Introduction to Earthquakes mccengg Plate Tectonics Theory mccengg mccengg 15 Major Tectonic Plates: 1. 2. 3. 4. 5. 6. 7. 8. 9. African Plate Antarctic Plate Arabian Plate Australian Plate Caribbean Plate Cocos Plate Eurasian Plate Indian Plate Juan de Fuca Plate 10. Nazea Plate 11. North American Plate 12. Pacific Plate 13. Philippine Plate 14. Scotia Plate 15. South American Plate mccengg mccengg Plate Tectonics • The earth’s outer shell (lithosphere) consists of several large and fairly stable rock slabs called plates. • Each plate extends to a depth of about 100-200km. • The plates move relative to each other at very slow speeds (a few cm per year) • The plates are considered rigid, there is little or no deformation within them. The deformation occurs at the boundaries. • The plate movements takes place over the weaker layer called the asthenosphere. mccengg Plate Boundaries: 1. Divergent boundaries – where 2 plates are moving apart and new lithosphere is produced or old lithosphere is thinned. 2. Convergent boundaries – where 2 plates are moving toward each other & the lithosphere is thickened or consumed by sinking into the mantle, i.e., Subduction zones. 3. Transform Boundaries – where plates move past one another without either convergence or divergence, i.e. Strikeslip faults. mccengg mccengg mccengg mccengg mccengg mccengg mccengg mccengg Interplate and Intraplate Earthquakes • Plate tectonic theory provides a simple and general explanation for plate boundary or inter-plate earthquakes, which contribute 95% of worldwide seismic energy release. • However, earthquakes are not confined to plate boundaries. • Local intra-plate earthquakes may occur virtually anywhere and may cause considerable damage. • Intra-plate earthquakes generally fall into two groups: plate boundary-related and mid-plate. The former takes place near plate edges and are tectonically linked. Mid-plate earthquakes are not related to plate edges. mccengg mccengg Faults • Faults are defined as a form of discontinuity in the bedrock and are associated with relative displacement of two large lblocks of rock masses. • They are zones of relative weakness in the earth’s crust. • After the stresses build up in the rock, the focus of the fault ruptures first. • The rupture then propagates at a very high speed forming a fault plane. • The vertical projection of the focus to the surface is the epicenter. mccengg mccengg • Shallow earthquakes – have focal depths of 5-15km • Intermediate earthquakes – have focal depths at about 20-50km • Deep earthquakes – occur at about 300-700km mccengg mccengg mccengg mccengg mccengg mccengg mccengg Elastic Rebound Theory • Movement of the tectonic plates relative to each other leads to an accumulation of strain, both at the plate boundaries and inside the plates. • The strain energy is the elastic energy that is stored due to the straining of the rocks. • When the strain reaches its limiting value along a weak region, existing fault or plate boundary, a sudden movement or slip occurs releasing the accumulated strain energy. • The action generates elastic waves within the rock mass which propagate and eventually reach the surface of the earth. mccengg mccengg • Illustration of the formation of a left lateral strike slip fault • The fault is the vertical line and is not yet ruptured mccengg • After a period of time the fault plane is still intact, but elastic stresses have built up considerably causing the rock to deform elastically. • A new road has been built recently on the deformed rock. mccengg • When the strained rock is stressed to its limit, the fault ruptures suddenly releasing a tremendous amount of strain energy. • The release of strain energy is accompanied by a variety of seismic waves that propagate in all directions. • After a time,the process will begin anew and the stresses will begin building up again. mccengg mccengg Seismic Waves mccengg mccengg mccengg mccengg mccengg mccengg mccengg mccengg Earthquake Measurement Parameters mccengg mccengg Magnitude is a measure of the size of an earthquake related to the total strain energy released. Types of Earthquake Magnitude: 1. Body Wave Magnitude (Mb) The Mb magnitude is measured as the common logarithm displacement amplitude in microns of the P-wave with period near one second. Developed to measure the magnitude of deep focus earthquakes, which do not ordinarily set up detectable surface waves with long periods. mccengg 2. Local magnitude (ML) The original magnitude definition by Richter. The magnitude of an earthquake measured as the common logarithm of the displacement amplitude, in microns, defined by a standard Wood-Anderson seismograph located on firm ground 100 km from the epicenter and having a magnification of 2800, a natural period of 0.8 second, and a damping coefficient of 80%. The definition itself applies strictly only to earthquakes having focal depths smaller than about 30 km. Empirical charts and tables are available to correct to an epicentral distance of 100 km for other types of seismographs and for various conditions of the ground The correction charts are suitable up to epicentral distances of about 600 km. The correction charts are site dependent and have to be developed for each recording site. mccengg 3. Surface wave magnitude (MS) This is measured as the common logarithm of the resultant of the maximum mutually perpendicular horizontal displacement amplitudes, in microns, of the 20-second period surface waves. The scale was developed to measure the magnitude of shallow focus earthquakes at relatively long distances. Magnitudes can be assigned from any suitable instrument whose constants are known. 4. Richter magnitude (M) Richter magnitude is a general usage that is usually ML up to 5.9, MS for 5.9 to about 8.0, and MW up to 8.3. mccengg 5. Seismic Moment Magnitude (MW) Defines magnitude based on the seismic moment Mo. MW = 2/3 log Mo – 10.7 where Mo, the seismic moment, is computed as Mo = G A D where G = rigidity modulus A = area of fault movement D = average static displacement The values are in dyne-cm units. mccengg mccengg mccengg Earthquake Measurement Parameters Frequency of occurrence of earthquakes (based on observations since 1900) mccengg Earthquake Measurement Parameters Magnitudes of Some Recent Damaging Earthquakes mccengg Intensity A subjective numerical index describing the effects of an earthquake on humans, on their structures, and on the earth’s surface at a particular place. The commonly used scale is the Modified Mercalli Intensity scale. It is expressed as a Roman numeral. mccengg Modified Mercalli Intensity Scale (MMI) of 1931 mccengg MMI Scale (with descriptive name and equivalent magnitude) An empirical relation between magnitude and maximum intensity was proposed by Gutenberg and Richter: mccengg mccengg mccengg The PHIVOLCS’ Earthquake Intensity Scale (PEIS) Intensity Scale Description I Scarcely Perceptible II Slightly Felt - Felt by few individuals at rest indoors. Hanging objects swing slightly. Still Water in containers oscillates noticeably. III Weak - Felt by many people indoors especially in upper floors of buildings…Still water in containers oscillates moderately. IV Moderately Strong - Felt generally by people indoors and by some people outdoors. Light sleepers are awakened. V Strong - Generally felt by most people indoors and outdoors. Many sleeping people are awakened. VI Very Strong - Many people are frightened; run outdoors. Some people lose their balance. Very old or poorly built houses and manmade structures are slightly damaged though well-built structures are not affected. VII Destructive - Most people frightened and run outdoors. Old or poorly-built structures suffer considerably damage. Some well-built structures are slightly damaged. Limited liquefaction, lateral spreading and landslides are observed. VIII Very Destructive - People panicky. People find it difficult to stand even outdoors. Many well-built buildings are considerably damaged. 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. IX Devastating - People forcibly thrown to ground. Many cry and shake with fear. Most buildings are totally damaged. bridges and elevated concrete structures are toppled or destroyed. 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. 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. mccengg mccengg mccengg Earthquake Hazards mccengg mccengg mccengg mccengg mccengg mccengg mccengg mccengg mccengg mccengg mccengg mccengg mccengg
