Slip, Sliding Away
Earthquakes are caused by forces deep within the Earth's crust. As continents collide, ocean floors split, magma flows, volcanoes erupt and other natural processes occur pressure is released making vibrations within the ground which we call earthquakes. Where one block of the Earth's crust interacts with another they may slip, slide or push each other resulting in a fault.
A fault is a fracture in the Earth's crust where two blocks of crust have slipped against each other. In the diagram at right figure A is a Normal Fault where the blocks are pulled apart allowing for one block to slip down on the other. Figure B indicates a Thrust Fault which occurs when blocks are pushed against each other raising one of the blocks. A Strike-Slip Fault, figure C, occurs when stress causes the blocks to move horizontally past one another.
Scientists have developed a theory called "Plate Tectonics" to explain why earthquakes continue to occur. In this theory the continents float on the surface of the Earth on a continental plate and slide, collide or push other continental plates. The heat and pressure from this movement causes rock deep within the Earth to melt (magma) and force its way to the surface to create volcanoes.
Plate movements are also believed to cause about 90 percent of all earthquakes. Less than 10 percent of earthquakes occur within plate interiors. In Washington the pressure between the North American Plate and the Juan de Fuca Plate (center of diagram) causes earthquakes to occur around Puget Sound and the Seattle area. It is believed that at one time in the distant past all of the plates formed one huge continent called Pangea. In the diagram the yellow lines indicate plate boundaries and the red lines mark areas of volcanic action.
Click on image for a detail view of Main Continental Plates diagram.
The severity of an earthquake is related to magnitude, that is, the seismic energy recorded on a seismograph and intensity meaning the observed effects that the ground shaking has on people, buildings, man-made structures and natural features. The epicenter of an earthquake is the spot on the surface directly above the area where the quake took place. The focus or hypocenter is the area inside the Earth where the event happened.
In 1935 Charles F. Richter developed the Richter Magnitude Scale to mathematically rate how much force an earthquake releases based on seismograph readings. The scale takes into account the distance between various seismographs, the density of the ground and other factors to give a number for reference. Each increase in whole number on the scale is ten times the measured amplitude (strength) of the previous and corresponds to about 31 times more energy released by the quake.
Uncounted numbers of quakes less than 2.0 happen each year with several thousand over 4.5 on the scale. The highest on the scale and most damaging have had magnitudes of 8.0 to 8.9. Quakes in this range occur about once per year. The simplified version below gives an idea of how the scale works.
|Magnitude||Richter Number||Relative Intensity|
|Low||Less than 2.0||Barely felt, minimal or no damage.|
|Moderate||2.0 to 4.4||Noticable, damage to buildings, sidewalks, weak structures.|
|Strong||4.5 to 7.0||Heavy shaking, falling walls and bridges.|
|Very strong||Greater than 7.0||Massive shaking, ground moving, landslides, great damage to structures.|
Earthquake intensity has been measured by various scales over the years but the one currently in use in the US is the Modified Mercalli Intensity Scale. Developed in 1931 by Harry Wood and Frank Neumann, this scale has 12 increasing levels of intensity ranging from "Not felt except by a few" to "Felt by all, damage slight" all the way up to "Few, if any masonry structures remain standing". The scale takes into account how people "felt" the quake and the observed structural damage.
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Here are some basic terms used in the tour. Find more geology terms in the Glossary.
- Volcanic rock caused by partial melting of the Earth's crust.
- The spot on the surface of the Earth directly above the area where an earthquake took place.
- A split or fracture in the Earth's crust where two blocks of crust have slipped, slid or pushed against each other.
- Focus (Earthquake)
- The area inside the Earth where an earthquake happened. Also known as the Hypocenter.
- The area inside the Earth where an earthquake happened. Also known as the Focus.
- The observed effects that an earthquake shaking the ground has on people, buildings, man-made structures and natural features.
- The seismic (earthquake) energy recorded on a seismograph.
- Mercalli Intensity Scale, Modified
- Earthquake intensity measured on a scale that has 12 increasing levels. The scale takes into account how people "felt" the quake and the observed structural damage.
- Richter Magnitude Scale
- A scale that mathematically rates how much force an earthquake releases based on seismograph readings.
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