Three urban shear-velocity transects using the refraction...
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Three urban shear-velocity transects using the refraction microtremor method James B. Scott*, John N. Louie, Tiana Rasmussen, Weston A. Thelen, Aasha Pancha, Matthew Clark, Hyunmee Park, Christopher T. Lopez, Nevada Seismological Laboratory. Summary These surveys of shallow ground shear velocity provide information that will assist in mitigating earthquake hazards in the Reno, Los Angeles and Las Vegas urban areas, and in responding to a damaging earthquake. This work is a reconnaissance, looking for ``hot spots'' in the seismic shaking hazard. As shallow shear velocity decreases, shaking hazard increases (Borcherdt, et al., 1991). The three transects were completed quickly and economically using the refraction microtremor method of Louie (2001). The results were tested for consistency with physical processes by applying a fractal analysis. Introduction Shallow velocity is traditionally estimated with engineering surveys in boreholes (ASTM, 2003). Such boreholes are costly, with results available from less than 50 sites in Los Angeles and about five sites in Las Vegas. Shaking hazard is best measured by recording damaging earthquakes on strong-motion seismometers, from which data are similarly sparse in all three cities. Hazard mitigation and earthquake emergency response planning both need complete mapping of the shaking hazard. To date these hazard maps have been predicted from geologic map units and sparse downhole shear-velocity measurements (Wills, et al. 2000), but our results from Reno (fig. 4, below) show that existing maps may not predict shallow velocities. Methods The three transects were completed by University of Nevada, Reno students using rolled arrays of IRIS/PASSCAL ``Texan'' single-channel recorders, Data analysis was with Optim's SeisOpt® ReMi ΤΜ package, a method developed by Louie (2001). Each team of 3 students placed an array of 45 (in Reno), 30 (in LA) or 40 (in LV) channels spaced at 20 m for a half-hour recording period. Each channel recorded a 4.5-Hz vertical geophone. Mostly ambient seismic ``microtremor'' noise was recorded, although some arrays were supplemented with hammer blows for P-wave refraction analysis. Geophones were not buried, but were leveled carefully. The 15 km Reno transect was completed in 2001 by 3 students in 9 days. With the 12-member crew, the 60-km-long San Gabriel River transect of 99 (600 m) array placements was completed in 4.5 days; the 14-km-long Las Vegas transect of 17 (800 m) array placements was completed in 2 days. Several months of permitting, preparation, and training activities preceded the July 4-16, 2003 field exercises. Conclusions Preliminary estimates of 30-m depth averaged velocities are plotted in figs. 1-3. The curves are unexpectedly smooth. In all three cities the lowest velocities observed are close to the NEHRP C/D class boundary, just under 350 m/s. Several Los Angeles deployments around Alamitos Bay, Long Beach, did not find any NEHRP-E class (very low) velocities, despite measuring artificial fill in a lagoon. Reno− Along the Truckee River, most of the Reno urban area shows NEHRP class C shallow shear velocities. Very little of the profile shows any significant chance that velocities are in the NEHRP class D range. Neither geological nor agricultural soil map units can serve to predict NEHRP hazard class or shallow velocities in the Reno area (fig. 4). Basin depths were estimated based on a Gravity-depth survey (Abbott and louie, 2000). Where the depth to a Tertiary diatomite is small, velocities increase. The deepest part of the Tertiary sedimentary basin (1.5-2.0 km), near the RF05 strong-motion station, shows relatively high shallow velocities. Low velocities near the UNRX station on the east side of the profile are above only moderately deep (0.5 km) parts of the basin. Lower velocities than along the river were found on the UNR campus 2 km north of downtown Reno, 365 m/s instead of 500 m/s. The campus has deeply weathered clay soil derived from volcanic alluvium, where the Truckee River has removed clay and deposited clean gravel and boulders along its course through downtown.
Transcript of Three urban shear-velocity transects using the refraction...
Three urban shear-velocity transects using the refraction microtremor methodJames B. Scott*, John N. Louie, Tiana Rasmussen, Weston A. Thelen, Aasha Pancha, Matthew Clark, Hyunmee Park,Christopher T. Lopez, Nevada Seismological Laboratory.
Summary
These surveys of shallow ground shear velocity provide information that will assist in mitigating earthquake hazards in the Reno,Los Angeles and Las Vegas urban areas, and in responding to a damaging earthquake. This work is a reconnaissance, looking for``hot spots'' in the seismic shaking hazard. As shallow shear velocity decreases, shaking hazard increases (Borcherdt, et al.,1991). The three transects were completed quickly and economically using the refraction microtremor method of Louie (2001).The results were tested for consistency with physical processes by applying a fractal analysis.
Introduction
Shallow velocity is traditionally estimated with engineering surveys in boreholes (ASTM, 2003). Such boreholes are costly, withresults available from less than 50 sites in Los Angeles and about five sites in Las Vegas. Shaking hazard is best measured byrecording damaging earthquakes on strong-motion seismometers, from which data are similarly sparse in all three cities. Hazardmitigation and earthquake emergency response planning both need complete mapping of the shaking hazard. To date these hazardmaps have been predicted from geologic map units and sparse downhole shear-velocity measurements (Wills, et al. 2000), butour results from Reno (fig. 4, below) show that existing maps may not predict shallow velocities.
Methods
The three transects were completed by University of Nevada, Reno students using rolled arrays of IRIS/PASSCAL ``Texan''single-channel recorders, Data analysis was with Optim's SeisOpt® ReMiΤΜ package, a method developed by Louie (2001). Eachteam of 3 students placed an array of 45 (in Reno), 30 (in LA) or 40 (in LV) channels spaced at 20 m for a half-hour recordingperiod. Each channel recorded a 4.5-Hz vertical geophone. Mostly ambient seismic ``microtremor'' noise was recorded, althoughsome arrays were supplemented with hammer blows for P-wave refraction analysis. Geophones were not buried, but were leveledcarefully. The 15 km Reno transect was completed in 2001 by 3 students in 9 days. With the 12-member crew, the 60-km-longSan Gabriel River transect of 99 (600 m) array placements was completed in 4.5 days; the 14-km-long Las Vegas transect of 17(800 m) array placements was completed in 2 days. Several months of permitting, preparation, and training activities precededthe July 4-16, 2003 field exercises.
Conclusions
Preliminary estimates of 30-m depth averaged velocities are plotted in figs. 1-3. The curves are unexpectedly smooth.
In all three cities the lowest velocities observed are close to the NEHRP C/D class boundary, just under 350 m/s. Several LosAngeles deployments around Alamitos Bay, Long Beach, did not find any NEHRP-E class (very low) velocities, despitemeasuring artificial fill in a lagoon.
Reno− Along the Truckee River, most of the Reno urban area shows NEHRP class C shallow shear velocities. Very little of theprofile shows any significant chance that velocities are in the NEHRP class D range. Neither geological nor agricultural soil mapunits can serve to predict NEHRP hazard class or shallow velocities in the Reno area (fig. 4). Basin depths were estimated basedon a Gravity-depth survey (Abbott and louie, 2000). Where the depth to a Tertiary diatomite is small, velocities increase. Thedeepest part of the Tertiary sedimentary basin (1.5-2.0 km), near the RF05 strong-motion station, shows relatively high shallowvelocities. Low velocities near the UNRX station on the east side of the profile are above only moderately deep (0.5 km) parts ofthe basin. Lower velocities than along the river were found on the UNR campus 2 km north of downtown Reno, 365 m/s insteadof 500 m/s. The campus has deeply weathered clay soil derived from volcanic alluvium, where the Truckee River has removedclay and deposited clean gravel and boulders along its course through downtown.
Three urban shear-velocity transects using the refraction microtremor method
Figure 1: Reno− Our measured values of V30 for points along the transect. Points above 760 m/s are classified NEHRP B,between 760 and 360 m/s NEHRP C and 360 to 150 m/s NEHRP D. Approximate locations along the transect of strong motionmonitoring sites RF05, UNRX and SWTP are shown in bold type.
Los Angeles− Along the San Gabriel River, velocities stay near the B/C boundary, just below 760 m/s, through Azusa and theSanta Fe Dam area, despite many kilometers of basin thickness. The bouldery and cobbly nature of the surface there suggests alarge average clast size, and a steep hydraulic gradient, may correlate with higher shallow velocities. Some increases in velocity,still within NEHRP C, appear near Whittier Narrows Dam. These may correlate with the presence of stiff gravel lenses, orperhaps with the presence of older sediments, uplifted by the Whittier blind thrust. The lowest velocities along the San Gabrielriver are near Carson Street and Bellflower, corroborated by a cross-line extending 2 miles west of the river. Velocities riseslightly at Seal Beach, perhaps due to added littoral sand, or perhaps due to the Holocene uplifts between strands of the Newport-Inglewood fault zone.
Figure 2: Los Angeles− V30 values for 99 transect array measurements are shown in blue. Corroborating measurements are alsoshown with various symbols as they plot back upon the transect line. All values are normalized by the distance of the midpointof the array from the mouth of the San Gabriel Canyon. For comparison, NEHRP classifications from a study by Wills et al.(2000) are shown in purple shaded blocks.
Las Vegas− The lowest velocities in the transect, at the NEHRP C/D boundary, are found near Interstate 15 and Lake MeadBlvd. Velocities then rise smoothly to the middle of the NEHRP-C range near Sahara Blvd. to the south. There appears to be aslight decline further south to Tropicana Blvd.
Three urban shear-velocity transects using the refraction microtremor method
Las Vegas TransectPreliminary & Modeled Results
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Distance South of Cheyenne Ave., km
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Las Vegas Transect, Prelim.Prelim. Station AveragesLas Vegas Transect, FinalCCSN Cheyenne CampusSGS Rock Site, Frenchman Mtn.Las Vegas SpringsDesert Inn and The StripUNLV Campus
Figure 3: Las Vegas− V30 values for transect array measurements are shown in blue. ReMi transect velocities agree well withnearby SASW and crosshole measurements. Only 3 km out of the 15-km-long transect have velocities near or below the NEHRP-C/D boundary (350 m/s). The transect shows significant lateral variations, but velocities do not correlate with surface mapping.
Figure 4: Reno− V30 vs. geologic formation and soil type showing the lack of correlation with either classification.
Spatial variability− Following the analysis method of Mela and Louie (2001), we examined the spatial variability of the shear-velocity data from the three transects. Never before had shallow shear-velocity datasets sampled this well in space. Weperformed a Fourier transform of each transect to a power spectrum over a spatial frequency axis. Again after Mela and Louie,we identified linear slopes on a log-log plot of the spectrum as evidence of fractal variations. We derived a fractal dimension,(D), from the slope values following the formula of Berry (1979). Fractal variations are probably geologic in origin. Our twoshorter transects (fig. 6) show spatial peaks perhaps due to smaller basin size and shorter transect length. Our Los Angeles plot(fig. 5) departs from fractal behavior at the upper spatial frequencies, showing the limit of resolving spatial variations with thistechnique.
Three urban shear-velocity transects using the refraction microtremor method
San Gabriel River Vs30 Transect Spatial Power Spectra
SGRiv: y = 20823x-1.5913
R2 = 0.6705 D=1.70
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Nevada Urban Vs30 Transects Spatial Power Spectra
LVprof: y = 10526x-2.0597
R2 = 0.5691 D=1.47
Reno: y = 28019x-1.3705
R2 = 0.4787 D=1.81
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Figure 5: Los Angeles transect spatial power spectra Figure 6: Reno and Las Vegas transects spatial power spectra
References
Abbott, R.E. and Louie, J.N. Case history: depth to bedrock using gravimetry in the Reno and Carson City, Nevada, area basins,Geophysics, 65, no. 2, 340-350, 2000.
Borcherdt, R. D., Wentworth, C. M., Janssen, A., Fumal, T., and Gibbs, J., Methodology for predictive GIS mapping of specialstudy zones for strong ground motion in the San Francisco Bay region, CA, in Proc. Fourth Int. Cont. on Seismic Zonation,Earthquake Engineering Research Institute, Oakland, California, 545-552, 1991.
ASTM: American Society for Testing and Material D4428: Standard test methods for crosshole seismic testing, Annual Book ofASTM Standards Vol. 04.08 Soil and Rock (I), 2003.
ASTM: American Society for Testing and Materials D5778: Standard test method for performing electronic friction cone andpiezocone penetration testing of soils, Annual Book of ASTM Standards Vol. 04.08 Soil and Rock (I) 2003.
Berry, M.V. Diffractals, J. Phys. A Math Gentile 12, 207-220, 1979
Mela, K. and Louie, J.N., Correlation length and fractal dimension interpretation from seismic data using variograms and powerspectra, Geophysics, 66, no. 5, 1372-1378, September-October 2001.
Louie, J.N., Faster, better: shear-wave velocity to 100 meters depth from refraction microtremor arrays, Bull. Seis. Soc. Amer.,91, no. 2, 347-364, 2001.
Wills, C.J., Petersen, M., Bryant, W.A., Reichle, M., Saucedo, G.J., Tan, S., Taylor, G., and Treiman, J. A site-conditions mapfor California based on geology and shear-wave velocity, Bull. Seis. Soc. Amer., 90, no. 6B, S187-S208, 2000.
Acknowledgements
Thanks to the IRIS PASSCAL Instrument Center at UNM for lending us the “Texan” equipment. Many thanks to Bob Greschkeof PASSCAL for his invaluable assistance in the field. Some velocity results in Las Vegas were obtained by Prof. Barbara Lukeof the UNLV Civil Engineering Dept.; all work there was in collaboration with her and with Prof. Cathy Snelson and Prof.Wanda Taylor of the UNLV Dept. of Geosciences.
