Earthquakes
An earthquake is the shaking of the ground caused by the sudden movement of large blocks of rock along a fault. All earthquakes occur in the crust. All earthquakes occur along faults. The strength of an earthquake depends on:
how much stress builds up before the rocks move
the distance the rocks move along the fault
A fault is a fracture or break in the Earth's crust, along which blocks of rock move past each other. The blocks of rocks move in different directions depending in the stress they are under. There are 3 main types of faults.
Normal Faults - as the rocks are being pulled (apart) the block of rock above the fault plane slides down. Earthquakes occur and mountains can form here
Reverse Fault - as the rocks are pushed together (stress pushes the rocks together), the block of rock above the fault plane moves up. Earthquakes occur and mountains can form here
Strike-Slip Fault - As the rocks are pushed horizontally, in opposite directions, the rocks slide or move sideways along the fault plane. The most famous strike - slip fault in the Unites Staes is the San Andreas Fault that runs along the west coast.
Seismic Waves
Earthquake energy travels in all directions (up, down, & sideways). The energy travels as seismic waves. Seismic waves are vibrations caused by earthquakes. These waves can be recorded by sensitive instruments. All earthquakes start below the Earth's surface at the focus. The focus of the earthquake is the point underground where rocks first begin to move. The epicenter is the point on the Earth's surface directly above the focus.
**The deeper the focus, the less damage on the surface because seismic waves lose energy as they travel up Earth's surface.**
Primary waves (P waves) - travel the fastest through rock materials causing the rock to vibrate in the same direction the waves are moving. They can travel through a solid, liquid, and a gas. They are compressional waves.
Secondary waves (S waves) - move through rock materials by causing particles to vibrate at right angles. S-waves can only travel through solids. These are transverse waves.
Primary and secondary waves travel through the Earth's interior. Studying these waves reveal information about the Earth's interior like an earthquake's depth.
Surface waves are the largest and slowest of the seismic waves but they cause the most destruction during an earthquake. Some surface waves move the crust itself in a side to side motion. If a building is on the crust that begins to move, the foundation will break and the building will fall.
A seismograph is an instrument that constantly records ground movement. The output looks like a bunch of wiggles in a line. The height of the wiggle indicates the amount of ground movement produced by seismic waves at the seismic location.
Evidence of Strike Slip Faults
Earthquakes are most dangerous when they occur where people live. Most injuries and deaths are directly caused by ground movement. Earthquakes can cause fires, explosions (due to broken gas lines, electrical power lines, or overturned stoves)
An earthquake's magnitude is the strength of the earthquake. Geologists use the Richter Scale to assign magnitude to earthquakes by assessing the amplitude (height) of the largest seismic wave each earthquake creates. Each additional unit of magnitude denotes a tenfold increase in the power of the earthquake (e.g., a magnitude 7.0 earthquake is ten times more powerful than a magnitude 6.0). A Moment Magnitude Scale is more accurate than the Richter Scale and is based on the amount of energy released by an earthquake. An increase of 1 whole number indicates an increase of 32x more energy.
Earthquakes move blocks of rock and other structures on the fault. The aftershock is a smaller earthquake(s) that follow a bigger quake. Liquefaction is when the shaking of the ground causes the soil to act like a liquid.
Tsunami
- a series of water waves triggered by an earthquake, volcanic eruption, or a landslide. Sometimes called tidal waves, tsunamis can rise more than 20 stories high and travel at speeds of 430 miles per hour. Tsunamis can be several waves that are different heights and arrive hours apart.
Finding the Epicenter
Use p11 of the ESRT
To locate the epicenter of an earthquake, scientists need seismograph readings from at least 3 different seismic locations.
Three steps to locating an epicenter:
Find the difference between the arrival times of the primary and secondary waves at each of the 3 seismic stations
The time difference is used to determine the distance of the epicenter from each station. The greater the distance in time = the farther away the epicenter is. Therefore an earthquake's depth can be determined by the difference of arrival times
A circle is drawn, on a map, around each station with a radius corresponding to the epicenter's distance from that station. The point where the 3 circles meet is the epicenter.