Lesson 1: Intro to Earthquakes

Focus: The zone within the Earth where rock movement produces an earthquake

Epicenter: The location on the Earth’s surface directly above the focus

Global Distribution of Earthquakes

1980-1990

Plate Boundaries

Elastic Rebound Theory

Stick-slip sliding

•  Alternating time periods of no-slip and sudden-slip.

•  Elastic energy builds during no-slip.

•  Elastic energy is released (earthquake) through sudden-slip.

Fault creep (stable sliding)

•  Slip is slow and gradual •  For extremely smooth fault

surfaces. •  No sudden release of energy

(no earthquakes).

Magnitude

•  Measure of earthquake size. •  It’s a logarithmic scale.

Intensity •  Strength of ground shaking •  Related to damage and

perception of people.

Richter’s Magnitude Scale Richter Magnitude Approximate TNT Equivalent Example

1.0 32 kg Construction site blast

2.0 1 metric ton Large mine blast

3.0 32 metric tons Massive ordnance air blast bomb

4.0 1 kiloton Small nuclear weapon

5.0 32 kilotons Nagasaki atomic bomb

6.0 1 megaton 1994 Dbl. Spring Flat EQ

7.0 32 megatons Largest thermonuclear weapon

8.0 1 gigaton 1906 San Francisco EQ

9.0 32 gigaton 1960 Chilean EQ

10.0 1 teraton Never recorded

Mercalli’s Intensity Scale

Types of Earthquake

Waves

Primary “P Waves” -- Compression

Secondary “S Waves” -- Shear

Rayleigh Waves

Love Waves

Seismograph

Seismogram

Activity 1

Distance = Velocity x Time

Train 1: 30 mph

Train 2: 60 mph

How long will it take for Train 1 to travel 120 miles? Train 2?

Train 1: 4 hours

Train 2: 2 hours 2 hr Diff

Distance = Velocity x Time

Train 1: 30 mph

Train 2: 60 mph

How long will it take for Train 1 to travel 240 miles? Train 2?

Train 1: 8 hours

Train 2: 4 hours 4 hr Diff

The travel time of the P wave is

distance from earthquake / (P-wave speed)

The travel time of the S wave is

distance from earthquake / (S-wave speed )

The difference in the arrival times of the waves is

distance from earthquake / (S-wave speed) - distance from earthquake / (P-wave speed )

which equals

distance from earthquake * ( 1/ (S-wave speed) - 1 / (P-wave speed) )

We can measure that difference from a seismogram and if we also know the speed that the waves travel, we could calculate the distance by equating the measured time difference and the expression. For the distance range 50 to 500 km, the S-waves travel about 3.45 km/s and the P-waves around 8 km/s. The value in parentheses is then equal to about (1/3.45 - 1/8) or about 1/8. Thus the simple rule of thumb for earthquakes in this distance range is the distance is about eight times the arrival time of S-wave less the arrival time of the P-wave.

Picking P and S waves on real data Example 1

Where is the P and S wave?

Picking P and S waves on real data Example 1

What is the S-P time?

Picking P and S waves on real data Example 2

Where is the P and S wave?

Picking P and S waves on real data Example 2

What is the S-P time?