Uncertainties in probability of occurrence of strong earthquakes for fault sources in the Central Apennines,Italy |
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Authors: | Aybige Akinci David Perkins Anna Maria Lombardi Roberto Basili |
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Institution: | 1.Istituto Nazionale di Geofisica e Vulcanologia,Rome,Italy;2.US Geological Survey,Denver,USA |
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Abstract: | Using the characteristic earthquake model, we calculate the probability of occurrence of earthquakes M
w > 5.5 for individual fault sources in the Central Apennines for the 30-year period (2007–2037). We show the effect of time-dependent
and time-independent occurrence (Brownian passage time (BPT) and Poisson) models together with uncertain slip rates and uncertain
maximum magnitudes and, hence, uncertain recurrence times. In order to reduce the large prior geological slip rate uncertainty
distribution for most faults, we obtain a posterior slip rate uncertainty distribution using a likelihood function obtained
from regional historical seismicity. We assess the uncertainty of maximum magnitude by assuming that the uncertainty in fault
width and length are described by a normal distribution with standard deviation equal to ±20% of the mean values. We then
estimate the uncertainties of the 30-year probability of occurrence of a characteristic event using a Monte Carlo procedure.
Uncertainty on each parameter is represented by the 16th and the 84th percentiles of simulated values. These percentiles bound
the range that has a 68% probability of including the real value of the parameter. We do these both for the Poisson case and
for the BPT case by varying the aperiodicity parameter (α value) using the values 0.3, 0.5, and 0.7. The Bayesian posterior slip rate uncertainties typically differ by a factor of
about 2 from the 16th to the 84th percentile. Occurrence probabilities for the next 30 years at the 84th percentile typically
range from 1% to 2% for faults where the Poisson model dominates and from 2% to 21% where one of the BPT models dominates.
The uncertainty in occurrence probability under the time-dependent hypothesis is very large, when measured by the ratio of
the 84th to the 16th percentile, frequently being as much as two orders of magnitude. On the other hand, when measured by
standard deviation, these standard deviations range from 2% to 6% for those faults whose elapsed time since previous event
is large, but always 2% or less for faults with relatively recent previous occurrence, because the probability of occurrence
is always small. |
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