This document discusses different aspects of measuring and understanding earthquakes. It describes seismic waves like P and S waves and how their travel times are used to locate the epicenter of an earthquake. It also discusses the Richter scale and Modified Mercalli scale for measuring earthquake magnitudes, and some of the destructive effects of earthquakes like liquefaction, tsunamis, landslides, and fires. While short term predictions remain difficult, long term forecasts try to identify seismic gaps along faults.

7. Focus, Epicenter, and Fault

15. Elastic Rebound Hypothesis

20. Elastic Rebound Hypothesis

29. Seismograph

31. Seismogram

33. Seismic Waves Paths Through the
Earth

38. Measuring Earthquakes
P Waves
- Are push-pull waves that push (compress)
and pull (expand) in the direction that
the waves travel
- Travel through solids, liquids, and gases
- Have the greatest velocity of all
earthquake waves

41. Measuring Earthquakes
S Waves
- Seismic waves that travel along Earth’s
outer layer
- Shake particles at right angles to the
direction that they travel
- Travel only through solids
- Slower velocity than P waves

42. Earth’s Interior Showing
P and S Wave Paths

45. Measuring Earthquakes
Locating an Earthquake
 Earthquake Distance
• The epicenter is located using the difference
in the arrival times between P and S wave
recordings, which are related to distance.
 Earthquake Direction
• Travel-time graphs from three or more
seismographs can be used to find the exact
location of an earthquake epicenter.
 Earthquake Zones
• About 95 percent of the major
earthquakes occur in a few narrow zones.

46. Locating an Earthquake

49. Pusan Seismic Station S-P Interval = seconds

50. Tokyo Seismic Station S-P Interval = seconds

51. Akita Seismic Station S-P Interval = seconds

52. Station S-P Interval Epicentral Distance
Pusan 60 seconds KM
Tokyo 50 seconds KM
Akita 70 seconds KM

53. RECORDING S-P EPICENTER
STATION INTERVAL DISTANCE
PUSAN 56 sec 549 km
TOKYO 44 sec 434 km
AIKITA 71 sec 697 km

54. Location You Found Actual Location

59. Measuring Earthquakes
 Richter Scale
• Based on the amplitude of the largest
seismic wave
• Each unit of Richter magnitude equates to
roughly a 32-fold energy increase
• Does not estimate adequately the size
of very large earthquakes

60. Measuring Earthquakes
 Modified Mercalli
• scales measures how people feel and
react to the shaking of an earthquake.
• It is a relative scale, because people
experience different amounts of shaking
in different places.
• It is based on a series of key responses
such as people awakening, the
movement of furniture, and damage to
structures.

61. Earthquake Magnitudes

68. Destruction from Earthquakes
Seismic Vibrations
 Liquefaction
• Saturated material turns fluid
• Underground objects may float to
surface

71. Tsunami

72. Tsunami
 Cause of Tsunamis
• A tsunami triggered by an earthquake
occurs where a slab of the ocean floor is
displaced vertically along a fault.
• A tsunami also can occur when the
vibration of a quake sets an underwater
landslide into motion.
• Tsunami is the Japanese word for “seismic
sea wave.”

74. Destruction from Earthquakes
 Landslides
• With many earthquakes, the greatest
damage
to structures is from landslides and ground
subsidence, or the sinking of the ground
triggered by vibrations.
 Fire
• In the San Francisco earthquake of 1906,
most
of the destruction was caused by fires that
started when gas and electrical lines were
cut.

76. Predicting Earthquakes
 Short-Range Predictions
• So far, methods for short-range predictions
of earthquakes have not been successful.
 Long-Range Forecasts
• Scientists don’t yet understand enough
about how and where earthquakes will
occur to make accurate long-term
predictions.
• A seismic gap is an area along a fault
where there has not been any earthquake
activity for a long period of time.