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Within a few days or weeks from the main shock and near its
location, some shaking and vibrating occur. These shakings
which are due to rock deformation against stress are called
foreshock.
Every main shock is not accompanied and some large
earthquakes have happened without any foreshock activity,
but sometimes a foreshock is destroyer than the main
earthquake.
Crust thickness
The thickness of the earth's crust is not the same
everywhere. It is thick in plains and continental shield,
but it becomes thinner in continental shelf. Near the axis
of mid-oceanic rifts, the crust thickness is lowest. In
continental mountain ranges, thickness of the crust is more
than any where on the earth.
Conrad discontinuity
Seismic waves change in the continental crust at the
boundary of “SIMA” and “SIAL”. This boundary is called
Conrad discontinuity. It has been proven that this boundary
does not extend every where in the Earth.
The Earth's Crust
Crust is a very thin layer on the Earth. Its thickness is
20 to
60 km in the
continent and 8 to
12 km in the ocean. It is assumed that crust is
divided into two horizontal layers with the same thickness.
Upper layer is granite stone and lower layer is basalt one.
The boundary between these two layers is indicated by Conrad
discontinuity.
Partial melted mantle of the Earth is below the basalt
layer. Boundary between the crust and the mantle was
indicated by Mohorovicic in
1910. This
boundary is called Mohorovicic boundary or Moho.
At this boundary compressional wave velocity changes from
5.6 Km/s to 8
Km/s which is an indication of the difference between
composition of the layers. Likely this boundary is solid
because S-wave propagates in it.
P and S waves
P-waves arrive to seismograms before S-wave in
0° to
103° from the
Earth’s center because P-wave velocity is more than S-wave
velocity. Because of multiple reflections, P-waves and
S-waves travel a curved path in this interval. Neither
P-waves nor S-waves are received directly from the
hypocenter in 103°
to 143°. It
is assumed that there is a shield in this interval that
prevents waves to be recorded directly. Because of core
density no P-wave crosses it and it is not observed in this
interval (P-wave shadow zone). After
143°, P-waves are observed without any evidence of
S-waves. It occurs because outer core is liquid (S-wave
shadow zone).
Moho discontinuity
By studying P and S waves of a shallow earthquake,
Mohorovicic observed that within 800 km away from the
earthquake two groups of P and S waves are recorded. But in
further distances only one group of waves is recorded. For
explaining this matter, Moho assumed boundary with two
different types of rocks. In the first group of waves, an
internal group of waves travel a curved path in low density
rocks and the other group travel longer path and arrive at
the surface with delay. The second group of waves penetrates
to the Core and Mantle interface and reflects. Core and
Mantle interface was determined based on the difference in P
and S wave velocity because Compression wave velocity is
5.6 Km/s in
the Crust and about
8 Km/s in the lower layers.
Upper Crust
The density of this layer is between
7.2 to
82.5 gr/cm3.
It is composed of Igneous, Sedimentary and Metamorphic
rocks. Its composition is similar to Granite therefore the
source of Granite magma is upper Crust. This section is rich
of Silica (Sio2)
and Alumina (Al2o)
and is called (SIAL). For modeling Crust under deep oceans,
the granite layer is omitted.
Lower Crust
This layer has a composition similar to Basalt, which
density is between1.3
to 85.2 gr/cm3
and it is rich of Silica, Iron and Magnesium and it is
called "SIMA" for this reason. No Basalt layer has been
observed beneath granite layer. Therefore Oceanic layer is a
rock layer similar to "SIMA" and there is no such material
to produce Granite magma in the oceans.
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