Seismic Intensity: a measure of violence of ground shaking(based on damage done to human-made structures, surface changes, and felt reports).Intensity varies with distance from the hypocenter and ground conditions.Isoseismals-lines of equal seismic intensity.Modified Mercalli Intensity Scale: I-XII (no units)_________________________________

Seismic Magnitude - a unique measure of the energy released or size(based on logarithm of maximum body waves or surface wave amplitude on seismographs).ML Richter Magnitude S-waves (no units)mb Body wave magnitude P-waves “MS Surface wave magnitude Rayleigh waves “Mo Seismic moment (units of Newton-meters) = (shear modulus * area * slip)MW Moment Magnitude (no units)

Modified Mercalli Intensity Scale (1902) [MMI-Unitless]

I. People generally do not feel any shaking. II. A few sensitive people might feel shaking. III. Many people feel shaking; hanging objects sway.IV. Most people indoors feel shaking; hanging objects swing; dishes rattle. V. Almost everyone feels shaking; doors swing; dishes break; pictures move;

no structural damage.VI. Everyone feels shaking; objects fall from shelves; pictures fall off walls; furniture moves. Plaster in walls might crack.VII. People have difficulty standing. Drivers feel their cars shaking.

Some furniture breaks; bricks fall from buildings; buildings damaged.VIII. Houses not bolted down may shift on their foundations;

towers and chimneys twist and fall; buildings damaged.IX. Well-built buildings suffer considerable damage; pipes break. X. Most buildings & foundations destroyed; bridges destroyed;

dams damaged; large landslides.XI. Most buildings collapse; bridges destroyed; large cracks in ground;

pipelines destroyed; railroad tracks bent.XII. Almost everything totally destroyed; objects thrown into air;

ground moves in waves or ripples.

MMI peak velocity acceleration (cm/sec ) near epicenter

( fraction of g)

I-IV. 1-2 0.015-0.02gV. 2-5 0.03-0.05gVI. 5-8 0.06-0.09gVII. 8-12 0.1-0.19gVIII. 20-30 0.2-0.3gIX. 50 0.3-0.5gX. 60-80 0.6-0.9gXI.-XII. >100 >1g

Richter Magnitude Nomogram

1994 Northridge CA Earthquake

VII

VI

13Aug1886: Charleston, SC Mw=7.5

Do sleeping pets sense earthquakes?It depends on their size…

Similar Magnitude Eqs butTotally different IsoseismalsDue to differences in lithospherestructure & rigidity

New York City Earthquake of August 10, 1884

Earthquake mb MS FSArea offset Mo Mw

(km) (m) (N-m) Loma Prieta 1989 6.2 7.1 40x15 1.7 3.0e19 6.9San Francisco 1906 -- 7.8 450x10 4 5.4e20 7.8Alaska 1964 6.2 8.4 500x300 7 5.2e22 9.1Chile 1960 -- 8.3 800x200 21 2.4e23 9.5

Moment Max MM # EQ EQ Name & EffectsMagnitude Intensity per year1-1.9 I ~3,000,000 Tiny-recorded if instrument is close

2-2.9 II ~350,000 Very Minor-Generally not felt indoors, but recorded. 3-3.9 III ~50,000 Minor-Generally not felt, but recorded. 4-4.9 IV-V ~6,000 Light-Often felt, but rarely causes damage. 5-5.9 VI-VII ~800 Moderate-Slight to major damage to buildings. 6-6.9 VII-VIII ~300 Strong-Can be destructive within ~80 km.7-7.9 IX-X ~20 Major-Can cause serious damage. >8 XI-XII ~1 Great-Can cause very serious damage.

The Moment Magnitude scale is not a simple logarithmic scale (factor of ten) with respect to energy-

MW 4---------5--------6--------7--------8Energy Ratio 48 43 39 35

Mw6 EQ = the energy of ~2050 * Mw4 EQ