Seismic origin: The place where an earthquake occurs
within the earth.
Depth of seismic origin：Consider the seismic origin as
a point，the vertical distance from this point to the
ground, known as the focal depth.
Epicenter: The projection point of the seismic origin
on the ground, known as the epicenter.
The meizoseismal area: The most severely damaged area
on the ground.
The epicentral distance：The direct distance from the
epicenter to any point on the ground.
Aftershock: a series of earthquakes after the main shock
Main earthquake type: Earthquake sequences with prominent
The swarm type of the earthquake: There are no prominent
main earthquake, the main energy is released from multiple
earthquakes of similar magnitude.
The isolation type of the earthquake：There were very few
foreshocks or aftershocks，seismic energy was essentially
released once by the main earthquake.
Seismic wave：Seismic waves are elastic waves that propagating
the earth's medium from the seismic origin. Seismic waves are
the only waves which known to penetrate the earth's interior.
When the earthquake occurs, the fracture and dislocation of
underground release enormous energy, motivate an elastic wave
and propagate all around, this is a seismic wave. Seismic waves
from the seismic origin propagating to all directions through
the earth's medium, therefore, we can record earthquakes by
seismograph around the world. People's understanding of the
earth's interior comes from seismic waves.
Seismic waves are mainly divided into body wave and surface wave.
Body waves can propagate to any direction in three–dimension space,
it can also be divided into longitudinal wave and shear wave. The
longitudinal wave (also known as the P wave) means that the vibration
direction is consistent with the wave propagation direction, we feel
jolt when they reach the ground, objects bouncing up and down. The
shear wave (as known as the S wave) means that the vibration direction
is perpendicular to the wave propagation direction, we feel shaky when
they reach the ground, objects swinging back and forth. The longitudinal
waves travel faster than the shear waves inside the earth, so when an
earthquake occurs, the longitudinal wave always reach the surface first.
Then, when a major earthquake occurs, we feel jolt up and down first, a
few seconds to a dozen seconds later, we feel a strong horizontal wobble.
When the body wave reaches the rock layer interface or the earth's surface,
it will create a border, or the earth surface propagates large range waves,
as known as surface waves
The longitudinal wave is a propulsive wave, the propagation velocity in the
crust is 5.5 ~ 7.0km/s, first arrival, which makes the ground vibrate up and
down, less destructive. When the shear wave propagates, the particle’s motion
direction is perpendicular to the shear wave’s direction of propagation, shear
stress is produced in the medium, as known as the shear waves. Because the
fluid cannot withstand shear stress, therefore, shear waves can only travel
in solids, cannot travel in liquid or gas. The velocities of p-waves and s-waves
are determined by the density and elastic constants of the medium. The propagation
velocity of shear wave in the crust is 3.2 ~ 4.0km/s, second arrival, it makes
the ground jitter before and after, left and right, more destructive. Surface
waves are mixing waves, that stimulate the produce the waves by the longitudinal
and shear waves meeting at the surface, the wave is long and big, has strong
amplitude. Surface waves are elastic waves that travel along the surface of
the earth, surface waves travel slower than body waves, it is the main factor
that destructed the building strongly. There are two kinds of the most important
surface waves; Rayleigh wave(R wave) and love wave (L wave), they were named for
in honor of their discoverer, a British scientist, LordRayleigh and A.E.H.Love.
An important aspect of utilizing seismic waves is seismic
exploration, the history of seismic exploration can goes back to
the mid-19th century. Mallette measured the propagative velocity
of the elastic waves in the earth's crust, according to utilize
the artificially motivated seismic waves, during the first world
war, both warring sides utilized seismic waves to locate each other's
emplacement by the recoil from a heavy cannon, these are the seeds
of seismic exploration. Because seismic exploration has precision
and resolution which other geophysical prospecting methods cannot
reach, so in the exploration of oil and other mineral resources,
therefore, seismic wave is one of the most important and effective
methods to explore oil and other mineral resources. All kinds of
mineral resources have certain characteristics in structure, for
example, petroleum and natural gas can only be formed and preserved
in a certain closed structure. Seismic wave will produce reflection
and refraction when it pass through these structures, according to
analyze the signals which received on the surface, we can infer about
the structure, depth and shape of the underground strata, therefore,
it can provide accurate positioning for future drilling work.
The utilization of seismic waves can also serve national defense.
Such as monitoring the underground nuclear explosions. Underground
nuclear explosions will produce seismic waves as well as earthquakes,
will leave a trail on the seismographic record in different regions.
However, the recording waveforms of underground nuclear explosions and
natural earthquakes are different, therefore, we can not only be
distinguished from natural earthquakes and underground nuclear explosions
by its waveform, but also can be given its time, location, energy, etc.