Earthquake Frequently Asked Questions

An earthquake is the sudden release of strain energy in the Earth's crust resulting in waves of shaking that radiate outwards from the earthquake source. When stresses in the crust exceed the strength of the rock, it breaks along lines of weakness, either a pre-existing or new fault plane. The point where an earthquake starts is termed the focus or hypocentre and may be many kilometres deep within the earth. The point at the surface directly above the focus is called the earthquake epicentre.

Earthquake !!! What it to do ???

DUCK LOW NEXT TO A LARGE ITEM such as a desk, car, couch, etc. The item will take the crush impact and although it may collapse, will leave a void next to the item that may allow survival. This is the TRIANGLE OF LIFE SURVIVAL TECHNIQUE... Teach children TOLST...!

Where do earthquakes occur?

Anywhere! However, they are unevenly distributed over the earth, with the majority occurring at the boundaries of the major crustal plates. These plate boundaries are of three types: destructive, where the plates collide; constructive, where the plates move apart; and conservative plate boundaries, like the San Andreas Fault, where the plates slide past each other. Earthquakes also occur, less frequently, within the plates and far from the plate boundaries.

Which countries have the largest and most frequent earthquakes?

Around 75% of the world's seismic energy is released at the edge of the Pacific, where the thinner Pacific plate is forced beneath thicker continental crust along "subduction zones". This 40,000 km band of seismicity stretches up the west coasts of South and Central America and from the Northern USA to Alaska, the Aleutians, Japan, China, the Philippines, Indonesia and Australasia.
Around 15% of the total seismic energy is released where the Eurasian and African plates are colliding, forming a band of seismicity which stretches from Burma, westwards to the Himalayas to the Caucasus and the Mediterranean.

What is the biggest earthquake that has ever happened?

One of the largest earthquakes ever was the Chile event of 22 May 1960 with moment magnitude of 9.5 Mw.
Other large earthquakes include Lisbon, 1 November 1755, magnitude 8.7 Ms; Assam, 12 June 1897, magnitude 8.7 Ms; Alaska, 28 March 1964, moment magnitude 9.2 Mw. Although the magnitude scale is open ended, the strength of the crustal rocks prior to fracturing limits the upper magnitude of earthquakes.

What is earthquake magnitude?

It is a measure of earthquake size and is determined from the logarithm of the maximum displacement or amplitude of the earthquake signal as seen on the seismogram, with a correction for the distance between the focus and the seismometer. This is necessary as the closer the seismometer is to the earthquake, the larger the amplitude on the seismogram, irrespective of the size or magnitude of the event. Since the measurement can be made from P, S or surface waves, several different scales exist, all of which are logarithmic because of the large range of earthquake energies (for example a magnitude 6 ML is 30 times larger, in terms of energy than a magnitude 5 ML). The Richter local magnitude (ML) is defined to be used for 'local' earthquakes up to 600 km away, and is the magnitude scale used by BGS when locating UK earthquakes.

Surface wave magnitude (Ms) is based on the maximum amplitude of the surface wave having a period of 20 + 2 s. It is used for observations near the earthquake epicentre where the surface wave is larger than the body wave. This scale applies to any epicentral distance or type of seismograph.

Body wave magnitude (mb) is calculated from the body waves (P,PP,S) and are usually used at larger distance from the earthquake epicentre (P-wave attenuation is less than surface waves, with distance). It can be used for any earthquake of any depth.

Moment magnitude (Mw) is considered the best scale to use for larger earthquakes as the Ms saturates at about magnitude 8. Moment magnitude is measured over the broad range of frequencies present in the earthquake wave spectrum rather than the single frequency sample that the other magnitude scales use.

For comparison purposes, a magnitude 5 ML earthquake is equivalent to the explosion of 1,000 tons of TNT whereas a magnitude 6 ML earthquake is the energy equivalent of 30,000 tons of TNT or a 30 kilotonne nuclear explosion.

Why do we need more than one earthquake magnitude scale?

The Richter magnitude scale (ML), described above is the best known magnitude scale. Charles Richter developed it in the 1930s for use on earthquakes in southern California, using high-frequency data from nearby or 'local' stations. Other magnitude scales include body-wave magnitude (mb), and surface wave magnitude (Ms). One of these three scales is generally used, depending on the frequency range and type of signal. Values for the magnitude of a given event may, therefore, vary according to the monitoring agency and preferred scale used. Although moment magnitude (Mw) is considered the most reliable measure of earthquake size, especially for the largest events, it is more difficult to routinely calculate and requires analysis of the frequency spectra of the earthquake.

What is the difference between magnitude and intensity?

Magnitude is a measure of earthquake size and remains unchanged with distance from the earthquake. Intensity, however, describes the degree of shaking caused by an earthquake at a given place and decreases with distance from the earthquake epicentre. We can, therefore talk about a magnitude 5.4 ML event with intensity of 6 EMS in the epicentral area, on the Lleyn Peninsula, but intensity 3 EMS at Carlisle. Magnitude measurement requires instrumental monitoring for its calculation, however, assigning an intensity requires a sample of the felt responses of the population. This is then graded according to the EMS intensity scale. For example, Intensity 1, Not felt, 2, Scarcely perceptible, 3, weak, felt by a few, up to 12 assigned for total devastation. Study of intensity and the production of isoseismal maps, contouring areas of equal intensity, is particularly important for the study of earthquakes which occurred prior to instrumental monitoring.

How many global major earthquakes occur each year ?

Are earthquakes on the increase?

NO. There is no evidence that earthquakes are becoming more frequent, we are simply recording larger numbers, especially of small earthquakes. The number of larger events remains stable. As extensive world-wide monitoring networks continue to expand, more events are located each year. The table below details USGS data for the frequency of earthquakes since 1900:

Can earthquakes be predicted?

Although it is known that most global earthquakes will concentrate at the plate boundaries, there is no reliable method of accurately predicting the time, place and magnitude of an earthquake. Most current research is concerned with minimising the risk associated with earthquakes, by assessing the combination of seismic hazard and the vulnerability of a given area. Many seismic countries, however, have research programs based on identifying possible precursors to major earthquakes. This includes the study of dilatancy, how rocks crack and expand under the increased stress associated with the earthquake. Some major earthquakes, but not all, are heralded by the occurrence of foreshocks. which can be detected by dense local monitoring networks. Other instruments can measure changes in the levels of radon gas, electrical and magnetic properties, velocity changes of seismic waves and changes in topography. Long term monitoring and examination by these sensors is required as some or all of these factors may change due to the opening of cracks prior to the earthquake.

All attempts to predict earthquakes have, however, been generally considered as failures and it is unlikely that accurate prediction will occur in the near future. Efforts will, instead, be channelled into hazard mitigation. Earthquakes are difficult or impossible to predict because of their inherent random element and their near-chaotic behaviour

Glossary of Terms Used in Seismology

Aftershock	An earthquake which follows a larger earthquake or main shock and originates at or near the focus of the larger earthquake. Generally, major earthquakes are followed by a larger number of aftershocks, decreasing in frequency with time.
Amplitude	The maximum height of a wave crest or depth of a trough.
Array	An ordered arrangement of seismometers or geophones, the data from which feeds into a central receiver.
Arrival	The appearance of a seismic wave on the seismic record.
Arrival time	The time at which a particular wave phase arrives at a detector.
Aseismic area	An area that is almost free of earthquakes.
Body wave	A seismic wave that travels through the interior of the earth and is not related to a boundary surface.
Crust	The outer layer of the Earth's surface.
Earthquake	Shaking of the earth caused by a sudden movement of rock beneath its surface.
Earthquake swarm	A series of minor earthquakes, none of which may be identified as the main shock, occurring in a limited area and time.
Elastic wave	Rock is an elastic material that when strained by normal external forces can return to its original state. When the strength of the rock is exceeded, the rock ruptures, generating elastic seismic or earthquake waves.
EMS	The currently used 12-grade European macroseismic scale (EMS-92) is the updated version of the MSK-scale intensity scale.
Epicentre	That point on the Earth's surface directly above the hypocentre of an earthquake.
Fault	A weak area in the Earth's crust where two sides of a fracture or fracture zone move relative to each other.
First arrival	The first recorded signal on a seismogram is the direction of the first P-wave, where upward ground motion is compressional and downward motion is dilatational.
Focus	The point where earthquake rupture or fault movement originates.
Foreshock	A small earthquake that may precede a larger earthquake or main shock and that originates at or near the focus of the larger event.
Frequency	The frequency of a wave (Hz) is the number of wave cycles per second.
Hypocentre	The calculated location of the focus of an earthquake.
Induced seismicity	Non-natural events induced by man's activity. These include mining induced events, events caused by loading of dams or pumping of water in geothermal areas.
Intensity	A measure of the effects of an earthquake at a particular place on humans and (or) structures. The intensity at a point depends not only upon the strength of the earthquake (magnitude) but also upon the distance from the earthquake to the epicentre and the local geology at that point.
Isoseismal line	A line enclosing points on the Earth's surface at which earthquake intensity is the same. It is usually elliptical in shape
Love wave	A major type of surface wave having a horizontal motion that is shear or transverse to the direction of propagation. It is named after A.E.H. Love, the English mathematician who discovered it.
Magnitude	A measure of the strength of an earthquake. There are several scales depending on which part of the seismogram is examined. These include Richter local magnitude (ML), Body wave magnitude (mb) and surface wave magnitude (Ms). Moment magnitude (Mw) is calculated from spectral analysis.
Mantle	The layer that lies between the crust and the core of the earth.
Microseism	A motion in the Earth that is unrelated to an earthquake. It is caused by a variety of natural and artificial agents, for example wave action, wind, traffic and industrial noise.
MSK	MSK intensity is the intensity scale used in Europe before the introduction of the EMS scale. It is a 12-grade scale ranging from not felt to complete devastation.
P wave	The first and faster of the body waves which moves by a series of compressions and dilatations, similar to a sound wave. They can travel through both solid and liquid.
Phase	The onset of a displacement on a seismogram indicating the arrival of the different types of seismic wave.
Plate	One of the segments which make up the Earth's crust. The plates are continuously moving relative to each other.
Plate boundary	The place where two or more plates in the Earth's crust meet.
Prediction	Predicting the time, place and magnitude of an earthquake.
Rayleigh wave	A type of surface wave having a retrograde, elliptical motion at the free surface. It is named after Lord Rayleigh, the English physicist who predicted its existence.
Reflected wave	A wave that has turned back from a boundary or discontinuity in the earth's crust.
Refraction	The change in direction of a wave on reaching a boundary of different density and velocity.
Richter scale	A popular name for the local magnitude scale (See Magnitude).
S wave	The second arrival on a seismogram, the S wave, is slower than the P-wave. It is a shear wave and cannot travel through liquids.
Seismogram	A record of an earthquake or ground vibration. The wave trace is made up of P-waves, S-waves and surface waves, the pattern of onsets of the first two arrivals help to determine the location. The seismogram can be either a paper record or a digital record that is analysed by computer.
Seismograph	An instrument that registers the occurrence of an earthquake and the time it occurred as a written record.
Seismologist	A scientist who studies earthquakes.
Seismometer	An instrument that not only measures the time of the arrival of earthquake waves, but also allows the exact motion of the ground to be computed from the record.
Seismoscope	An instrument that registers the occurrence of an earthquake, but not the time.
Signal-to-noise ratio	The comparison between the amplitude of the seismic signal and the amplitude of noise caused by seismic unrest and (or) the seismic instruments.
Subduction zone	An elongated region along which a crustal plate descends relative to another crustal block, for example, the descent of the Pacific plate beneath the Andean plate along the Andean trench.
Surface waves	Seismic waves with motion restricted to near the ground surface (Love and Rayleigh)
Teleseism	An earthquake that is distant from the recording station.
Travel time	The time required for a wave train to travel from its source to a point of observation.
Tsunamis	A huge sea wave caused by earthquakes. (Referred to by many as a tidal wave.)
Volcanic earthquake	Earthquakes associated with volcanic activity.
Wavelength	The distance between two successive crests or troughs of a wave.

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This page is constructed to give information's about earthquake preparedness. It should give answers to questions what to do before - during and after an earthquake.
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The informations above are taken from a book of the Global Seismology and Geomagnetism Group – London.

Descriptor	Magnitude	Average Annually
Great	8 and higher	1
Major	7 - 7.9	18
Strong	6 - 6.9	120
Moderate	5 - 5.9	800
Light	4 - 4.9	6,200 (estimated)
Minor	3 - 3.9	49,000 (estimated)
Very Minor	2 - 3.0	about 1,000 per day
Very Minor	1 - 2	about 8,000 per day

Earthquake Questions & Answers (FAQ's)

Table of Contens - Içerik:

What is an earthquake?