Measuring an Earthquake

Geology Department:

An earthquake is quantified in terms of intensity and magnitude.

Intensity varies depending on the location of the observer. It is based on the result of the earthquake on both built and natural structures. It quantifies the effects of an earthquake in a particular place. If you are near to an earthquake you will feel more than if you are far from it.

Magnitude is a single measured value for the earthquake as a whole. It quantifies the amount of energy released at the hypocenter (where the break that caused the earthquake occurred). It is calculated from measurements taken by calibrated instruments.

Earthquakes result from forces deep within the Earth. The energy released in an earthquake was energy stored various ways within the rocks before the earthquake. It can be suddenly released, for example by shearing movements along faults in the crust. This is called the hypocenter or focus of the earthquake; the epicenter is the point on the Earth's surface above the focus.

## The Richter Magnitude Scale

What we see and feel in an earthquake are seismic waves (vibrations) traveling through the Earth. They are recorded on seismographs. These draw an oscillating line that varies in amplitude with that of the vibrations in the ground. Sensitive seismographs can detect large earthquakes from very distant places in the planet.

The Richter magnitude scale was developed as a way to compare the magnitudes of earthquakes. This is calculated from the logarithm of the amplitude of waves recorded by seismographs. Adjustments are made to compensate for the distance between the seismographs and the epicenter. The magnitude is expressed in whole numbers with decimal fractions. 5.4 might be a moderate earthquake; and a magnitude 6.2 might be a strong earthquake. Because of the logarithmic scale, each whole number increase represents a tenfold increase in the strenght measured. Each whole number step up in the magnitude scale corresponds to the release of about 31 times more energy.

Microearthquakes, which will probably not be felt by people, would measure less than two. Thousands of larger, though still minor, earthquakes occur each year. Major earthquakes have magnitudes of 8.0 or more, and occur somewhere about once a year. The Richter Scale has no upper limit but, the largest known earthquakes have had magnitudes of about 8.8.

## The Modified Mercalli Intensity Scale

The effect of an earthquake on the Earth's surface is called it's intensity. The intensity scale is constructed from a set of key indicators such as people awakening, movement of furniture, damage to chimneys. Although there have been many intensity scales over the last several hundred years proposed to evaluate the effects of earthquakes, the one currently most in use is the Modified Mercalli (MM) Intensity Scale. It was developed in 1931 by the Harry Wood and Frank Neumann. This scale comprises of 12 levels of increasing intensity that range from imperceptible shaking to catastrophic destruction and these are designated by Roman numerals. Note that these numbers do not have a mathematical basis, they are instead an arbitrary ranking based on observed effects.

The Modified Mercalli Intensity value given to an earthquake at a specific place is a more meaningful measure of severity as experienced by an observer at that place than the overall magnitude. This is because intensity refers to the effects at that place, while magnitude refers to the whole earthquake. After the occurrence of widely-felt earthquakes, people in the disturbed area are canvassed for their experience so that intensity values can be assigned. The results and information furnished by other sources (such as surveying the resultant damage)are used to assign an intensity value. Then isoseismal maps are compiled that show the extent of various levels of intensity experienced in the earthquake area. The maximum intensity will generally occur near the epicenter.

The lowest numbers of the scale mainly deal with the way the earthquake is felt by people. The higher numbers of the scale are based on structural damage. Structural engineers usually contribute information for values of Vlll or above.

The following is an abbreviated description of the 12 levels of Modified Mercalli intensity.

I. Not apparent to people, can only by measured by instruments

II. Can be apparent to people in quiet situations but only just. Some suspended light objects might swing slightly.

III. Noticeable (especially on top of high buildings) as a vibration like that of a passing heavy vehicle.

IV. Some things like windows, doors, dishes and loose ornaments rattle, Walls creak and stationary cars rock.

V. Felt by everyone and many people are woken up at night. Windows break and some free standing objects fall over.

Vl. Heavy furniture moves, trees sway, weak structures crack slightly and some plaster falls but mostly damage is slight.

Vll. Well constructed buildings may crack but there can be considerable damage in poor constructions particularly at vunerable points like chimneys.

Vlll. There is considerable damage in ordinary buildings. Some walls as well as chimneys collapse. There is considerable danger for people in poorly built or older structures.

IX. The ground begins to crack, pipes break, houses begin to collapse and substantial buildings can be moved off their foundations.

X.The ground cracks, there may be landslides, most buildings are destroyed. Even specially designed buildings are damaged. Railway lines are bent.

Xl. Few structures remain standing, bridges and railways are destroyed. Electricity, water and telephone networks fail.

Xll. The damage is total. The ground is heavily distorted and there is violent heaving and shaking. Substantial objects are thrown into the air.