Earthquakes and the Earth’s Interior

Ag Earth Science – Chapter 8.1

earthquake The vibration of the Earth produced by the rapid release of energy.

focus The point within the Earth where the earthquake starts

epicenter The location on the surface directly above the focus

fault Fractures in the Earth where movement has occurred

elastic rebound hypothesis The explanation stating that when rocks are deformed, they break, releasing the stored energy that results in the vibrations of an earthquake.

aftershock A small earthquake that follows the main earthquake

foreshock A small earthquake that often precedes a major earthquake

Earthquakes Each year, more than 30,000 earthquakes occur worldwide that are strong enough to be felt. Most earthquakes are minor tremors, and about 75/year are considered major earthquakes.

Earthquakes Earthquakes – the vibration of the Earth produced by the rapid release of energy  The point within (inside) the Earth where the earthquake starts is called the FOCUS.  The EPICENTER is the location on the surface directly above the focus.

Earthquakes Earthquakes are usually associated with large fractures in the earth’s crust and mantle called FAULTS. Faults – fractures in the earth where movement has occurred.

Elastic Rebound Hypothesis The springing back of the rock into its original form is called “elastic rebound” (example – a stretched rubber band). The explanation says that when rocks are deformed, they first bend and then break, releasing stored energy. Most earthquakes are produced by the rapid release of elastic energy stored in rock that has been subjected to great forces. When the strength of the rock is exceeded, it suddenly breaks, causing the vibration of the earthquake.

Earthquakes The movements that follow a major earthquake often produce smaller earthquakes called aftershocks. Small earthquakes called foreshocks often come before a major earthquake.

Ag Earth Science – Chapter 8.2

seismograph An instrument that records earthquake waves

seismogram A record made by a seismograph

surface wave A seismic wave that travels along the surface of Earth

S wave A seismic wave that shakes particles perpendicular to the direction the wave is traveling

P wave Earthquake wave that pushes and pulls rocks in the direction of the wave, also known as a compression wave.

moment magnitude A more precise measure of earthquake magnitude than the Richter scale which is derived from the amount of displacement that occurs along the fault zone and estimates the energy released by an earthquake

Measuring Earthquakes The study of earthquake waves, or seismology, dates back almost 2000 years. Seismographs – instruments that record earthquake waves. Seismogram – modern seismographs that amplify and electronically record ground motion.

Earthquake Waves The energy from an earthquake spreads outward as waves in all directions from the focus. Seismograms show that two main types of seismic waves are produced by an earthquake – surface waves and body waves.

Earthquake Waves Surface Waves  Seismic waves that travel along Earth’s outer layer.

Earthquake Waves Body Waves  Waves that travel through the Earth’s interior  P waves – “push – pull” waves. The push (compress) and pull (expand) rocks in the direction the waves travel.  S waves – shake the particles at right angles to the direction of travel.

Earthquake Waves A seismogram shows all three types of seismic waves – surface waves, P waves, and S waves.

Locating an Earthquake The difference in velocities of P and S waves provides a way to locate the epicenter.

Locating an Earthquake The winning car is always faster than the losing car. The P wave always wins the race, arriving ahead of the S wave. The longer the race, the greater the difference in arrival times of the P and S waves at the finish line (seismic station). The greater the interval measured on a seismogram between the arrival of the first P wave and the first S wave, the greater the distance to the earthquake source.

Locating an Earthquake Travel-time graphs from three or more seismographs can be used to find the exact location of an earthquake epicenter.

Locating an Earthquake Earthquake Zones  95% of the major earthquakes occur in a few narrow zones  circum-Pacific belt: Japan, Philippines, Chile, and Alaska’s Aleutian Islands  Mediterranean – Asian belt

Measuring Earthquakes Historically, scientists have used two different types of measurements to describe the size of an earthquake.  Intensity – measure of the amount of shaking at a given location / based on damage  Magnitude – measure of the seismic waves or the amount of energy released at the source of the earthquake

Measuring Earthquakes Richter Scale  Based on amplitude of largest seismic wave (P, S, or surface) recorded on a seismogram.  A tenfold of increase in wave amplitude equals an increase of 1 on the magnitude scale.  Example – a 5.0 magnitude is 10 times greater than a 4.0 magnitude

Measuring Earthquakes Moment Magnitude  Derived from the amount of displacement that occurs along a fault zone.  Moment magnitudes is the most widely used measurement for earthquakes because it is the only magnitude scale that estimates the energy released by earthquakes.

Measuring Earthquakes Mercalli Scale  Rates an earthquake’s intensity in terms of the earthquake’s effects at different locations.  Example – Earthquake barely felt is a “I”  Earthquake that causes near/total damage is a “XII”

Ag Earth Science – Chapter 8.3

tsunami Japanese word for a seismic sea wave

liquefaction A phenomenon, sometimes associated with earthquakes, in which soils and other unconsolidated materials saturated with water are turned into a liquid that is not able to support buildings

seismic gap An area along a fault where there has not been any earthquake activity for a long period of time

Seismic Vibrations The damage to buildings and other structures from earthquake waves depends on several factors. These factors include the intensity and duration of the vibrations, the nature of the material on which the structure is built, and the design of the structure. Liquefaction - soils and other unconsolidated materials saturated with water are turned into a liquid that is not able to support buildings.

Tsunamis Tsunami – seismic sea wave A tsunami triggered by an earthquake occurs where a slab of the ocean floor is displaced vertically along a fault. A tsunami can also occur when the vibration of a quake sets an underwater landslide into motion.

Other Dangers With many earthquakes, the greatest damage to structures is from landslides and ground subsidence, or the sinking of the ground triggered by vibrations.

Predicting Earthquakes The goal of short-range prediction is to provide an early warning of the location and magnitude of a large earthquake. So far, methods for short- range predictions of earthquakes have not been successful. Long-range forecasts give the probability of a certain magnitude earthquake occurring within years.

Predicting Earthquakes Scientists study historical records and look for seismic gaps (area of fault where no activity has taken place for a long time). Scientists don’t yet understand enough about how and where earthquakes will occur to make accurate long-term predictions.

Ag Earth Science – Chapter 8.4

crust The thin, rocky outer layer of the Earth

mantle The 2890 km – thick layer of earth located below the crust

lithosphere The rigid outer layer of earth including the crust and upper mantle

asthenosphere A weak plastic layer of the mantle situated below the lithosphere

outer core A layer beneath the mantle about 2260 km thick

inner core The solid innermost layer of the earth, about 1220 km in diameter

Moho It is the boundary separating the crust from the mantle, discernable by an increase in the velocity of seismic waves

Layers Defined by Composition Earth’s interior consists of three major zones defined by its chemical composition – the crust, mantle, and core. Crust – the thin, rocky outer layer of Earth, is divided into oceanic and continental crust. Mantle – 82%+ of the earth’s volume is contained in the mantle – a solid, rocky shell that extends to a depth of 2890 km. Core – the core is a sphere composed of iron-nickel alloy

Layers Defined by Physical Properties Earth can be divided into layers based on physical properties – the lithosphere, asthenosphere, outer core, and inner core. Lithosphere – Earth’s outermost layer that is about 100 km in thickness. Consists of the crust and uppermost mantle. Asthenosphere – lies beneath the lithosphere and is a softer, weaker layer due to it’s high temperature “waxy” texture. The core is divided into the outer (liquid) core and inner (solid) core.

Discovering Earth’s Composition Early seismic data and drilling technology indicate that the continental crust is mostly made of lighter, granitic rocks. The crust of the ocean floor has a basaltic composition Earth’s core is thought to be mainly dense iron and nickel, similar to metallic meteorites. The surrounding mantle is believed to be composed of rocks similar to stony meteorites.