Coasts
Deltas
Hurricanes and Typhoons
Plate Motion
Relative Sea-level (RSL) Cycle
Sea Walls/Revetments
Tsunamis
Wadden Sea
Waves
EPICENTER, HYPOCENTER
Nina Kukowski
Institute of Geosciences, Friedrich-Schiller-University,
Jena, Germany
Synonyms
Focus (hypocenter)
Definition
The hypocenter of an earthquake is the subsurface location
at which energy stored along a locked fault is first
released. The epicenter is that location at the surface,
which is closest to the hypocenter.
During an earthquake, rupture propagates from the
hypocenter to both sides along the originally locked fault
(see “Earthquakes,” Figure 1). The hypocenter is directly
located beneath the epicenter, and the distance or depth
of the hypocenter, respectively, is the focal depth. Seismic
waves initially propagate spherically away from the hypocenter and will be reflected and refracted at, e.g., the
boundaries of geological layers, which show a contrast
in impedance, which is the product of seismic velocity
and rock density. This leads to complicated travel paths
of seismic waves arriving at seismic stations.
One of the most important inverse problems of seismology is to find the hypocenter, i.e., the location, where the
earthquake nucleates, from the arrival times of the different seismic waves at seismic stations (Stein and
Wysession, 2003). Whereas the distance to the epicenter,
the epicentral distance (Lowrie, 2007), can be estimated
from the time difference of the arrivals of p- and s-waves
at one seismic station, to localize the epicenter, distances
determined from three stations are necessary, from which
it can be found through triangulation. As Earth’s velocity
structure is not homogenous, there will be an error in the
epicenter estimate, which will be quite small, if data from
many seismic stations are available and if these stations
are not too far away.
Focal depth can range from close to the surface of the
Earth to several hundred kilometers depth (see “Earthquakes,” and “Wadati-Benioff-Zone”). It can be estimated
from the epicentral distance and the distance traveled by
the direct wave. Again, combining estimates from several
seismic stations improves the accuracy of determining
focal depth.
In case of shallow earthquakes, the epicenter usually
also is within the area of greatest damage. However, in
case of deeper earthquakes, when rupture is propagating
all along the fault to the surface, damage usually is greatest
where the fault reaches the surface. This is especially true
for seismogenic zone (see “Seismogenic Zone”) earthquakes along subduction zones (see “Subduction”), which
can reach the largest magnitudes observed. For such seismic events, the seismogenic fault often reaches the seafloor, which may lead to destructive tsunamis (see
“Tsunamis”).
Bibliography
Lowrie, W., 2007. Fundamentals of Geophysics, 2nd edn. New
York: Cambridge University Press. 38199.
Stein, S., and Wysession, M., 2003. An Introduction to Seismology,
Earthquakes and Earth Structure. Malden, MA: Blackwell,
498 pp.
Cross-references
Earthquakes
Fault-Plane Solutions of Earthquakes
Seismogenic Zone
Subduction
Wadati-Benioff-Zone
EROSION-HIATUSES
William W. Hay
Department of Geological Sciences, University of
Colorado at Boulder, Estes Park, CO, USA
Definition
Erosion is the processes whereby the detritus and solutes
resulting from the weathering of sediments and rock are
transported to a site of deposition. Hiatus derives from
the Latin, meaning a gap in something. In its geological
sense, a hiatus represents either a cessation in deposition
of sediments, resulting in an interval during which no
strata form, or an erosional surface within the stratigraphic
sequence representing a gap in the rock record.
History
Prior to the introduction of the ideas of seafloor spreading
and plate tectonics, documented by the recovery of long
sequences of ocean basin sediments by the Deep Sea Drilling Project (DSDP) in the late 1960s and 1970s, it was
thought that the deep sea was the repository of an essentially undisturbed monotonous but continuous
multibillion-year record of Earth history. It was thought
that sedimentation had been slow but continuous in a
EROSION-HIATUSES
231
Deltas
Hurricanes and Typhoons
Plate Motion
Relative Sea-level (RSL) Cycle
Sea Walls/Revetments
Tsunamis
Wadden Sea
Waves
EPICENTER, HYPOCENTER
Nina Kukowski
Institute of Geosciences, Friedrich-Schiller-University,
Jena, Germany
Synonyms
Focus (hypocenter)
Definition
The hypocenter of an earthquake is the subsurface location
at which energy stored along a locked fault is first
released. The epicenter is that location at the surface,
which is closest to the hypocenter.
During an earthquake, rupture propagates from the
hypocenter to both sides along the originally locked fault
(see “Earthquakes,” Figure 1). The hypocenter is directly
located beneath the epicenter, and the distance or depth
of the hypocenter, respectively, is the focal depth. Seismic
waves initially propagate spherically away from the hypocenter and will be reflected and refracted at, e.g., the
boundaries of geological layers, which show a contrast
in impedance, which is the product of seismic velocity
and rock density. This leads to complicated travel paths
of seismic waves arriving at seismic stations.
One of the most important inverse problems of seismology is to find the hypocenter, i.e., the location, where the
earthquake nucleates, from the arrival times of the different seismic waves at seismic stations (Stein and
Wysession, 2003). Whereas the distance to the epicenter,
the epicentral distance (Lowrie, 2007), can be estimated
from the time difference of the arrivals of p- and s-waves
at one seismic station, to localize the epicenter, distances
determined from three stations are necessary, from which
it can be found through triangulation. As Earth’s velocity
structure is not homogenous, there will be an error in the
epicenter estimate, which will be quite small, if data from
many seismic stations are available and if these stations
are not too far away.
Focal depth can range from close to the surface of the
Earth to several hundred kilometers depth (see “Earthquakes,” and “Wadati-Benioff-Zone”). It can be estimated
from the epicentral distance and the distance traveled by
the direct wave. Again, combining estimates from several
seismic stations improves the accuracy of determining
focal depth.
In case of shallow earthquakes, the epicenter usually
also is within the area of greatest damage. However, in
case of deeper earthquakes, when rupture is propagating
all along the fault to the surface, damage usually is greatest
where the fault reaches the surface. This is especially true
for seismogenic zone (see “Seismogenic Zone”) earthquakes along subduction zones (see “Subduction”), which
can reach the largest magnitudes observed. For such seismic events, the seismogenic fault often reaches the seafloor, which may lead to destructive tsunamis (see
“Tsunamis”).
Bibliography
Lowrie, W., 2007. Fundamentals of Geophysics, 2nd edn. New
York: Cambridge University Press. 38199.
Stein, S., and Wysession, M., 2003. An Introduction to Seismology,
Earthquakes and Earth Structure. Malden, MA: Blackwell,
498 pp.
Cross-references
Earthquakes
Fault-Plane Solutions of Earthquakes
Seismogenic Zone
Subduction
Wadati-Benioff-Zone
EROSION-HIATUSES
William W. Hay
Department of Geological Sciences, University of
Colorado at Boulder, Estes Park, CO, USA
Definition
Erosion is the processes whereby the detritus and solutes
resulting from the weathering of sediments and rock are
transported to a site of deposition. Hiatus derives from
the Latin, meaning a gap in something. In its geological
sense, a hiatus represents either a cessation in deposition
of sediments, resulting in an interval during which no
strata form, or an erosional surface within the stratigraphic
sequence representing a gap in the rock record.
History
Prior to the introduction of the ideas of seafloor spreading
and plate tectonics, documented by the recovery of long
sequences of ocean basin sediments by the Deep Sea Drilling Project (DSDP) in the late 1960s and 1970s, it was
thought that the deep sea was the repository of an essentially undisturbed monotonous but continuous
multibillion-year record of Earth history. It was thought
that sedimentation had been slow but continuous in a
EROSION-HIATUSES
231