Earthquakes An earthquake is the shaking and trembling that results from the sudden movement of part of the Earth’s crust. The most common cause of earthquakes is faulting, when energy is released.
San Andreas Fault The land to the west is moving north. The land to the east of the fault is moving south. All the rocks do not move at the same time so earthquakes occur in one area and then another.
San Francisco Quake In 1906, movement along the San Andreas fault caused an extremely destructive earthquake in San Francisco.
Tsunamis Earthquakes which occur on the ocean floor produce giant sea waves called tsunamis. Tsunamis can travel at speeds of 700 to 800 km per hour. As they approach the coast, they can reach heights of greater than 20 meters.
Focus The point beneath the surface where the rocks break and move is called the focus. The focus is the underground origin of an earthquake. Most faults occur between the surface and a depth of 70 kilometers.
Epicenter Directly above the focus, on the Earth’s surface is the epicenter. Earthquake waves reach the epicenter first. During an earthquake, the most violent shaking is found at the epicenter.
Seismic Waves There are three main types of seismic waves. Primary Secondary Surface
Primary Waves Seismic waves that travel fastest are P waves. They travel through solids, liquids and gases. They move at different speeds depending on the density of the material through which they are moving. As they move deeper in the Earth they move faster. P waves are compression waves.
Secondary Waves Seismic waves that do not travel through the Earth as fast as P waves do are called secondary or S waves. S waves travel through solids but not liquids or gases. S waves cause particles to move from side to side. They move at right angles to the direction of the wave.
Surface Waves The slowest moving seismic waves are called surface waves or L waves. L waves originate on the Earth’s surface at the epicenter. They move along the surface the way waves travel in the ocean. The Earth’s surface moves up and down with each L wave. L waves cause most of the damage.
The Seismograph Invented in 1893 by John Milne, a seismograph detects and measures seismic waves. A weight attached to a spring remains nearly still even when the Earth moves. A pen attached to the weight records any movement on a roll of paper on a constantly rotating drum. The drum moves with the Earth and affects the line.
Seismologists Seismologists study earthquakes. They can determine the strength of an earthquake by the height of the wavy line recorded on the paper. The seismograph record of waves is called a seismogram.
Measuring Earthquakes There are at least three different measures for rating earthquakes: –Richter Scale –Moment Magnitude –Modified Mercalli
Richter Scale Developed by Charles F. Richter in 1935. Determines the magnitude of an earthquake by measuring the amplitude of the largest seismic wave. Each one step increase goes up 10 times in intensity. Not used to express damage.
Moment Magnitude Measures the magnitude of an earthquake based on the amount of energy released. Now most widely accepted by the USGS for all modern large earthquakes. For each one number increase, the magnitude increases 10 times.
Modified Mercalli Scale Composed of 12 increasing levels of intensity. –Range from imperceptible shaking to catastrophic destruction. Based on observations of an earthquake’s effects. The epicenter of an earthquake usually receives the highest ranking.
Predicting Earthquakes Scientists have identified warning signals the help predict earthquakes with greater accuracy. Often changes occur in the speed of P and S waves before an earthquake occurs. Sometimes slight changes in the tilt of the Earth’s surface can be detected. Some scientists believe animals’ behavior is affected.
Magma Deep within the Earth, under tremendous pressure and at great temperatures, rock exists as a hot liquid called magma. This molten rock is found in pockets called magma chambers.
Lava When the magma reaches the surface, it is called lava.
Volcanic Eruptions During volcanic eruptions, many rock fragments are blown into the air. The smallest particles are called volcanic dust. (less than 0.25 mm) Volcanic Ash (0.25 -5mm) falls to the Earth and forms small rocks. Volcanic bombs (a few cm to several meters) are molten and harden as they travel through the air.
Volcanic Activity Volcanoes are rather unpredictable. Some erupt regularly, others have not erupted in modern history. Scientists classify them as: –Active –Dormant –Extinct.
Active Volcanoes An active volcano is one that erupts wither continually or periodically such as Mount Katmai in Alaska and Mount St. Helens in the Cascade Range.
Dormant Volcano A volcano that has been known to erupt within modern times but is now inactive is classified as a dormant volcano. Mount Rainier in Washington state is an example.
Extinct Volcano A volcano not known to have erupted within modern history is classified as an extinct volcano. They have been worn away almost to the level of their magma chamber. Mount St. Helens was considered to be dormant but erupted after long periods of inactivity.
Volcano and Earthquake Zones Most major earthquakes and volcanic eruptions occur in three zones of the world. Scientists believe that there is a great deal of movement and activity in the Earth’s crust in these three zones.
Ring of Fire One major earthquake and volcano zone extends nearly all the way around the edge of the Pacific Ocean. This zone goes through New Zealand, the Philippines, Japan, Alaska and along the western coasts of North and South America. The San Andreas fault is part of this zone.
Mediterranean Zone A second Major earthquake and volcano zone is located near the Mediterranean Zone and extends across Asia into India. Many countries in the zone, including Italy, Greece and Turkey, have violent earthquakes. Many volcanic eruptions also occur in this zone.
Mid-Atlantic Ridge Zone The third major earthquake and volcano zone extends through Iceland and to the middle of the Atlantic Ocean. There is under the ocean a long range of volcanic mountains called the Mid-Atlantic Ridge. Scientists believe that the volcano and earthquake activity are due to the formation of new parts of the Earth’s crust along the ridge. The volcanic island of Iceland is part of this zone.