Читать книгу Statistical Methods and Modeling of Seismogenesis - Eleftheria Papadimitriou - Страница 33

The next versions of the simulation algorithm developed by our group included the following improvements that make the physics of the model more realistic (Console et al. 2017, 2018a,b): The occurrence time for the nucleation of a new rupture is computed for the cell of the fault model that has the shortest time for reaching a stress exceeding the respective strength threshold; this speeds up the computations made by the computer code between two consecutive events. The fault geometry allows faults with any orientation and size in a three-dimensional space and their source mechanisms may be different from fault to fault: an expanding rupture may jump from one fault to another, according to a criterion of maximum distance from a cell on the border of a fault to a cell on the border of another fault.

Finally, as an example of a possible use of synthetic catalogs, an attenuation law has been applied to all the events reported in the synthetic catalog for the production of maps showing the exceedance probability of given values of Peak Ground Acceleration (PGA) on the territory under investigation.

The new version of the simulator code containing the above-mentioned improvements was first applied to a set of the Italian database of seismic sources (DISS 3.2.0), selecting all the faults that are recognized in the Calabria region, for a total of 22 fault segments. The application of our simulation algorithm provides typical features in time, space and magnitude behavior of the seismicity, which can be compared with those of the real observations. Figures 2.8 and 2.9 show examples of the results obtained in this study.

A temporal feature of the synthetic catalogs obtained from our simulation algorithm was explored by analyzing the statistical distribution of the time by which an event of any magnitude can precede or follow a strong earthquake of M ≥ 6.0. This study was aimed to assess the existence in the synthetic catalogs of some kind of time-dependent occurrence rate as a long-term precursor of strong earthquakes. The analysis was carried out by a stacking technique on the catalog obtained from the 0.25 × 0.25 km discretization starting from a minimum magnitude M = 3.0, and its results are displayed in Figure 2.10. This figure shows an outstanding trend of acceleration of seismic activity in a 150 years period before the strong earthquakes, as well as a sort of quiescence with a slow recovering to the normality after such earthquakes.

In order to explore the potential use of a synthetic catalog obtained by the simulation algorithm for seismic hazard assessment, we adopted the Cornell (1968) method applied to the M ≥ 4.5, 100 ky simulated catalog. The PGA at a dense grid of points covering the territory of the Calabria region was estimated for each earthquake of the catalog through a typical attenuation law (Sabetta-Pugliese 1987):

where M is the earthquake magnitude, d is the epicentral distance, and S₁ and S₂ are parameters taking into account the soil dynamic features at the site. At each node of the grid we obtained the distribution of the number of times that a given PGA was exceeded in 100 kyrs, and from that, the probability of exceedance of the given PGA in 50 years. The results for PGA = 0.2 g are shown in Figure 2.11, as an example of how the method can work.

Figure 2.8. a) Magnitude-frequency distribution of the earthquakes in the synthetic catalog obtained from the simulation algorithm described in the text using a discretization of 1 × 1 km and an aspect ratio (A-R) coefficient equal to 2 and various values of the stress reduction (S-R) coefficient (based on Console et al. 2017). b) As in (a), using various values of the aspect ratio (A-R) coefficient and a stress reduction (S-R) coefficient equal to 0.7 (based on Console et al. 2017). For a color version of this figure, see www.iste.co.uk/limnios/statistical.zip