EARTHQUAKE INTENSITY & THEIR SCALES

 

EARTHQUAKE INTENSITY & THEIR SCALES

 

A measure of the effects of an earthquake at a particular place on humans and structure gives information about the intensity of earthquakes. Intensity of earthquake at a point depends not only upon the strength of the earthquake but also upon the distance from the earthquake to the epicentre and the local geology at that point. There are three main reasons for the intensified destructive Seismology

earthquake: location (an earthquake with a focus under an ocean at a large distance from land, forexample, is not usually destructive, unless it generates a tsunami), depth (shallower earthquakes can be more devastating), but, most importantly, the size of the earthquake. A straightforward way to measure the size of an earthquake is to look at the damage it caused.

This is a measure of the strength of the ground shaking caused by an earthquake, and is called theearthquake intensity.

It is important to realise that intensity refers to the damage caused at a particular place by an earthquake. An earthquake will have different intensities in different places. It is therefore wrong to refer to the intensity of an earthquake; it is necessary to state the intensity in, say, Los Angeles, or "the maximum intensity'. The intensity at the epicentre of the 1989 Loma Prieta earthquake was intensity 8, but the maximum intensity (and damage) was in parts of San Francisco and Oakland, about 100 km to the northwest of the epicentre, where intensity 9 was recorded. This was mainly due to the presence of a loose sand layer at the ground surface in the San Francisco Bay area which did not support structures as well as the solid rock nearer to the epicentre.

 

(1) Intensity Scale

 

The intensity scale is a qualitative scale because it is not based on specific measurements,even though it has numbers.

 

I Not Noticeable

 

The intensity of the observation is below the limit of sensibility; the tremor is detected and recorded by seismographs only.

 

II Scarcely Noticeable (Very Slight)

 

Vibration is felt only by individual people at rest in houses, especially on upper floors ofbuildings.

 

III Weak, Partially Observed Only

 

The earthquake is felt indoors by a few people, outdoors in favorably circumstances. The vibrations is felt like that due to side passing of a light truck. Attentive observers notice a slight swinging of hanging objects, somewhat more heavily on higher stories.

 

IV Largely Observed

 

The earthquake is felt indoors by many people out door by few. Here and there people awake, but no one is frightened. The vibration is like that due to the passing of a heavily loaded truck. Windows, doors and dishes rattle. Floors and walls crack. Furniture begins to shake. Hanging objects swing slightly. Liquids in open vessels are slightly shaking. In standing motorcars the shock is noticeable.

 

V Awakening

 

(i) The earthquake is felt indoors by all, outdoors by many. Many sleeping people awake. A few run outdoors. Animals become uneasy. Buildings tremble throughout. Hanging objects swing considerably. Pictures knock against walls or swing out of place. Occasionallypendulum clocks stop. A few unstable objects may overturn or shift. Open doors and windows are thrust open and slam back again. Liquids spill in small amounts from well filled open containers. The sensation of vibration is like that of a heavy object falling inside the building

 

(ii) Slight damage of Grade-I in buildings of Type-A is possible.

 

(iii) Sometimes changes in flow of springs.

 

VI Frightening

 

(i) Felt by most people indoors and outdoors. Many people frightened and run outdoors. A few persons loose their balance. Domestic animals run out of the their stalls. In a few instances dishes and glassware may break, books fall down. Heavy fumiture may moveand small steeple bells may ring.

 

(ii) Damage of Grade I is sustained in single buildings of Type B and many of Type A. Damagein a few buildings of Type A is of Grade 2.

 

(iii)  In a few cases cracks up to width of 1 cm possible in wet ground; in mountains occasional landslides, change in flow of springs and in level of well water is observed.

 

VII Damage to Building

 

(i) Most people are frightened and run outdoors. Many find it difficult to stand. Personsdriving motor cars notice the vibration. Large bells ring.

 

(ii) In many buildings of Type C damage of Grade I is caused; in many buildings of Type Bdamage is of Grade 2. Many buildings of Type A suffer damage of Grade 3, few of Grade4. In single instances, landslides of roadway on steep slopes; Cracks in roads, seams ofpipelines damaged; cracks in stone walls.

 

(iii) Waves are formed on water, and water is made turbid by mud stirred up. Water level in wells may change, and the flow of springs changes. In a few cases dry springs have their flow restored and existing springs stop flowing. In isolated instances parts of sandy or gravel banks sup off.

 

VIII Destruction of Building

 

(i) Fright and panic, also persons driving motor cars are disturbed. Here and there branchesof trees break off. Even heavy furniture moves and partly overturns. Hanging lamps are damaged in part.

 

(ii) Many buildings of Type C suffer damage of Grade 2, a few of Grade 3. Many buildings ofType B suffer damage of Grade 3. Many buildings of Type A suffer damage of Grade 4and some of Grade 5. Occasional breakage of pipe seams. Memorials and monumentsmove and twist. Tombstones overturn. Stone walls collapse.

 

(iii) Small landslips in hollows and on banked roads on steep slopes: cracks in ground up to width of several centimetres. Water in lakes becomes turbid. New reservoirs come into existence. Dry wells refill and existing wells become dry. In many cases changes in flow and level of water.

 

IX General Damage of Buildings

 

(i) General panic: considerable damage to furniture. Animals run to and fro in confusion and cry

 

(ii) Many building of Type C suffer damage of Grade 3, a few of Grade 4. Many buildings of Type B show damage of Grade 4, a few of Grade 5. Many buildings of Type A suffer damage of Grade 5. Monuments and columns fall. Considerable damage to water reservoirs, Underground pipe lines partly damages. In individual cases railway lines are bent and roadway damaged.

 

(iii) On flat land overflow of water sand and mud is often observed. Ground cracks up to widths of 10 cm, on slopes and river banks more than 10 cm. Furthermore, a large number of slight cracks in ground: fall of rocks, many landslides and earth flows; large waves in water. Dry wells renew their flow and existing wells dry up.

 

X General Destruction of Buildings

 

(i) Many buildings of Type C suffer damage of Grade 4, a few of Grade 5. Many buildings ofType B show damage of Grade 5. Most of Type A have destruction of Grade 5. Criticaldamage to dams and dykes and severe damage to bridges. Railway lines are bent slightly.Underground pipes are broken or bent. Road paving and asphalt shows waves.

 

(ii) In ground, cracks up to widths of several tens of centimetres, sometimes up to 1 meter. Broad fissures occur parallel to watercourses. Loose ground slides from steep slopes. Considerable landslides are possible from river-banks and steep slopes. In coastal areas displacement of sand and mud, change of water levels in wells: water from canals, lakes, rivers etc. thrown on land, new lakes are formed.

 

XI Destruction

 

(i) Severe damage even to the well built buildings, bridges, water dams and railway lines. highways become useless: underground pipes destroyed.

 

(ii) Ground considerably distorted by broad cracks and fissures as well as by movement in horizontal and vertical directions. Numerous landslips and rock falls. The intensity of earthquake requires to be investigated specially.

 

XII Landscape Change

 

(i) Practically all structures above and below ground are greatly damaged or destroyed.

 

(ii) The surface of the ground is radically changed. Considerable ground cracks with extensive vertical and horizontal movements are observed. Falling of rocks and slumping of river banks over wide areas; lakes are dammed; waterfalls appear and rivers are deflected. The intensity of earthquake requires to be investigated specially.

 

Intensity scales have two major disadvantages; they are no use for earthquakes under the oceans and no use for uninhabited areas. Even in inhabited areas, building standards vary in different areas, so earthquake effects are different. To overcome these problems in measuring the size of earthquakes, a different way of measuring earthquake size, using instruments, is often used instead of intensity.