Shallow Shear-Velocity Structure Below Strong-Motion and
Precarious-Rock Sites Near the San Andreas Fault

John N. Louie and Robert E. Abbott
Seismological Laboratory and Dept. of Geological Sciences
Mackay School of Mines, The University of Nevada, Reno, NV 89557-0141
Phone: 775-784-4219 Email: louie@seismo.unr.edu Web: www.seismo.unr.edu/ftp/pub/louie/larse/

Abstract

Microtremor noise recordings made on 200-m-long lines of seismic refraction equipment can estimate shear velocity with 20% accuracy, often to 100 m depths. With no source, densely urbanized locations give better results than quiet rural settings. We have performed hammer refraction and passive microtremor surveys at eight sites near the Mojave segment of the San Andreas. Thirty-meter average velocities estimated from P-wave hammer refraction agreed with surface-wave results to better than 10%, under the assumption that soils have Poisson's ratios of 0.25-0.30. Surface-wave dispersion modeling cannot duplicate the detail in the velocity profile yielded by a suspension logger. However, we were able to match the average velocity of 10-20 m depth ranges and suggest structure below the 100 m depth of the Rosrine hole at the Newhall Fire Station. The so-called ``rock'' sites of strong-motion recording we have surveyed at Newhall, Mill Creek Summit, and Palmdale have 30-meter average shear velocities well below the 760 m/s standard used in creating the predictive ground-motion maps. The precarious-rock sites all have 30-meter velocities significantly above the 760 m/s standard.

Site Map
Fig. 1 - Map of sites measured for shallow velocities.

Velocities at Piute and Lovejoy Buttes
Fig. 2 - Velocities at Piute and Lovejoy Buttes.

Velocities at Aliso Can. and Gleason Rd.
Fig. 3 - Velocities at Aliso Canyon and Gleason Road.

Velocities at Antelope Buttes and Palmdale
Fig. 4 - Velocities at Antelope Buttes and Palmdale.

Velocities at Newhall and Mill Creek Summit
Fig. 5 - Velocities at Newhall Fire Station and Mill Creek Summit, compared against velocity logs of the Rosrine borehole.

Comparison of 30-m Velocity Estimates
Fig. 6 - Comparison of 30-m averaged velocity estimates; with shear-velocities derived from microtremor modeling on the horizontal, and estimated from P-wave refraction results on the vertical, assuming Poisson's ratio = 0.25. Hard-rock sites are consistent with higher Poisson's ratios, approaching 0.37. An alternative interpretation is that the linear microtremor arrays are underestimating larger 30-m shear velocities by 22%.

Download a summary of shallow velocity data.

Summary

Site Name 30-m Shear Velocity from Microtremor 30-m Shear Velocity Est. from Refraction Comments
Piute Butte 1465 m/s 1627 m/s Precarious Rocks 20 km from SAF, Weak-Motion Records
Aliso Canyon 1246 m/s 1585 m/s Semi-Precarious Rocks 15 km from SAF
Lovejoy Butte 1046 m/s 1266 m/s Semi-Precarious Rocks (0.4-0.5 g overturning accel.) 16 km from SAF, Weak-Motion Records
Gleason Road 795 m/s 760 m/s Semi-Precarious Rocks 15 km from SAF
Antelope Buttes 778 m/s 862 m/s Semi-Precarious Rocks 11 km from SAF
Palmdale 740 m/s not available PDL Strong-Motion Records
Mill Creek Summit 462 m/s 486 m/s MCS Strong-Motion, and Weak-Motion Records
Newhall Fire Station 327 m/s 309 m/s Strong- and Weak-Motion Rec., Rosrine Log
       
Refraction microtremor techniques effectively found 30-m average shear velocity at a variety of urban and rural locations. Some of the rural locations were too quiet to yield excellent-quality microtremor surface-wave dispersion results. However, the excellent hammer refraction results from quiet locations corroborated the less certain microtremor results. Also, with 200-m refraction arrays, refraction never constrained velocities below 35 m depth, while the microtremor records usually constrained velocity to 100 meters. At each site the microtremor refraction setup, recording, and tear-down took about 2 hours with 3 people. Refraction recording added another two hours. See the demo analysis on the laptop, or a UNR class lab exercise on the method.

Last Updated on 9/17/00
By John Louie