GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 2.1 281 So the second step is to represent it as P S = exp (– dr ( d )), where d is the inter-event distance, and r ( d ) is the instantaneous occurrence, death, rate. The shape of r ( d ) (Fig. 2) is rather peculiar, but it can be represented as the sum of a double exponential, that describes efficiently the functions at lower distances, and a power law, that explains it at greater distances. The double exponential decays rapidly to zero, while the power law still yields a significant component at d = 0 . It is obvious that at very short distances d is very poorly constrained, since the epicentral location is not determined well enough for this purpose, and since the correct locations should be those of the centroid. The latter however has often serious problems, so it cannot be used. Given all the above considerations, r ( d ) can be expressed as r ( d ) = – c 1 exp ( c 3 / d C 2 – d C 2 / c 4 ) – c 5 (2 R – d ) C 6 , where R is the Earth radius and c 1 ,..., c 6 ��� ���������� are constants. As far as uncertainties are concerned, the most meaningful approach seems to be an empirical one. The data set has been subdivided in four different sets, each of about 2200-2300 events. Specifically they consider four time intervals: 1927-1949, 1950-1969, 1970-1992, and 1993-2013. It can be observed that the second and the fourth interval contain the five largest earthquakes, namely the 1952, 1960, 1964, 2004, and 2011 ones. These two intervals yields survival functions that consistently are below the average, while the other two, characterized by lesser moment release, lays above. The final consideration is linked to intermediate distance correlations. The survival function above 1600-2000 km could be modeled in terms of a quadratic. This means that below this thresholds a correlation do exist. This obviously poses a question in hazard investigations. In fact if zoning is obviously a correct way to estimate occurrence rate in geodynamically homogeneous areas, it must be remembered that this could be completely overturned in the short term, if a nearby more active area would start a higher level of activity. References Corral A.; 2005; Time-decreasing hazard and increasing time until the next earthquake. Physical Review E, 71, 1, DOI 10.1103/PhysRevE.71.017101. Storchak D.A., Di Giacomo D., Bondár I., Engdahl E. R., Harris J., Lee W.H.K., Villaseñor A., Bormann P.; 2013: Public Release of the ISC-GEM Global Instrumental Earthquake Catalogue (1900-2009). Seism. Res. Lett., 84, 5, 810-815, DOI 10.1785/0220130034. Fig. 2 - Occurrence rate. Blu: observed data; red modeled.