ERTH 360 - Earthquakes

What are the two main seismic waves used to locate earthquakes?

P-waves and S-waves.

Which seismic wave arrives first at a station?

S-Wave

Which seismic wave arrives second at a station?

S-Wave

What is the S–P time?

The difference between the arrival time of the S-wave and the P-wave

What does the S–P time tell you?

It tells you the distance from the seismic station to the earthquake.

If the S–P time is larger, is the earthquake closer or farther away?

Farther Away

Can one seismic station locate an earthquake exactly?

No. One station only gives the distance, not the direction.

What shape does one station define on a map for earthquake location?

A circle around the station.

How many stations are needed to locate an earthquake epicenter?

At least 3

What is the epicenter?

The point on Earth’s surface directly above the earthquake.

What is the hypocenter (focus)?

The actual point within the Earth where the earthquake rupture begins.

What information improves earthquake location accuracy?

• more stations

• better station coverage

• accurate picking of arrivals

• a good velocity model

What is meant by first motion?

The first direction of ground movement recorded when the P-wave arrives.

What are the two possible first-motion types?

Compressional (up/push) and dilatational (down/pull).

What does a compressional first motion mean?

The station records an initial upward/pushing motion.

What does a dilatational first motion mean?

The station records an initial downward/pulling motion.

Why do different stations record different first motions for the same earthquake?

Because stations lie in different parts of the earthquake’s radiation pattern.

What is a focal mechanism?

A representation of the pattern of P-wave first motions around an earthquake.

What is another common name for a focal mechanism diagram?

A beach ball diagram.

What does a focal mechanism show?

• possible fault orientation

• areas of compression

• areas of tension/dilatation

What do the shaded parts of a beach ball usually represent?

Compression

What do the unshaded parts of a beach ball usually represent?

Tension or dilatation.

What are nodal planes?

The two planes that separate compressional and dilatational quadrants in a focal mechanism.

How many nodal planes are shown in a focal mechanism?

Two.

What do the two nodal planes represent?

One is the actual fault plane and the other is the auxiliary plane.

Can first motions alone tell you which nodal plane is the real fault plane?

No

What extra information can help identify the real fault plane?

• aftershock distribution

• mapped faults

• geological information

• rupture imaging

What is the P-axis in a focal mechanism?

The pressure/compression axis.

What is the T-axis in a focal mechanism?

The tension/extension axis.

What does the P-axis indicate?

The direction of maximum shortening/compression.

What does the T-axis indicate?

The direction of maximum extension/tension.

What kind of tectonic regime is associated with a normal fault focal mechanism?

Extension

What kind of tectonic regime is associated with a reverse fault focal mechanism?

Compression.

What is the basic shape of the first-motion radiation pattern?

Four quadrants:

• two compressional

• two dilatational

In earthquake location, what are you solving for overall?

• latitude

• longitude

• depth

• origin time