Earthquakes

Definition Vibrations in the ground caused by a release of energy along faults, (breaks), in the Earth’s lithosphere as a result of plate tectonics.

Main Causes of Earthquakes Plate movements Volcanic Eruptions Mining Meteor Impacts

Rock Deformation When a force is applied to a body of a rock at plate boundaries, the rock may bend or break.

Faults As stress builds in the deformed rock, a fault can form. Definition: Faults are a break in the Earth’s lithosphere where one block of rock moves toward, away from, or past another.

Types of Faults Strike-slip Normal Reverse

Strike-Slip A strike-slip fault occurs when two blocks of rock slide horizontally past each other in opposite directions. Location: Transform Plate Boundaries

Strike-slip Fault

Normal Fault Tectonic forces pull two blocks of rock apart. The block of rock above the fault moves down relative to the block of rock below the fault. Location: Divergent Plate Boundary

Normal Fault

Reverse Fault Tectonic forces push two blocks of rock together. The block of rock above the fault moves up relative to the block below the fault. Location: Convergent Plate Boundaries.

Reverse Fault

Focus & Epicenter Focus: The location inside the Earth where the rock first begins to move or break. Epicenter: The location on the earth’s surface which is directly above the focus.

Focus & Epicenter

Seismic (Body) Waves When rocks move along a fault, a release of energy travels out in all directions as waves from the focus.

Types of Seismic Waves Primary Waves: P-waves Secondary Waves: S-waves Surface Waves

Primary Waves P-waves cause rock in the ground to move in a push-pull motion similar to spring. P-waves are the fastest moving of all seismic waves.

Secondary (Shear) Waves S-waves cause the rocks to move up and down at right angles relative to the direction the wave is traveling.

Surface Waves Surface waves travel along the Earth’s surface moving rocks up and down in a rolling motion. Surface waves are the slowest type of seismic waves.

Types of Surface Waves Love Waves Rayleigh Waves

Love Waves Love waves causes the rock to move side to side in a directions perpendicular to the waves’ direction of travel.

Rayleigh Waves Rayleigh waves are slow moving waves that travel along the surface in a rolling motion similar to ocean waves.

Locating the epicenter An instrument called a seismometer measures and records ground motion and can be used to determine the distance seismic waves have traveled.

Seismometer

Seismogram

Analyzing the Data Step 1: Determine the number of seconds between the arrival time of the first P-wave and the first S- wave. This time difference is know as “Lag Time”. The lag time must be calculated from at least 3 different recording stations.

Analyzing the Data Continued Step 2: Plot the lag times found from the three recording stations on a (S-P) Lag Time Graph in order to determine the distance to the epicenter.

Analyzing the Data Continued Step 3: Using a ruler and a map scale to measure the distance between the seismometer and the earthquakes epicenter. Step 4: Place the point of a compass on the seismometer location and the pencil on the distance away from the epicenter. Next, draw a circle around the seismometer. Repeat this step for the other two recording stations.

The epicenter Where the three triangulated circles intersect is the location of the epicenter of the earthquake.

Determining an earthquakes magnitude Scientists use three different scales to measure and describe an earthquakes magnitude. Magnitude is a measure of the amount of energy released during an earthquake. Richter Magnitude Scale Moment Magnitude Scale Modified Mercalli Scale

Richter Scale The Richter scale is used to interpret two different values recorded by the seismometer. The amount of ground motion. The amount of energy released by the earthquake

Ground Motion The Richter scale begins at zero but has no upper limits. Each increase of 1 unit represents ten times the amount of ground motion. Example: A magnitude 8 produces ten times greater ground shaking than a magnitude 7. The ground motion value is the measure used to alert the general public.

Energy Release The Richter scale also measures the energy released by the earthquake. On this scale each increase of one whole number, is represented by a 31-fold increase in energy. Example: A magnitude 8 is 31 times more powerful than a magnitude 7

Moment magnitude scale The moment magnitude scale measures the total amount of energy released by an earthquake. The energy released depends on three factors: The size of the fault The amount of motion that occurs along the fault The strength of the rocks that break during the earthquake.

Moment magnitude scale continued On this scale for each increase of one unit the earthquake releases 31.5 times more energy. Example: A magnitude 8 earthquake releases more than 992 times more energy than a magnitude 6.

Modified Mercalli Scale On the Modified Mercalli scale, an earthquakes magnitude is measured by the amount of damage caused by ground motion. This scale ranges from 1 – 12. 1 being minimal and 12 being total destruction.

Largest recorded earthquake The largest ever recorded earthquake occurred in Chile. May 22nd 1960 at 11:19:14 UTC a magnitude 9.5 earthquake was recorded. Approximately 1,655 killed, 3,000 injured, 2,000,000 homeless, and $550 million damage in southern Chile; tsunami caused 61 deaths, $75 million damage in Hawaii; 138 deaths and $50 million damage in Japan; 32 dead and missing in the Philippines; and $500,000 damage to the west coast of the United States.

Associated Earthquake Hazards Aftershocks Fires Tsunamis Liquefaction

Liquefaction Liquefaction occurs when loose soil temporarily takes on some properties of a liquid. (Quicksand)

Results of Liquefaction

How can earthquakes be used Seismologists have used earthquakes to map the Earth’s interior. P and S waves change speed and direction depending on the density of different materials. Scientists have found that S-waves cannot travel through the outer core, proving it is liquid. By studying the P-waves scientists were also able to determine that the inner core is composed of iron and nickel.

Earth’s Interior using Seismology