Abstract Several drilling projects have been conducted through active faults with the aim of learning about the geology of the fault zones and tentatively correlating the structure and mineralogy of the fault zones with their seismological behaviour during recent earthquakes. Here we present the major results obtained from structural and mineralogical studies of core samples retrieved from the dextral reverse strike-slip Nojima Fault (Japan) within granitic rocks following the Kobe earthquake (1995), and from the Chelungpu Thrust Fault (Taiwan) within alternating silts and shales following the Chi-chi earthquake (1999). We show how these projects, despite not fullfilling all their objectives, have still contributed to a better geological knowledge of the fault zones, to a better characterization of the slip zones related to the recent earthquakes particularly of their thickness, microstructures and deformation mechanisms, and to a better understanding of the nature and role of fluids within the fault zone. They have also led to new questions, and to new approaches, for studying fault-rock samples. For all of these reasons, they have stimulated international scientific research into fault-zone geology.

Abstract Statistical properties of crack-seal veins are investigated with a view to assessing stress release fluctuations in crustal rocks. Crack-seal patterns correspond to sets of successive parallel fractures that are assumed to have propagated by a subcritical crack mechanism in the presence of a reactive fluid. They represent a time-sequence record of an aseismic and anelastic process of rock deformation. The statistical characteristics of several crack-seal patterns containing several hundreds of successive cracks have been studied. Samples were collected in three different areas, gold-bearing quartz veins from Abitibi in Canada, serpentine veins from the San Andreas system in California and calcite veins from the Apennine Mountains in Italy. Digitized pictures acquired from thin sections allow accurate measurement of crack-seal growth increments. All the samples show the same statistical behaviour regardless of their geological origin. The crack-seal statistical properties are described by an exponential distribution with a characteristic length scale and do not show any spatial correlation. They differ from other fracture patterns, such as earthquake data, which exhibit power-law correlations (Gutenberg-Richter relationship). Crack-seal series represent a natural fossil record of stress release variations (less than 50 bars) in the crust that show a characteristic length scale, associated with the resistance of rock to effective tension, and no correlation in time.