalities, 42, occurred in the collapse of the Cypress Street Viaduct on the Nimitz Freeway (Interstate 880), where a double-decker portion of the freeway collapsed, crushing the cars on the lower deck. One 50-foot (15 m) section of the San Francisco-Oakland Bay Bridge also collapsed, causing two cars to fall to the deckbelow, leading to the single fatality on the bridge After the 1906 earthquake much of the rubble was bulldozed into San Francisco Bay. This reclaimed land was built upon and was extremely unstable. In the Loma Prieta earthquake many buildings on this reclaimed land were destroyed. Because this quake occurred during the evening rush hour, there could have been a large number of cars on the freeways at the time, which on the Cypress Street Viaduct could have endangered many hundreds of commuters. Very fortunately, and in an unusual convergence of events, the two local Major League Baseball teams (the Oakland Athletics and the San Francisco Giants) were about to start their third game of the World Series (the game was scheduled to start shortly after 5:30 p.m.). Many people had left work early or were participating in early after-work group viewings and parties. The quake also caused an estimated $6 billion in property damage, the costliest natural disaster in U.S. history at the time. It was the largest earthquake to occur on the San Andreas Fault since the great 1906 San Francisco earthquake.
KOBE • When did the earthquake happen? • 5.46 a.m., on Tuesday January 17th, 1995. What happened? • An earthquake measuring 7.2 on the Richter scale hit the city. The epicentre was on Awaji island in Osaka Bay. The earthquake's focus was very shallow, at a depth of 15 - 30 kilometres. This resulted in extremely violent shaking of the ground. • Some 75,000 buildings were damaged or destroyed during the earthquake and its aftershocks. Road and rail links were damaged, making it difficult for fire engines and ambulances to reach the worst affected parts of the city. Gas and water supplies were cut off, as were telecommunications links. • Nearly 6,000 people were killed, with another 26,000 injured. 310,000 people were made homeless. The cost of repairing the damage was put at £60 billion. • Most deaths happened in the old residential areas where buildings had been constructed before the introduction of designs to help withstand earthquakes. Many of the oldest buildings were wooden and caught fire very quickly. • Although many newer buildings in Kobe were built to withstand earthquakes some of them still collapsed. The Hanshin Expressway, an elevated road, collapsed although it was built to withstand earthquakes measuring up to 8.3 on the Richter scale. Other modern buildings with steel frames were designed to sway in the event of an earthquake, yet some of these also collapsed. Others were so badly damaged they had to be demolished. • The earthquake hit Japanese industry. The Toyota motor company had two factories in Kobe, both of which made vehicle parts which no other factory could make. Once these factories were put out of production Toyota's whole assembly system ground to a halt. • The Japanese Government was heavily criticised for its response to the earthquake. Relief efforts were condemned for being slow, uncoordinated and badly equipped.
STRUCTURAL DAMAGE • The cities of Kobe and Osaka are connected by an elevated highway. The earthquake caused several portions of this highway to collapse. • Most of the deaths and injuries occurred when older wood-frame houses with heavy clay tile roofs collapsed. Note that homes and buildings are designed to be very strong in the vertical direction because they must support their own static weight. On the other hand, buildings can be very susceptible to horizontal ground motion.Furthermore, many of the structures in Kobe built since 1981 had been designed to strict seismic codes. Most of these buildings withstood the earthquake. In particular, newly built ductile-frame high rise buildings were generally undamaged.Unfortunately, many of the buildings in Kobe had been built before the development of strict seismic codes. • The collapse of buildings was followed by the ignition of over 300 fires within minutes of the earthquake. The fires were caused by ruptured gas lines. Response to the fires was hindered by the failure of the water supply system and the disruption of the traffic system.
Effects of an earthquake Effects of an earthquake The impact of a strong earthquake can be devastating. Earthquakes can destroy settlements and kill many people. Aftershocks can cause even more damage to an area. It is possible to classify the impacts of an earthquake, by taking the following factors into account: short-term (immediate) impacts long-term impacts social impacts (the impact on people) economic impacts (the impact on the wealth of an area) environmental impacts (the impact on the landscape)
In pictures: Mexican earthquake Thursday, 23 January, 2003, 10:51 GMT • A powerful earthquake struck Mexico's Pacific coast on Tuesday night, killing at least 25 people and destroying at least 500 homes. • About 10,000 people have been left homeless • Most of the fatalities were caused by buildings collapsing in the state of Colima, the site of the earthquake's epicentre For many, the quake was a terrible reminder of another that hit in 1985, which killed at least 9,500 people
The 2004 Indian Ocean earthquake, known by the scientific community as the Sumatra-Andaman earthquake, was an undersea earthquake that occurred at 00:58:53 UTC (07:58:53 local time) December 26, 2004 with an epicentre off the west coast of Sumatra, Indonesia. The earthquake triggered a series of devastating tsunamis that spread throughout the Indian Ocean, killing large numbers of people and inundating coastal communities across South and Southeast Asia, including parts of Indonesia, Sri Lanka, India, and Thailand. Although initial estimates have put the worldwide death toll at over 275,000 with thousands of others missing, recent analysis compiled lists a total of 229,866 persons lost, including 186,983 dead and 42,883 missing.[1] The figure excludes 400 to 600 people who are believed to have perished in Myanmar which is more than that government's official figure of only 61 dead.[2] The catastrophe is one of the deadliest disasters in modern history. The disaster is known in Asia and in the international media as the Asian Tsunami, and also called the Boxing Day Tsunami in Australia, Canada, New Zealand, and the United Kingdom as it took place on Boxing Day. Disturbingly enough, the tsunami occurred exactly one year after the 2003 earthquake that devastated the southern Iranian city of Bam. • The magnitude of the earthquake was originally recorded as 9.0 on the Richter scale, but has been upgraded to between 9.1 and 9.3. At this magnitude, it is the second largest earthquake ever recorded on a seismograph. This earthquake was also reported to be the longest duration of faulting ever observed, lasting between 500 and 600 seconds, and it was large enough that it caused the entire planet to vibrate at least half an inch, or over a centimetre.[3] It also triggered earthquakes in other locations as far away as Alaska.[4]
Causes of Tsunamis • Generation of a tsunami • Tsunamis can be generated when the sea floor abruptly deforms and vertically displaces the overlying water. Such large vertical movements of the Earth’s crust can occur at plate boundaries. Subduction earthquakes are particularly effective in generating tsunamis. As an Oceanic Plate is subducted beneath a Continental Plate, it sometimes brings down the lip of the Continental with it. Eventually, too much stress is put on the lip and it snaps back, sending shockwaves through the Earth’s crust, causing a tremor under the sea, known as an Undersea Earthquake. • Sub-marine landslides (which are sometimes triggered by large earthquakes) as well as collapses of volcanic edifices may also disturb the overlying water column as sediment and rocks slide downslope and are redistributed across the sea floor. Similarly, a violent submarine volcanic eruption can uplift the water column and form a tsunami.
