Understanding Earthquakes

Not realistic!

Mexico City Earthquake, September 19, 1985 Mexico City Earthquake, September 19, 1985. Eight-story frame structure with brick infill walls broken in two. The foundation also came off.

Earthquake damage in Panama

8.2 magnitude Earthquake damage in Chile.

Loma Prieta, California, Earthquake October 17, 1989. San Francisco.

Loma Prieta, California, Earthquake October 17, 1989. San Francisco.

Loma Prieta, California, Earthquake October 17, 1989. San Francisco.

Loma Prieta, California, Earthquake October 17, 1989. San Francisco. AKA: World Series EQ. 6.9 magnitude. 60 people killed.

Northridge Earthquake, Orange County, California, January, 21, 1994 6.7 magnitude

Northridge Earthquake, Orange County, California, January, 21, 1994

Northridge Earthquake, Orange County, California, January, 21, 1994

Northridge Earthquake, Orange County, California, January, 21, 1994

Kaoiki, Hawaii, Earthquake November 16, 1983

Alaska’s Good Friday earthquake, 1964

Alaska’s Good Friday earthquake, 1964

Japan, 2016

What are Earthquakes? Earthquakes are defined as movements of the ground that are caused by a sudden release of energy when rocks along a fault move. Earthquakes usually occur when rocks under stress suddenly shift along a fault.

What is a fault? A fault is a break in a body of rock along which one block moves relative to another.

Fault on the surface of the Earth San Andreas fault

Earthquakes around the World What do you notice about the locations of earthquakes?

Recent Earthquakes (last 24 hours) https://earthquake.usgs.gov/earthquakes Red= deepest EQ, yellow = shallowest

Fact: About 95% of earthquakes occur along plate boundaries.

Hanging wall - Footwall You could hang a lantern on the hanging wall. You can stand on the footwall Foot Wall Hanging Wall

Types of Faults: Normal When forces in the Earth pull rocks apart, this causes tension. Tension causes Normal faults. Label the Hanging Wall and Footwall UP DOWN Foot Wall Hanging Wall

Types of Faults: Reverse Compressional forces occur when rocks are pushed together. Compression causes rocks to move up and over other rocks. Label the Hanging Wall and Footwall UP DOWN Hanging Wall Foot Wall

Normal versus Reverse Faults

Types of Faults: Strike-slip Rocks on either side of the fault move sideways past each other, without any upward or downward movement. Strike-slip

3. Anatomy of an earthquake Focus - The subsurface point at which rock has fractured and movement has occurred. (One focus, two foci). Epicenter - The point on Earth’s surface directly above the focus.

Anatomy of an earthquake Although earthquake depths vary, 90% occur about 70 km (50 miles) below Earth’s surface. Very deep earthquakes are those with depths between 300 - 700 km below the surface. These earthquakes often occur in subduction zones.

Fill in the focus, epicenter and fault

The sounds of an Earthquake https://phys.org/news/2014-01-landslide-triggered-quakes.html

4. Seismic Waves As rocks move along faults, the rocks release energy in the form of vibrations, called seismic waves. These waves travel outward in all directions from the focus through the surrounding rock.

Seismic Waves

How are earthquakes like a pebble dropped into a pool of water? SAME because the pebble sends out ripples or vibrations in the water. DIFFERENT because the ripples don’t travel up or down.

Types of Seismic Waves a. Primary waves (P-waves) - Waves that cause rocks to move back and forth in the same direction as the wave. They are the fastest seismic waves and are the first ones to be detected on a seismograph. P-waves can travel through solids, liquids and gases!

P-Waves = primary= compressional Wave traveling in this direction Rocks moving in this direction

S-Waves (Secondary, Shear) Secondary waves (S-waves) - Waves that cause rocks to move in a direction that is perpendicular to the wave direction. These waves are the second fastest waves. S-waves can only travel through solid materials.

S-Waves = Secondary= Shear Wave traveling in this direction Rocks moving in this direction

Surface Waves c. Surface waves - Travel on the Earth’s surface only. They are the slowest moving waves, but often cause the most damage. ►Why do you think that is?

Surface Waves Complex waves that move back and forth or in an elliptical shape.

Example of Surface Waves

5. Locating an Earthquake a. Seismograph - An instrument that detects and records vibrations in the ground, including earthquakes and explosions. The seismograph records both the horizontal and vertical motion of the ground. b. Seismogram - The paper that the earthquake signal is traced onto and analyzed

Quick Review What is the fastest wave? What is the next fastest wave? What is the slowest wave? fastest = p-wave, next fastest = s-wave, slowest = surface wave

Label this seismogram S-wave background noise P-wave Surface wave

c. Epicenter Location If seismic wave information is obtained at 3 seismograph stations, the epicenter of the earthquake can be determined. Each seismograph records the arrival of an earthquake. Since time (t) is known, and the velocity (v) of earthquakes is known...scientists can calculate how far away (d) the earthquake occurred from the seismograph station.

Epicenter Location

How a seismograph works

ADD THESE NOTES TO YOUR TABLE OF CONTENTS! Earthquake Notes