Earthquakes and Earth’s Interior - The Changing Earth: Exploring Geology and Evolution - page 193

Pacific Ocean

Indian
Ocean

Atlantic
Ocean

Deep-focus earthquake Intermediate-focus earthquake Shallow-focus earthquake

◗

Figure 8.4 Earthquake Epicenters and Plate Boundaries This map of earthquake epicenters
shows that most earthquakes occur within seismic zones that correspond closely to plate boundaries.
Approximately 80% of earthquakes occur within the circum-Pacifi c belt, 15% within the Mediterranean-
Asiatic belt, and the remaining 5% within plate interiors and along oceanic spreading ridges. The
dots represent earthquake epicenters and are divided into shallow-, intermediate-, and deep-focus
earthquakes. Along with many shallow-focus earthquakes, nearly all intermediate- and deep-focus
earthquakes occur along convergent plate boundaries.

slide past one another rather than converge. The October 17,
1989, Loma Prieta earthquake in the San Francisco area
(Figure 8.6b) and the January 17, 1994, Northridge earthquake
(Figure 8.6c) occurred along this plate boundary.
The second major seismic belt, accounting for 15% of all
earthquakes, is the Mediterranean-Asiatic belt. This belt extends

westward from Indonesia through the Himalayas, across Iran
and Turkey, and westward through the Mediterranean re-
gion of Europe. The devastating 1990 and 2003 earthquakes
in Iran that killed 40,000 and 43,000 people, respectively; the
1999 Turkey earthquake that killed approximately 17,000; the
2001 India earthquake that killed more than 20,000 people;

Tonga
volcanic arc

Tonga
Trench

Oceanic
lithosphere

TONGA
ISLANDS

0

AUSTRALIA

200

Magma

NEW ZEALAND

Oceanic
lithosphere

Mantle

400

Earthquake
focus

600

◗ Figure 8.5 Benioff Zones Focal depth increases in a well-defi ned zone that dips approximately
45 degrees beneath the Tonga volcanic arc in the South Pacifi c. Dipping seismic zones are called
Benioff or Benioff-Wadati zones.