12.4. DEFINITIONS
12.4.1. Earthquake
An EARTHQUAKE is a feeble to violent trembling of the ground produced by sudden displacement of rocks or rock materials below the earth’s surface. Sudden displacement along fault fissures in the solid and rigid layer of the earth generates TECTONIC EARTHQUAKES. Those induce by rising lava or magma beneath active volcanoes generates VOLCANIC EARTHQUAKES.
12.4.2. Intensity versus Magnitude
There are two general ways to describe the strength of an earthquake. One method is through INTENSITY which is based on how people, manmade structures, natural objects and land surfaces behave and react in the areas affected by an earthquake. By observing and documenting all these behaviors and reactions, one arrives at the felt INTENSITY of an earthquake within a given area.
The Philippine Institute of Volcanology and Seismology (PHILVOLCS) EARTHQUAKE INTENSITY SCALE (PEIS) was adopted in 1996 to replace the nine-point Adapted Rossi-Forel Scale. The PEIS is a ten-point scale with Intensity I described as scarcely perceptible and Intensity X as completely devastating (Refer to Table 1 below)
PHILVOLCS Earthquake Intensity Scale (Table 1)
Intensity Scale Description
I Scarcely Perceptible. Perceptible to people only under favorable circumstances. Delicately balanced objects are disturbed slightly. Still water in containers oscillates.
II Slightly Felt. Felt by few individuals at rest indoors. Hanging objects swing slightly. Still water in containers oscillates noticeably.
II Weak. Felt by many people indoors especially in upper floors of building. Vibration is felt like the passing of a light truck. Dizziness and nausea are experienced by some people. Hanging objects swings moderately. Still water in containers oscillates moderately.
IV Moderately Strong. Felt generally by people indoors and some people outdoors. Light sleepers are awakened. Vibration is felt like the passing of a heavy truck. Hanging objects swing considerably. Dinner plates, glasses, windows and doors rattle. Floors and walls of wood framed buildings creak. Standing motor cars may rock slightly. Water in containers oscillates strongly.
V Strong. Generally felt by most people indoors and outdoors. Many sleeping people are awakened. Some are frightened; some run outdoors. Strong shaking and rocking are felt throughout the building. Hanging objects swing violently. Dining utensils clatter and clink; some are broken. Small, light unstable objects may fall or overturn. Liquids spill from filled open containers. Standing vehicles rock noticeably. Shaking of leaves and twigs of trees are noticeable.
VI Very Strong. Many people are frightened, many run outdoors. Some people lose their balance. Motorists feel like driving with flat tires. Heavy object and furniture move or may shifted. Small church bell ring. Wall plaster may crack. Very old poorly-built houses and manmade structures are slightly damaged though well built structures are not affected. Limited rock falls and rolling boulders occur on hilly to mountainous areas and escarpments. Trees are noticeably shaken.
VII Destructive. Most people are frightened and run outdoors. People find it difficult to stand in upper floors. Heavy objects overturn or topple. Big churches bells may ring. Old poorly-built structures suffer considerable damage. Some well built structures are slightly damaged. Some crack may appear in dikes, fish ponds, road surface, or concrete hollow block walls. Limited liquefaction, lateral spreading and landslides are observed. Trees are shaken strongly. (Liquefaction is a process by which loose saturated sand loses strength during an earthquake and becomes a liquid).
VIII Very Destructive. People are panicky. People find it difficult even outdoors. Many well-built buildings are considerably damaged. Concrete dikes and foundation of bridges are destroyed by ground setting or toppling. Railway tracks are bent or broken. Tombstones may be displaced, twisted or broken. Liquefaction and lateral spreading cause man-made structures to sink, tilt or topple. Numerous landslides and rock falls occur in mountainous and hilly areas. Boulders are thrown out from their positions particularly near the epicenter. Fissures and fault ruptures may be observed. Trees are violently shaken. Water splashes or slops over dikes or banks of rivers.
IX Devastating. People are forcibly thrown to the ground. Many cry and shake with fear. Most buildings are totally damaged. Bridges and elevated structures are toppled or destroyed. Numerous utility posts, towers and monuments are tilted, toppled or broken. Water and sewer pipes are bent, twisted or broken. Landslides and liquefaction with lateral spreading and sand boils are wide spread. Boulders are commonly thrown out. River water splashes violently or slops over dikes and banks.
X Completely Devastating. Practically all man-made structures are destroyed. Massive landslides and liquefaction, large scale subsidence and uplifting of land forms and many ground fissures observed. Channels in river courses and destructive seiches in large lakes occur. Many trees are toppled, broken or uprooted.
The other method of describing the strength of an earthquake is based on instrumentally-derived information and correlated strength with the amount of total energy released at the earthquake’s point of origin. MAGNITUDE is calculated mathematically using the amount and duration of movements that ground vibration causes on the needle of a standard seismograph. (Refer to Table 2)
Richter Magnitude Scale (Table 2)
Magnitude Scale Description
1 Earthquakes with Magnitude (M) below 1 are only detectable when an ultrasensitive seismometer is operated under favorable condition.
2 Most earthquake with M below 3 are the “hardly perceptible shocks’ and are not felt. They are only recorded by seismographs of nearby stations.
3 Earthquakes with M3 to 4 are the “very feeble shocks” where damages are not usually reported.
4 Earthquakes with M4 to 5 are the “feeble shocks” where damages are not usually reported.
5 Earthquake with M5 to 6 are the “earthquakes of moderate strength” and are felt over the wide areas; some of them cause small local damages near the epicenters.
6 Earthquakes with M6 to 7 are the “strong earthquakes” and are accompanied by local damages near the epicenter. First class seismological station can observe them wherever they occur within the earth.
7 Earthquakes with M7 to 8 are the “major earthquakes” and can cause considerable damages near the epicenters. Shallow seated-or near-surface major earthquakes when they occur under the sea, may generate tsunamis. First class seismological stations can observe them wherever they occur within the earth.
8 Earthquakes with M8 to 9 are the “great earthquakes” occurring once or twice a year. When they occur on land areas, damages affect wide areas. When they occur under the sea, considerable tsunamis are produced. Many aftershocks occur in areas approximately 100 to 1,000 kilometers in diameter.
9 Earthquakes with M over 9 have never occurred since the data based on the seismographic observation became available.
12.4.3. What Happens During an Earthquake
a. Ground Shaking
Most of the damages incurred during earthquakes mainly results from strong ground vibrations that are caused by the passage of seismic waves from the earthquake source to the ground surface. Generally, the intensity of ground shaking in a given area is influenced by the magnitude of the earthquake, distance of the site from the earthquake generator, and the modifying effects of subsoil conditions. Usually, though not invariably, the shallower the earthquake source and the closer the area from the epicenter, the stronger is the felt intensity within a particular site. The resulting damage to buildings and similar structures however, are exacerbated by age and exposure, the type and quality of materials used the quality of design, and the mode of construction. Majority of casualties during earthquakes are caused by the collapse of buildings.
b. Ground Rupture
Surface rupturing or the breaking and movement of the ground along an active fault trace could result to horizontal/vertical shifting of the ground or a combination of both. Damage can be severe for structure directly straddling and located within a narrow zone of the active fault trace. For the 1990 Luzon earthquake, the deformation zone was within five (5) meters from the surface rupture. The location, pattern and style of surface faulting generally appear to occur along pre-existing active fault traces, hence, the precise delineation of these traces is very important in mitigating damages due to surface rupturing. Faults occurring directly within the city could cause additional damage through rupturing.
c. Tsunami
Tsunamis are giant sea waves generated by under-the-sea earthquakes and volcanic eruptions. Not all submarine earthquakes, however, can cause the occurrence of tsunamis. Tsunami can only occur when the earthquake is shallow-seated and strong enough (M7.0) to displace part of the sea bed and disturb the mass of water over it. Other sources of tsunamis include submarine or coastal landslides, pyroclastic flows and large volume debris avalanches from oceanic and partly submerged volcanoes, and caldera collapse.
d. Fire
One hazard which may be indirectly caused by earthquakes, especially in heavily built-up areas, is fire. Fire can be sparked from power or gas lines or other flammable facilities that are damaged during earthquakes. Earthquake-related fires are common occurrence in countries when gases for domestic and commercial use are piped. It should be borne in mind that the hotel has a centralized LPG system which is distributed to different kitchens through a network of pipes.
e. Other Complications
• Weather Condition
• Chemical Spillage
• Nuclear Spillage
f. What Will Happen To You During An Earthquake?
What happens to you during an earthquake depend primarily on two factors: how big is the earthquake is and where you are when it strikes. Where your are, in particular, involves how close you are at the epicenter of the earthquake, what type of soil or rock is under you and what happens to it when it moves, how close your are to the coastal and inland shores, the level of the ground water under you, the type of building you are in or near, its design and quality of construction, and the furnishings and other objects that surround you.
g. Earthquake-related injuries are commonly caused by:
• Partial building collapses, such as toppling chimneys, falling bricks from wall facings, collapsing walls, falling ceiling plaster, light fixtures, and pictures.
• Flying or falling glass from broken windows, with greater risk around modern high-rise buildings.
• Overturned bookcases, fixtures, and other furniture and appliances.
• Fires from broken chimneys, broken gas and electric lines, or spilled flammable liquids. The danger may be aggravated by lack of water to fight fires due to broken water mains.
• Fallen power lines.
• Human panic reactions, such as crowds stampeding for stairways or exit
Originally Posted by higanstolsdawen
Posting Permissions
