What is an Earthquake?

What is an Earthquake ?

Earthquake is the sudden shaking or vibration of part of earth's surface as a result of seismic energy released along the fault or by other natural processes.

Earthquake Seismology: Is the study of vibration within the Earth caused by sudden movement along faults or other natural processes.

Earthquake is natural geology hazard, and As it has been reported on 6 Feb, 2023 that about 7.8 magnitude earthquake struck south Turkey and Syria, while left serious destruction of properties including loss of life. I have decided to produce this post to let you know about this phenomena. Let's be together till the end of this post.

Geologically for an Earthquake to occur two criterions should be met,

(i) Sort of movement, to stress the materials (rocks) beyond their elastic limit.

(ii) Material failure with brittle fracture.

ORIGIN OF EARTHQUAKE.

It explained by "Elastic Rebound Theory" as follows,

Consider, three (3) stages for an Earthquake development

a/ Unstrained state of the fault segment

b/ Strain growth close to the fault due to relative motion of adjacent crustal blocks.

c/ Strain release "Rebound" of strained segment as earthquake.

Terms used in Earthquakes

Focus (Hypocenter): Is the point where seismic waves originated.

Epicentre: Is the point on the Earth's surface vertically above the focus.

Epicentral Distance: Is the distance from the epicentre to the seismic recording station.

Shock occurs when strain energy is partially released locally. Main quakes (Shocks), occurs when most of the stored strain energy is released.

After Shock, are numerous shocks after the main shock. It can proceed after a week or month

GLOBAL SEISMICITY

Refers to the world wide distribution of earthquakes.

Seismicity map is plotted as the evidence in support plate tectonics theory and delineates the presently active plate margins.

Categories of earthquakes based on plate margins and Focal depth.

Based on Plate margins

(i) Interplate Earthquakes

It occurs at narrow wadati benioff zones with interplate seismic activities about 99% of global seismicity.

Globally it comprises three (3) major zones,

a/ Circum-pacific zones: It releases about 75-80% of seismic energy annually.

b/ Mediterranean transasiatic zone: It release about 15-20% of seismic energy annually.

c/ Oceanic ridges and rises zones: It releases about 3 -7% of seismic energy annually.

Each of these zones are characterized by Active Volcanism

(ii) Intraplate Earthquake

It occurs in places other than active plate margins. It comprises about 1% of global seismicity.

Based on Focal depth.

(i) Shallow Focus Earthquakes

It occurs at a focal depth of less than 70 km. Examples; Oceanic ridges and all active seismically zones. It contributes about 85% of seismic energy annually.

(ii) Intermediate Focus Earthquakes

It occurs at a focal depth between 70-300 km. It contributes about 12% of seismic energy annually.

(iii) Deep Focus Earthquakes

It occurs at a focal depth greater than 300km. It contributes about 3% of seismic energy annually.

Examples, in circum pacific ,mediterranean transasiatic zones and accompanied at subduction of plates

EARTHQUAKES SIZE

There are two (2) major methods of describing how large an earthquake is,

Figure: A simple seismogram (Lowrie, 2007)

i/ Intensity

It describes the extent of local damage it causes at the location of the observer.

It is subjective parameter that is based on an assessment of visible effects

It depends on three (3) factors rather than the actual size of the earthquakes

Magnitude: High amplitudes of ground motion results in high damage and vice versa is true.

Proximity to focus : Seismic Waves near focus points always have higher amplitudes than far away points, hence high damage is suspected near these areas.

Loose soil as opposed to firm bedrock: Firm bedrocks shaken less than loose soil hence, high damage is expected at built structures over loose soils than over firm bedrocks.

It is expressed in Mercalli Scale (I-XII).

ii/ Magnitude

It describes the amplitudes of ground motion inferred from signals on seismographs.

It is a more objective measure of its size that is determined instrumentally.

It is expressed in Richter Scale (1 - 9/10). It is a logarithmic scale such as the size MI 4 on the Richter scale is 10 times greater than the size of MI 3.

EFFECTS OF EARTHQUAKES

Earthquakes can cause either primary or secondary effects.

(i) Primary Effects

Ground Motion: Ground shaking intensity depends on Local geologic conditions, for example unconsolidated sediments will have more shaking than solid firm bedrock, Size of an Earthquakes, and Distance from the epicenter.

Faulting and Ground rupture: Ground rupture occurs only along the fault zone.

(ii) Secondary Effects

Fire Outbreak: This is because ground shaking may trigger rupture and falling of power lines and natural gas lines that may cause an explosion of fire if they come in contact with flammable materials or naked fire flames.

Landslides: In mountainous regions, where slopes is unstable ground shaking may trigger landslides,

rocks and debris falls, slumps and avalanche.

Changes in Ground Level: As a result of strain build up along the Fault that may cause some part of ground level to become elevated /rise.

Tsunami: Giant ocean waves that can rapidly travel across oceans. The Giant Waves moves Up and down due to rapid Vertical up and down motion of propagated seismic waves. These Waves may travel far across coastal areas and reach human settlements and result in deadly destruction, for both human lives, animals and their property.

Flooding: Due to rupture of human dams and reservoirs that may contribute much water into river channels to above their flooding level.

Liquefaction: Occurs when strong ground shaking causes waterlogged packed soils to lose their stiffness (strength). This may cause damage to built structures due to induced weak ground strength.

Prediction of Location of Earthquakes

Prime areas for prediction are seismically active zones plate margins, Although this may combine two (2) events, Knowledge of historical seismicity pattern and the elastic Rebound model of what causes an Earthquake.

What is the Seismic Gap?

Seismic Gap, refers to local Gap (zone/section) for location of Earthquake that is in the process of accumulating the strain energy necessary to cause rupture.