Topic 2 CETS 1

Engineering Seismology

Epicenter

the geographical point on the ground surface where an earthquake is estimated to be centered

Focus or Hypocenter

directly below the epicenter along with the focal depth gives the location where the rock ruptures at a fault (fault rupture) that generates the main earthquake

Fault Plane

the plane along which the rock ruptures and slips

Dip Angle

angle with respect to the ground surface

Strike Angle

the angle the fault plane makes with respect to the north direction along the surface

Fault Slip

the relative displacement between the two sides of the fault plane

Epicentral Distance

The radiating seismic waves are recorded using a seismometer at an observation station located at a distance known as the

Shallow Earthquakes

more destructive than others because the mass of the rock above deeper earthquakes attenuates their shock waves. focal depths less than 70 km (43 miles)

Intermediate Earthquakes

focal depths between 70 km (43 miles) and 300 km (186 miles)

Deep Earthquakes

focal depths greater than 300 km (186 miles).

foreshocks

are precursors of the impending fault rupture. Most moderate-to-large shallow earthquakes are preceded by smaller quakes, called.

aftershocks

result from adjustments to the stress imbalance in the rocks produced during the rupture. foreshocks followed by smaller quakes called as

Fault slip

used to classify faults depending on the direction of the movement of rocks on one side relative to the other side

dip slip

vertical movement of rocks

strike slip

horizontal movement of rocks

Normal Faults & Reversed Faults

Dip-slip faults are further subdivided based on the relative vertical movement of the tectonic plates

Normal Fault

occur at plate boundaries where tectonic plates spread apart at divergent zones causing the hanging wall to move down relative to the footwall.

Reversed Faults

occur at plate boundaries where tectonic plates collide at convergent zones causing the hanging wall to move up relative to the footwall

Thrust faults

are reverse faults with a small dip angle

fault’s scarp

When these faults break through to the surface, surface rupture, they produce an exposed steep slope known as the

right lateral or left-lateral faults

At transform zones, tectonic plates move horizontal relative to each other; the faults associated with this motion are strike-slip, which can be further subdivided into

oblique slip

It should be noted that dip-slip and strike-slip faults can occur simultaneously, in which case the fault is classified as an

Body Waves

The portion of the energy released from a fault rupture as shaking first travels through the interior of the earth as

surface waves.

When body waves reach the ground surface, they are classified as

P-Waves & S-Waves

Body waves naturally divide into two different types,

P-Waves

are longitudinal waves that move through earth’s interior in successive compression and rarefaction

Rayleigh and Love

Surface waves are also divided into two types

Rayleigh waves

combine effects of both P- and S-waves on the earth’s surface and move disturbed material vertically and horizontally

Love waves

move the ground side to side on the surface plane, perpendicular to the direction of the wave propagation, similar in character and speed to S-waves.

Intensity

is a non- instrumental perceptibility measure of damage to structures, ground surface effects, e.g. fractures, cracks and landslides, and human reactions to earthquake shaking

Mercalli– Cancani– Seiberg (MCS)

12- level scale used in southern Europe

Modified Mercalli (MM)

12- level scale proposed in 1931 by Wood and Neumann, who adapted the MCS scale to the California data set. It is used in North America and several other countries

Medvedev– Sponheuer– Karnik (MSK)

12- level scale developed in Central and Eastern Europe and used in several other countries

European Macroseismic Scale (EMS):

12- level scale adopted since 1998 in Europe. It is a development of the MMscale

Japanese Meteorological Agency (JMA)

7- level scale used in Japan. It has been revised over the years and has recently been correlated to maximum horizontal acceleration of the ground

Magnitude

is a quantitative measure of earthquake size and fault dimensions

Local (or Richter) magnitude

measures the maximum seismic wave amplitude A (in microns) recorded on standard Wood– Anderson seismographs located at a distance of 100 km from the earthquake epicentre.

Bodywave magnitude

This scale is suitable for deep earthquakes that have few surface waves. Moreover, can measure distant events, e.g. epicentral distances not less than 600 km.

Surface wave magnitude

is used for large earthquakes. However, it cannot be used to characterize deep or relatively small, regional earthquakes

Moment magnitude

accounts for the mechanism of shear that takes place at earthquake sources. It is not related to any wavelength

INTENSITY I

Scarcely Perceptible- Perciptible to people under favorable circumstance. Delicately balanced objects are disturbed slightly. Still Waater in containers oscillates slowly

INTENSITY II

Slightly Felt - Felt by few individuals at rest indoors. Hanging objects swing slightly. Still Water in containers oscillates noticeably.

INTENSITY III

Weak - Felt by many people indoors especially in upper floors of buildings. Vibration is felt like one passing of a light truck. Dizziness and nausea are experienced by some people. Hanging objects swing moderately. Still water in containers oscillates moderately.

INTENSITY IV

Moderately Strong - Felt generally by people indoors and by some people outdoors. Light sleepers are awakened. Vibration is felt like a passing of heavy truck. Hanging objectsswing considerably. Dinner, plates, glasses, windows and doors rattle. Floors and walls of wood framed buildings creak. Standing motor cars may rock slightly. Liquids in containers are slightly disturbed. Water in containers oscillate strongly. Rumbling sound may sometimes be heard.

INTENSITY 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 felt throughout building. Hanging objects swing violently. Dining utensils clatter and clink; some are broken. Small, light and 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.

INTENSITY VI

Very Strong - Many people are frightened; many run outdoors. Some people lose their balance. motorists feel like driving in flat tires. Heavy objects or furniture move or may be shifted. Small church bells may ring. Wall plaster may crack. Very old or poorly built houses and man-made structures are slightly damaged though well-built structures are not affected. Limited rockfalls and rolling boulders occur in hilly to mountainous areas and escarpments. Trees are noticeably shaken.

INTENSITY VII

Destructive - Most people are frightened and run outdoors. People find it difficult to stand in upper floors. Heavy objects and furniture overturn or topple. Big church bells may ring. Old or poorly-built structures suffer considerably damage. Some well-built structures are slightly damaged. Some cracks may appear on 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 lose strength during an earthquake and behave like liquid).

INTENSITY VIII

Very Destructive - People panicky. People find it difficult to stand even outdoors. Many well-built buildings are considerably damaged. Concrete dikes and foundation of bridges are destroyed by ground settling or toppling. Railway tracks are bent or broken. Tombstones may be displaced, twisted or overturned. Utility posts, towers and monuments mat tilt or topple. Water and sewer pipes may be bent, twisted or broken. Liquefaction and lateral spreading cause man- made structure to sink, tilt or topple. Numerous landslides and rockfalls occur in mountainous and hilly areas. Boulders are thrown out from their positions particularly near the epicenter. Fissures and faults rapture may be observed. Trees are violently shaken. Water splash or stop over dikes or banks of rivers.

INTENSITY IX

Devastating - People are forcibly thrown to ground. Many cry and shake with fear. Most buildings are totally damaged. bridges and elevated concrete structures are toppled or destroyed. Numerous utility posts, towers and monument are tilted, toppled or broken. Water sewer pipes are bent, twisted or broken. Landslides and liquefaction with lateral spreadings and sandboils are widespread. the ground is distorted into undulations. Trees are shaken very violently with some toppled or broken. Boulders are commonly thrown out. River water splashes violently on slops over dikes and banks.

INTENSITY 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 are observed. Changes in river courses and destructive seiches in large lakes occur. Many trees are toppled, broken and uprooted.