Stefano Santini

Asperity distribution of the 1964 Great Alaska earthquake and its relation to subsequent seismicity in the region

Abstract The 1964 Alaska earthquake was the second largest seismic events in the 20th century. The aim of this work is the use of surface deformation data to determine asperity and slip distributions on the fault plane of the Alaska earthquake: these distributions are calculated by a Monte Carlo method. To this aim, we decompose the fault plane in a large number of small square asperity units with a side of 25 km; this allows us to obtain plane surfaces with an irregular shape. In the first stage, each asperity unit is allowed to slip a constant amount or not to slip at all, providing the geometry of the dislocation surface that best reproduces the observed displacements. To this purpose, a large number of slip distributions have been tried by the use of the Monte Carlo method. The slip amplitude is the same for all the asperities and is equal to the average fault slip inferred from the seismic moment. In the second stage, we evaluate the slip distribution in the dislocation area determined by the Monte Carlo inversion: in this case, we allow unit cells to undergo different values of slip in order to refine the initial dislocation model. The results confirm the previous finding that the slip distribution of the great Alaska earthquake was essentially made of two dislocation areas with a higher slip, the Prince William Sound and the Kodiak asperities. Analysis of the post-1964 seismicity in the rupture region shows a strong correlation between the larger earthquakes (Mw≥6) and the distribution of locked asperities following the 1964 event, which can be considered as an independent test of the validity of the model. We do not find slip values higher than 25 m for any of the patches, and we determine two separate high-slip zones: one correspondent to the Prince William Sound asperity, and one (∼18 m slip) to the Kodiak asperity. The slip distribution connected with the 1964 shock appears to be consistent with the following seismicity in the region.

Subduction and continental collision events in the southern Apennines: constraints from two crustal cross-sections

Subduzione e collisione continentale in Appennino meridionale: vincoli da due sezioni crostali.Mediante l’integrazione di dati geologici di superficie e sottosuolo, sono state realizzate due sezioni crostali attraverso il sistema catena-avanfossa-avampaese dell’Appennino meridionale. Di tali sezioni, quella settentrionale e stata elaborata dall’interpretazione del profilo sismico a riflessione CROP 04, mentre quella meridionale attraversa i giacimenti petroliferi della Val d’Agri e di Tempa Rossa, in Basilicata. Le due sezioni mostrano la presenza, nel sottosuolo del Cilento, di unita di basamento continentale coinvolte nella strutturazione della catena appenninica. Queste evidenze suggeriscono che, dopo l’iniziale fase di subduzione oceanica, l’evoluzione tettonica dell’Appennino meridionale e stata caratterizzata da due eventi di subduzione continentale alternati a due stadi di collisione continentale.

An analytical model for the geotherm in the Basilicata oil fields area (southern Italy)

A geothermal model for the area of the Basilicata oil fields has been obtained by an analytical procedure. The model takes into account both the temperature variation due to the re-equilibrated conductive state after thrusting and frictional heating. Input parameters include heat flow density data and a series of geologically derived constraints – thrust depth, timing of thrusting, slip rate – obtained by the integration of surface and subsurface datasets. For the top 5 km of the crust, the resulting geothermal curve shows a remarkably good fit with temperatures recorded from deep oil wells.The new geotherm provides a fundamental constraint for rheological and stress accumulation modelling in the seismically active study area. Furthermore, the analytical solution provided in this study may be used as a basis to calculate the relevant geotherm for further areas and/or tectonic settings.

A viscoelastic shear zone model of compressional and extensional plate boundaries

A model is proposed describing the mechanical evolution of a shear zone along compressional and extensional plate boundaries, subject to constant strain rate. The shear zones are assumed as viscoelastic with Maxwell rheology and with elastic and rheological parameters depending on temperature and petrology. Stress and strain are computed as functions of time and depth. For both kinds of boundaries the model reproduces the existence of a shallow seismogenic zone, characterized by a stress concentration. The thickness of the seismogenic layer is evaluated considering the variations of shear stress and frictional strength on faults embedded in the shear zone. Assuming that a fault dislocation takes place, the brittle-ductile transition is assumed to occur at the depth at which the time derivative of total shear stress changes from positive to negative values. The effects of different strain rates and geothermal gradients on the depth of the brittle-ductile transition are studied. The model predictions are consistent with values inferred from seismicity data of different boundary zones.