Geology Reference
In-Depth Information
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Figure 8.3
The Focus and Epicenter of an Earthquake
Asia
India
Epicenter
Africa
Sri Lanka
Borneo
Sumatra
Wave ray
Focus
Madagascar
Australia
Wave front
Fault
The focus of an earthquake is the location where the rupture
begins and energy is released. The place on the surface vertically
above the focus is the epicenter. Seismic wave fronts move out in
all directions from their source, the focus of an earthquake.
a
The epicenter of the December 26, 2004 earthquake that caused
the devastating tsunami in the Indian Ocean. The epicenter was
located 160 km off the west coast of northern Sumatra and had a
focal depth of 30 km.
b
ground will shake. During some very large earthquakes, the
ground might shake for 3 minutes, a seemingly brief time, but
interminable if you are experiencing the earthquake fi rsthand.
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convergent margins (
Figure 8.4). Furthermore, a pattern
emerges when the focal depths of earthquakes near island
arcs and their adjacent ocean trenches are plotted. Notice in
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The location within Earth's lithosphere where fracturing
begins—that is, the point at which energy is fi rst released—is
an earthquake's
focus
,or
hypocenter
. What we usually hear in
news reports, however, is the location of the
epicenter
, the
point on Earth's surface directly above the focus (Figure 8.3a).
For example, according to the U.S. Geological Survey, the
December 26, 2004, earthquake that triggered the devastating
tsunami in the Indian Ocean had an epicenter 160 km off the
west coast of northern Sumatra (3
o
18' N and 95
o
52' E) and a
focal depth of 30 km (Figure 8.3b).
Seismologists recognize three categories of earthquakes
based on focal depth.
Shallow
-
focus
earthquakes have focal
depths of less than 70 km from the surface, whereas those with
foci between 70 and 300 km are
intermediate-focus
, and the foci
of those characterized as
deep-focus
are more than 300 km deep.
However, earthquakes are not evenly distributed among these
three categories. Approximately 90% of all earthquake foci are
at depths of less than 100 km, whereas only about 3% of all
earthquakes are deep. Shallow-focus earthquakes are, with few
exceptions, the most destructive, because the energy they release
has little time to dissipate before reaching the surface.
A definite relationship exists between earthquake foci
and plate boundaries. Earthquakes generated along diver-
gent or transform plate boundaries are invariably shallow
focus, whereas many shallow-focus earthquakes and nearly
all intermediate- and deep-focus earthquakes occur along
Figure 8.5 that the focal depth increases beneath the Tonga
Trench in a narrow, well-defi ned zone that dips approximately
45 degrees. Dipping seismic zones, called
Benioff
or
Benioff-
Wadati zones
, are common to convergent plate boundaries
where one plate is subducted beneath another. Such dipping
seismic zones indicate the angle of plate descent along a con-
vergent plate boundary.
AND HOW OFTEN?
No place on Earth is immune to earthquakes, but almost 95%
take place in seismic belts corresponding to plate boundaries
where plates converge, diverge, and slide past each other.
Earthquake activity distant from plate margins is minimal,
but can be devastating when it occurs. The relationship between
plate margins and the distribution of earthquakes is readily ap-
parent when the locations of earthquake epicenters are super-
imposed on a map showing the boundaries of Earth's plates.
The majority of all earthquakes (approximately 80%)
occur in the
circum-Pacific belt
, a zone of seismic activity
nearly encircling the Pacifi c Ocean basin. Most of these earth-
quakes result from convergence along plate margins, as in
the case of the 1995 Kobe, Japan, earthquake (
Figure 8.6a).
The earthquakes along the North American Pacific Coast,
especially in California, are also in this belt, but here plates
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