IV. Measuring the size of Earthquakes Intensity vs. Magnitude

1. Earthquakes 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

2. 2) Earthquake Intensity and magnitude • Mercalli intensity scale • Magnitude Intensity of shaking & damage at a specific location Depends on distance to earthquake & strength of earthquake A measure of the energy released in an earthquake Depends on size of fault that breaks

3. Earthquakes • IV. Measuring the size of Earthquakes • Intensity vs. Magnitude • 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

4. Modified Mercalli Intensity Scale • 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.

5. 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.

6. Earthquakes • IV. Measuring the size of Earthquakes • Intensity vs. Magnitude • 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

7. 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)

8. 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.

9. Earthquakes 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 mm M2 = amplitude of 0.01 mm M3 = amplitude of 0.1 mm M4 = amplitude of 1 mm M5 = amplitude of 10 mm M6 = amplitude of 10 cm M7 = amplitude of 1 meter!

10. 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!

11. 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

12. Earthquakes 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!

13. Earthquake Magnitude 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

14. Earthquakes • IV. Measuring the size of Earthquakes • 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

15. 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) The shaded regions on the fault surface are the areas that rupture during different size events M0 = A x D x μ Seismic moment (M0) = fault area x displacement x strength of rock

16. Earthquakes 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

17. Most Destructive Known Earthquakes on Record in the World (> 50,000 deaths) (Listed in order of greatest number of deaths) Date Location Deaths M Comments January 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* 8.0 August 9, 1138 Syria, Aleppo 230,000 May 22, 1927 China, near Xining 200,000 8.3 Large fractures. December 22, 856+ Iran, Damghan 200,000 December 16, 1920 China, Gansu 200,000 8.6 Major fractures, landslides. March 23, 893+ Iran, Ardabil 150,000 September 1, 1923 Japan, Kwanto 143,000 8.3 Great Tokyo fire December 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 80,000 November 18, 1727 Iran, Tabriz 77,000 November 1, 1755 Portugal, Lisbon 70,000 8.7 Great tsunami December 25, 1932 China, Gansu 70,000 7.6 May 31, 1970 Peru 66,000 7.8 Great rock slide and flood 1268 Asia Minor, Silicia 60,000 January 11, 1693 Italy, Sicily 60,000 May 30, 1935 Pakistan, Quetta 30,000 7.5 Quetta almost completely destroyed (~60,000) February 4, 1783 Italy, Calabria 50,000 June 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)