No abstract

Plate tectonics successfully describes the surface of Earth as a mosaic of moving lithospheric plates. But it is not clear what happens at the base of the plates, the lithosphere-asthenosphere boundary (LAB). The LAB has been well imaged with converted teleseismic waves, whose 10-40-kilometre wavelength controls the structural resolution. Here we use explosion-generated seismic waves (of about 0.5-kilometre wavelength) to form a high-resolution image for the base of an oceanic plate that is subducting beneath North Island, New Zealand. Our 80-kilometre-wide image is based on P-wave reflections and shows an approximately 15° dipping, abrupt, seismic wave-speed transition (less than 1 kilometre thick) at a depth of about 100 kilometres. The boundary is parallel to the top of the plate and seismic attributes indicate a P-wave speed decrease of at least 8 ± 3 per cent across it. A parallel reflection event approximately 10 kilometres deeper shows that the decrease in P-wave speed is confined to a channel at the base of the plate, which we interpret as a sheared zone of ponded partial melts or volatiles. This is independent, high-resolution evidence for a low-viscosity channel at the LAB that decouples plates from mantle flow beneath, and allows plate tectonics to work.

Multichannel analysis of surface waves (MASW) and refraction microtremor (ReMi) are two of the most recently developed surface acquisition techniques for determining shallow shear-wave velocity. We conducted a blind comparison of MASW and ReMi results with four boreholes logged to at least 260 m for shear velocity in Santa Clara Valley, California, to determine how closely these surface methods match the downhole measurements. Average shear-wave velocity estimates to depths of 30, 50, and 100 m demonstrate that the surface methods as implemented in this study can generally match borehole results to within 15% to these depths. At two of the boreholes, the average to 100 m depth was within 3%. Spectral amplifications predicted from the respective borehole velocity profiles similarly compare to within 15% or better from 1 to 10 Hz with both the MASW and ReMi surface-method velocity profiles. Overall, neither surface method was consistently better at matching the borehole velocity profiles or amplifications. Our results suggest MASW and ReMi surface acquisition methods can both be appropriate choices for estimating shearwave velocity and can be complementary to each other in urban settings for hazards assessment.

Sedimentary basins can trap earthquake surface waves and amplify the magnitude and lengthen the duration of seismic shaking at the surface. Poor existing gravity and well-data coverage of the basins below the rapidly growing Reno and Carson City urban areas of western Nevada prompted us to collect 200 new gravity measurements. By classifying all new and existing gravity locations as on seismic bedrock or in a basin, we separate the basins' gravity signature from variable background bedrock gravity fields. We find an unexpected 1.2-km maximum depth trough below the western side of Reno; basin enhancement of the seismic shaking hazard would be greatest in this area. Depths throughout most of the rest of the Truckee Meadows basin below Reno are less than 0.5 km. The Eagle Valley basin below Carson City has a 0.53-km maximum depth. Basin depth estimates in Reno are consistent with depths to bedrock in the few available records of geothermal wells and in one wildcat oil well. Depths in Carson City are consistent with depths from existing seismic reflection soundings. The well and seismic correlations allow us to refine our assumed density contrasts. The basin to bedrock density contrast in Reno and Carson City may be as low as −0.33 g/cm 3. The log of the oil well, on the deepest Reno subbasin, indicates that Quaternary deposits are not unusually thick there and suggests that the subbasin formed entirely before the middle Pliocene. Thickness of Quaternary fill, also of importance for determining seismic hazard below Reno and Carson City may only rarely exceed 200 m. * Numbers beginning 277 are the order in the Moana Hot Springs listing in Garside and Schilling (1979), pp. 134-138. Other numbers are the final five digits of the American Petroleum Institute (API) well number (which would be preceded by 27-031); Nevada State Department of Mineral Resources permit numbers, starting with NV; and Nevada State Department of Water Resources permit numbers, starting with DWR. * * May be the active Sierra Pacific Power Co. municipal water supply well at Harvard Way and Marker Street, Reno. † NBMG = Nevada Bureau of Mines and Geology. † † Authors' reinterpretation of driller's log puts the top of the Hunter Creek Sandstone at 168-m depth.

This study assesses a 60 km NNE-SSW transect along the San Gabriel River for shallow shear velocities, in San Gabriel Valley and the Los Angeles Basin of southern California. We assessed a total of 214 sites, 199 along the transect at 300 m spacing, during a one-week field campaign with the refraction microtremor (ReMi) technique. The transect's maximum 30-meter shear velocity (V s 30) occurs in coarse alluvium of San Gabriel Valley where the San Gabriel River exits the San Gabriel Mts.; at 730 m/s, upper NEHRP site class C. Much of the northeast section of the transect (in San Gabriel Valley) is also NEHRP class C, or near the CD class boundary. The section of the transect south from Whittier Narrows to Seal Beach shows NEHRP-D velocities in active alluvium. The transect's lowest V s 30 , 230 m/s at the Alamitos Bay estuary, is also classed as NEHRP-D. An increase toward the NEHRP CD class boundary occurs at the shoreline beach outside Alamitos Bay, confirmed by additional measurements on Seal Beach. Our measured V s 30 values generally show good correlation with published site-classification maps and existing borehole data sets. There is no evidence in our data for an increase in velocity predicted by Wills et al. (2000) at their "CD" to "BC" site classification boundary at the San Gabriel Mountains front, nor for any decrease at their "D" to "DE" class boundary at Alamitos Bay. Very large V s 30 variations exist in soil and geologic units sampled by our survey. The V s 30 variations we measured are smaller than V s 30 variations of 30% or more we found between closely spaced (<0.5 km) downhole measurements in the Los Angeles Basin, which are not uncommon within a community data set we examined showing hundreds of boreholes. We find the San Gabriel River's hydraulic gradient to be a good predictor of minimum V s 30 , based on the expected effect of the hydraulic gradient on the grain size of sediments deposited by a river. The V s 30 data show a fractal spatial dependence, which appears at distances greater than 700 m. The unprecedented number of shearvelocity measurements we have made suggests that large measurement populations may be necessary to properly characterize V s 30 trends within any surficial geological unit.