Earthquake Hazard Surveys Across Urban Basins Nevada Seismological Laboratory

• Final Technical Report to the USGS on ``Improving Next-Generation Attenuation Models with Shear-Velocity Measurements at All TriNet and Strong-Motion Stations in LA,'' grant 05HQGR0078, Jan. 1, 2007 (1.2 Mb PDF)
• Shallow Vs measurements in Hutt and Wellington: Incorporation into a community seismic model presentation for GNS Science, New Zealand, April 27, 2006. (6.4 Mb Quicktime movie) The animations in the presentation are available as:
  • Benites & Olsen model: 14.4 Mb Quicktime movie, or 2.9 Mb MPEG-4 for iPods.
  • New model: 13.3 Mb Quicktime movie, or 2.9 Mb MPEG-4 for iPods.
• "Shallow Shear Velocity and Seismic Microzonation of the Urban Las Vegas, Nevada Basin" by James B. Scott, Tiana Rasmussen, Barbara Luke, Wanda Taylor, J. L. Wagoner, Shane B. Smith, and John N. Louie, BSSA, vol. 96, no. 3 (June 2006), pp. 1068-1077, doi: 10.1785/0120050044. (600 kb PDF preprint)
• "A Transect of 200 Shallow Shear Velocity Profiles Across the Los Angeles Basin" by Thelen et al., BSSA, vol. 96, no. 3 (June 2006), pp. 1055-1067, doi: 10.1785/0120040093. (920 kb complete PDF preprint; 7.1 Mb high-res figures in PDF)
• James B. Scott, Matthew Clark, Christopher T. Lopez, Aasha Pancha, Tiana Rasmussen, Shane B. Smith, Weston A. Thelen, and John N. Louie, 2004, Three urban transects of shallow shear velocity using the refraction microtremor method: Proceedings of the Managing Risk in Earthquake Country Conference Commemorating the 100th Anniversary of the 1906 Earthquake, April 18-22, San Francisco, Calif., 10 pp.

  Waves hit Las Vegas PDF

  (1.8 Mb PDF)
• Refraction Microtremor for Shallow Shear Velocity in Urban Basins presentation for the Royal Society of Wellington, New Zealand, Aug. 18, 2005; also as a 14.2 Mb Powerpoint file, or a 9.6 Mb PDF. The animation in the presentation is available as a:
  • 40 Mb Quicktime movie, or
  • 24 Mb AVI movie.
  Associated are an analysis of the effects of geotechnical details on 10-sec and 2-sec synthetics (1.5 Mb PPT; 2.4 Mb PDF), and an analysis of the effects of complex basin edges on 2-sec synthetics (848 kb PPT, 2.1 Mb PDF).
• Refraction Microtremor for Shear Velocity in California Urban Basins presentation for the California Geological Survey, Feb. 24, 2005; also as a 12.4 Mb Powerpoint file.
• 2004 Fall AGU Poster on Las Vegas velocity and response models by Louie et al.: JPEG, or 3.7 Mb PDF.
• Louie, J. N., Scott, J. B., Rasmussen, T., Thelen, W. A., Pancha, A., Clark, M., Park, H., and Lopez, C. T., 2004, Shallow shear-velocity transects of urban areas, and seismic-hazard mapping: presented at Seismol. Soc. Amer. Ann. Mtg., Palm Springs, April 15.
• John N. Louie, Aasha Pancha, Glenn P. Biasi, Weston Thelen, James B. Scott, Mark F. Coolbaugh, and Shawn Larsen, 2005 in prep., Tests and applications of 3-d geophysical model assembly: for submittal to Geophys. Res. Lett.. (385 kb PDF)
• James B. Scott, John N. Louie, Tiana Rasmussen, Weston A. Thelen, Aasha Pancha, Matthew Clark, Hyunmee Park, and Christopher T. Lopez, 2004, Three urban shear-velocity transects using the refraction microtremor method: Expanded Abstracts, Soc. of Explor. Geophys. 74th Annual Internat. Meeting, Oct. 10-15, Denver, Colorado. (2.1 Mb PDF)
• Surveying Seismic Microzonation in Urban Areas presentation; also as Powerpoint Package and Powerpoint file.
• Surveying Seismic Microzonation in Urban Areas presentation for Ben Gurion Univ., Dec. 23 2003.
• Poster on the Las Vegas transect for the 2003 Geological Society of America meeting, Seattle: 472 kb JPEG; 2.4 Mb Powerpoint.
• Scott, J. B., M. Clark, T. Rennie, A. Pancha, H. Park and J. N. Louie, 2004, A shallow shear-wave velocity transect across the Reno, Nevada area basin: Bull. Seismol. Soc. Amer., 94(6), 2222-2228.

• Excel workbook of final Vs30 results from the San Gabriel River transect>
• Ascii table of longitude, latitude, Vs30 for San Gabriel River transect: sgriv-remi.asc.
• Ascii table of longitude, latitude, Vs30 for Las Vegas transect, plus data from B. Luke, UNLV: lv-v30-meas+prof.asc.
• Aug. 4, 2004 final report to USGS-NEHRP, ``Improving Southern California Seismic Hazard Models With a 45-km Shear Velocity Profile Along the San Gabriel River'' (2.5 Mb PDF)
• Las Vegas Valley Seismic Response Project orientation page at UNLV
• Excel spreadsheets of preliminary plus final analyses for: San Gabriel River, Las Vegas.
• Link to San Gabriel River and Las Vegas Fieldwork Photo Albums
• May 2002 proposal to NEHRP program: RTF, HTML, budget
• More information on the refraction microtremor surveying technique.

Final Results from San Gabriel River, Los Angeles

Preliminary Plus Final Results from Las Vegas

Some velocity results in Las Vegas were obtained by Prof. Barbara Luke of 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.

Objectives: These surveys of shallow ground shear velocity provide information that will assist in mitigating earthquake hazards in the Los Angeles and Las Vegas urban areas, and in responding to a damaging earthquake. This work is a reconnaissance, looking for any ``hot spots'' in the seismic shaking hazard. As shallow shear velocity decreases, shaking hazard increases.

Background: Shallow velocity is traditionally estimated with engineering surveys in boreholes. 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 both cities. Hazard mitigation and earthquake emergency response planning both need continuous mapping of the shaking hazard. To date these hazard maps have been predicted partly from geologic map units, but our results from Reno (below) show that existing maps may not predict shallow velocities.

Methods: Both transects were completed by UNR students using rolled arrays of IRIS/PASSCAL ``Texan'' single-channel recorders, as for the Reno transect. Data analysis was with Optim's SeisOpt<sup>®</sup> ReMi<sup>TM</sup> package, a method developed by Louie (April 2001 BSSA). Each team of 3 students placed an array of 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. 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.

Results: Preliminary estimates of 30-m depth averaged velocities, raw Rayleigh phase velocities, are plotted above. The curves are unexpectedly smooth. Results of dispersion-curve modeling will be posted here in August, and we expect modeling results to show more variance.

• In both cities the lowest velocities observed are close to the NEHRP C/D class boundary, just under 350 m/s. Several deployments around Alamitos Bay, Long Beach, did not find any very low NEHRP-E class velocities, despite being made on artificial fill in a lagoon.
• Along the San Gabriel River, velocities stay near the B/C boundary, just below 760 m/s, through Azusa and the Santa Fe Dam area, despite many kilometers of basin thickness. The bouldery and cobbly nature of the surface there suggests a large 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, or perhaps with the presence of older sediments, uplifted by the Whittier blind thrust.
• The lowest velocities along the San Gabriel river are near Carson Street and Bellflower, corroborated by a cross-line extending 2 miles west of the river. Velocities rise slightly at Seal Beach, perhaps due to added littoral sand, or perhaps due to the Holocene uplifts between strands of the Newport-Inglewood fault zone.
• In Las Vegas the lowest velocities in the transect, at the NEHRP C/D boundary, are found near Interstate 15 and Lake Mead Blvd. Velocities then rise smoothly to the middle of the NEHRP-C range near Sahara Blvd. to the south. There appears to be a slight decline further south to Tropicana Blvd.

• Ascii table of longitude, latitude, Vs30 for Reno transect, plus additional measurements by Aasha Pancha: reno-v30-meas.asc.

• Poster presented at the American Geophysical Union Fall Meeting, San Francisco, Dec. 9 (page size PDF, 3 Mb).
• A poster for the 2002 Society of Exploration Geophysicists Annual International Meeting (page size PDF, 5.1 Mb).
• A poster for the 2002 IRIS Workshop describing the results (page size PDF, 6.7 Mb).
• An article describing the experiment, intended for the IRIS Newsletter.
• Photo album from 3 of the 9 field days.
• A flyer to inform property owners and cooperating officials.
• More information on the refraction microtremor surveying technique.

Example data files of results from:

• Preliminary Reno amplification map, incorporating a V₃₀ model and the Reno transect measurements. Click on the images for PDF versions.


• Prior Reno amplification map, using Wills et al. (2000) geologic-map classifications and Abbott & Louie (2000) basin depths from gravity only, no local V₃₀ measurements. Click on the images for PDF versions.


• Plots of preliminary results; distance is from west on the left to east on the right. Click on the images for PDF versions.


Results:
1. This 15-km transect, resulting in 50 shear-velocity vs. depth profiles, was completed in 9.5 days by a crew of 3. Acquisition used 45 IRIS/PASSCAL Texan recorders and followed a river bicycle path.
2. 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.
3. Neither geological nor agricultural soil map units can serve to predict NEHRP hazard class or shallow velocities in the Reno area. Where the depth to a Tertiary diatomite is small, velocities increase.
4. 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.
5. 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.
• Oct. 1 cluster calibration test
• Oct. 11 survey, to 1.2 km W of Sewer Plant on Cleanwater Wy.
• Oct. 15 survey, 1.2 km W of Sewer Plant to E. McCarran Blvd on Cleanwater Wy.

• Excel workbook of Vs30 results for Wellington and Lower Hutt: wnlh-remi.xls.

• Text table of Vs30 results for Hutt City: hutt-remi.txt.

• Ascii table of Vs30 results for Wellington and Lower Hutt: wnlh-remi.asc

• Ascii table of Z0.6 results for the Parkway neighborhood, Hutt City: Parkway-remi-depth.asc

• Bedrock-surface map:
  
• Vs30 map:
  
• Maximum horizontal ground velocity map computed with E3D at 0.5 Hz, dx=100 m: