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). M L Richter Magnitude S-waves (no units) m b Body wave magnitude P-waves “ M S Surface wave magnitude Rayleigh waves “ M o Seismic moment (units of Newton-meters) = (shear modulus * area * slip) M W 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. MMIpeak velocity acceleration (cm/sec ) near epicenter ( fraction of g) I-IV g V g VI g VII g VIII g IX g X g XI.-XII. >100>1g

Richter Magnitude Nomogram

1994 Northridge CA Earthquake

VII VI

13Aug1886: Charleston, SC M w =7.5

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

Similar Magnitude Eqs but Totally different Isoseismals Due to differences in lithosphere structure & rigidity New York City Earthquake of August 10, 1884

Earthquake m b M S FSArea offset M o M w (km) (m) (N-m) Loma Prieta x e San Francisco x e Alaska x e Chile x e Moment Max MM# EQEQ Name & Effects Magnitude Intensityper year 1-1.9I ~3,000,000 Tiny- recorded if instrument is close 2-2.9II~350,000Very Minor- Generally not felt indoors, but recorded III~50,000Minor- Generally not felt, but recorded IV-V ~6,000 Light- Often felt, but rarely causes damage VI-VII~800Moderate- Slight to major damage to buildings VII-VIII~300Strong- Can be destructive within ~80 km IX-X~20Major- Can cause serious damage. >8 XI-XII~1Great- Can cause very serious damage. The Moment Magnitude scale is not a simple logarithmic scale (factor of ten) with respect to energy- M W Energy Ratio M w 6 EQ = the energy of ~2050 * M w 4 EQ