Seismogenic zoning based on the geothermal gradient, strenght and depth of the fragile-ductile transition in the uppercrust. Methodological approach for seismic hazard calculations in southeast Spain

摘要

In this work we present a methodological approach to define seismogenic zones for seismic hazard analysisbased on the relationships which can be drawn from the geothermal gradient, total strength of the uppercrust and the depth of the fragile-ductile transition, and their relation with the mean focal depth and aparameter hereby defined: Seismic Density Index (number of events per km3 x 1000). A revision of thedeep geophysical data available in SE Spain has led to the definition of 6 distinctive crustal blocks. Twomain types of crustal blocks are identified: hot-and-weak and cold-and-strong. Hot-and-weak blocks showconsistently higher geothermal gradients, lower strengths and shallower fragile-ductile depths than coldand-strong blocks. Furthermore hot-and-weak blocks show higher seismic density indexes and shallowermean focal depths. However, two blocks biased from this relationship and so are called anomalous.According to these results four seismogenic zones can be defined in SE Spain. The a and b parameters ofthe Gutenberg-Richter law of each zone are consistent with the thermal and strength properties of theblocks. Zones based on the hot-and-weak blocks show higher a and b values than the cold-and-strongones, while the anomalous blocks show values in between these extremes