Preliminary Studies of Seismic Risk in Turkey, and the Occurrence of Upper Bounded and Other Large Earthquake Magnitudes

Summary

Gumbel’s third distribution of extreme values is discussed and applied to two files of Turkish earthquakes created from diverse agencies and catalogues. The forecasting ability of both is in agreement and a preliminary map of seismic risk is produced.

Some alternative approaches to evaluating seismic risk, especially through upper bounded and other large magnitude earthquake occurrences, are illustratrd using a 4° cell of seismicity centred on 39N, 41E, in Anatolia. Gumbel upper bounds w to earthquake magnitude occurrence are asymptotic, uncertain, and theoretically correspond to infinite return periods whereas strain energy release diagrams lead to an empirical estimate of the large magnitude M₃ which is equivalent to the total strain energy which may be accumulated in a region. M₃ is less than w and shows a finite ‘waiting time’ during which the energy equivalent to M₃ may be accumulated. Combining Gumbel III statistics with an intensity attenuation law defines the ‘most perceptible’ earthquake magnitude which has the maximum probability of being felt at any intensity level. These ‘most perceptible’ earthquakes will vary between regional seismicity cells in Turkey, but local values may help as a criterion for choosing engineering design time histories.