Unusual earthquakes
Immediately after a large earthquake, the so-called main
shock, usually, many smaller events, the so-called aftershocks, occur in the same region, i.e., along parts of the
fault plane that had ruptured before. This seismic activity
often decays over some weeks or months, depending on
the size of the main earthquake.
If seismic activity is observed over weeks or months,
but without any major event, this is called an earthquake
swarm. Further, there are also unusual types of seismic
events like those causing a tsunami larger than expected
from their magnitude (Okal and Newman, 2001). Since
about 15 years, another type of seismic events, called slow
slip events, has been identified along certain plate
boundaries.
Earthquake swarms
Earthquake swarms are periods of seismic activity of more
or less equal size without dominating events, an abrupt
onset and ending of the seismic activity. Individual magnitudes scarcely exceeding three or four, which are occurring over weeks or months in a certain region limited in
space (e.g., Scholz, 2002; Hainzl et al., 2012). Earthquake
swarms usually consist of several hundreds to several
thousands detected individual events. Often, they start
and cease with gradually increasing and decreasing magnitudes, respectively. Whereas earthquake swarms were
first mainly detected in volcanic regions, they now have
been identified in nearly all seismotectonic settings, such
as subduction zones close to the seismogenic thrust, transform faults, or intraplate regions (Holtkamp and
Brudzinski, 2011; Hainzl et al., 2012). B-values of earthquake swarms often are between 1.5 and 2, and therefore
considerably larger than those obtained from global or
regional earthquake data sets (Scholz, 2002; Holtkamp
and Brudzinski, 2011). Fluids have been proposed to play
an important role for earthquake swarms, e.g., pore pressure changes or magma transport. Further, earthquake
swarms may be potentially related to slow slip events.
Slow earthquakes
Recently, a special type of release of seismic energy has
been identified from GPS records in several subduction
zones around the Pacific, like Cascadia offshore the
USA and Canada west coast (Dragert et al., 2001) or
Hikurangi offshore the east coast of New Zealand’s north
island (Wallace et al., 2012). Well-studied regions with
SSEs recurring in relatively regular intervals, like
Cascadia or Hikurangi show that these events may nucleate at very shallow depths, about 5–6 km beneath the seafloor, but also at depths of 30–40 km indicating they can
occur in very different thermal regimes. Several types of
slow earthquakes are known, e.g., tremors, slow slip
events, or very low-frequency earthquakes. Mostly, they
seem to occur in subduction zones with a young
downgoing plate, but have also been detected beneath
the San Andreas fault system (Beroza and Ide, 2011).
It seems that fluids are invoked in their occurrence,
possibly at very high fluid pressures. However, SSEs’
mechanisms so far are largely not understood (Schwartz
and Rokosky, 2007). An important role of Love waves
also has been hypothesized. Slow earthquakes seem to
occur episodically, often quite frequent, with recurrence
times of only a few years or even less at the same location
and typically show durations of hours to several months.
Slow earthquakes cannot be detected with seismometers
and so far only have been identified from geodetic
(GPS) records. Slip caused by slow earthquakes suggests
that usually their size is comparable to that of usual
earthquakes with magnitudes around 6–6.5.
Seismic hazard
Most seismic hazard is related to major and great earthquakes, as one magnitude >8 earthquake releases more
energy than all other earthquakes during an entire year. It
describes the probability of a certain level of ground shaking caused by recurring earthquakes (Shedlock et al.,
2000; Stein and Wysession, 2003).
Damage caused by an earthquake mainly is the consequence of ground acceleration, which describes the
amount of shaking or strong motion, and is used as a measure for seismic risk. Other hazardous phenomena occurring in relation to earthquakes are, e.g., slumping in
regions of steep slope, liquefaction, or tsunamis (see
Wadati-Benioff-Zone).
The size of an earthquake does not necessarily directly
relate to damage and fatalities caused by it as the depth of
the hypocenter, and consequently ground motion as well
as bedrock in the region affected by an earthquake largely
control destruction. When nonconsolidated, watersaturated sand, which is typical, e.g., for river estuaries
or shores, is repeatedly shaken, especially at resonance
period, e.g., during ground motion caused by a large earthquake, it may behave like a liquid, i.e., it completely
looses its shear strength and therefore no longer is able
to carry, e.g., buildings. As a consequence of this liquefaction, buildings may sink several meters and become
severely damaged, like during the 1906 San Francisco,
1964 Alaska, 2010 Canterbury, or 2011 Christchurch
earthquakes (Quigley et al., 2013). Strike-slip earthquakes
seem to be especially prone to a high degree of damage
relative to their magnitudes as their hypocenters usually
are quite shallow. If sediments redistributed by liquefaction keep their geotechnical properties, they may be indicative of paleoearthquakes (Green et al., 2005).
Therefore, globally, seismic hazard differs considerably
between different tectonic settings and to a large degree
mirrors the distribution of brittle strain rate, i.e., deformation (Triep and Sykes, 1997). To quantify seismic hazard,
data of historical and instrumentally recorded earthquakes, their source characteristics, and related strong
ground motions have been compiled to compute global
seismic hazard and assign a level of seismic hazard to
every location worldwide (Giardini et al., 1999).
EARTHQUAKES
215
Immediately after a large earthquake, the so-called main
shock, usually, many smaller events, the so-called aftershocks, occur in the same region, i.e., along parts of the
fault plane that had ruptured before. This seismic activity
often decays over some weeks or months, depending on
the size of the main earthquake.
If seismic activity is observed over weeks or months,
but without any major event, this is called an earthquake
swarm. Further, there are also unusual types of seismic
events like those causing a tsunami larger than expected
from their magnitude (Okal and Newman, 2001). Since
about 15 years, another type of seismic events, called slow
slip events, has been identified along certain plate
boundaries.
Earthquake swarms
Earthquake swarms are periods of seismic activity of more
or less equal size without dominating events, an abrupt
onset and ending of the seismic activity. Individual magnitudes scarcely exceeding three or four, which are occurring over weeks or months in a certain region limited in
space (e.g., Scholz, 2002; Hainzl et al., 2012). Earthquake
swarms usually consist of several hundreds to several
thousands detected individual events. Often, they start
and cease with gradually increasing and decreasing magnitudes, respectively. Whereas earthquake swarms were
first mainly detected in volcanic regions, they now have
been identified in nearly all seismotectonic settings, such
as subduction zones close to the seismogenic thrust, transform faults, or intraplate regions (Holtkamp and
Brudzinski, 2011; Hainzl et al., 2012). B-values of earthquake swarms often are between 1.5 and 2, and therefore
considerably larger than those obtained from global or
regional earthquake data sets (Scholz, 2002; Holtkamp
and Brudzinski, 2011). Fluids have been proposed to play
an important role for earthquake swarms, e.g., pore pressure changes or magma transport. Further, earthquake
swarms may be potentially related to slow slip events.
Slow earthquakes
Recently, a special type of release of seismic energy has
been identified from GPS records in several subduction
zones around the Pacific, like Cascadia offshore the
USA and Canada west coast (Dragert et al., 2001) or
Hikurangi offshore the east coast of New Zealand’s north
island (Wallace et al., 2012). Well-studied regions with
SSEs recurring in relatively regular intervals, like
Cascadia or Hikurangi show that these events may nucleate at very shallow depths, about 5–6 km beneath the seafloor, but also at depths of 30–40 km indicating they can
occur in very different thermal regimes. Several types of
slow earthquakes are known, e.g., tremors, slow slip
events, or very low-frequency earthquakes. Mostly, they
seem to occur in subduction zones with a young
downgoing plate, but have also been detected beneath
the San Andreas fault system (Beroza and Ide, 2011).
It seems that fluids are invoked in their occurrence,
possibly at very high fluid pressures. However, SSEs’
mechanisms so far are largely not understood (Schwartz
and Rokosky, 2007). An important role of Love waves
also has been hypothesized. Slow earthquakes seem to
occur episodically, often quite frequent, with recurrence
times of only a few years or even less at the same location
and typically show durations of hours to several months.
Slow earthquakes cannot be detected with seismometers
and so far only have been identified from geodetic
(GPS) records. Slip caused by slow earthquakes suggests
that usually their size is comparable to that of usual
earthquakes with magnitudes around 6–6.5.
Seismic hazard
Most seismic hazard is related to major and great earthquakes, as one magnitude >8 earthquake releases more
energy than all other earthquakes during an entire year. It
describes the probability of a certain level of ground shaking caused by recurring earthquakes (Shedlock et al.,
2000; Stein and Wysession, 2003).
Damage caused by an earthquake mainly is the consequence of ground acceleration, which describes the
amount of shaking or strong motion, and is used as a measure for seismic risk. Other hazardous phenomena occurring in relation to earthquakes are, e.g., slumping in
regions of steep slope, liquefaction, or tsunamis (see
Wadati-Benioff-Zone).
The size of an earthquake does not necessarily directly
relate to damage and fatalities caused by it as the depth of
the hypocenter, and consequently ground motion as well
as bedrock in the region affected by an earthquake largely
control destruction. When nonconsolidated, watersaturated sand, which is typical, e.g., for river estuaries
or shores, is repeatedly shaken, especially at resonance
period, e.g., during ground motion caused by a large earthquake, it may behave like a liquid, i.e., it completely
looses its shear strength and therefore no longer is able
to carry, e.g., buildings. As a consequence of this liquefaction, buildings may sink several meters and become
severely damaged, like during the 1906 San Francisco,
1964 Alaska, 2010 Canterbury, or 2011 Christchurch
earthquakes (Quigley et al., 2013). Strike-slip earthquakes
seem to be especially prone to a high degree of damage
relative to their magnitudes as their hypocenters usually
are quite shallow. If sediments redistributed by liquefaction keep their geotechnical properties, they may be indicative of paleoearthquakes (Green et al., 2005).
Therefore, globally, seismic hazard differs considerably
between different tectonic settings and to a large degree
mirrors the distribution of brittle strain rate, i.e., deformation (Triep and Sykes, 1997). To quantify seismic hazard,
data of historical and instrumentally recorded earthquakes, their source characteristics, and related strong
ground motions have been compiled to compute global
seismic hazard and assign a level of seismic hazard to
every location worldwide (Giardini et al., 1999).
EARTHQUAKES
215