IV. Measuring the size of Earthquakes

Intensity vs. Magnitude

A. Intensity: A measure of the degree of earthquake shaking at a given locale based on the damage to human structures, extent of ground rupture, and human/animal reactions

Earthquakes

2) Earthquake Intensity and magnitude

Mercalli intensity scale

Magnitude

Intensity of shaking & damage at a specific location

A measure of the energy released in an earthquake

Depends on distance to earthquake& strength of earthquake

Depends on size of fault that breaks

IV. Measuring the size of Earthquakes

Intensity vs. Magnitude

A. Intensity

1. Modified Mercalli Intensity Scale: after volcanologist Guiseppe Mercalli who developed the original form in 1902

12 levels of intensity, I, lowest to XII. highest

Compares historical EQ’s, pre-instrumental time

Earthquakes

I. Not felt except by a very few under especially favorable conditions.

II. Felt only by a few persons at rest, especially on upper floors of buildings.

III. 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. 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. Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop.

VI. Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.

Modified Mercalli Intensity Scale

VII. 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. 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. 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. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent.

XI. Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly.

XII. Damage total. Lines of sight and level are distorted. Objects thrown into the air.

IV. Measuring the size of Earthquakes

Intensity vs. Magnitude

A. Intensity

1. Modified Mercalli Intensity Scale:

Problems: cannot find epicenter exactly

EQ either far away & strong or close & weak

Intensity of shaking depends on rock type

Damage: depends on quality of building construction

Can’t be used in unpopulated areas or oceans

Earthquakes

IV. Measuring the size of Earthquakes

Intensity vs. Magnitude

B. Magnitude: more quantitative, based on ground motion/amount of energy released

1. Richter Scale

1935 Charles Richter, Caltech

Based on the amplitude of the largest seismic wave recorded (P, S, or surface)

Earthquakes

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 measures the largest wiggle on the seismogram; other magnitude scales measure different parts of the earthquake.

Intensity is a measure of the shaking and damage caused by the earthquake, and this value changes from location to location.

IV. Measuring the size of Earthquakes

1. Richter Scale

a. Vary so much in strength: nonlinear scales

Based on logrithmic scale—

each unit corresponds to 10 fold increase in amplitude

At a distance of 100 kilometers:

M1 = amplitude of 0.001 mmM2 = amplitude of 0.01 mmM3 = amplitude of 0.1 mmM4 = amplitude of 1 mmM5 = amplitude of 10 mmM6 = amplitude of 10 cmM7 = amplitude of 1 meter!

Earthquakes

IV. Measuring the size of Earthquakes

1. Richter Scale

Magnitude 6 is 10 times larger than a M5 or

Magnitude 7 is 100 times larger than M5

b. Each magnitude unit corresponds to 32 fold increase in energy

Magnitude 6 released 32 times more energy than a M5 or

Magnitude 7 releases >1000 times more energy than a M5!

Earthquakes

Richter Magnitude How many kilograms of TNT would have this much energy?

0 0.6

1.0 20 2.0 600 Smallest EQ people can normally feel

3.0 20 000 Most people near epicenter feel the quake, Nearly 100, 000 occur every year of size 2.5 - 3.0

4.0 60 000 A small fission atomic bomb; EQ’s above 4.5 can cause local damage

5.0 20 000 00 A standard fission bomb, similar to the first bomb tested

in New Mexico, U.S.

6.0 60 000 000 A hydrogen bomb; can cause great damage locally, About 100 shallow EQs of size 6.0 every year

7.0 20 billion Major earthquake; about 14 every year. Enough energy to heat New York City for 1 year. Large enough to

be detected all over globe

8.0 60 billion Largest known: 8.9 in Japan and in Chile/Ecuador; San Francisco destroyed by 7.8 in 1906

9.0 20 trillion Roughly the world’s energy usage in a year, Chile, 9.5 in 1960 and Alaska, 9.2 in 1964

IV. Measuring the size of Earthquakes

Comparison: Southern California has ≥ M 8 every ~160 years

Last large EQ in 1857 = 146 years ago

So does the occasional M6 release energy to alleviate this problem?

Energy released in M8 is 1000 times greater than mag 6

Thus, LA needs 1000 M6’s to equal an M8 (LA has a M6 every ~5 years)

So LA can expect a seismic event 100 times as violent as any in recent memory!

Earthquakes

Earthquakes release a tremendous amount of energy, which is why they can be so destructive.

The table below shows magnitudes with the approximate amount of TNT needed to release the same amount of energy.

Magnitude Approximate Equivalent TNT Energy 4.0 1,010 tons 5.0 3,1800 tons 6.0 1,010,000 tons 7.0 31,800,000 tons 8.0 1,010,000,000 tons 9.0 31,800,000,000 tons

Earthquake Magnitude

IV. Measuring the size of Earthquakes

1. Richter Scale

c. Problems:

Limited to M7 or smaller M8 wave amplitude is 10 meters =

too big to record on seismometer

Developed for California rocks—not necessarily good everywhere

Amount of shaking is partially due to rock type, so while two EQs may shake the same, they may actually release different amounts of energy

IV. Measuring the size of Earthquakes

1. Richter Scale

c. Problems:

Limited to M7 or smaller M8 wave amplitude is 10 meters =

too big to record on seismometer

Developed for California rocks—not necessarily good everywhere

Amount of shaking is partially due to rock type, so while two EQs may shake the same, they may actually release different amounts of energy

Earthquakes

IV. Measuring the size of Earthquakes

2. Moment-Magnitude Scale—the newest scale—used today by seismologists—measures the EQ total energy (measures the “cause” not the “effect”)

Seismic Moment = (fault rupture area or L x W)

X (total amount of slip along the fault)

X (strength of rock) (the force that was required to overcome the friction sticking)

Earthquakes

The shaded regions on the fault surface are the areas that rupture during different size events

Seismic moment (M0) = fault area x displacement x strength of rock

M0 = A x D x μ

IV. Measuring the size of Earthquakes

2. Moment-Magnitude Scale

Calibrated so small and moderate quakes equal Richter scales, but moment-magnitude much better for large quakes

A more direct measure of EQ

Data are taken directly from the physical fault that produced the quake

Reflects the energy released in a EQ

Strongest recorded EQ, 1960 Chilean quake with Moment Magnitude of 9.5

Earthquakes

Most Destructive Known Earthquakes on Record in the World (> 50,000 deaths) (Listed in order of greatest number of deaths)

Date Location Deaths M CommentsJanuary 23, 1556 China, Shansi 830,000 October 11, 1737 India, Calcutta** 300,000 2004, Sumatra >300,000 people, M = 9.3 July 27, 1976 China, Tangshan 255,000* 8.0August 9, 1138 Syria, Aleppo 230,000May 22, 1927 China, near Xining 200,000 8.3 Large fractures.December 22, 856+ Iran, Damghan 200,000December 16, 1920 China, Gansu 200,000 8.6 Major fractures, landslides.March 23, 893+ Iran, Ardabil 150,000September 1, 1923 Japan, Kwanto 143,000 8.3 Great Tokyo fireDecember 28, 1908 Italy, Messina 70,000 7.5 Deaths from to 100,000 earthquake & tsunami. September, 1290 China, Chihli 100,000 November, 1667 Caucasia, Shemakha 80,000November 18, 1727 Iran, Tabriz 77,000November 1, 1755 Portugal, Lisbon 70,000 8.7 Great tsunami December 25, 1932 China, Gansu 70,000 7.6May 31, 1970 Peru 66,000 7.8 Great rock slide and flood1268 Asia Minor, Silicia 60,000January 11, 1693 Italy, Sicily 60,000May 30, 1935 Pakistan, Quetta 30,000 7.5 Quetta almost completely destroyed (~60,000)February 4, 1783 Italy, Calabria 50,000June 20, 1990 Iran 50,000 7.7 Landslides.* Official casualty figure--estimated death toll as high as 655,000.+ Note that these dates are prior to 1000 AD. No digit is missing.** Later research has shown that this was a typhoon, not an earthquake. (1737 Calcutta Earthquake Bilham, 1994)