Spectrogram Evaluation of Seismic Risk in Managua, Nicaragua
2005
Hedberg, Jonas
Managua has experienced two devastating earthquakes during the last century resulting in major human loss. The destruction in the earthquake of 1972 was particularly severe in defined areas of the city, including the former city centre, which is now unused terrain. This may have been because of amplification in those areas due to impedance contrasts and resonance effects in the surface layers. The aim is to test a method of evaluating resonance and attenuation in the surface layers by imaging the traffic impulse response in a spectrogram. This would be a fast method of identifying earthquake-sensitive sites for more thorough investigation of the dynamic properties with other methods. The traffic impulse response method uses heavy vehicles as a seismic source. This is done by placing an obstacle over the road which causes the vehicles to vertically impact the ground. The ground response to that impact is then recorded and represented in a spectrogram image, where the attenuation with time on each frequency can be seen. Measurements are carried out at 128 sites along the Pan-American Highway crossing the city in the low area close to Lake Xolotlán where the sediments can be expected to be the thickest. These include the area where the former city centre was located. The time-frequency hazard analysis is developed as an evaluation method for this study. It convolves the spectrogram at each time instant with a typical earthquake spectrum and a building response spectrum in order to obtain an estimate of the local seismic risk. In the measurements resonances are generally not seen in the spectrograms. Only five sites exhibit clear prevailing frequencies. The transient nature of the impulse response may provide resonance peaks less distinct than in microtremor analysis, as the frequency resolution improves with a longer time window. Weak resonances may thus be hard to interpret from the spectrogram but resonances that can considerably amplify earthquake motion will show, as long as the vehicle impact contains power at sufficiently low frequencies to not miss the surface layer resonance peak. The time-frequency hazard analysis gives an integrate measure of resonance and attenuation without having to rely on subjective interpretation. The spectrogram interpretation and the time-frequency hazard analysis show that the surface layers along the Pan-American Highway are generally competent enough not to give rise to any resonance amplification, with the exception of very localized zones and in the area of the city centre. (Ett examensarbete "Minor Field Study" utfört vid Avd f Teknisk Geologi / A Master Thesis, publ. by the Division of Engineering Geology)
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