FUNDAMENTAL LINES AND DESIRED RELATIONS

 

Earthquake

• All natural earthquakes occur in the lithosphere.
• Seismic wave studies offer a full picture of the layered interior.
• An earthquake is, simply put, shaking of the earth’s crust.
• It is caused due to the energy release, which triggers waves that travel in all directions.
• The emanation of energy occurs along a fault.
• A fault is a sharp break in the crustal rocks.
• Rocks along a fault generally move in opposing directions.

Classification of Earthquakes

• Tectonic earthquakes: The most common form of earthquakes, is caused by the movement of loose fragmented pieces of land on earth’s crust knowns as tectonic plates.
• Volcanic earthquake: The less prevalent compared to the tectonic variety, these earthquakes happen before or after the eruption of a volcano. It is caused when magma leaving the volcano is filled by rocks being pushed to the surface.
• Collapse earthquake: This earthquake occurs in underground mines. The main cause is pressure generated within the rocks.
• Explosion earthquakes: The occurrence of this type of earthquake is artificial. High-density explosion such as nuclear explosions is the primary cause.

Causes of Earthquakes

• It is caused due to the tectonic movements of the earth.
• The energy release produces waves that travel in all directions.
• The point where energy is released is called the focus or hypocentre. It is generally located at a depth of 60 km.
• This causes a release of energy, and the energy waves travel in all directions.
• The point where the energy is released is called the focus of an earthquake or hypocentre.
• The point on the surface of the earth which is vertically above the focus is called the epicentre. It is the first place to experience the waves.

Measurement

All earthquakes are different in their intensity and magnitude. The instrument for the measurement of the vibrations is known as Seismograph.

Magnitude scale

• Richter scale is used to measure the magnitude of the earthquake
• The energy released during a quake is expressed in absolute numbers of 0-10.

 

Intensity scale

• The Mercalli scale is used to measure the intensity of an earthquake
• It measures the visible damage caused due to the quake.
• It is expressed in the range of 1-12.

Effects of Earthquake.

The following are the immediate hazardous effects of Earthquake:

• Shaking of ground
• The disparity in ground settlement
• Natural disasters like Tsunami, landslide, mudslides, and avalanches
• Soil liquefaction
• Ground lurching and displacement
• Floods and fires
• Infrastructure collapse.

 

Seismic belts

Seismicbelt, narrow geographic zone on the Earth’s surface along which most earthquake activity occurs. The outermost layer of the Earth (lithosphere) is made up of several large tectonic plates. The edges where these plates move against one another are the location of interplate earthquakes that produce the seismic belts. Island arcs, mountain chains, volcanism, deep ocean troughs, and oceanic ridges are often features of seismic belts. The two major seismic belts are the Circum-Pacific Belt, which surrounds the Pacific Ocean, and the Alpide Belt, which stretches from the Azores through the Mediterranean and Middle East to the Himalayas and Indonesia, where it joins the Circum-Pacific Belt. A purely oceanic seismic belt lies along the mid-Atlantic ridge.

Earthquakes can strike any location at any time, but history shows they occur in the same general patterns year after year, principally in three large zones of the earth:

The world's greatest earthquake belt, the circum-Pacific seismic belt, is found along the rim of the Pacific Ocean, where about 81 percent of our planet's largest earthquakes occur.

It has earned the nickname "Ring of Fire".

The belt exists along boundaries of tectonic plates, where plates of mostly oceanic crust are sinking (or subducting) beneath another plate. Earthquakes in these subduction zones are caused by slip between plates and rupture within plates.

The Alpide earthquake belt (mid Continental belt) extends from Java to Sumatra through the Himalayas, the Mediterranean, and out into the Atlantic.

This belt accounts for about 17 percent of the world's largest earthquakes, including some of the most destructive.

The third prominent belt follows the submerged mid-Atlantic Ridge. The ridge marks where two tectonic plates are spreading apart (a divergent plate boundary).

Most of the mid-Atlantic Ridge is deep underwater and far from human development.

Seismic Waves (Earthquake Waves)

§  Seismic waves are the waves of energy caused by earthquakes or an explosion. They are the energy that travels through the earth and is recorded on seismographs.

§  Earthquake waves are basically of two types — body waves and surface waves.

§  Body waves are generated due to the release of energy at the focus and move in all directions travelling through the body of the earth. Hence, the name body waves.

There are two types of body waves. They are called P and S-waves.

·       P-waves move faster and are the first to arrive at the surface. These are also called ‘primary waves’. The P-waves are similar to sound waves. They travel through gaseous, liquid and solid materials.

·       S-waves arrive at the surface with some time lag. These are called secondary waves. An important fact about S-waves is that they can travel only through solid materials.

The body waves interact with the surface rocks and generate a new set of waves called surface waves. These waves move along the surface.

The surface waves are the last to report on seismographs. These waves are more destructive. They cause displacement of rocks, and hence, the collapse.

Thus, the characteristics of the seismic waves are quite important. It has helped scientists to understand the structure of the interior of the earth.

 

Civil Engineering considerations in seismic areas

·        Objective of Earthquake-Resistant Design is to make such buildings that can resist effect of ground motion and would not collapse during the strong Earthquake.

·        All the components of the structure and foundations are designed to resist inertia forces in addition to the normal forces.

·        All the walls must be jointed properly, so that they can act together against the earthquake shaking at the time of seismic force transferring to the ground.

Base Isolation Method

·        The idea behind the Base Isolation is to detach (isolate) the building from the ground in such a way that earthquake motions are not transmitted up through the building, or at least greatly reduced.

·        If a building is rested on flexible pads that offer resistance against lateral movements, then some effect of the ground shaking will be transferred to the building above.

·        If the flexible pads are properly chosen, the forces induced by ground shaking can be a few times smaller than that experienced by the building built directly on ground.

Seismic Dampers

Seismic Dampers are fixed in place of structural elements as diagonal braces.

They absorb a part of seismic energy when it is transmitted through them.

Types of seismic Dampers

a. Viscous Dampers Energy is absorbed by silicone-based fluid passing between piston-cylinder arrangement.

b. Friction Dampers Energy is absorbed by surfaces with friction between them rubbing against each other

c. Yielding Dampers Energy is absorbed by metallic components.

 

Earthquake in India

§  India is one of the highly earthquake affected countries because of the presence of technically active young fold mountains - Himalaya.

§  India has been divided into four seismic zones (II, III, IV, and V) based on scientific inputs relating to seismicity, earthquakes occurred in the past and tectonic setup of the region.

 

 

 

 

 

LAND SLIDES

The term landslide or, less frequently, landslip refers to several forms of mass wasting that may include a wide range of ground movements, such as rockfalls, deep-seated slope failures, mudflows, and debris flows.

 Landslides occur in a variety of environments, characterized by either steep or gentle slope gradients, from mountain ranges to coastal cliffs or even underwater, in which case they are called submarine landslides. 

Gravity is the primary driving force for a landslide to occur, but there are other factors affecting slope stability that produce specific conditions that make a slope prone to failure.

In many cases, the landslide is triggered by a specific event (such as a heavy rainfall, an earthquake, a slope cut to build a road, and many others), although this is not always identifiable.

Classification of Landslides

Individual landslides are often formed by more than one type of movement.

Different types of features are grouped below according to where they occur and whether they mainly involve bedrock or regolith (rock and soil debris).

All landslides are potential hazards, but most occur in remote mountain areas away from people. Debris flows in particular are a regular hazard that disrupts transport routes in the Highlands during severe weather conditions.

Landslides in bedrock

Rock falls

Single and small rock falls from cliffs build up to form aprons of scree or talus, sometimes developing over long time periods. Some scree slopes are relict, where the scree apron almost buries the crags that once released rock falls that formed them.

A trait of actively forming rock fall screes is the sorting of rock debris, with the largest stones at the base of the scree and the smallest at the top. Reworking of the scree slopes by other processes

e.g. snow avalanching, debris flows and gully erosion by water – disrupts this sorting.

Rock slope failures

This group of landslides varies greatly in features. Many rock slope failures in Scotland are so large (involving whole mountainsides) that most people don’t even notice them.

For example, the rock slope failure on Beinn Fhada in Kintail is huge – 3km long, from near sea level to 1,000m, with a failed mass of perhaps more than 100 million cubic metres.

Rotational landslides

Rotational landslides occur where more resistant rocks founder over underlying weaker rocks. Multiple failures may produce spectacular whole mountainside collapse, as at Trotternish in Skye and at Hallaig in Raasay. At Hallaig, the landslide complex continues under water, down to the seabed.

Landslides in regolith

Debris flows

Debris flows are Scotland’s most common type of landslide hazard. They range in character from sloppy wet mudflows to slurries of rock debris similar in texture to wet concrete.

Debris flows begin on steep slopes of more than 20 degrees. But they can continue to travel over much gentler ground that slopes at only 10 degrees. How far a debris flow can travel depends on how much debris it carries compared to the volume of water.

A debris flow often starts off as a translational slide, but the water and rubble mix as the slide moves downslope, forming a slurry that flows. Debris flows often leave a trail of rubble in their wake, forming distinctive ridges or levees.

Debris flows range in size from 1m to 10m across, and may carry up to several cubic metres of debris. One-off debris flows may occur on open hillsides, but repeated debris flows are more common below gullies.

Debris cones form where repeated debris flows build up. Many debris cones in the Highlands have periods of activity and quiet. To trigger them, suitable weather conditions are needed and enough mud and rock must have collected higher up in the gully floor.

Creep

Creep is the slow downslope movement of material under gravity. It generally occurs over large areas.

Three types of creep occur:

• seasonal movement or creep within the soil – due to seasonal changes in soil moisture and temperature, e.g. frost heave processes
• continuous creep – where the soil and rock debris isn’t strong enough to resist gravity
• progressive creep – where the soil and rock debris suddenly reaches a critical state such that it will fail due to other factors

Solifluction

Solifluction is a mixture of creep and flow, which forms distinctive sheets, terraces and lobes of debris and boulders. Solifluction sheets and lobes are found on steeper slopes where the process has moved loosened boulders and soil downslope.

Scotland’s larger boulder lobes were last active during the Loch Lomond Readvance (12,900 to 11,500 years ago). Some smaller solifluction features are still active most winters.

Translational slides

Such slumps and slides occur in very wet weather, when the near-surface soil and rock debris gets saturated with water, and slides and flows downslope. They can develop into more dangerous debris flows on steep ground, where the landslide may travel a long distance, churning up the debris into a slurry.