Upper-Mantle Shear Velocities beneath Southern California Determined from Long-Period Surface Waves

We used long-period surface waves from teleseismic earthquakes recorded by the TERRAscope network to determine phase velocity dispersion of Rayleigh waves up to periods of about 170 sec and of Love waves up to about 150 sec. This enabled us to investigate the upper-mantle velocity structure beneath southern California to a depth of about 250 km. Ten and five earthquakes were used for Rayleigh and Love waves, respectively. The observed surface-wave dispersion shows a clear Love/Rayleigh-wave discrepancy that cannot be accounted for by a simple isotropic velocity model with smooth variations of velocity with depth. Separate isotropic inversions for Love- and Rayleigh-wave data yield velocity models that show up to 10% anisotropy (transverse isotropy). However, tests with synthetic Love waves suggest that the relatively high Love-wave phase velocity could be at least partly due to interference of higher-mode Love waves with the fundamental mode. Even after this interference effect is removed, about 4% anisotropy remains in the top 250 km of the mantle. This anisotropy could be due to intrinsic anisotropy of olivine crystals or due to a laminated structure with alternating high- and low-velocity layers. Other possibilities include the following: upper-mantle heterogeneity in southern California (such as the Transverse Range anomaly) may affect Love- and Rayleigh-wave velocities differently so that it yields the apparent anisotropy; higher-mode Love-wave interference has a stronger effect than suggested by our numerical experiments using model 1066A. If the high Love-wave velocity is due to causes other than anisotropy, the Rayleigh-wave velocity model would represent the southern California upper-mantle velocity structure. The shear velocity in the upper mantle (Moho to 250 km) of this structure is, on average, 3 to 4% slower than that of the TNA model determined for western North America

Anisotropy beneath California: shear wave splitting measurements using a dense broadband array

We have determined the shear wave splitting parameters for a dense network of broad-band stations in the western United States using high-quality SKS and SKKS waveforms, with particularly high spatial resolution in the southern California region covered by the TriNet seismic network. The alignment of most fast polarization directions can be explained by plate-tectonic, extensional and compressional events. We find that the overall pattern of fast directions agrees well with the Pn anisotropy model by Hearn that images the uppermost mantle. Furthermore, the measured fast directions are generally orthogonal to the maximum horizontal compressive stress directions as determined from shallow crustal stress indicators (World Stress Map). This suggests that the pattern of anisotropy in the western US is predominantly uniform throughout the crust and upper mantle and that a 100–150 km thick layer (as estimated from the SKS delay time, assuming 4 per cent anisotropy) of anisotropic material has experienced coherent strain conditions and has undergone a similar deformation history. A more detailed investigation reveals small-scale lateral variations in anisotropy that are manifested by minor differences in splitting parameters between closely located stations as well as between SKS and SKKS for the same station-event pairs. We also identify a contrast in splitting parameters between central (the greater Bay area) and southern California. In central California, our measurements show evidence for variation of splitting parameters with backazimuth, while in southern California the pattern of measurements can be fit adequately with a single-layer anisotropy model. This contrast dominates any consistent effect of the San Andreas Fault (SAF). We can fit the variation of splitting parameters as a function of polarization azimuth for some stations in the vicinity of the SAF better with a two-layer anisotropy model than a single layer model, with one thin layer having a fast direction parallel to the SAF. However, many alternative models, which could incorporate dipping axes of anisotropy, lateral variation of anisotropy or a more continuous variation of fast direction with depth, would be able to produce a similar fit

Empreintes du passé

Les empreintes du passé présentées dans ce numéro témoignent de la diversité et de la richesse des nations du Sud. Le fil d'Ariane qui relie ces textes consiste à retrouver les traces de populations disparues ou parties ailleurs. Ces traces peuvent alors prendre des formes très diverses : gravures, tertres révélant des poteries, meules dormantes, citernes, constructions anciennes "parcs" fossiles d'arbres remarquables ou encore "pierres errantes" qui se fixent, deviennent des lieux sacrés, et déterminent des espaces magiques. Des auteurs de disciplines différentes (archéologie, géographie, ethnologie, économie) procèdent à une lecture du passé qui s'inscrit dans la longue durée et leurs témoignages vont souvent rejoindre les données de la tradition orale. (Résumé d'auteur

Shallow subduction zone earthquakes and their tsunamigenic potential

We have examined the source spectra of all shallow subduction zone earthquakes from 1992 to 1996 with moment magnitude 7.0 or greater, as well as some other interesting events, in the period range 1-20 s, by computing moment rate functions of teleseismic P waves. After comparing the source spectral characteristics of ‘tsunami earthquakes’ (earthquakes that are followed by tsunamis greater than would be expected from their moment magnitude) with regular events, we identified a subclass of this group: ‘slow tsunami earthquakes’. This subclass consists of the 1992 Nicaragua, the 1994 Java and the February 1996 Peru earthquakes. We found that these events have an anomalously low energy release in the 1-20 s frequency band with respect to their moment magnitude, although their spectral drop-off is comparable to those of the other earthquakes. From an investigation of the centroid and body wave locations, it appears that most earthquakes in this study conformed to a simple model in which the earthquake nucleates in a zone of compacted and dehydrated sediments and ruptures up-dip until the stable sliding friction regime of unconsolidated sediments stops the propagation. Sediment-starved trenches, e.g. near Jalisco, can produce very shallow slip, because the fault material supports unstable sliding. The slow tsunami earthquakes also ruptured up-dip; however, their centroid is located unusually close to the trench axis. The subduction zones in which these events occurred all have a small accretionary prism and a thin layer of subducting sediment. Ocean surveys show that in these regions the ocean floor close to the trench is highly faulted. We suggest that the horst-and-graben structure of a rough subducting oceanic plate will cause contact zones with the overriding plate, making shallow earthquake nucleation and up-dip propagation to the ocean floor possible. The rupture partly propagates in sediments, making the earthquake source process slow. Two factors have to be considered in the high tsunami-generating potential of these events. First, the slip propagates to shallow depths in low-rigidity material, causing great deformation and displacement of a large volume of water. Second, the measured seismic moment may not represent the true earthquake displacement, because the elastic constants of the source region are not taken into account in the standard CMT determination