Earth Quake Ground Motion

EARTH QUAKE GROUND MOTION – BASIC CONCEPTS AND ESTIMATION TECHNIQUES

An earthquake is a sudden trembling or shaking of the ground caused by movement of the earth surface (crust).

The earthquake is caused by a portion of the rigid crust of the earth giving way or getting fractured, some distance below its surface.

Consequently, sudden slipping of the resulting portion or 'fault slipping' occurs.

It is just a landslide on a large scale. The earthquake occurs due to a re-adjustment of the earth's crust in response to a change of pressure deep distance at the depth of about 100 miles in the earth's crust.

The earthquake results due to variety of causes like erosion, deposition, tidal forces, centrifugal forces, etc. Thus it represents the energy released by the relative motion of portions of the earth crust.

Focus

The place where the actual fracture occurs is called the focus of the earthquake. It is not a geometrical point but an extended region.

Epicentre

The point nearest to the focus on the surface of the earth, is called the epicentre (Fig 5.1).

Causes of earthquake

The following are some important causes of earthquake:

• Sudden movement of hot gases and magma

• Volcanic activities

• Stress caused by water pressure in dams

• Tectonic plates movement

Types of earthquakes

There are '2' types of earthquakes

(i) Interplate earthquakes

(ii) Intraplate earthquakes

(i) Interplate earthquakes

Most of the earthquake occur at narrow belts along tectonic plate boundaries. These earthquakes are known as interplate earthquakes.

(ii) Intraplate earthquakes

The earthquakes that occur within a plate are known as intraplate earthquakes.

Classification of earthquake

The earthquakes can also be classified into three categories based on its depth of focus. They are:

(i) Shallow focus earthquakes

(ii) Intermediate focus earthquakes

(iii) Deep focus earthquakes

(i) Shallow focus earthquakes:

These earthquake have depth of focus less than 70 km. Nearly 80% of total earthquakes are shallow focus earthquakes. These types of earthquakes are of greater concern for earthquake resistant design.

(ii) Intermediate focus earthquakes:

These are earth- quakes with depth ranging from 70 km to 300 km.

(iii) Deep focus earthquakes:

These are earthquakes having focal depth greater than 300 km.

Intensity

It is a qualitative measure of the strength of an earthquake. It gives a strength of the earthquake using observed damage to structures or ground and magnitude of the earthquake shaking.

The intensity of an earthquake varies greatly according to distance from the earthquake, ground conditions and other factors

The earthquake intensity is highest near the maximum fault displacement. It progressively decreases to lower value at further away.

The popular intensity scale is the Modified Mercalli Intensity (MMI) scale with twelve gradation denoted by Roman numerals from I to XII.

Magnitude of earthquake

It is a quantitative measure of the size of an earthquake. It can be correlated to the amount of wave energy released at the source of an earthquake.

There are various magnitude scales in use. These scales differ from each other because these are derived from measuring different wave components of an earthquake.

Richter scale

The magnitude of earthquake is usually measured in Richter scale.

It was developed by Charles F. Richter. In this scale, the magnitude of the earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs.

That is an increase of 1 magnitude unit represents a factor of ten times in amplitude.

Just The seismic waves of a magnitude 6 earthquake are 10 times greater in amplitude than those of a magnitude 5 earthquake. However, in terms of energy release, a magnitude 6 earthquake is about 31 times greater than a magnitude 5.

The earthquakes are classified from minor to great, depending on their magnitude as shown in Table 5.1.

Table 5.1

Classification of earthquakes

Effects of earthquake

The earthquakes really pose danger to a person. Some of the main categories of earthquake effects are mentioned below.

1. Ground shaking

Ground shaking is a term used to describe the vibration of the ground during an earthquake.

Ground shaking is caused by body and surface seismic waves. Buildings can be damaged by the ground shaking.

2. Liquefaction of ground

When water and soil are mixed, the ground becomes very soft and acts similar to quick sand. If liquefaction occurs under a building, it may start to lean, tip over, or sink several feet.

3. Ground displacement

If a structure (building, road, etc.) is built across a fault, the ground displacement during an earthquake could seriously damage that structure.

4. Land slides

Land slide causes sliding of rocks and soil on the sides of an hilly area.

5. Flood

An earthquake can cause damage to dams or levees along a river. The water from the river or the reservoir would then flood the area, damaging buildings and may be sweeping away.

6. Fire

Fire can be started by broken gas lines and power lines, or tipped over wood or coal stoves.

7. Tsunami

Tsunami are water waves that are caused by sudden vertical movement of a large area of the sea during an undersea earthquake. The height of a tsunami in the deep ocean is typically about 1 feet.

As tsunami reach shallow water or a continental shelf, the height of the waves increases many times (sometimes reaching as much as 30 m). These are more destructive near the shore.