We present a technique that greatly improves the precision in measuring temporal variations of crustal velocities using an earthquake doublet, or pair of microearthquakes that have nearly identical waveforms and the same hypocenter and magnitude but occur on different dates. We compute differences in arrival times between seismograms recorded at the same station in the freqency domain by cross correlation of short windows of signal. A moving-window analysis of the entire seismograms, including the coda, gives g(t), the difference in arrival times versus running time along the seismogram. The time resolution of the method is an order of magnitude better than the digitization interval. The g(t) technique is illustrated with a pair of microearthquakes, M = 1.7 and 2.0, that occurred before and after the Coyote Lake, California, earthquake (M = 5.9) of August 6, 1979, and on the same segment of the Calaveras fault that ruptured during the earthquake. The coda wave arrivals for some stations are progressively delayed for the second earthquake in the doublet, so that its seismogram appears as a stretched version of the earlier event. We interpret this systematic variation in 6(t) along the coda as a change in the average S velocity in the upper crust in the time interval between the two doublets. S wave velocities appear to have decreased by 0.2% in an oblong region 5-10 km in radius at the south end of the aftershock zone.

Dense microearthquake swarms occur in the upper south flank of Kilauea, providing multiplets composed of hundreds of events. The similarity of their waveforms and the quality of the data have been sufficient to provide accurate relative relocations of their hypocenters. A simple and efficient method has been developed which allowed the relative relocation of more than 250 events with an average precision of about 50 m horizontally and 75 m vertically. Relocation of these events greatly improves the definition of the seismic image of the fault that generates them. Indeed, relative relocations define a plane dipping about 6° northward, although corresponding absolute locations are widely dispersed in the swarm. A composite focal mechanism, built from events providing a correct spatial sampling of the multiplet, also gives a well‐constrained northward dip of about 5° to the near‐horizontal plane. This technique thus collapses the clouds of hypocenters of single‐event locations to a plane coinciding with the slip plane revealed by previous focal mechanism studies. We cannot conclude that all south flank earthquakes collapse to a single plane. There may locally be several planes, perhaps with different dips and depths throughout the south flank volume. The 6° northward‐dipping plane we found is too steep to represent the overall flexure of the oceanic crust under the load of the island of Hawaii. This plane is probably an important feature that characterizes the basal slip layer below the upper south flank of Kilauea volcano. Differences in seismicity rate and surface deformations between the upper and lower south flank could be related to the geometry of this deep fault plane. The present work illustrates how high precision relative relocations of similar events in dense swarms, combined with the analysis of geodetic measurements, can help to describe deep fault plane geometry. Systematic selection and extensive relative relocation of similar earthquakes could be attempted in other well‐instrumented, highly seismic areas to provide reliable basic information, especially useful for understanding of earthquake generation processes.

The western Alps are an active collision belt whose current stress field is inhomogeneous [Müller et al., 1992]. We report new seismological data which significantly improve our knowledge of this stress field. About 1600 earthquakes which occurred in the western Alps during the last 10 years were precisely located, and 79 new focal solutions were computed. The analysis of this database shows that widespread extension affects all the internal zones of the belt. To better constrain the associated stress regime, six stress tensors were computed using the Gephart and Forsyth [1984] method. They show that the current tectonics of the western Alps are contrasted with close variation in the stress regime (transpression to the front of the belt contrasting with extension in the core of the belt). The extensional direction is radial to the arcuate geometry of the belt and bounded outboard by the former thrust of the internal zones onto the external zone, suggesting extensional reactivation of this inherited crustal discontinuity. Such widespread extension within the inner part of an actually ongoing collision belt cannot be explained by simple collision‐related tectonics. We propose that intrabelt buoyancy forces, such as those produced by a slab retreat or slab break‐off, interfere with the boundary forces driven by the ongoing Europe‐Africa convergence.

Abstract.A

SUMMARY In the French western Alps, east of Grenoble, we identify the Belledonne Border Fault as an active seismic fault. This identification is based on the seismic monitoring of the Grenoble area by the Sismalp seismic network over the past 12 yr (1989–2000). We located a set of earthquakes with magnitudes ranging from 0 to 3.5 along a ∼50 km long alignment which runs in a N30°E direction on the western flank of the Belledonne crystalline massif. Available focal solutions for these events are consistent with this direction (N36°E strike‐slip fault with right‐lateral displacement). These events along the Belledonne Border Fault have a mean focal depth of ∼7 km (in the crystalline basement), with a probably very low slip rate. The Belledonne Border Fault has never been mapped at the surface, where the otherwise heavily folded and faulted Mesozoic cover makes this identification difficult. Historical seismicity also shows that, over the past two and a half centuries, a few events located mainly along the southern part of the Belledonne Border Fault caused damage, with the magnitude 4.9 1963 Monteynard earthquake reaching intensity VII. The most recent damaging event in the study area is the magnitude 3.5 1999 Laffrey earthquake (intensity V–VI). Although its epicentre lies at the southern tip of the Belledonne Border Fault, there is clear evidence that aftershocks were activated by the left‐lateral slip of a N122°E‐striking fault. The length of the Belledonne Border Fault, which could easily accommodate a magnitude 6 event, as well as the proximity to the Isère valley with its unlithified Quaternary deposits up to 500 m thick known to generate marked site effects, make the identification of the Belledonne Border Fault an important step in the evaluation of seismic risk in the Grenoble area. Besides, the activity observed on the fault will now have to be taken into account in future geodynamic models of the western Alps.