Strong ground motion (Engineering Seismology). Earthquake shaking capable of causing damage to structures. The release of the accumulated elastic strain energy by the sudden rupture of the fault is the cause of the earthquake shaking.
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Earthquake shaking capable of causing damage to structures
The release of the accumulated elastic strain energy by the sudden rupture of the fault is the cause of the earthquake shaking
P wave arrives before S wave. S-Trigger time = 3.2 sec, hypocentral distance between approx. 5*3.2= 16 km and 8*3.2= 26 km
P-motion much higher frequency than S, and predominately on vertical component.
Is the horizontal S-wave motion polarized?
Elastic response spectra (many structures can be idealized as SDOF oscillators)
convert displacement spectrum into acceleration spectrum (multiply by (2/T)2). For velocity spectrum, multiply by 2π/T.
Acceleration or velocity spectra usually used in engineering
I Barely felt
II Felt by only few people
III Felt noticeably, standing autos rock slightly
IV Felt by many, windows and walls creak
V Felt by nearly everyone, some dished and windows broken
VI Felt by all, damaged plaster and chimneys
VII Damage to poorly constructed buildings
VIII Collapse of poorly constructed buildings,
slight damage to well built structures
IX Considerable damage to well constructed buildings,
buildings shifted off foundations
X Damage to well built wooden structures, some masonry
buildings destroyed, train rails bent, landslides
XI Few masonry structure remain standing, bridges
destroyed, ground fissures
XII Damage total
Example of observed directivity effects in the M7.3 Landers earthquake ground motions near the fault.Directivity played a key role in the recent San Simeon, CA, earthquake
Cracking in river levee
Ground motion generally decreases with increasing epicentral distance
Seismic waves lose amplitude with distance traveled - attenuation
So the amplitude of the waves depends on distance from the earthquake. Therefore unlike magnitude, intensity is not a single number.