Monitoring Faults

1
Monitoring Faults

• S E C T I O N 2 - 4

2
Objectives

• How do geologists monitor faults?
• How do geologists determine earthquake
• risk?

3

• Geologists have tried to predict when and where
  an earthquake will occur.

4

• They hypothesize that an approaching earthquake
  is signaled by changes in the elevation or tilt
  of the land.

5

• To observe these changes, geologists put in place
  instruments that measure stress and deformation
  in the crust.

6

• Four instruments that are used to monitor faults
  are creep meters, laser-ranging devices,
  tiltmeters, and satellites

7

• A creep meter uses a wire stretched across a
  fault to measure horizontal movement of the
  ground.
• On one side of the fault, the wire is anchored to
  a post.

8

• On the other side, the wire is attached to a
  weight that can slide if the fault moves.
• Geologists can measure the amount that the fault
  has moved by measuring how much the weight has
  moved against a measuring scale.

9

• A laser-ranging device uses a laser beam to
  detect even tiny fault movements.

10

• The device calculates any change in the time
  needed for the laser beam to travel to a
  reflector and bounce back.
• Thus, the device can detect any change in
  distance to the reflector.

11

• A tiltmeter measures tilting of the ground.
• The tiltmeters used by seismologists consist of
  two bulbs that are filled with a liquid and
  connected by a hollow stem.

12

• If the land rises or falls even slightly, the
  liquid will flow from one bulb to the other.
• Each bulb contains a measuring scale to measure
  the depth of the liquid in that bulb

13

• Geologists read the scales to measure the amount
  of tilt occurring along the fault.

14

• Besides ground based instruments, geologists use
  satellites equipped with radar to make images of
  faults.
• The satellite bounces radio waves off the ground.

15

• As the waves echo back into space, the satellite
  records them.
• The time it takes for the radio waves to make
  their round trip provides precise measurements of
  the distance to the ground.

16

• The distance from the ground to the satellite
  changes with every change in the ground surface.

17

• By comparing different images of the same area
  taken at different times, geologists detect small
  changes in elevation.
• These changes in elevation result when stress
  deforms the ground along a fault.

18

• Even with fault-monitoring devices, scientists
  cannot yet predict earthquakes accurately.
• However, they do know that earthquakes are likely
  wherever plate movement stores energy in rocks.

19

• Geologists can determine earthquake risk by
  locating where faults are active and where past
  earthquakes have occurred.

20

• In the United States, earthquake risk is highest
  on the west coast and Alaska.

21

• Other regions of the United States also have some
  risk of earthquakes.
• Scientists have created maps showing earthquake
  risk in different regions of the country.

22
End