A seismograph or seismometer measures the magnitude of an earthquake. The seismograph actually measures of seismic waves caused by the earthquake. The horizontal pendulum seismograph was invented in 1880, but the first seismograph showed up around 132A.D. Today's seismographs use electronics and computers and are more accurate.

Earthquake magnitude is measured using the Richter scale which ranges from one to ten. A magnitude of two can barely be measured and is not felt. A magnitude of four causes mainly shaking and rattling but no major damage. A magnitude of six can cause minor destruction up to 100 miles across. A magnitude of eight can cause major destruction several hundred miles across. Finally, a magnitude of 10 would be catastrophic, but has never been recorded in modern times.

When a major earthquake occurs, especially below the ocean surface, a tsunami he can occur. One of the most famous earthquake induced tsunamis occurred in 2004 in the Indian Ocean. Tsunamis can move up to 500 mph but are not visible in mid ocean as much of the energy and motion are below the surface of the water. A tsunami does not become a large destructive wave until it encounters shallow water closer to land. Tsunamis can penetrate several miles inland and cause widespread destruction often more than the original earthquake itself.

Earthquakes occur in the lithosphere which contains the Earth's outer crust. The lithosphere contains several plates that cause earthquakes when they rub against each other. One reason these plates rub together is because of the athenosphere below it, which is very soft due to the high heat of the Earth's core and allows a plates to move above it. The troposphere and mesosphere are part of the atmosphere and are not related to earthquakes.

The great San Francisco earthquake occurred in 1906 and lasted a full minute which is quite long for an earthquake. This longer duration, along with the 8.0 magnitude, caused widespread destruction and fires. The epicenter was two miles off shore and occurred along nearly 300 miles of the San Andreas fault. The earthquake was felt from Oregon and Los Angeles and caused an estimated 3000 deaths. Nearly two thirds of the population were left homeless due to the widespread destruction from the earthquake and subsequent fires. This earthquake is still the worst natural disaster in California history.

California earthquakes are caused by the San Andreas fault. The San Andreas fault is 800 miles long and runs through southwestern California. It is part of the boundary between the North American plate and the Pacific plate. The Pacific plate is the most active tectonic plate on the planet and is responsible for the earthquakes and volcanoes in the Pacific Rim of fire. At the San Andreas fault the Pacific plate is moving north and the North American plate is moving south. It is this movement that causes the rubbing and California's famous earthquakes.

The Pacific plate causes the most earthquakes and this area is known as the Pacific Ring of fire. About 90 percent of the world's earthquakes in his horseshoe shaped region. The main areas in the Pacific Ring of fire are Western coasts of North and South America, Eastern Asia and the island chains of the South Pacific. This area is also responsible for most of the world's volcanoes.

Oregon is north of the San Andreas fault and is in less danger of experiencing a major earthquake. Central California experienced a massive earthquake in 1857. Northern California experienced the great San Francisco earthquake in 1906. However, Southern California has not experienced a major earthquake in over 300 years. Most scientists agree that Southern California could experience a major earthquake at any time. Palm Springs and San Bernardino are especially vulnerable due to their proximity to the San Andreas fault.

The earthquake that caused the 2004 Indonesian tsunami altered the Earth's speed of rotation and caused the day to shorten by the few milliseconds. However, the tidal effects of the moon offset the changes within a few months. Water levels in the Indian Ocean did change, but only while the tsunami passed. Some tiny islands in Indonesia sank as much as a meter which may have decreased their size slightly. The earthquake also caused the Earth to wobble by a few centimeters, but this too was offset by another phenomenon called the Chandler wobble.

The vast majority of earthquakes last 10 seconds or less. However, the great San Francisco earthquake of 1906 lasted nearly a full minute. The longest earthquake ever recorded was the Indonesian earthquake of 2004. Most famous for causing the Indonesian tsunami, killing nearly a quarter million people, this earthquake lasted almost 10 minutes.

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