A test of a physically-based strong ground motion prediction methodology with the 26 September 1997, Mw = 6.0 Colfiorito (Umbria-Marche sequence), Italy earthquake.

Abstract

We test the physically-based ground motion hazard prediction methodology of Hutchings et al. [Hutchings, L., Ioannidou, E., Kalogeras, I., Voulgaris, N., Savy, J., Foxall, W., Scognamiglio, L., and Stavrakakis, G., (2007). A physically-based strong ground motion prediction methodology; Application to PSHA and the 1999 M = 6.0 Athens Earthquake. Geophys. J. Int. 168, 569–680.] through an a posteriori prediction of the 26 September 1997, Mw 6.0 Colfiorito (Umbria–Marche, Italy) earthquake at four stations. By “physically-based” we refer to ground motion synthesized with quasi-dynamic rupture models derived from physics and an understanding of the earthquake process. We test five hypotheses proposed by Hutchings et al. [Hutchings, L., Ioannidou, E., Kalogeras, I., Voulgaris, N., Savy, J., Foxall, W., Scognamiglio, L., and Stavrakakis, G., (2007). A physically-based strong ground motion prediction methodology; Application to PSHA and the 1999 M = 6.0 Athens Earthquake. Geophys. J. Int. 168, 569–680.] that support application of the methodology to physically-based probabilistic seismic hazard or risk analysis. We use two methods to test the hypotheses. First, we test whether observed records fall within the 68% log-normal confidence interval for the distribution of absolute acceleration response (AAR), pseudo velocity response (PSV), and Fourier amplitude spectra (FFT) created by a suite of source models. We also used the godness of fit between synthesized seismograms to verify whether at least one of the source models in the suite generates seismograms that match the observed waveforms, and if good fits to seismograms are due to source models that are close to what is actually known about the Colfiorito earthquake.

We tested the hypotheses with a range of source parameters proposed by Hutchings et al. [Hutchings, L., Ioannidou, E., Kalogeras, I., Voulgaris, N., Savy, J., Foxall, W., Scognamiglio, L., and Stavrakakis, G., (2007). A physically-based strong ground motion prediction methodology; Application to PSHA and the 1999 M = 6.0 Athens Earthquake. Geophys. J. Int. 168, 569–680.]. We synthesized records from 100 rupture scenarios that were generated by a Monte Carlo selection of parameters within the range. This range was based upon having some prior knowledge of where the earthquake would occur. Observed values of AAR, PSV and FFT fit within the 68% confidence interval for all four stations, and one of the models generated seismograms that had a good fit compared to the observations.

Moreover, a strict test for validating a physically-based ground motion hazard prediction methodology is that as more information is known about the source, the uncertainty of the prediction should narrow, but still include the actual ground motion. Then, we tightened the source parameters to be centered about the known parameters for the Colfiorito earthquake, and allowed for less uncertainty in their values. We found this to be true for this test. While the 68% confidence interval narrowed from a factor of ± about 4 to ± about 2 for the distributions, observed values of AAR, PSV and FFT still fit within the distributions for all four stations. Ultimately, we have calculated peak ground velocity (PGV) and peak ground acceleration (PGA) for all the synthetic seismograms obtained from the computed scenarios, and we have found that they are comparable with the actual and with those from the attenuation relation.

We conclude that the methodology of Hutchings et al. [Hutchings, L., Ioannidou, E., Kalogeras, I., Voulgaris, N., Savy, J., Foxall, W., Scognamiglio, L., and Stavrakakis, G., (2007). A physically-based strong ground motion prediction methodology; Application to PSHA and the 1999 M = 6.0 Athens Earthquake. Geophys. J. Int. 168, 569–680.] is promising in giving ground motion hazard prediction estimates.