ABSTRACT
Laboratory experiments report that detectable seismic velocity changes should occur in the vicinity of fault zones prior to earthquakes. However, operating permanent active seismic sources to monitor natural faults at seismogenic depth is found to be nearly impossible to achieve. We show that seismic noise generated by vehicle traffic, and especially heavy freight trains, can be turned into a powerful repetitive seismic source to continuously probe the Earth's crust at a few kilometers depth. Results of an exploratory seismic experiment in Southern California demonstrate that correlations of train-generated seismic signals allow daily reconstruction of direct P body waves probing the San Jacinto Fault down to 4-km depth. This new approach may facilitate monitoring most of the San Andreas Fault system using the railway and highway network of California.
ABSTRACT
We present the results of a series of experiments whose goal was the measurement of the quality factor in the frecuency range relevant for the seismic risk. A crustal model is set up that allows to explain the observed attenuation of the short period regional records in the frecuency range 0.5 to 10 Hz. Taking into account the seismic layering of the lower continental crust revealed by many deep reflection surveys allows to simulate the early code of the seismograms. A detailed study of the attenuation of regional phases indicates very clearly the increase of the mean Q value of the crust with the frecuency. Several arguments indicate the importance of the crustal heterogeneity for the apparent wave attenuation. The depth dependance of Q is investigated through theoretical simulations corresponding to extreme cases. The comparison with observations shows that the apparent quality factor seems to be almost independent of depth in the crust. We compute synthetic accelerations in the distance range 30-200 km for the crustal model deduced from the analysis of regional data. The influence of the lower crustal layering can be seen at close distance from the source and partly explains the long durations observed. The simulations predict a reliable decay of peak ground acceleration with distance, particularly in the vicinity of the critical distance of the Moho reflection. Another important feature of regional wave propagation is that several zones of extreme attenuation of crustal guided waves have been discovered at major structural boundaries. We show that these extinctions are not explained by the large scale geometry of the Moho but are likely to be due to the properties of the crustal material themselves. This may imply that anomalies can affect propagation even in short distance ranges (AU)
