Measuring the Severity of Quakes
The Richter Scale (1935) is used to measure the amount of energy released in a given earthquake. The Richter reading won't be affected by the observer's distance from the earthquake, although the perceived strength will be greater near to the epicenter and generally decrease as the distance increases. Richter readings are useful in comparing comparing earthquakes that occur in different times and places. There are many other factors that contribute to the damage, such as the underlying rocks, building construction and
population density. The Richter reading by itself does not give enough information to tell what the effects will be in any particular place. That said, however, in general, the larger the Richter reading, the greater the damage will be close to the epicenter. In recent years, scientisits have used a variety of magnitude scales to measure different aspects of the waves produced by an earthquake. These different magnitude scales reflect a greater complexity than can be can be represented by Richter's original scale. These different scales sometimes lead to confusion when different magnitude readings are reported for the same quake. These different readings reflect different aspects of the quake. Especially, in large quakes, these differences can be substantial. For instance, the 1964 Alaska quake was originally recorded as 8.6 Magnitude. Now scientists think that a 9.2 Magnitude more accurately reflects that quake's intensity. The important thing to remember is that these are not necessarily contradictory but are on scales and measure different things.
An observer near the epicenter of an earthquake will generally experience a magnitude 3 or 4 on the Richter Scale as a mild shaking, 5 or 6 will be enough to cause some damage, 7 will cause a lot of damage and 8 or higher (classified as a Great Earthquake) will cause large scale description.
The Modified Mercalli Scale (1931) of earthquake damage measures the intensity of an earthquake at a particular place. It uses the type and amount of damage. Unlike the Richter Scale, it does not measure the absolute strength of the earthquake but
how strongly it is felt at a particular place. This makes the Mercalli Scale useful in comparing damage from different quakes and in plotting damage patterns from a given quake. This type of comparison has helped scientists figure out the factors that contribute to earthquake damage. A map showing the Mercalli intensity at different locations for the same quake can be quite revealing. These maps can be related to geological maps to see what effect the underlying rocks have on the intensity of the quake. They show that softer porous soils shake very violently while bedrock is less affected.
The earthquake hazard maps show the expected severity of ground shaking in the San Francisco Bay Area based on the underlying soils and distance from major faults.
Another factor that has a major effect on the damage is the building method and materials used. Unreinforced masonry has the worst record since it has little ability to flex or move without collapsing. Wood frame buildings, or reinforced buildings, on the other hand, can hold together under quite severe shaking- Third world countries with masonry buildings often have huge numbers of casualties from quakes that, had they occurred in California, with its stringent building codes and predominantly wooden buildings, would have been considered relatively minor.
People can be trapped in collapsed buildings or under rubble that collapses into the street. This is the type of damage that leads to the worst casualties. The worst thing to do in a quake is to rush out into the street during the quake. The danger from being hit by falling glass and debris is many times greater in front of the building than inside. In the 1989 Loma Prieta quake, the streets of San Francisco's financial district were covered by broken glass and people were buried under the facade of a brick building that fell forward into the street. Likewise in the 1964 Alaska quake, a huge concrete facade fell off of a department store onto pedestrians passing by.
Buildings that can otherwise withstand the quake can be knocked off their foundations and severly damaged. This type of damage can be largely prevented by bolting the frame securely to the
foundation, so it will remain in place.
Buildings can be damaged when the ground gives way beneath them. This can be in the form of a landslide down a hill, or liquefaction of soils that can cause severe settling of the ground. Ground movement can change the whole landscape, as in the New Madrid (Missouri) Quake that changed the course of the Mississippi River. A landslide into a lake or reservoir can cause flooding downstream. This kind of damage is not unique to earthquakes, but can be triggered by a quake.
Fires often break out following earthquakes. They can be caused by flammable materials being thrown into a cooking or heating fire or broken gas lines. Fires can easily get out of control since the earthquake may have broken water mains or blocked roads firefighters need to use. There are many demands made on the emergency response systems that slow down response to fires. In the 1906 San Francisco Earthquake, for example, the fire that followed the quake caused more damage than the earthquake, itself.
Underwater earthquakes, volcanoes, or landslides can produce a tsunami or tidal wave. This wave can travel very rapidly thousands of miles across the ocean. In deep water the tsunami may only raise the ocean level by a few inches, hardly enough to notice. But as it approaches land, the shallower water causes the wave to build in height to as much as 50 feet or more and suddenly flood coastal areas. Tsunamis carry a lot of energy and when they hit the coast strong currents can cause massive erosion of the coastline as well as tearing apart buildings it encounters. Typically a tsunami will last for a period of hours with successive waves drastically lowering and raising the sea level. Although scientists now understand the causes of tsunamis, there are many local factors including the slope of the seafloor at a given location, the distance and direction of travel from the earthquake that will determine the severity of the resulting wave.
A tsunami is a large wave caused by an abrupt displacement of water, very similar to the ripples from a rock thrown into the water. Water at any given point does not travel with the wave, but moves up and down as the wave passes. Unlike normal ocean waves, tsunamis have a very long period between the waves. While normal waves on the beach might come in every half-minute or so, a tsunami's waves will be separated by many minutes or hours. Not only crests of the waves very high but also the troughs between the waves will be very low. Usually as the tsunami comes onshore, the wave trough arrives first, causing the sea level to drop, exposing the seabed. Soon however, the crest arrives, flooding coastal areas. Anybody living near the coast should be aware of tsunamis and ready to move to higher ground immediately after an earthquake or when tsunami warnings are in effect.
Most tsunamis are too small to be noticed but the Tsunami Warning Center in Hawaii tracks them and sends out warnings when there is a chance of a large one. Unfortunately, some of the worst tsunamis occur near the epicenter of the originating earthquake and, therefore, give little warning. This was true of the devastating 1998 New Guinea tsunami, which was caused by an earthquake just off the coast.
Tsunamis contributed to significant damage in the 1755 Lisbon and the 1964 Alaska quake, among others.
More advanced information is contained in The International Journal of The Tsunami Society.
Factors that Affect Damage
Earthquakes cause many different kinds of damage depending on the strength of the quake, distance, type of underlying rock or soil and the building construction. A given Richter reading will produce vastly different amounts of damage in different parts of the world. Even the same quake can have very different effects in neighboring areas. For instance, in the 1989 Loma Prieta earthquake, some of the worst damage was in the Marina neighborhood of San Francisco, seventy miles from the epicenter. Many areas much closer to the quake suffered only minimal damage. The difference in this case was that the Marina was built on loose soil that had been used to fill in the edge of San Francisco Bay. This combination of uncompacted soil with a lot of water in it led to a phenomenon called 'liquefaction'. Liquefaction occurs when the ground loses its cohesion and behaves like a liquid. When this happens during an earthquake it can result in increased intensity of the shaking, or landslides. It can also cause rapid settling and collapse of buildings. Buildings on solider ground do much better.Types of Damage
Building Collapse
Buildings Knocked Off Their Foundations
Landslides
Fire
Tsunami
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