continuation of plate tectonics- convection in the asthenosphere is still the driving force of...

Continuation of plate tectonics- convection in the

asthenosphere is still the driving force of moving

lithospheric plates.

At plate boundaries where pressure builds up and eventually breaks rock

• Usually not in the middle of plates (1%)• We call these Intraplate earthquakes

Faults occur where boundaries meet and rocks “pass their elastic limit”

• Earthquakes often occur near these fault lines• Three types of faults

See Exploring Earth Website

1. Normal faults• Caused by TENSIONAL forces• Forces PULL OR STRETCH rocks

2. Reverse- - caused by COMPRESSIONAL forces- rocks are PUSHED toward each other

3. Strike-slip fault- Caused by SHEAR forces- Rocks move PAST EACH OTHER with little up or down motion.

For each type of fault:•NAME•DEMONSTRATE•DESCRIBE how the rocks move at the fault line – away, toward, past

•FORCES - compressional, tensional or shear

•Type of BOUNDARY•FOOTWALL/HANGING WALL

Point where plate movement occurs and energy is originated is called the focus (can be miles below the surface).

Point on Earth’s surface above the focus is called the epicenter

Let’s see if we can figure out from our data first!!!

Now let’s consult a textbook’s website and see what you think.

Normal (divergent): shallow Strike-slip (transform): shallow Intraplate: shallow Reverse (convergent)

• C/C - shallow• C/O - all depths (up to 700 km – 435 mi) Focus Depths increase farther from the coast Due to subduction

All types of seismic waves (s,p and surface) are detected by seismographs and recorded on seismograms.

An earthquake creates seismic waves that travel away from the epicenter of an earthquake.• Remember, epicenter is the place on the

Earth’s surface directly above where the earthquake occurred.

Seismic waves are detected by an instrument called a seismograph, which our book calls a seismometer.

Energy shown on a seismogram. Studied by seismologist!

Three types of seismic waves:

Primary waves (P waves) travel fastest• ~6 km/sec in the upper crust• Move particles back and forth in the

same direction• Cause little destruction

Secondary waves (S-waves) travel slower, cause more damage• ~3.5 km/sec in the upper crust• Move particles back and forth at a ninety

degree angle to wave motion Surface waves travel slowest, cause the

most damage• Move particles side-to-side and in a

swaying motion

Where does each wave type originate?

The difference in arrival time between p-waves and s-waves can be timed to determine how far away from the seismograph station the earthquake occurred.

With at least three stations reporting, we can pinpoint the earthquake’s location using TRIANGULATION (see next slide or page 312 for example).

How far away is the epicenter of an earthquake if there is 5 minutes between wave arrival time?

If an earthquake occurs 4700 miles away from a seismograph station:• How long after the earthquake will the P-wave

arrive?• How long after the earthquake will the S-wave

arrive?• What will be the difference in arrival time?

In three steps, seismologists can find the epicenter of an earthquake.

• 1. Find the time difference between P&S wave arrival

• 2. Use the time difference to find the distance from the seismograph station to the epicenter Gives a radius around the city

• 3. Compare with minimum three locations to find epicenter

Richter Scale – 1935 Measures earthquake magnitude Determined by height of waves from

seismogram Scale is infinite (depends on

sensitivity of equipment) Largest recorded - 9.5 1960 Chile

Earthquake Base-10 logarithm - √1000 ≈ 31.6 x Quantitative

Mercalli Scale – late 1800’s - modified• Measures earthquake intensity• Determined by human observation and

structural damage• I – XII• Qualitative

Mercalli Scale

What happens to Mercalli Intensity as distance from epicenter increases?

Magnitude Shallow/deep Liquefaction Tsunami Time of day Shadow zone Landslides/mudslides/avalanches - terrain Structures (codes, poverty, expected?) Gas Pipes/Fires Warning – Pacific Tsunami Warning Center Duration (how long did shaking occur?) Government Aid

By Permission Only Location, date and time Pictures Damage (cost), deaths, injuries Magnitude Difference in P/S waves in Punxsy Search USGS Top Ten for list of

earthquakes. Also a list on p. 318. Other research Plates involved

Occur far from plate boundaries

Faults can still exist, even where two tectonic plates do not meet

Example: New Madrid Fault

Explain how Primary and Secondary waves are used to pinpoint the location of an earthquake’s epicenter

Calculate the difference in earthquake magnitude using the Richter Scale