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 ActualitÉ > Sismology
Recent
seismic activity
Seismicity
maps
Earthquakes
regularly occur in the South West Pacific which
present a third of the world seismicity.
We present three maps where figure the last earthquakes
recorded, the South West Pacific viewed
as a whole and two sub-zones: the arc
of the New Hebrides and
the Fiji-Tonga
zone.
In order to have a clear idea of the seismicity, these maps
have to be compared with those showing the seismicity in these regions
over a longer period of time.
This
seismic information (location, depth, magnitude)
is released by the NEIC (National
Earthquake Information Center, http://neic.usgs.gov/).
Based in Denver, Colorado, USA, this centre measures with a
short delay all the epicentres of earthquakes occuring in the world
with a magnitude over 4. Recording and analysing earthquakes implies
varying timing, so the information can be a few hours delayed.
Shallow earthquakes
which have a focal depth located between the surface and 70 km are
represented by a red disk whose diameter is proportionnal to the
magnitude of
the earthquake. The values of the magnitudes are
detailed in the right top corner of the picture.
Deep earthquake whose hypocentre is located between 70 and 600 km are represented
by black disks. The
date the map is created is indicated in the left down corner. The last earthquake
is represented by a yellow disk with a thin border if it is shallow (0–70
km deep) or a thick border if it is deep (over 70 km).
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For
further information on the geology and geophysical studies
conducted by IRD in the South West pacific, visit the web page
of the Géosciences
Azur unit.
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In
the South-West Pacific
The
map of recent seismicity in the South West Pacific plots
the 40 last earthquakes that occurred in this region over
a given period of time (indicated
in the title).This map is actualised every hour.
The date the map is created is indicated in the left down
corner.
The South West Pacific is a boundary zone between the Australian and
the Pacific tectonic plates. The Pacific plate dipps under the Australian
plate between New Zealand and Samoa, this boundary zone being named
the Tonga Kermadec subduction zone. The Australian plate dipps under
the arc of the New Hebrides and the Solomon Islands. In
the region of New Guinea, the plates boundaries become really complex.
In this region of the world the convergence rates reach around 10 cm/year,
with a maximum of 24 cm/year recorded North of Tonga.
This
map of recent seismicity can be compared with the map describing seismicity
in the region over more than the past 20 years.
This latter only plots earthquakes with a magnitude over
5.
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In
Vanuatu (Arc of
the New
Hebrides)
The
map of recent seismicity in the Vanuatu plots
the 20 last earthquakes that occurred in this region over
a given period of time (indicated in the title).This map
is actualised every hour. The date the map is created
is indicated in the left down
corner.
The
arc of the New Hebrides is a boundary zone between the Australian
plate and the North Fiji Bassin. The Australian plate (on
which New Caledonia and the Loyalty Islands are based) is
thrusted and dipps under the arc of the New Hebrides. In
the southern part of this arc, the subduction rate reaches
around 10 cm/year, meaning that the Lifou and Tanna Islands
(respectively in New Caledonia and Vanuatu) are getting closer
at a speed of 10 cm per year.
This
map of recent seismicity can be compared with the map describing seismicity
in the region over more than the past 20 years.
This latter only plots earthquakes with a magnitude over
5.
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In
Fiji-Tonga
The
map of recent seismicity in the Fiji-Tonga zone plots
the 20 last earthquakes that occurred in this region over
a
given
period of time (indicated in the title).This map is actualised
every hour. The date the
map is created is indicated in the left down corner.
The
Fiji-Tonga seismic zone is the subduction boundary where
the Pacific plaque dipps under the Australian plate, therefore
resulting
in an intensive seismicity which extends to 600
km depth.
At the Northern end of Tonga, the subduction rate reaches the maximum
of 24cm/year. The seismicity is more diffused and very shallow in the
North Fiji bassin.
This
map of recent seismicity can be compared with the map describing seismicity
in the region over more than the past 20 years.
This latter only plots earthquakes with a magnitude over
5.
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| Magnitude
of an earthquake |
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The
magnitude of an earthquarke is a scale giving a measure of
the energy released by an earthquake. To calculate this magnitude,
seismologists use the amplitudes of the three types of seismic
waves:
– P waves are longitudinal vibrations,
compressive waves. Their velocity of propagation can reach
3.5 to 14 km/s, depending on the nature of the rocks and
the depth of their propagation.
– S waves are transversal vibrations,
or shear waves, perpendicular to the direction of the propagation.
These waves are around 1,7 times slower than the P waves.
– Surface waves are superficial
and have a large wave length. They are even slower than
the S waves.
Seismic
waves are not generated in an isotropic way, ie identical
in all the space directions, and they progressively
lessen along the distance they cover. Seismic stations
will record different amplitudes. Even taking into
account the epicentral distance (distance between the
epicenter and a seismic station) the magnitudes calculated
from these amplitudes will be slightly different. The
final magnitude will then be the mean of all magnitudes
calculated by the seismic stations.
According
to the seismic waves studied, three types of magnitudes
can be calculated:
– Mb or Body Wave Magnitude,
which is based on the P body waves generated by the
earthquake
– Ms or Surface Wave Magnitude
– Mw or Moment Magnitude calculated
from the global seismic record.
Sometimes they can differ from one magnitude unit to each other.
The
meanings of magnitude and intensity should not be confused.
The intensity is a value rating the damages caused
by an earthquake. For example, an intensity VI on the
Mercalli scale describes an earthquake that causes
cracks in walls. An earthquake of low magnitude, but
occuring near a city, can causes lots of damages and
therefore have a very high intensity.
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