ALBERT

Notes: The tectonic stress orientation is estimated in the lithosphere of northern Sicily, the southern Tyrrhenian sea and southern Calabria, and in the Wadati-Benioff zone below the Tyrrhenian, by inversion of fault-plane solutions of earthquakes covering a magnitude range from 2.5 to 7.1. Focal mechanisms of 97 earthquakes are taken from the literature, after a critical evaluation of their data quality. an average misfit of F= 13° indicates that the set of all shallow (〈50km) earthquakes is generated by a heterogeneous stress field. For three subsets, based on regional and magnitude separation, F was small enough (2.8°〈inlineGraphic alt="leqslant R: less-than-or-eq, slant" extraInfo="nonStandardEntity" href="urn:x-wiley:0956540X:GJI857:les" location="les.gif"/〉F〈inlineGraphic alt="leqslant R: less-than-or-eq, slant" extraInfo="nonStandardEntity" href="urn:x-wiley:0956540X:GJI857:les" location="les.gif"/〉 5.9°) to support the assumption of a homogeneous stress direction; for an additional subset, with F= 7.4°, such a condition is close to being fulfilled even though some heterogeneity appears to be present. The number of earthquakes in these subsets ranged from nine to 22, and the uncertainties of the principal stress directions were generally of the order of 20° at the 90 per cent confidence level. The earthquakes with M〈inlineGraphic alt="geqslant R: gt-or-equal, slanted" extraInfo="nonStandardEntity" href="urn:x-wiley:0956540X:GJI857:ges" location="ges.gif"/〉 5 define a regional stress field with the greatest principal stress, σ1, dipping at a shallow angle to the south. In north-eastern Sicily and south-western Calabria the stress field estimated by earthquakes is extensional, with σ3 in a direction of WNW, and a near-vertical σ1, in agreement with the graben tectonics mapped geologically in this area. In western Sicily the σ1 direction is oriented WNW, but this result is judged less reliable than the others, based on the broader confidence limits of the solution and the average misfit of 7.4°. The earthquakes in the Wadati-Benioff zone define σ1 dipping at about 70° to the NW, subparallel to the zone, with σ2 horizontal and striking parallel to the zone.

Notes: Abstract In order to determine how reliably one can invert accelerograms to determine the rupture process details, when the station configuration is less than optimal, we use the vertical component of synthetic accelerograms for a Haskell-type earthquake rupture model, at stations in the vicinity of a dip-slip fault and solve the inverse problem. Of the various station configurations used, one is a uniform distribution and the others are very non-uniform. Faults of two different aspect ratios are considered. We mainly use much larger spatial and temporal cell sizes in the inversion than we use to construct the artificial data. The fault mechanism and the fault area are taken as known in the inversions. To solve the inverse problem, we use the method of linear programming and stabilize the solution by the use of physical constraints. The constraints of positivity of the slip rates on the fault is used in all cases in this study. In some cases, additional physical constraints such as preassigning the final moment, the rupture speed, and so on, are also used. We find that using a cell size almost double the wavelength of interest, we are able to reproduce the solution of the problem, even when we add a small amount of random noise to the artificial data, provided the source medium structure is known. We show that the best station configuration is when the stations are on the hanging wall, due to the fact that they provide the best illumination of the fault surface. This provides an incentive to install permanent ocean bottom strong ground motion stations in subduction zones. We also analyzed the effect of the rupture propagation direction on the results of the inversion showing that even four stations are sufficient to retrieve the rupture process if they are in the forward direction of the rupture propagation; the results for this case are better than when the four stations are placed in the backward direction, even when their positions are such that they illuminate the fault in exactly the same way as the four stations in the forward direction. Thus azimuthal distribution and the resulting illumination of the fault as well as the relation of the position of the stations to the direction of rupture propagation are more important than simply the number of stations. Finally, we find that proper knowledge of source medium structure is essential to recover the source process details reliably and that poor knowledge of crustal structure cannot be compensated by adding stations or by additional constraints.