ALBERT

Description: We attempt to distinguish between structure-related and stress-induced shear-wave velocity anisotropy in the upper crust using microearthquakes (M〈 or =2.6) that occurred before and after the Tres Pinos, California earthquake (M 5.5) of 26 January 1986. The Tres Pinos earthquake occurred on the Quien Sabe fault, a north-west-southeast trending right-lateral strike-slip fault, approximately 10 km east of the Calaveras fault in central California. Digital, three-component seismograms are available from 1984 to 1992 from the Quien Sabe Ranch station (HQR), located only 7.3 km north-northeast of the epicenter of the main event. We studied polarization directions and delay times using a method that combines cross-correlation techniques and particle-motion plots. To enhance the significance of our interpretations we reduced uncertainties related to the hypocenter-location of the earthquakes by source relocation based on joint-hypocenter determination and cross correlation. In the study area we found two zones with different but stable polarization directions. Around the Quien Sabe fault system, shear-wave polarizations of over 30 microearthquakes clearly indicate that the slow shear-wave direction is normal to the northwest-southeast trend of this vertical right-lateral strike-slip fault. Thus, velocity anisotropy appears to be caused by the fault system itself. However, anisotropy associated with a cluster of microearthquakes located about 4 km to the east is quite different. In this zone the fast and slow shear-polarization directions are correlative with the maximum and minimum horizontal-stress directions, respectively. Thus in this case, it appears the crustal velocity anisotropy is controlled by the state of stress in the crust. The observation of these two polarization directions at one station suggests that the shallow crust below the station HQR does not influence shear-wave polarization directions.

Description: Coral reefs are biologically diverse ecosystems threatened with effective collapse under rapid climate change, in particular by recent increases in ocean temperatures. Coral bleaching has occurred during major El Niño warming events, at times leading to the die-off of entire coral reefs. Here we present records of stable isotopic composition, Sr/Ca ratios and extension rate (1940–2004) in coral aragonite from a northern Venezuelan site, where reefs were strongly impacted by bleaching following the 1997–98 El Niño. We assess the impact of past warming events on coral extension rates and geochemical proxies. A marked decrease in coral (Pseudodiploria strigosa) extension rates coincides with a baseline shift to more negative values in oxygen and carbon isotopic composition after 1997–98, while a neighboring coral (Siderastrea siderea) recovered to pre-bleaching extension rates simultaneously. However, other stressors, besides high temperature, might also have influenced coral physiology and geochemistry. Coastal Venezuelan reefs were exposed to a series of extreme environmental fluctuations since the mid-1990s, i.e. upwelling, extreme rainfall and sediment input from landslides. This work provides important new data on the potential impacts of multiple regional stress events on coral isotopic compositions and raises questions about the long-term influence on coral-based paleoclimate reconstructions.