Magnitude, Intensity, & Energy MUSE 11B

Who’s That? How did this get here?

Modified Mercalli Intensity Intensity = effect of an EQ on the Earth's surface Numerous intensity scales have been developed over the last several hundred years, but the one currently used in U.S. is the Modified Mercalli (MM) Intensity Scale. Developed in 1931 by the American seismologists Harry Wood and Frank Neumann, based on scale developed in 1902 by Italian seismologist Giuseppe Mercalli. Scale is composed of 12 increasing levels of intensity that range from imperceptible shaking to catastrophic destruction. Roman numerals. It does not have a mathematical basis; instead it is an arbitrary ranking based on observed effects. MMI assigned to a specific site after an EQ has a more meaningful measure of severity to the nonscientist than the magnitude because intensity refers to the effects actually experienced at that place.

Modified Mercalli Intensity 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. 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.

MMI Activity Using the handout, develop an isoseismal map

Isoseismal Map

Frequency of Occurrence of EQ DescriptorRichter Magnitudes Earthquake EffectsFrequency of Occurrence MicroLess than 2.0Microearthquakes, not felt.About 8,000 per day Very minor Generally not felt, but recorded.About 1,000 per day Minor Often felt, but rarely causes damage.49,000 per year (est.) Light Noticeable shaking of indoor items, rattling noises. Significant damage unlikely. 6,200 per year (est.) Moderate Can cause major damage to poorly constructed buildings over small regions. At most slight damage to well-designed buildings. 800 per year Strong Can be destructive in areas up to about 100 miles across in populated areas. 120 per year Major Can cause serious damage over larger areas.18 per year Great Can cause serious damage in areas several hundred miles across. 1 per year Rare great9.0 or greaterDevastating in areas several thousand miles across. 1 per 20 years

Richter Magnitude Approximate TNT for Seismic Energy Yield Example kg (12.4 lb)Hand grenade kg (70 lb)Construction site blast kg (392 lb)WWII conventional bombs 2.01 metric tonlate WWII conventional bombs metric tonsWWII blockbuster bomb metric tonsMassive Ordnance Air Blast bomb metric tonsChernobyl nuclear disaster, kilotonSmall atomic bomb kilotonsAverage tornado (total energy) kilotonNagasaki atomic bomb kilotonsLittle Skull Mtn., NV Quake, megatonDouble Spring Flat, NV Quake, megatonsNorthridge quake, megatonsLargest thermonuclear weapon megatonsLanders, CA Quake, gigatonSan Francisco, CA Quake, gigatonsAnchorage, AK Quake, gigatons2004 Indian Ocean earthquake teratonestimate for a 100 km rocky bolide impacting at 25 km/s

How Richter magnitude (M L ) was measured M L = log 10 of the maximum ground motion (in millimeters) recorded on a Wood-Anderson short-period seismometer 100 km from the earthquake

Source Study of the 1906 San Francisco Earthquake by David J. Wald David J. Wald, Hiroo Kanamori and Donald V. HelmbergerDavid J. Wald Bull. Seism. Soc. Am., 83, , 1993

Other magnitude scales MagnitudeSymbolWave Local (Richter)MLML S or Surface Wave* Body-Wavembmb P Surface-WaveMsMs Rayleigh MomentMwMw Rupture Area, Slip

Moment Magnitude - M w M w = (2/3)log 10 M o – 10.7 M o = Seismic Moment M o = μAu o μ = shear modulus (typically 30 x 10 9 N/m 2 or 30 x dyne/cm 2 ) o A = area of fault rupture o u =average displacement along fault

Radiated Seismic Energy = E s Conservation of Energy Total energy before = Total energy after P.E. = E s + crushing of rocks + heat P.E. built up from strain in rocks as two sides of faults move past each other E s = radiated seismic energy

Radiated Seismic Energy = E s E s = M o (1.6 x ) where Es is measured in ergs and Mo in dyne-cm E s = M o (1.6 x ) Note: 1 kilowatt hour = 3.6 x ergs typical house 15 KW hours

From Kramer, S.L. (1996). Geotechnical Earthquake Engineering, Prentice Hall, Inc., Upper Saddle River, New Jersey

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