# Earthquake Geophysics Essay

explanatory Essay
3184 words
3184 words

EARTHQUAKE
Introduction:-
1 Definition:
An earthquake is what happens when two blocks of the earth suddenly slip past one another. The surface where they slip is called the fault or fault plane. The location below the earth’s surface where the earthquake starts is called the hypocenter, and the location directly above it on the surface of the earth is called the epicenter.

Figure 1; Showing the fault plane, point of origination and epicenter
(http://earthquake.usgs.gov/learn/kids/eqscience.php)
2 Earthquake Geophysics:

The earth has the following structure, a 30 km thick crust, a 2900 km thick mantle, and a radius of 3470 km core, which forms the total radius 6370 km. The crust mantle and core are further divided in to subtypes like core is divided in to outer core and inner core. There is elasticity present in the earth’s crust and it produces strain when it under goes some stresses. Elastic rebound theory of Reid is based on this property.
Reid suggests that ‘the crust, in many parts of the earth, is being slowly displaced, and the difference between displacements in neighboring regions sets up elastic strains, which may become greater than the rock can endure. A rupture then takes place, and the strained rock rebounds under its own elastic stresses, until the strain is largely or wholly relieved’.
The point where the earthquake is originated is called focus or hypocenter and on the earth surface directly above the focus, the point is called epicenter. Earthquakes are divided in to the following types according to their depths
1) Shallow focus (focus depth < 70 km)
2) Intermediate focus (depth 70-300 km)
3) Deep focus (depth > 300km)
Wave’s passes through earth and produce motion to the ground during an earthquake. There a...

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...akes.

7.4 Results:

The moment magnitude of KME was the smallest from the all the four earthquakes but the range of groundwater level change is highest from all the four earthquakes from -0.2m to 0.67m among the 54 wells. The short distance of study area to the hypocenter can be the reason for the big range of change. FOE had hypocenter which was located at shortest distance to the study area at the second number and this earthquake had also second strongest effect. Hypocenter for GYE was far from study area and thus it also had less effect on the water level, same case for the SCE earthquake its hypocenter was located 2500 Km from the study area.
The range of ground water level changes due to FOE earthquake was 0.01m – 0.15m which was at second number. The range of water level change for GYE earthquake was 0.01m – 0.05m while for SCE earthquake was 0.01m – 0.04m.

#### In this essay, the author

• Explains that an earthquake occurs when two blocks of the earth suddenly slip past one another. the surface where they slip is called the fault or fault plane.
• Explains earthquake geophysics, which consists of a 30 km thick crust, 2900 km mantle, and 3470 km core. elastic rebound theory of reid is based on this property.
• Explains that reid suggests that the crust, in many parts of the earth, is being slowly displaced and the difference between displacements in neighboring regions sets up elastic strains, which may become greater than the rock can endure.
• Explains that the point where the earthquake originated is called focus or hypocenter and on the earth surface directly above the focus, the epicenter. earthquakes are divided into the following types according to their depths
• Explains that wave's pass through earth and produce motion to the ground during an earthquake.
• Describes the effects of earthquakes, such as shaking of ground, displacement, fire, tsunami, flooding, soil liquefaction, and slope failure.
• Explains the magnitude of the tohoku earthquake, which is also called great east japan earthquake.
• Describes some major earthquakes worldwide with their damages to the life.
• Explains that soils and ground have significant role in earthquake assessment or seismic hazard assessment and helps to form earthquake resistant design.
• Explains that fluid induced seismicity is observed in oil-well stimulation, hydro thermal fields, filling of dams, and mine flooding.
• Explains that fluid induced seismicity is happening in oil field wells when there is less production from the oil well then the high pressured water is introduced underground in to stimulation well.
• Describes a well drilled by the us army in rocky mountain arsenal in the denver basin in 1961, where fluids were injected at the rate of 7.5-21 million per month.
• Explains that there was no seismicity in the area before the wellbore injection project started and after that a lot of seismic events happened.
• Explains that the malpasset dam collapsed due to rock foundation failure. water flowed in the fractures of bed rock at the bottom or toe of the dam, forcing it to lean downward.
• Analyzes the correlation between seismicity and increasing water level in the flooding area of a south african mine.
• Explains that the delay in seismic activity in mining was similar to that in dam impoundment. the delay fluid induced seismicity was observed in the rocky mountain arsenal case.
• Explains that earthquake prediction by environmental isotopes and tracing techniques has been noticed since 1980s. the mechanism of deep migration and to determine fault activity is used.
• Explains tracing techniques application of stable isotopes o18 and h2 can be measured with the help of mass spectrometer. investigation of circulation depth of geothermal water, water rock interactions and ground water recharge could evolve seismicity.
• Explains that groundwater shows abnormal changes due to observation environment changes or tectonic activity in the distant area. by tracing techniques of h2 and o18 the changes in water can be determined.
• Explains that earthquake gestation and occurrence is closely associated with the geothermal fluid's circulation depth.
• Explains that the greater the circulation depth of water more negative is the values.
• Explains that fault intensity becomes smaller and fault tends to move if the depth of circulation of geothermal water is large.
• Explains that wang studied the area south of zhangjiakou city for the seismicity and hydrothermal activity.
• Explains that water-rock interaction and fluid rock interaction change isotopic composition with rocks through which they are flowing. anomaly in ground water can be determined by long-term measurements of deep wells and hot springs.
• Explains that earthquakes and groundwater level fluctuations correlation can give us sign that how crust responses in response to tectonic stress and strain.
• Explains the effects of earthquakes on groundwater level changes in unconsolidated sedimentary basins. japan is one of the seismically active regions in the world.
• Describes the geological setting of kumamoto city, which is composed of unconsolidated sediments of upper quaternary.
• Explains that four earthquakes affected the groundwater level in kumamoto city during the study period of 10 years.
• Explains wang and manga (2010) and wang (2007) proposed a parameter (e) for the responses of seismic energy.
• Explains that the moment magnitude of kme was the smallest from all the four earthquakes, but the range of groundwater level change is highest among the 54 wells.
• Explains the range of ground water level changes due to foe earthquake was 0.01m – 0.15m which was at second number.