A seismic shadow zone is an area of the Earth‘s surface where seismographs cannot detect an earthquake after its seismic waves have passed through the Earth. When an earthquake occurs, seismic waves radiate out spherically from the earthquake’sfocus. The primary seismic waves are refracted by the liquid outer core of the Earth and are not detected between 104° and 140° (between approximately 11,570 and 15,570 km or 7,190 and 9,670 mi) from the epicenter. The secondary seismic wavescannot pass through the liquid outer core and are not detected more than 104° (approximately 11,570 km or 7,190 mi) from theepicenter.P waves that have been converted to s-waves on leaving the outer core may be detected beyond 104 degrees.
The Mercalli intensity scale is a seismic scale used for measuring the intensity of an earthquake. It measures the effects of an earthquake, and is distinct from the moment magnitude 
usually reported for an earthquake (sometimes misreported as theRichter magnitude), which is a measure of the energy released. The intensity of an earthquake is not entirely determined by its magnitude. It is not based on first physical principles, but is, instead, empirically based on observed effects.[1]
The Mercalli scale quantifies the effects of an earthquake on the Earth’s surface, humans, objects of nature, and man-made structures on a scale from I (not felt) to XII (total destruction).[2][3] Values depend upon the distance from the earthquake, with the highest intensities being around the epicentral area. Data gathered from people who have experienced the quake are used to determine an intensity value for their location. The Italian volcanologist Giuseppe Mercalli revised the widely used simple ten-degreeRossi–Forel scale between 1884 and 1906, creating the Mercalli Intensity scale which is still used nowadays.
In 1902, the ten-degree Mercalli scale was expanded to twelve degrees by Italian physicist Adolfo Cancani. It was later completely re-written by the German geophysicist August Heinrich Sieberg and became known as the Mercalli–Cancani–Sieberg (MCS) scale.
The Mercalli–Cancani–Sieberg scale was later modified and published in English by Harry O. Wood and Frank Neumann in 1931 as the Mercalli–Wood–Neumann (MWN) scale. It was later improved by Charles Richter, the father of the Richter magnitude scale.
The scale is known today as the Modified Mercalli scale (MM) or Modified Mercalli Intensity scale (MMI).
| I. Not felt | Not felt except by a very few under especially favorable conditions. |
|---|---|
| II. Weak | Felt only by a few persons at rest, especially on upper floors of buildings. |
| III. Weak | Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibrations similar to the passing of a truck. Duration estimated. |
| IV. Light | Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably. |
| V. Moderate | Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop. |
| VI. Strong | Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight. |
| VII. Very Strong | Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken. |
| VIII. Severe | Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned. |
| IX. Violent | Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations. |
| X. Extreme | Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent. |
| XI. Extreme | Few, if any, (masonry) structures remain standing. Bridges destroyed. Broad fissures in ground. Underground pipe lines completely out of service. Earth slumps and land slips in soft ground. Rails bent greatly. |
| XII. Extreme | Damage total. Waves seen on ground surfaces. Lines of sight and level distorted. Objects thrown upward into the air. |
The Intensity Scale differs from the Richter Magnitude Scale in that the effects of any one earthquake vary greatly from place to place, so there may be many Intensity values (e.g.: IV, VII) measured for the same earthquake. Each earthquake, on the other hand, should have only one Magnitude, although the various methods of calculating it may give slightly different values (e.g.: 4.5, 4.6).
The Richter Scale is designed to allow easier comparison of earthquake magnitudes, regardless of the location.
C.F.Richter was a geologist living and working in California, U.S.A, an area subjected to hundreds of ‘quakes every year. He took the existing Mercalli scale and tried to add a ‘scientific’ scale based on accurate measurements that could be recorded by seismographs ( instruments used to measure vibration) regardless of their global location.
By measuring the speed, or acceleration, of the ground when it suddenly moves, he devised a scale that reflects the ‘magnitude’ of the shock.
The Richter scale for earthquake measurements is logarithmic. This means that each whole number step represents a ten-fold increase in measured amplitude. Thus, a magnitude 7 earthquake is 10 times larger than a 6, 100 times larger than a magnitude 5 and 1000 times as large as a 4 magnitude.
This is an open ended scale since it is based on measurements not descriptions.
An earthquake detected only by very sensitive people registers as 3.5 on his scale, whilst the worst earthquake ever recorded reached 8.9 on the ‘Richter Scale’.
The Richter magnitude scale (also Richter scale) assigns a magnitude number to quantify the energy released by an earthquake. The Richter scale, developed in the 1930s, is a base-10 logarithmic scale, which defines magnitude as the logarithm of the ratio of the amplitude of the seismic waves to an arbitrary, minor amplitude.
As measured with a seismometer, an earthquake that registers 5.0 on the Richter scale has a shaking amplitude 10 times that of an earthquake that registered 4.0, and thus corresponds to a release of energy 31.6 times that released by the lesser earthquake.[1] The Richter scale was succeeded in the 1970s by the moment magnitude scale. This is now the scale used by the United States Geological Survey to estimate magnitudes for all modern large earthquakes
The Mercalli scale isn’t considered as scientific as the Richter scale, though. Some witnesses of the earthquake might exaggerate just how bad things were during the earthquake and you may not find two witnesses who agree on what happened; everybody will say something different. The amount of damage caused by the earthquake may not accurately record how strong it was either.