Earthquake Origins

An earthquake is the vibration of earth produced by rapid release of energy.

This energy radiates in all directions from its source (FOCUS) in the form of waves. (much like the waves created when a stone is dropped into a calm pond of water) The media usually only discusses the EPICENTER of an earthquake, because it’s the place on the ground above the focus that people can actually see.

Earthquakes are the result of movement along large fractures (cracks) called faults in the middle of a plate, but mostly along plate boundaries. San Andreas Fault

We are in the “Mother Lode” (Gold Rush area) by an old subduction zone called the Melones Fault. San Andreas Fault System (there are many smaller ones Attached to it. . . )

The force that is applied to the rock material on each side of the fault is called STRESS. (An example would be if you put your hands together and pushed them toward each other)

As movement occurs, tectonic forces (stress) slowly deform the rock on both sides of the fault. As rocks are bending (STRAIN) they are storing elastic energy (much like a wooden stick would if you bent it) Eventually, the rocks bend beyond their breaking point and snap (ELASTIC LIMIT) The stored energy is released in the form of movement , and the rocks return to their original shape.(ELASTIC REBOUND)

Elastic Rebound

ENERGY TRAVELS THROUGH MATERIAL IN WAVES. The greater the force is, the higher the peaks (amplitude) and the faster it “wiggles” (oscillates)

There are Three Types of Earthquake Waves P waves (push) S waves (wave/ripple) Surface waves (spiral/corkscrew)

P WAVES P = primary waves are the fastest earthquake waves. Move through liquids, solids and gases (all layers of the earth). When they hit layers of different density, they slow down/bend

S WAVES S = Secondary S waves travel through solid material only, so they do not pass through the outer core of the earth at all. S waves can only travel through material that will return back to its former shape (lithosphere and mantle)

Surface Waves Surface waves travel along the surface of the earth only (crust). Surface waves are the slowest and most destructive of all earthquake waves.

Destruction from surface waves

Locating an Earthquake (1) First,earthquakes are detected using seismographs, which is a drawing device on a swinging arm anchored in bedrock that detect the vibrations of the P and S waves on a roll of paper.

There are ways that scientists and the media “rate” earthquakes according to intensity and damage: RICHTER SCALE: A scale of 1-10 that records the largest wave that was emitted from an earthquake in terms of force. Each number is 10x the next. MERCALI SCALE: A scale of 1-12 that lists a range of damage done by an earthquake.

The way to detect the location of origin of an earthquake, is by a process called TRIANGULATION. You need 3 seismographs in different locations to do this. .

The difference in P and S wave arrival time is called the LAG TIME The further away a station is from the focus the greater the difference in the arrival times of the P and S waves, like a lightning flash and thunder clap.

(2) The tape’s lag time is placed on a TIME-TRAVEL graph for both wave types. You can then follow the lag time down to the bottom to find your distance from the epicenter

Station 1 Station 3 Station 3 A circle is drawn from each epicenter, since you don’t know which direction it came from. Three seismic stations are needed to locate an earthquake epicenter (where the circles cross) Station 3 Station 3