Can someone help me in explaining why, when speaking about directivity effects, it is spoken only on velocity time history and not using acceleration time history?
I could understand why fling effects are spoken using only displacement time history, but need to understand why velocity profile is used for directivity? Also, looking at the velocity profile, can we say if it is backward rupture directivity or forward rupture directivity?
By velocity, particle or ground motion velocity is meant rather than propagation velocity. Ground velocities of near a mb = 6 earthquake are typically on the order of cm/sec. Directivity enhances the amplitude of ground motions in the direction of slip propagation, assuming there is a preferred direction of slip growth during an earthquake (there may not be if slip grows at an equal rate in all directions from point of intiatiion). The amplitude enhancement factor is frequency dependent, enhancing higher frequency radiation in the direction of rupture propagation. The ground motion can be measured either in displacement, velocity, or acceleration. Since seismometers at longer distances from earthquakes are either narrow or broadband filters of particle velocity, the directivity factor may be reported by particle velocity, but it can easily be converted to an acceleration if that's what you need to predict for a strong ground motion accelerometer.
The directivity effect is a phenomenon which appears when the velocity of the fault rupture is very close to the velocity of the shear wave. This leads to the appearance of long period pulses which may or may not be visible in the acceleration traces, but they are readily distinguishable in the velocity traces (which are basically low-passed versions of the accelerograms). These velocity pulses appear to be stronger when the rupture propagates towards the site (forward directivity).
@ Prof. Vernon Cormier: Thank you for your answer... its an useful explanation
Jagan, Anca is right and in fact you can recover the long period, fling acceleration pulses, velocity pulses and the displacement fling-step, in the wavelet domain, using linear phase transforms, have a look at the two papers given below:
A A Chanerley, N A Alexander, J Berrill, H Avery, B Halldorsson, R Sigbjornsson (2013), “Concerning baseline errors in the form of acceleration transients when recovering displacements from strong motion records using the un-decimated wavelet transform”, BSSA, Vol 103, pp283-295
A A Chanerley, N A Alexander (2010), “Obtaining Estimates of low-frequency fling, instrument tilts and displacement timeseries using wavelet decomposition”, Bulletin of Earthquake Engineering, Vol 8, pp231
Cheers
Andrew
- Badges
- Science topic
- Similar topics
- Philosophy
- Philosophy Of Science
- Reasoning