Amplitude earthquake

Earthquake magnitude is a measure of the “size,” or amplitude, of the seismic waves generated by an earthquake source and recorded by seismographs. (The types and nature of these waves are described in the section Seismic waves.) Because the size of earthquakes varies enormously, it is necessary for purposes of comparison to compress the range of wave amplitudes measured on seismograms by means of a mathematical device. In 1935 the American seismologist Charles F. Richter set up a magnitude scale of earthquakes as the logarithm to base 10 of the maximum seismic wave amplitude (in thousandths of a millimetre) recorded on a standard seismograph (the Wood-Anderson torsion pendulum seismograph) at a distance of 100 km (60 miles) from the earthquake epicentre. Reduction of amplitudes observed at various distances to the amplitudes expected at the standard distance of 100 km is made on the basis of empirical tables. Richter magnitudes ML are computed on the assumption that the ratio of the maximum wave amplitudes at two given distances is the same for all earthquakes and is independent of azimuth.

Richter first applied his magnitude scale to shallow-focus earthquakes recorded within 600 km of the epicentre in the southern California region. Later, additional empirical tables were set up, whereby observations made at distant stations and on seismographs other than the standard type could be used. Empirical tables were extended to cover earthquakes of all significant focal depths and to enable independent magnitude estimates to be made from body- and surface-wave observations. A current form of the Richter scale is shown in the table.

At the present time a number of different magnitude scales are used by scientists and engineers as a measure of the relative size of an earthquake. The P-wave magnitude (Mb), for one, is defined in terms of the amplitude of the P wave recorded on a standard seismograph. Similarly, the surface-wave magnitude (Ms) is defined in terms of the logarithm of the maximum amplitude of ground motion for surface waves with a wave period of 20 seconds.

As defined, an earthquake magnitude scale has no lower or upper limit. Sensitive seismographs can record earthquakes with magnitudes of negative value and have recorded magnitudes up to about 9.0. (The 1906 San Francisco earthquake, for example, had a Richter magnitude of 8.25.)

A scientific weakness is that there is no direct mechanical basis for magnitude as defined above. Rather, it is an empirical parameter analogous to stellar magnitude assessed by astronomers. In modern practice a more soundly based mechanical measure of earthquake size is used—namely, the seismic moment (M0). Such a parameter is related to the angular leverage of the forces that produce the slip on the causative fault. It can be calculated both from recorded seismic waves and from field measurements of the size of the fault rupture. Consequently, seismic moment provides a more uniform scale of earthquake size based on classical mechanics. This measure allows a more scientific magnitude to be used called moment magnitude (Mw). It is proportional to the logarithm of the seismic moment; values do not differ greatly from Ms values for moderate earthquakes. Given the above definitions, the great Alaska earthquake of 1964, with a Richter magnitude (ML) of 8.3, also had the values Ms = 8.4, M0 = 820 × 1027 dyne centimetres, and Mw = 9.2.

Earthquake energy

Energy in an earthquake passing a particular surface site can be calculated directly from the recordings of seismic ground motion, given, for example, as ground velocity. Such recordings indicate an energy rate of 105 watts per square metre (9,300 watts per square foot) near a moderate-size earthquake source. The total power output of a rupturing fault in a shallow earthquake is on the order of 1014 watts, compared with the 105 watts generated in rocket motors.

The surface-wave magnitude Ms has also been connected with the surface energy Es of an earthquake by empirical formulas. These give Es = 6.3 × 1011 and 1.4 × 1025 ergs for earthquakes of Ms = 0 and 8.9, respectively. A unit increase in Ms corresponds to approximately a 32-fold increase in energy. Negative magnitudes Ms correspond to the smallest instrumentally recorded earthquakes, a magnitude of 1.5 to the smallest felt earthquakes, and one of 3.0 to any shock felt at a distance of up to 20 km (12 miles). Earthquakes of magnitude 5.0 cause light damage near the epicentre; those of 6.0 are destructive over a restricted area; and those of 7.5 are at the lower limit of major earthquakes.

The total annual energy released in all earthquakes is about 1025 ergs, corresponding to a rate of work between 10 million and 100 million kilowatts. This is approximately one one-thousandth the annual amount of heat escaping from the Earth’s interior. Ninety percent of the total seismic energy comes from earthquakes of magnitude 7.0 and higher—that is, those whose energy is on the order of 1023 ergs or more.

Frequency

Global seismicity patterns had no strong theoretical explanation until the dynamic model called plate tectonics was developed during the late 1960s. This theory holds that the Earth’s upper shell, or lithosphere, consists of nearly a dozen large, quasi-stable slabs called plates. The thickness of each of these plates is roughly 80 km (50 miles). The plates move horizontally relative to neighbouring plates at a rate of 1 to 10 cm (0.4 to 4 inches) per year over a shell of lesser strength called the asthenosphere. At the plate edges where there is contact between adjoining plates, boundary tectonic forces operate on the rocks, causing physical and chemical changes in them. New lithosphere is created at oceanic ridges by the upwelling and cooling of magma from the Earth’s mantle. The horizontally moving plates are believed to be absorbed at the ocean trenches, where a subduction process carries the lithosphere downward into the Earth’s interior. The total amount of lithospheric material destroyed at these subduction zones equals that generated at the ridges.

Seismological evidence (such as the location of major earthquake belts) is everywhere in agreement with this tectonic model. Earthquake sources are concentrated along the oceanic ridges, which correspond to divergent plate boundaries. At the subduction zones, which are associated with convergent plate boundaries, intermediate- and deep-focus earthquakes mark the location of the upper part of a dipping lithosphere slab. The focal mechanisms indicate that the stresses are aligned with the dip of the lithosphere underneath the adjacent continent or island arc.

Some earthquakes associated with oceanic ridges are confined to strike-slip faults, called transform faults, that offset the ridge crests. The majority of the earthquakes occurring along such horizontal shear faults are characterized by slip motions. Also in agreement with the plate tectonics theory is the high seismicity encountered along the edges of plates where they slide past each other. Plate boundaries of this kind, sometimes called fracture zones, include the San Andreas Fault in California and the North Anatolian fault system in Turkey. Such plate boundaries are the site of interplate earthquakes of shallow focus.

The low seismicity within plates is consistent with the plate tectonic description. Small to large earthquakes do occur in limited regions well within the boundaries of plates; however, such intraplate seismic events can be explained by tectonic mechanisms other than plate boundary motions and their associated phenomena.

Intensity scales

The violence of seismic shaking varies considerably over a single affected area. Because the entire range of observed effects is not capable of simple quantitative definition, the strength of the shaking is commonly estimated by reference to intensity scales that describe the effects in qualitative terms. Intensity scales date from the late 19th and early 20th centuries, before seismographs capable of accurate measurement of ground motion were developed. Since that time, the divisions in these scales have been associated with measurable accelerations of the local ground shaking. Intensity depends, however, in a complicated way not only on ground accelerations but also on the periods and other features of seismic waves, the distance of the measuring point from the source, and the local geologic structure. Furthermore, earthquake intensity, or strength, is distinct from earthquake magnitude, which is a measure of the amplitude, or size, of seismic waves as specified by a seismograph reading. See below Earthquake magnitude.

A number of different intensity scales have been set up during the past century and applied to both current and ancient destructive earthquakes. For many years the most widely used was a 10-point scale devised in 1878 by Michele Stefano de Rossi and Franƈois-Alphonse Forel. The scale now generally employed in North America is the Mercalli scale, as modified by Harry O. Wood and Frank Neumann in 1931, in which intensity is considered to be more suitably graded. A 12-point abridged form of the modified Mercalli scale is provided below. Modified Mercalli intensity VIII is roughly correlated with peak accelerations of about one-quarter that of gravity (g = 9.8 metres, or 32.2 feet, per second squared) and ground velocities of 20 cm (8 inches) per second. Alternative scales have been developed in both Japan and Europe for local conditions. The European (MSK) scale of 12 grades is similar to the abridged version of the Mercalli.

Modified Mercalli scale of earthquake intensity

• I. Not felt. Marginal and long-period effects of large earthquakes.
• II. Felt by persons at rest, on upper floors, or otherwise favourably placed to sense tremors.
• III. Felt indoors. Hanging objects swing. Vibrations are similar to those caused by the passing of light trucks. Duration can be estimated.
• IV. Vibrations are similar to those caused by the passing of heavy trucks (or a jolt similar to that caused by a heavy ball striking the walls). Standing automobiles rock. Windows, dishes, doors rattle. Glasses clink, crockery clashes. In the upper range of grade IV, wooden walls and frames creak.
• V. Felt outdoors; direction may be estimated. Sleepers awaken. Liquids are disturbed, some spilled. Small objects are displaced or upset. Doors swing, open, close. Pendulum clocks stop, start, change rate.
• VI. Felt by all; many are frightened and run outdoors. Persons walk unsteadily. Pictures fall off walls. Furniture moves or overturns. Weak plaster and masonry cracks. Small bells ring (church, school). Trees, bushes shake.
• VII. Difficult to stand. Noticed by drivers of automobiles. Hanging objects quivering. Furniture broken. Damage to weak masonry. Weak chimneys broken at roof line. Fall of plaster, loose bricks, stones, tiles, cornices. Waves on ponds; water turbid with mud. Small slides and caving along sand or gravel banks. Large bells ringing. Concrete irrigation ditches damaged.
• VIII. Steering of automobiles affected. Damage to masonry; partial collapse. Some damage to reinforced masonry; none to reinforced masonry designed to resist lateral forces. Fall of stucco and some masonry walls. Twisting, fall of chimneys, factory stacks, monuments, towers, elevated tanks. Frame houses moved on foundations if not bolted down; loose panel walls thrown out. Decayed pilings broken off. Branches broken from trees. Changes in flow or temperature of springs and wells. Cracks in wet ground and on steep slopes.
• IX. General panic. Weak masonry destroyed; ordinary masonry heavily damaged, sometimes with complete collapse; reinforced masonry seriously damaged. Serious damage to reservoirs. Underground pipes broken. Conspicuous cracks in ground. In alluvial areas, sand and mud ejected; earthquake fountains, sand craters.
• X. Most masonry and frame structures destroyed with their foundations. Some well-built wooden structures and bridges destroyed. Serious damage to dams, dikes, embankments. Large landslides. Water thrown on banks of canals, rivers, lakes, and so on. Sand and mud shifted horizontally on beaches and flat land. Railway rails bent slightly.
• XI. Rails bent greatly. Underground pipelines completely out of service.
• XII. Damage nearly total. Large rock masses displaced. Lines of sight and level distorted. Objects thrown into air.

With the use of an intensity scale, it is possible to summarize such data for an earthquake by constructing isoseismal curves, which are lines that connect points of equal intensity. If there were complete symmetry about the vertical through the earthquake’s focus, isoseismals would be circles with the epicentre (the point at the surface of the Earth immediately above where the earthquake originated) as the centre. However, because of the many unsymmetrical geologic factors influencing intensity, the curves are often far from circular. The most probable position of the epicentre is often assumed to be at a point inside the area of highest intensity. In some cases, instrumental data verify this calculation, but not infrequently the true epicentre lies outside the area of greatest intensity.

Magnitude

Sketch of a traditional seismometer. (Public domain.)

The time, location, and magnitude of an earthquake can be determined from the data recorded by seismometer. Seismometers record the vibrations from earthquakes that travel through the Earth. Each seismometer records the shaking of the ground directly beneath it. Sensitive instruments, which greatly magnify these ground motions, can detect strong earthquakes from sources anywhere in the world. Modern systems precisely amplify and record ground motion (typically at periods of between 0.1 and 100 seconds) as a function of time.

Types of Magnitudes

Magnitude is expressed in whole numbers and decimal fractions. For example, a magnitude 5.3 is a moderate earthquake, and a 6.3 is a strong earthquake. Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude as measured on a seismogram.

When initially developed, all magnitude scales based on measurements of the recorded waveform amplitudes were thought to be equivalent. But for very large earthquakes, some magnitudes underestimate true earthquake size, and some underestimate the size. Thus, we now use measurements that describe the physical effects of an earthquake rather than measurements based only on the amplitude of a waveform recording. More on that later.

From Richter’s (1958) book, Elementary Seismology.(Public domain.)

The Richter Scale (ML) is what most people have heard about, but in practice it is not commonly used anymore, except for small earthquakes recorded locally, for which ML and short-period surface wave magnitude (Mblg) are the only magnitudes that can be measured. For all other earthquakes, the moment magnitude (Mw) scale is a more accurate measure of the earthquake size.

Although similar seismographs had existed since the 1890’s, it was only in 1935 that Charles F. Richter, a seismologist at the California Institute of Technology, introduced the concept of earthquake magnitude. His original definition held only for California earthquakes occurring within 600 km of a particular type of seismograph (the Woods-Anderson torsion instrument). His basic idea was quite simple: by knowing the distance from a seismograph to an earthquake and observing the maximum signal amplitude recorded on the seismograph, an empirical quantitative ranking of the earthquake’s inherent size or strength could be made. Most California earthquakes occur within the top 16 km of the crust; to a first approximation, corrections for variations in earthquake focal depth were, therefore, unnecessary.

The Richter magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs. Adjustments are included for the variation in the distance between the various seismographs and the epicenter of the earthquakes.

Cross-section showing the fault area and the values that are used to compute the seismic moment. (Public domain.)

Moment Magnitude (MW) is based on physical properties of the earthquake derived from an analysis of all the waveforms recorded from the shaking. First the seismic moment is computed, and then it is converted to a magnitude designed to be roughly equal to the Richter Scale in the magnitude range where they overlap.

Moment (MO) = rigidity x area x slip

where rigidity is the strength of the rock along the fault, area is the area of the fault that slipped, and slip is the distance the fault moved. Thus, stronger rock material, or a larger area, or more movement in an earthquake will all contribute to produce a larger magnitude.

Moment Magnitude (MW) = 2/3 log10(MO) — 9.1

See the Magnitude Types Table (below)for a summary of types, magnitude ranges, distance ranges, equations, and a brief description of each.

Energy Release

Earthquake magnitudes and energy release, and comparison with other natural and man-made events. (Courtesy Incorporated Research Institutes for Seismology, IRIS.)

Another way to measure the size of an earthquake is to compute how much energy it released. The amount of energy radiated by an earthquake is a measure of the potential for damage to man-made structures. An earthquake releases energy at many frequencies, and in order to compute an accurate value, you have to include all frequencies of shaking for the entire event.

While each whole number increase in magnitude represents a tenfold increase in the measured amplitude, it represents an 32 times more energy release.

The energy can be converted into yet another magnitude type called the Energy Magnitude (Me). However, since the Energy Magnitude and Moment Magnitude measure two different properties of the earthquake, their values are not the same.

The energy release can also be roughly estimated by converting the moment magnitude, Mw, to energy using the equation log E = 5.24 + 1.44Mw, where Mw is the moment magnitude.

Did You Feel It? map for the M6.0 Napa, California earthquake on August 24, 2014. The earthquake epicenter is shown as a star, and the geocoded intensities are shown as small colored squares. The associated MMI value for each color is shown in the key at the bottom. (Public domain.)

Whereas the magnitude of an earthquake is one value that describes the size, there are many intensity values for each earthquake that are distributed across the geographic area around the earthquake epicenter. The intensity is the measure of shaking at each location, and this varies from place to place, depending mostly on the distance from the fault rupture area. However, there are many more aspects of the earthquake and the ground it shakes that affect the intensity at each location, such as what direction the earthquake ruptured, and what type of surface geology is directly beneath you. Intensities are expressed in Roman numerals, for example, VI, X, etc.

Traditionally the intensity is a subjective measure derived from human observations and reports of felt shaking and damage. The data used to be gathered from postal questionnaires, but with the advent of the internet, it’s now collected using a web-based form. However, instrumental data at each station location can be used to calculate an estimated intensity.

The intensity scale that we use in the United States is called the Modified Mercalli Intensity Scale, but other countries use other scales.

Examples

These examples illustrate how locations (and depth), magnitudes, intensity, and faults (and rupture) characteristics are dependent and related.

Intensity of Shaking Depends on the Local Geology

This shows the shaking amplitude recorded on 3 different seismometers from the M6.9 Loma Prieta, CA earthquake in 1989. All 3 stations are about the same distance from the earthquake to the south, but the type of local geology beneath the instrument influences the amount of shaking at that location. Bedrock shakes the least, and soft mud the most. (Public domain.)

Intensity of Shaking Depends on Depth of the Earthquake

Maps showing the shaking intensity from two different earthquakes with about the same magnitude. (Public domain.)

The shaking from the M6.7 Northridge, CA earthquake was more intense and covered a wider area than the slightly larger M6.8 Nisqually, WA earthquake.

The reason is shown by the two cartoon cross-sections below. There was more shaking in the Northridge earthquake because the earthquake occurred closer to the surface (3-11 miles), as opposed to the Nisqually earthquake’s deeper hypocenter (30-36 miles).

Image showing the location and depth of the Northridge and Nisqually earthquakes. (Public domain.)

Moment Release (Energy) of Many Small Earthquakes vs. One Large Earthquake

The small- and moderate-size earthquakes that occur frequently around the world release far less energy that a single great earthquake.

This graph demonstrates the logarithmic nature of earthquake magnitudes and energy release.  (Public domain.)

What Would it Take to Make a Magnitude N Earthquake?

If we sum all of the energy release from all of the earthquakes over the past ~110 years, the equivalent magnitude ~ Mw9.95.

If the San Andreas Fault were to rupture end-to-end (~1400km), with ~10m of average slip, it would produce an earthquake of Mw 8.47.

If the South American subduction zone were to rupture end-to-end (~6400km), with ~40m of average slip, it would produce an earthquake of Mw 9.86.

You would need ~14,000km fault length, with a seismogenic thickness averaging 40km (width of 100km), to slip and average of 30m to produce an Mw 10.

Map showing a fault with length ~14,000km, outlined in black, which would be needed to produce a Mw 10 earthquake. (Public domain.)

You would need ~80,000km of fault length with an average seismogenic width of 100km to produce an Mw10.5. All of the subduction zones in the World, plus some adjoining structures amount to ~40,000km, and the circumference of the Earth is ~40,000km, so an Mw 10.5 is highly unlikely.

Map outlining all subduction zones and other structures in the world, amounting to a length of ~40,000km, which would still not be enough to produce a Mw 10.5 earthquake. (Public domain.)

Thanks to Gavin Hayes and David Wald for providing much of the material for this page.

MwwW10OOMwcW10OOMwbW10OOMwrW10OOMs20 or MsS10A/T10mbb10A/TDhATDhDhMfaML Ml, or mlmb_Lg, mb_lg, or MLgMd or mdMi or MwpMee1010ESEs MhFinite FaultMint

There are many different ways to measure different aspects of an earthquake:

• Magnitude is the most common measure of an earthquake’s size. It is a measure of the size of the earthquake source and is the same number no matter where you are or what the shaking feels like. The Richter scale is an outdated method for measuring magnitude that is no longer used by the USGS for large, teleseismic earthquakes. The Richter scale measures the largest wiggle (amplitude) on the recording, but other magnitude scales measure different parts of the earthquake. The USGS currently reports earthquake magnitudes using the Moment Magnitude scale, though many other magnitudes are calculated for research and comparison purposes.
• Intensity is a measure of the shaking and damage caused by the earthquake; this value changes from location to location.

Wave Amplitude, Fault Size, Amount of Slip

There are a number of ways to measure the magnitude of an earthquake. Most scales
are based on the amplitude of seismic waves recorded on seismometers. These scales
account for the distance between the earthquake and the recording seismometer so that
the calculated magnitude should be about the same no matter where it is measured.
Another scale is based on the physical size of the earthquake fault and the amount
of slip that occurred. Then there are also measures of earthquake shaking intensity.
The intensity from one earthquake varies greatly from place to place.

What’s the difference between magnitude and intensity? This 8 minute video uses the
analogy of a lightbulb to explain the how an earthquake can have different intensities
at different places.

Earthquake Intensity

What Controls the Shaking You Feel?

A few of these scales are described in more detail below.

Charles Richter studying a seismogram.

The Richter Scale

The first widely-used method, the Richter scale, was developed by Charles F. Richter in 1934. It used a formula based on the amplitude
of the largest wave recorded on a specific type of seismometer and the distance between
the earthquake and the seismometer. That scale was specific to California earthquakes
and crust; other scales, based on wave amplitudes and total earthquake duration, were
developed for use in other situations and they were designed to be consistent with
Richter’s scale.

The Moment Magnitude Scale

Unfortunately, many scales, such as the Richter scale, do not provide accurate estimates
for large magnitude earthquakes. Today the moment magnitude scale, abbreviated MW, is preferred because it works over a wider range of earthquake sizes and is applicable
globally. The moment magnitude scale is based on the total moment release of the earthquake.
Moment is a product of the distance a fault moved and the force required to move it.
It is derived from modeling recordings of the earthquake at multiple stations. Moment
magnitude estimates are about the same as Richter magnitudes for small to large earthquakes.
But only the moment magnitude scale is capable of measuring M8 (read «magnitude 8»)
and greater events accurately.

Magnitudes are based on a logarithmic scale (base 10). What this means is that for
each whole number you go up on the magnitude scale, the amplitude of the ground motion
recorded by a seismograph goes up ten times. Using this scale, a magnitude 5 earthquake
would result in ten times the level of ground shaking as a magnitude 4 earthquake
(and about 32 times as much energy would be released). To give you an idea how these
numbers can add up, think of it in terms of the energy released by explosives: a magnitude
1 seismic wave releases as much energy as blowing up 6 ounces of TNT. A magnitude
8 earthquake releases as much energy as detonating 6 million tons of TNT. Pretty impressive,
huh? Fortunately, most of the earthquakes that occur each year are much too small
to be felt by most people.

Magnitude scales can be used to describe earthquakes so small that they are expressed
in negative numbers. The scale also has no upper limit. The largest recorded earthquake
occurred along the subduction zone in Chile in 1960. It was a magnitude 9.5 but larger
earthquakes may be possible.

Fortunately, large earthquakes are much less common than small ones. Here’s a table describing the magnitudes of earthquakes, their effects, and the estimated number of those earthquakes that occur each year.

The Mercalli Scale

Another way to measure the strength of an earthquake is to use the observations of
the people who experienced the earthquake, and the amount of damage that occurred,
to estimate its intensity. The Mercalli scale was designed to do just that The original scale was invented by Giuseppe Mercalli
in 1902 and was modified by Harry Wood and Frank Neumann in 1931 to become what is
now known as the Modified Mercalli Intensity Scale. To help distinguish it from magnitude scales, the MMI scale uses roman numerals.

Although the Mercalli scale does not use scientific equipment to measure seismic waves, it has been very useful
for understanding the damage caused by large earthquakes. It has also been used extensively
to investigate earthquakes that occurred before there were seismometers.

Some factors that affect the amount of damage that occurs are:

• the size (magnitude) of the earthquake
• the distance from the epicenter,
• the depth of the earthquake,
• the building (or other structure) design,
• and the type of surface material (rock or dirt) the buildings rest on.

Different building designs hold up differently in an earthquake and the farther you
are from the earthquake, the less damage you’ll usually see. Whether a building is
built on solid rock or sand makes a big difference in how much damage it sustains.
Solid rock usually shakes less than sand, so a building built on top of solid rock
shouldn’t be as damaged as it might if it was sitting on a sandy lot.

Благодаря современным технологиям, ученым удалось подсчитать, сколько ежегодно происходит землетрясений на нашей планете. Их фиксируется больше миллиона. Большая часть их них не ощущается людьми из-за своей малой магнитуды, но есть те, которые становятся настоящей катастрофой.

А что такое магнитуда землетрясений и в чем ее измеряют? Как ученым удается определять, какие из явлений нанесут ущерб, а какие останутся неощутимыми?

Магнитуда

Учеными были разработаны специальные шкалы, по которым измеряют силу подземных толчков. Чтобы понять, что такое магнитуда землетрясения, необходимо ознакомиться с величинами измерений этого явления.

Есть несколько типов шкал: Меркалли — Канкани, Медведева — Шпонхойера — Карника, Рихтера. Благодаря им понятно, что такое магнитуда. Это число, которое можно измерить по определенному эталонному показателю. Во время очередного землетрясения принято говорить о бальности и магнитуде.

Можно ли предупредить гибель людей

В 20-веке в опасных зонах началось строительство специальных сейсмоустойчивых зданий повышенной прочности. Проводится разъяснительная работа среди населения, как вести себя во время землетрясения. Создаются специальные безопасные участки, где лучше всего оставаться во время стихийного бедствия.

К сожалению, прогноз приближающегося землетрясения с хорошей точностью пока невозможен, однако научные изыскания в этом направлении ведутся. По всему миру расположены сейсмические станции. Ведутся сводки сейсмоактивности, составляются карты геотермических процессов в недрах земли, по этим статистическим данным строятся прогнозы.

Замечено, например, что перед бедствием из горных пород усиленно выделяется газ радон, который можно зафиксировать. Исследуется также аномальное поведение животных перед катастрофой. Основными предвестниками подземных толчков могут быть рыбы и насекомые.

Шкала определения магнитуды

Самой первой шкалой длительное время считали сетку Меркалли — Канкани. В наше время она является устаревшей моделью, так что значение подземных толчков ею не измеряют.

Однако на ее основе разработаны все современные методы оценки силы ударов, в числе которых международная шкала MSK 64 (Медведева — Шпонхойера — Карника). Ее берут в большинстве стран мира для анализа интенсивности явления.

Поверхностные волны

распространяются вдоль земной поверхности или параллельно ей и не проникают глубже 80-160 км. В этой группе выделяются волны Рэлея и волны Лява (названные по именам ученых, разработавших математическую теорию распространения таких волн). При прохождении волн Рэлея частицы породы описывают вертикальные эллипсы, лежащие в очаговой плоскости. В волнах Лява частицы породы колеблются перпендикулярно направлению распространения волн. Поверхностные волны часто обозначаются сокращенно как L

-волны. Скорость их распространения составляет 3,2-4,4 км/с. При глубокофокусных землетрясениях поверхностные волны очень слабые.

MSK 64

Данная система оценки представлена двенадцатибальной шкалой. По ней можно узнать, что характеризует магнитуда землетрясения:

• 1 балл. Такие явления не ощущаются людьми, но их фиксируют аппараты.
• 2 балла. В некоторых случаях могут наблюдаться людьми, чаще всего на верхних этажах зданий.
• 3 балла. Удары заметны тем, у кого высокая чувствительность.
• Землетрясение 4 балла. Отмечается дребезжание стекол.
• 5 баллов. Считается достаточно ощутимым землетрясением, при котором могут раскачиваться отдельные предметы.
• 6 баллов. Образование трещин на зданиях.
• 7 баллов. Возможно падение тяжелых предметов. В стенах зданий появляются крупные трещины.
• 8 баллов. Дома частично рушатся.
• 9 баллов. Здания и другие конструкции рушатся.
• 10 баллов. В грунте возникают глубокие трещины, старые строения полностью разрушаются.
• 11 баллов. На поверхности земли появляются многочисленные трещины, в горах происходят обвалы. Здания полностью разрушаются.
• 12. Рельеф серьезно изменяется, а строения полностью разрушаются.

Продольные и поперечные волны.

На сейсмограммах эти волны появляются первыми. Раньше всего регистрируются продольные волны, при прохождении которых каждая частица среды подвергается сначала сжатию, а затем снова расширяется, испытывая при этом возвратно-поступательное движение в продольном направлении (т.е. в направлении распространения волны). Эти волны называются также Р-

волнами, или первичными волнами. Их скорость зависит от модуля упругости и жесткости породы. Вблизи земной поверхности скоростьР -волн составляет 6 км/с, а на очень большой глубине — ок. 13 км/с. Следующими регистрируются поперечные сейсмические волны, называемые такжеS -волнами, или вторичными волнами. При их прохождении каждая частица породы колеблется перпендикулярно направлению распространения волны. Их скорость зависит от сопротивления породы сдвигу и составляет примерно 7/12 от скорости распространенияР- волн.

Оценка по системе Рихтера

В 1935 году ученый Ч. Рихтер предположил, что магнитуда – это энергия сейсмических волн. На основе этого утверждения он разработал особую шкалу, по которой до сих пор проводят оценку сотрясательной активности.

Шкала магнитуд Рихтера характеризует величину энергии, выделяемой во время сейсмологической активности. В ней используется логарифмический масштаб, где каждое значение указывает на толчок в десять раз больше предыдущего. К примеру, если фиксируется землетрясение 4 балла, то явление вызовет в десять раз более сильное колебание, чем магнитуда 3 балла по этой же шкале.

По Рихтеру, сейсмологическая активность измеряется следующим образом:

• 1.0-2.0 – фиксируется приборами;
• 2.0-3.0 – слабые ощущения толчков;
• 3.0 – раскачиваются люстры в домах;
• 4-5 – толчки слабые, но могут вызывать незначительные разрушения;
• 6.0 – толчки, способные вызвать умеренные разрушения;
• 7 – трудно устоять на ногах, по стенам начинают идти трещины, лестничные пролеты могут разрушаться;
• 8.5 – очень сильные землетрясения, способные вызывать изменения рельефа.
• 9 – вызывает цунами, почва сильно трескается.
• 10 – глубина разлома сто и более километров.

Развитие теории

И, наконец, начиная с середины 1960-х годов, сейсмологи добились довольно полного понимания того, как скользящий разлом порождает колебания грунта. Важной величиной, характеризующей прочность разлома, является сейсмический момент – алгебраическое произведение площади разлома, скольжения разлома и жесткости окружающей породы.

Как говорят сейсмологи, землетрясение с большой магнитудой соответствует разлому с большим моментом, причем увеличение на единицу величины соответствует увеличению момента примерно в 30 раз. Но эта связь неточна, есть много случаев, когда небольшие сдвиги вызывают неожиданно большое землетрясение или наоборот.

Землетрясения в истории

Одним из самых сильных землетрясений в мире стала сейсмологическая активность, зафиксированная в 1960 году в Чили. По шкале Рихтера, приборы указали на значительную активность. Тогда чилийцы узнали, что такое магнитуда 8.5 балла. Толчки вызвали цунами с десятиметровой высотой волн.

Через четыре года, в северной части Аляскинского залива, были зафиксированы сотрясания магнитудой 9 баллов. Из-за этой активности плит произошло сильное изменение береговой линии некоторых островов.

Еще одно мощное землетрясение произошло в 2004 году в Индийском океане. По шкале Рихтера ему присвоено 9 баллов. Толчки стали причиной возникновения сильнейшего цунами с высотой волны более пятнадцати метров.

В 2011 году, в Японии, произошло землетрясение, которое стало причиной огромной трагедии: погибли тысячи людей и была разрушена АЭС.

К сожалению, подобные катастрофы не большая редкость. Как предотвратить землетрясения, ученым пока неизвестно.

Сейсмические волны.

Колебания, распространяющиеся из очага землетрясения, представляют собой упругие волны, характер и скорость распространения которых зависят от упругих свойств и плотности пород. К упругим свойствам относятся модуль объемной деформации, характеризующий сопротивление сжатию без изменения формы, и модуль сдвига, определяющий сопротивление усилиям сдвига. Скорость распространения упругих волн увеличивается прямо пропорционально квадратному корню значений параметров упругости и плотности среды.

Амплитуда и период

характеризуют колебательные движения сейсмических волн. Амплитудой называется величина, на которую изменяется положение частицы грунта при прохождении волны по сравнению с предшествовавшим состоянием покоя. Период колебаний — промежуток времени, за который совершается одно полное колебание частицы. Вблизи очага землетрясения наблюдаются колебания с различными периодами – от долей секунды до нескольких секунд. Однако на больших расстояниях от центра (сотни километров) короткопериодные колебания выражены слабее: для Р

-волн характерны периоды от 1 до 10 с, а дляS -волн – немного больше. Периоды поверхностных волн составляют от нескольких секунд до нескольких сотен секунд. Амплитуды колебаний могут быть значительными вблизи очага, однако на расстояниях 1500 км и более они очень малы — менее нескольких микрон для волнР иS и менее 1 см – для поверхностных волн.

Под толщей вод

Причины возникновения землетрясений на дне океана те же, что и на суше — подвижки литосферных плит. Несколько отличаются их последствия для людей. Очень часто смещение океанических плит вызывает цунами. Зародившись над эпицентром, волна постепенно набирает высоту и у берега часто достигает десяти метров, а иногда и пятидесяти.

По статистике, свыше 80 % цунами обрушиваются на берега Тихого океана. Сегодня существует множество служб в сейсмоопасных зонах, трудящихся над прогнозированием возникновения и распространения разрушительных волн и оповещающих население об опасности. Однако человек по-прежнему мало защищен от подобных стихийных бедствий. Примеры землетрясений и цунами начала нашего века – лишнее тому подтверждение.

Местоположение Гипоцентра

Гипоцентристы землетрясений могут находиться на десятки до сотен километров ниже поверхности. По мере увеличения глубины гипоцентра землетрясения скалы вокруг него станут менее хрупкими и более пластичными. Из-за этого в определенный момент камень станет слишком слабым, чтобы землетрясения произошли или были значительными. Сила землетрясения зависит от того, сколько стрессов накапливается на неровностях, прежде чем они сломаются. В результате, если неровности разрушаются или деформируются до того, как могут накопиться большие количества стресса, землетрясение не будет значительным.

Литосфера — это жесткий внешний слой Земли, содержащий кору и части верхней мантии. Поскольку скала относительно хрупкая в литосфере, землетрясения происходят легко. Астеносфера — это область под литосферой. Скала в астеносфере менее хрупкая и более восприимчива к течению. Скала в астеносфере по-прежнему твердая, но пластичная, что она деформируется больше как мокрая глина или глупая замазка, когда к ней прикладывается давление. Так как землетрясения являются результатом хрупких разрывов вдоль разлома, они уменьшаются по частоте, потому что скала становится менее хрупкой и более пластичной по своей деформации по мере увеличения глубины.

География явления

Распределение землетрясений на планете достаточно неравномерно. Определяется оно главным образом взаимодействием и перемещением литосферных плит.

Основной сейсмический пояс, где выделяется около 80% всей сейсмической энергии, находится в Тихом океане. Здесь, в районах глубоководных желобов, происходят подвижки литосферных плит под континент. Остальная часть энергии выделяется в Евроазиатском складчатом поясе. Это происходит в местах столкновения Евроазиатской плиты с Индийской и Африканской плитами, а также в районах срединно-океанических хребтов.

Отражение и преломление.

Встречая на своем пути слои пород с отличающимися свойствами, сейсмические волны отражаются или преломляются подобно тому, как луч света отражается от зеркальной поверхности или преломляется, переходя из воздуха в воду. Любые изменения упругих характеристик или плотности материала на пути распространения сейсмических волн заставляют их преломляться, а при резких изменениях свойств среды часть энергии волн отражается (см

Определение эпицентра

Встряски планеты происходят фактически каждый день, только они настолько минимальны, что человек почти не может их прочувствовать. Поэтому зачастую довольно-таки трудно определить, где именно находится эпицентр землетрясения. Это делается на основании данных, полученных от сейсмографов, установленных на трех различных станциях. Либо эти приборы могут быть использованы в рамках одного научного предприятия.

Точное определение эпицентров – важная задача сейсмографов. Это обусловлено тем, что они время от времени повторяются. Поэтому, чем точнее получены данные, тем лучше можно будет вычислить вероятность следующих колебаний.

Сейсмология

Землетрясения изучает наука сейсмология. В разных странах мира ученые проводят наблюдения за поведением земной коры. В этом им помогают специальные приборы — сейсмографы. Они измеряют и автоматически записывают малейшие сотрясения, происходящие в любой точке земного шара. При колебаниях земной поверхности основная часть сейсмографа — подвесной груз — вследствие инерции приходит в движение относительно основания прибора, и самописец фиксирует сейсмический сигнал, передаваемый маркеру.

Важной задачей сейсмологии является прогноз землетрясений. К сожалению, современная наука еще не может точно их предвидеть. Сейсмологи могут более-менее достоверно определить район и силу землетрясения, но его начало спрогнозировать очень сложно.

You may not always feel the earth shaking, but California has earthquakes occurring all the time. Seismographic networks measure earthquakes by their magnitude, energy release and intensity.

Years ago, all magnitude scales were based on the recorded waveform lengths or the length of a seismic wave from one peak to the next. But for very large earthquakes, some magnitudes underestimated the true earthquake size. Now, scientists use earthquake measurements that describe the physical effects of an earthquake rather than measurements based only on the height of a waveform recording.

How are earthquakes recorded & detected?

When the Earth trembles, earthquakes spread energy in the form of seismic waves. A seismograph is the primary earthquake measuring instrument. The seismograph produces a digital graphic recording of the ground motion caused by the seismic waves. The digital recording is called a seismogram.

A network of worldwide seismographs detects and measures the strength and duration of the earthquake’s waves. The seismograph produces a digital graphic plotting of the ground motion of the event.

How is earthquake magnitude measured?

An earthquake has one magnitude unit. The magnitude does not depend on the location where measurement is made. Since 1970, the Moment Magnitude Scale has been used because it supports earthquake detection all over the Earth.

Earthquake Measurements

To get a better idea of the strength of the shaking and damage, the Moment Magnitude Scale was developed to capture all the different seismic waves from an earthquake to worldwide seismic networks.

Earthquake intensity scales describe the severity of an earthquake’s effects on the Earth’s surface, humans, and buildings at different locations in the area of the epicenter. There can be multiple intensity measurements. The Modified Mercalli Scale measures the amount of shaking at a particular location.

Earthquake Magnitude Scale

From 1935 until 1970, the earthquake magnitude scale was the Richter scale, a mathematical formula invented by Caltech seismologist Charles Richter to compare quake sizes.

The Richter Scale was replaced because it worked largely for earthquakes in Southern California, and only those occurring within about 370 miles of seismometers. In addition, the scale was calculated for only one type of earthquake wave. It was replaced with the Moment Magnitude Scale, which records all the different seismic waves from an earthquake to seismographs across the world.

Richter’s equations are still used for forecasting future earthquakes and calculating earthquake hazards.

Today, earthquake magnitude measurement is based on the Moment Magnitude Scale (MMS). MMS measures the movement of rock along the fault. It accurately measures larger earthquakes, which can last for minutes, affect a much larger area, and cause more damage.

The Moment Magnitude can measure the local Richter magnitude (ML), body wave magnitude (Mb), surface wave magnitude (Ms).

Earthquake Magnitude Classes

Earthquakes are also classified in categories ranging from minor to great, depending on their magnitude. What’s the difference between a light and moderate quake?

These terms are magnitude classes. Classes also provide earthquake measurement. The classification starts with “minor” for magnitudes between 3.0 and 3.9, where earthquakes generally begin to be felt, and ends with “great” for magnitudes greater than 8.0, where significant damage is expected.

How is earthquake intensity measured?

A second way earthquakes are measured is by their intensity. Earthquake Intensity measurement is an on-the-ground description. The measurement explains the severity of earthquake shaking and its effects on people and their environment. Intensity measurements will differ depending on each location’s nearness to the epicenter. There can be multiple intensity measurements as opposed to one magnitude measurement.

The Modified Mercalli Scale

The Modified Mercalli (MM) Intensity Scale is used in the United States. Based on Giuseppe Mercalli’s Mercalli intensity scale of 1902, the modified 1931 scale is composed of increasing levels of intensity that range from observable quake impacts from light shaking to catastrophic destruction. Intensity is reported by Roman numerals.

An earthquake intensity scale consists of a series of key responses that includes people awakening, movement of furniture, damage to chimneys and total destruction.

How to prepare for a high magnitude quake

An earthquake is a sudden, rapid shaking of the earth caused by the shifting of rock beneath the earth’s surface. The size of an earthquake and the energy released by an earthquake will affect how much you feel it. Major earthquakes strike without warning, at any time of year, day or night.

Prepare before the next big one:

• Create an earthquake safety plan. Discuss with your family what to do, where to meet if separated, and how you will communicate when an earthquake strikes. Check work, childcare and school emergency plans.
• Practice DROP, COVER, and HOLD ON with all members of your household.
• Don’t rely on doorways for protection. During an earthquake, get under a sturdy table or desk. Hold on until shaking stops.
• Pick safe places in each room of your home.
• Create an emergency survival kit that provides you and your pets with three days of nonperishable food and water, medicines, emergency radio and first aid materials. Keep in a reachable place.
• Identify an out-of-the-area friend or relative that family members can check in by mobile texting.
• Find out if your home is in need of earthquake retrofitting and eligible for a grant.
• Identify and fix potential earthquake hazards in your home.
• Protect your home investment and bounce back from a devastating earthquake with the best choices of earthquake insurance from CEA.

Whether you are a homeowner, mobilehome owner, condo-unit owner or renter, buy peace of mind with affordable and flexible earthquake insurance now.

Understanding Geologic & Structural Risks

Every day, Californians face earthquake danger. Our state has nearly 16,000 known faults and more than 500 active faults. Most of us live within 30 miles of an active fault risk.

Visit the CEA risk map for each county to learn about faults in your area. This information will help you survive an earthquake.

Then learn about your home’s structural risks, the steps you can take to seismically strengthen your house and the benefits of retrofitting. Make your home more resistant to earthquake damage by assessing its structure, contents and need for loss of use earthquake insurance.

Avoid financial disaster with loss of use coverage if your house sufferers extensive damage—get coverage with a CEA earthquake policy.

Personal Preparedness Guidelines

Get started today on preparing for a major earthquake. Top earthquake survival tips include:

• Create an earthquake safety plan for you and loved ones, including pets.
• Identify safe places in each room of your home.
• Practice Drop, Cover, and Hold On with each member of your household.
• Make or purchase an earthquake safety kit.
• Find out if your home is in need of earthquake retrofitting and eligible for a grant.
• Identify and fix potential earthquake hazards in your home.
• Protect your home investment and bounce back from a devastating earthquake with the best choices of earthquake insurance from CEA.

Is your house at risk for earthquake damage?

Preparing your home BEFORE an earthquake is important to your safety. Decrease your risk of damage and injury from an earthquake by identifying possible home hazards.

A seismic retrofit by strengthening your home’s foundation makes it more resistant to shaking. CEA offers earthquake home insurance premium discounts for houses and mobilehomes that have been properly retrofitted. Find out about grants to help for retrofits under the Earthquake Brace + Bolt Program, and the CEA Brace + Bolt program.

Is earthquake insurance worth it?

While it is wise to be prepared physically when the ground shakes, it’s also important to be financially protected. Without earthquake insurance, you place yourself at risk of losing everything or sustaining damages to your personal property that you cannot afford to repair or replace.

• Pay your mortgage for a house that may need to be rebuilt?
• Cover the costs bill for temporary accommodations?
• Repair or replace your personal belongings?

CEA earthquake insurance not only helps repair damages. Loss of use coverage covers the costs of temporary shelter and additional living expenses so that families can get back on their feet quicker.

Get a Free Earthquake Insurance Estimate!

Contact your home insurance agent today to discuss adding a separate earthquake policy to your home insurance. You can add the coverage now, no need to wait until your home policy comes up for renewal. For the best choice of CEA earthquake insurance policies, select deductibles from 5%-25%.

We work with 25 residential insurance companies that serve the majority of California homeowners. Get a free estimate now!