Government Engineering College, Bhavnagar. Sub:- Geology & Geotechnics

Earthquake

Earthquakes Introduction What is earthquake? Why do earthquake occures? Where do earthquakes occur? Types of earthquakes Types of earthquake waves How is an Earthquake’s Epicenter Located? Measuring of earthquake Earthquake size: two ways to measure Frequency of Occurrence of Earthquakes Effects of earthquake

Earthquake (also known as a quake, tremor or temblor) is the result of a sudden release of energy in the earth's crust that creates seismic waves.

What is an earthquake? Earthquakes are the Earth's natural means of releasing stress. The stress is released as energy which moves through the Earth in the form of waves, which we feel and call an earthquake. Earthquakes are a sudden release of strain progressively stored in rocks that bend until they finally break and move along a fault.

. The point within Earth where faulting begins is the focus, or hypocenter The point directly above the focus on the surface is the epicenter

Why do earthquakes occur? Fractures, faults Energy released and propagates in all directions as seismic waves causing earthquakes focus epicenter

Earthquakes are most common at plate boundaries, where different tectonic plates meet. 80 % of all recorded earthquakes occur at the circum-Pacific seismic belt (also known as The Ring of Fire). 15% occur in the Mediterranean-Asiatic belt. Remaining 5% occur in the interiors of plates and on spreading ridge centers. Where do earthquakes occur?

Types of earthquakes There are three different types of earthquakes: 1.Tectonic earthquakes : These occur when rocks in the Earth's crust break due to geological forces created by movement of tectonic plates.

2. Volcanic earthquakes : These occur in conjunction with volcanic activity. Collapse earthquakes are small earthquakes in underground caverns and mines

3. Explosion earthquakes : These result from the explosion of nuclear and chemical devices

Types of earthquake waves

primary wave, is the fastest of the three waves and the first detected by seismographs. They are able to move through both liquid and solid rocks. P waves, like sound waves, are compressional waves, which mean that they compress and expand matter as they move through it. P wave

secondary waves, are the waves directly following the P waves. As they move, S waves shear, or cut the rock they travel through. S waves cannot travel through liquid because, while liquid can be compressed, it can't shear. S waves are the more dangerous type of waves because they are larger than P waves and produce vertical and horizontal motion in the ground surface. S waves

Travel just below or along the ground’s surface Slower than body waves; rolling and side-to-side movement Especially damaging to buildings Surface wave

There are two types of surface waves : 1. Love waves-that move like S waves but only horizontally 2. Rayleigh waves-that move both horizontally and vertically in a vertical plane pointed in the direction of travel Love wavesRayleigh waves

How is an Earthquake’s Epicenter Located? Three seismograph stations are needed to locate the epicenter of an earthquake A circle where the radius equals the distance to the epicenter is drawn The intersection of the circles locates the epicenter TRIANGULATION!

Seismometers: Instruments that detect seismic waves. Seismographs: Record intensity, height and amplitude of seismic Waves. Measuring of Earthquakes

Earthquake size: two ways to measure 1.Magnitude: Richter Scale Measures the energy released by fault movement related to the maximum amplitude of the S wave measured from the seismogram Logarithmic-scale; quantitative measure For each whole number there is a 31.5 times increase in energy eg. an increase from 5 to 7 on the Richter scale = an increase in energy of 992 times!!

2) Intensity: The Mercalli Scale of earthquake damage measures the intensity of an earthquake at a particular place. It uses the type and amount of damage. Unlike the Richter Scale, it does not measure the absolute strength of the earthquake, but how strongly it is felt at a particular place.

Frequency of Occurrence of Earthquakes DescriptorMagnitudeAverage Annually Great8 and higher1 ¹ Major ² Strong ² Moderate ² Light ,000 (estimated) Minor ,000 (estimated) Very Minor ,300,000 (estimated) ¹ Based on observations since ² Based on observations since 1990.

Effects of earthquake Building collapseLand slide Tsunami

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