WHERE EARTHQUAKES OCCUR
WHERE EARTHQUAKES OCCUR
•Three general belts
–CIRCUM-PACIFIC
–ALPIDE OR TRANSASIATIC OR MEDITERRANEAN- TRANSASIATIC
–MID-OCEAN RIDGES
CLASSIFICATION
•Depth of Focus
–SHALLOW (NORMAL)—0-70 KM (●)
–INTERMEDIATE—70-300 KM (●)
–DEEP—>300 KM (●)
•In fact, no earthquakes are known from depths much greater than 700 km
EARTHQUAKE DISTRIBUTION
CIRCUM-PACIFIC BELT
•Virtually all deep focus earthquakes
•Most larger, intermediate focus earthquakes
–>90%
•90% of the large normal focus earthquakes
•At least 80% of the world’s seismic energy released each year
EARTHQUAKE DISTRIBUTION
TRANS-ASIATIC BELT
•The rest of the large intermediate focus earthquakes
•The rest of the large normal focus earthquakes
EARTHQUAKE DISTRIBUTION
MID-OCEAN RIDGES
•Small, normal focus earthquakes
•Occur along the ridge
•Also occur on transform faults offsetting ridge segments
EARTHQUAKE DISTRIBUTION
VERTICAL DISTRIBUTION
•Most earthquakes are shallow (normal focus)
•In places in the ocean Hugo Benioff noted a pattern of increasing focal depth
KURILE-KAMCHATKA ARC
CROSS SECTION
ALEUTIAN ARC
SOUTH PACIFIC
ANDEAN ARC
CONCLUSIONS
•Earthquake foci get deeper under volcanic island arcs
•Earthquake foci also get deeper under continents where trenches are adjacent to
continents
•Volcanoes are present at all of these sites
•There must be a connection among ocean trenches, earthquakes of increasing focal
depth, and volcanism
ANNUAL EARTHQUAKE FREQUENCY
WORLD EARTHQUAKES
SUMMARY
•Larger earthquakes are less common than smaller earthquakes
•Deeper earthquakes are less common than shallow earthquakes