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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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Strong ground motion engineering seismology l.jpg

Strong ground motion(Engineering Seismology)

Earthquake shaking capable of causing damage to structures


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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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Horizontal motions are of most importance for earthquake engineering

  • Shaking often strongest on horizontal component:

    • Earthquakes radiate larger S waves than P waves

    • Decreasing seismic velocities near Earth’s surface produce refraction of the incoming waves toward the vertical, so that the ground motion for S waves is primarily in the horizontal direction

  • Buildings generally are weakest for horizontal shaking


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Questions engineering

  • What are the most useful measures of ground motion?

  • What factors control the level of ground motion?


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Measures of ground-motion for engineering purposes engineering

  • PGA (peak ground acceleration)

  • PGV (peak ground velocity)

  • Response spectral acceleration (elastic, inelastic) at periods of engineering interest

  • Intensity (Can be related to PGA and PGV.)


Peak ground acceleration pga l.jpg
Peak ground acceleration (PGA) engineering

  • easy to measure because the response of most instruments is proportional to ground acceleration

  • liked by many engineers because it can be related to the force on a short-period building

  • convenient single number to enable rough evaluation of importance of records

  • BUT it is not a measure of the force on most buildings

  • and it is controlled by the high frequency content in the ground motion (i.e., it is not associated with a narrow range of frequencies); records can show isolated short-duration, high-amplitude spikes with little engineering significance


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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?


Peak ground velocity pgv l.jpg
Peak ground velocity (PGV) hypocentral distance between approx. 5*3.2= 16 km and 8*3.2= 26 km

  • Many think it is better correlated with damage than other measures

  • It is sensitive to longer periods than PGA (making it potentially more predictable using deterministic models)

  • BUT it requires digital processing (no longer an important issue)


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Large Recorded Ground Velocities hypocentral distance between approx. 5*3.2= 16 km and 8*3.2= 26 km


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Peak ground displacement (PGD) hypocentral distance between approx. 5*3.2= 16 km and 8*3.2= 26 km

  • The best parameter for displacement-based design?

  • BUT highly sensitive to the low-cut (high-pass) filter that needs to be applied to most records (in which case the derived PGD might not represent the true PGD, unlike PGA, for which the Earth imposes a natural limit to the frequency content). For this reason I (Dave Boore) recommend against the use of PGD.





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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


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Frequencies of ground-motion for engineering purposes (multiply by (2

  • 10 Hz --- 10 sec (usually below about 3 sec)

  • Resonant period of typical N story structure ~ N/10 sec

  • Corner periods for M 5, 6, and 7 ~ 1, 3, and 9 sec


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Frequency Response (multiply by (2

of Structures


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Modified Mercalli Intensity (multiply by (2

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


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What Controls the Level of Shaking? (multiply by (2

  • Magnitude

  • Directivity

    • Larger fault, more energy released and over a larger area

  • Distance from fault

    • Shaking decays with distance

  • Local site response (rock or soil)

    • amplify the shaking

    • Strongest shaking in rupture direction

    • Pockets of higher shaking (lens effect)


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Earthquake Magnitude (multiply by (2

  • Earthquake magnitude scales originated because of

    • the desire for an objective measure of earthquake size

    • Technological advances -> seismometers


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Modern Seismic Magnitudes (multiply by (2

  • Today seismologists use different seismic waves to compute magnitudes

  • These waves generally have lower frequencies than those used by Richter

  • These waves are generally recorded at distances of 1000s of kilometers instead of the 100s of kilometers for the Richter scale


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Teleseismic M (multiply by (2S and mb

  • Two commonly used modern magnitude scales are:

    • MS, Surface-wave magnitude (Rayleigh Wave)

    • mb, Body-wave magnitude (P-wave)


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Why use moment magnitude? (multiply by (2

  • It is the best single measure of overall earthquake size

  • It does not saturate

  • It can be estimated from geological observations

  • It can be estimated from paleoseismology studies

  • It can be tied to plate motions and recurrence relations


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(From J. Anderson) (multiply by (2


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(From J. Anderson) (multiply by (2


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Ground Motion (multiply by (2Important Factors

  • Source effects

    • Magnitude or moment

    • Rupture directivity

  • Path effects

    • Attenuation with distance: geometric, scattering, and anelastic

    • Critical reflections off Moho Discontinuity

  • Site effects

    • Local amplification

Bay Mud

25 km


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Directivity (multiply by (2

  • Directivity is a consequence of a moving source

  • Waves from far-end of fault will pile up with waves arriving from near-end of fault, if you are forward of the rupture

  • This causes increased amplitudes in direction of rupture propagation, and decreased duration.

  • Directivity is useful in distinguishing earthquake fault plane from its auxiliary plane because it destroys the symmetry of the radiation pattern.


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Rupture Directivity (multiply by (2

Seismic Waves

Hypocenter

Rupture direction


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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


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2003 San Simeon earthquake ground motions near the fault.M6.5 Earthquake

Pacific Ocean

SLO County


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Rupture Directivity earthquake ground motions near the fault.

SLO County


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Damage in Oceano earthquake ground motions near the fault.2003 San Simeon Earthquake

Cracking in river levee

Failed foundation


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Effect of Distance earthquake ground motions near the fault.

Ground motion generally decreases with increasing epicentral distance


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2003 San Simeon Earthquake Distance and directivity earthquake ground motions near the fault.


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Amplitude and Intensity earthquake ground motions near the fault.M7.6 Pakistan earthquake 2005

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.


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Site Amplification earthquake ground motions near the fault.

  • Ground shaking is amplified at “soft soil” (low velocity) sites

  • Shear-wave velocity is commonly used to predict amplification

    • VS30 ( time it takes for a shear wave to travel from a 30 m depth to the land surface, i.e., time-averaged 30-m velocity)


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Ground Motion Deconvolution earthquake ground motions near the fault.

(Steidl)


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Amplification of PGA earthquake ground motions near the fault.as a function of VS30


Velocities of holocene and pleistocene units oakland ca l.jpg

Holocene earthquake ground motions near the fault.

Pleistocene

Velocities of Holocene and Pleistocene Units – Oakland, CA


Damage distribution during the 1989 m6 9 loma prieta earthquake correlated quite well with vs30 l.jpg
Damage distribution during the 1989 M6.9 Loma Prieta earthquake correlated quite well with Vs30.


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Summary of Strong Ground Motion from Earthquakes earthquake correlated quite well with Vs30.

  • Measured using PGA, PGV, pseudo-spectral acceleration or velocity PSA or PSV, and intensity.

  • Increases with magnitude.

  • Enhanced in direction of rupture propagation (directivity).

  • Generally decreases with epicentral distance.

  • Low-velocity soil site gives much higher ground motion than rock site. Vs30 is a good predictor of site response.