Assessments of Conditions at 70 Rock Sites Having Vs30 near the NEHRP B-C Boundary with Measures of Heterogeneity J.B. Scott1, J.N. Louie1, D. Pei1, K. Heath1, G. Biasi1, A. Pancha1, S. Pullammanappallil2 1. Nevada Seismological Laboratory 2. Optim, Inc.

Vs30 From 600-1329 m/s at 70 sites (NEHRP B-C Boundary) Large addition to higher velocity database. Vs30 determined using Refraction Microtremor (ReMi) . Useful in developing attenuation relations. Lateral heterogeneity observed at some sites. Possible explanation for uncertainties in fitting attenuation relations to ground-motion data.

Lateral Heterogeneity Investigations at 15 Sites Vs30 determined from two orthogonal 285 m-long ReMi arrays originating at the same point: Vs30 #1 Vs30 #2

Orthogonal Line Differences Average difference = 17%; Standard Dev. = 16% Largest difference = 57%; smallest = 1%.

Spatial Variability Experiment Vs30 from four orthogonal ReMi line locations compared to 16 downhole measurements within an area of <1 km2. 285-m arrays are shown in next slide as lines of triangles representing geophone positions; dots are borehole positions.

EXHS 122 m MDVS

Substantial scatter in Vs profiles from downhole logs even though they are closely grouped (< 270 m radius circle)

Comparison of Vs from orthogonal lines originating at station position MDVS to nearest borehole (RF#19)

Observations The Vs30 averages for all boreholes: The Vs30 averages for ReMi: EXHS: Vs30 = 687 ± 29.1; MVDS: Vs30 = 601 ± 45.0. If surface layers only seen in the borehole logs are ignored, only small differences in Z0; These low-velocity surface layers may be laterally limited "soft spots" not prevalent over the 1 x 1-km area.

Effect of Soil cover on Vs30 ReMi Vs30 at CISN station PAS = 638 m/s Borehole Vs30 at PAS = 969 m/s

Note the shallow low velocity layers in this ReMi model; The start of the borehole log is below the surface measured by ReMi

Soil Effects The hill-top is topographically old and could have a Quaternary-age soil developed atop it. Removing the upper two layers from the stack (equal in thickness to the height of the ridge) would make Vs30=905 m/s, closer to the downhole Vs30=969 m/s. The dispersion in the p-f shows dispersive energy at velocities above the lowest velocity envelope picked (next slide). Under the 1/4-wave approximation the dispersion curve from the downhole Vs30 would intersect our picks at the star and cross on the plots in the next slide.

Conclusions Borehole logs are single point measurements; ReMi is a measure of a volume average: ReMi incorporates velocities found within the long 30- to 700-m wavelengths. Heterogeneity in soil cover can account for scatter in Vs30. Such factors also suggest how ReMi measured lower Vs30 values at southern California rock sites than were expected, with few above the NEHRP B-C boundary.