Printed in the Bulletin of the Seismological Society of America, December 2004, vol. 94, p. 2222-2228.
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In October and November 2001, we performed an urban shear-wave-velocity transect across 16 km of the Reno, Nevada area basin. Using the refraction microtremor method of Louie (2001) we determined shear-wave velocity versus depth profiles at 55 locations. Shear-wave velocity averaged to 30 m depth (Vs30) is one predictor of earthquake ground motion amplification in similar alluvium-filled basins, and it is the basis of site hazard classification under NEHRP-UBC provisions. A geologic map-based NEHRP classification along nearly all of our transect line would be NEHRP-D, but our measurements of Vs30 revealed that 82% of the transect is classified NEHRP-C. Relatively stiff Tertiary sediments underlie the surface of the Reno basin, and weaker soils occur east of downtown Reno in the floodplain of the Truckee River. Although 53 of our locations were on the geologically youngest and most active fluvial units, these sites showed Vs30 values ranging from 286 m/s (NEHRP-D) to 849 m/s (NEHRP-B). Mapped geologic and soil units are not accurate predictors of Vs30 measurements in this urban area. A test model based on gravity results showed Quaternary-alluvium depth can be combined with transect Vs30 measurements to predict Vs30 across the Reno basin.
|Fig. 1: Map showing the location of our transect across the Reno-Sparks, Nevada urban basin. Dots mark the locations of the endpoints of each 300-meter-long segment of the microtremor recording arrays. Large white numerals indicate distances along the transect from its west end. Regionally grouped geologic units from the NBMG Washoe County Digital Geologic Map (2000) are: mv, Peavine Sequence, metavolcanic rocks; mvs, Peavine Sequence, undifferentiated metavolcanics/metasediments; Qal, Stream deposits; Qls, Landslide deposits; Qtg, Pre-Lake Lahontan deposits; Ta, Alta Formation; Tab, Basalt, basaltic and andesite; Tk, Kate Peak formation, flows or flow breccia; Tst, Pliocene sedimentary rocks.|
Our transect nearly coincided with an earlier study of Neogene basin depth by Abbott and Louie (2000). They produced a 2-D depth profile of the Reno basin based on gravity modeling supported by borehole data. Their model depths for each of our transect points are shown in figure 2.
|Fig. 2: Depth-of-alluvium and depth-to-basement-rock profile for points along our transect (fig. 1) from the gravity model of Abbott and Louie (2000). The densities of each rock unit used in modeling are shown. Refer to fig. 1 for geographical reference of transect distances.|
The array data were processed according to Louie's (2001) refraction microtremor analysis to produce a shear-wave-velocity-versus-depth profile for each 15-instrument, 300-m segment. Figure 3 shows an example of the seismic records from a 900 meter-long array east of downtown Reno, and also a slowness-frequency spectrum from a 300-m array downtown. We picked Rayleigh dispersion points along a minimum-velocity envelope of the energy in the slowness-frequency image. Typical dispersion runs from small slowness values at low frequencies, down to the right toward larger slowness values at higher frequencies (fig. 3, right). The picks for this array run from frequencies of 1.16 to 10.55 Hz.
We interactively forward modeled the fundamental-mode Rayleigh dispersion curve for each 300-m segment to obtain a shear-wave-velocity versus depth relationship. As described by Louie (2001), the interactive forward-modeling process included analyses of the depth to which shear-wave velocities can be resolved from the Rayleigh dispersion picks. We computed Vs30 by arithmetically averaging slownesses to 30 m depth (by slowness in order to preserve bulk travel time). The total time to measure one location, from array layout to velocity modeling, can be as little as two hours.
|Fig. 3: a) Twenty seconds of example seismic data from a 900-meter-long E-W array of 45 recorders crossing Wells Avenue in Reno (at transect distance 6.1 km). Note the prominent waves radiating east and west from heavy traffic at 360 m/s. b) Example slowness-frequency (p-f) spectrum, in the manner of Louie (2001), for a 300-meter-long array extending west from Virginia St. in downtown Reno, at 4.9 km transect distance. Increased spectral ratio, defined as the ratio of the spectral power at a particular slowness and frequency over the average spectral power for all slownesses at that frequency, is represented as lighter parts of the p-f image. Square symbols mark the fundamental-mode Rayleigh-wave phase velocity dispersion picks for this array. With a slowness range on the vertical axis of 0-0.01 sec/meter, this p-f spectrum analyzes for apparent velocities (in the W-E direction of this linear array) of 100 m/s at the bottom edge of the plot to infinite velocity at the top edge.|
|Fig. 4: Our measured values of Vs30 for 55 points along the Reno transect. Vs30 values are slowness-averaged from 55 modeled velocity-depth profiles. Sites with Vs30 above 760 m/s are classified as NEHRP B; between 760 and 360 m/s as NEHRP C; and 360 to 150 m/s as NEHRP D (plot extends only to 200 m/s). Refer to fig. 1 for geographical references for transect distances.|
|Fig. 5: Shear-wave-velocity section assembled from 55 velocity-depth profiles that model the dispersion curves derived from all fifty-five, 300-m microtremor array segments. Velocities deeper than 200 m are least constrained by the dispersion data, and thus vary widely between the segments. To prevent this unconstrained variation from dominating the contour plot, the velocities were slowness-smoothed horizontally with a 300-meter moving window before contouring. Shallower velocities show more stability, but are also horizontally smoothed for this plot. This section has 20 times vertical exaggeration. Values contoured are in m/s. Contour interval is 100 m/s between 300 and 800 m/s. (Velocities not between 300-800 m/s are not contoured.) Refer to fig. 1 for geographical references for transect distances.|
Vs30 measurements become relatively high just east of where the transect crosses West McCarran Blvd. (1.6 km transect distance on figs. 1 and 4). This segment, identified as Qto on the basis of the geologic map, would previously have been classified as NEHRP-C/D. We found that location to have a Vs30 of 849 m/s, classifying it instead as NEHRP-B. The velocity section of figure 5 shows that, even after lateral smoothing, high-velocity contours (600 m/s and above) come very close to the surface at this location. Abbott and Louie (2000) proposed from gravity and well data that higher-density, more lithified Tertiary sandstones and diatomites (compared to young Quaternary sediments) are buried at about 200 m depth east of downtown Reno, but shallow and crop out frequently west of downtown. The 600 m/s velocity contour in fig. 5 may be tracing the top of the Tertiary sediments, in this cross section.
The correlation between adjacent measurements is good, as fig. 4 shows, for example, in the multi-point Vs30 gradient between 0 and 2 km distance. A separate measurement of Vs30 on the University of Nevada, Reno campus (see location in fig. 1) was 360 m/s, in contrast to the corresponding transect value at the 4.9 km distance of 507 m/s (fig. 4). We interpret this difference to be due to greater stiffness in the gravelly sediments adjacent to the Truckee River, contrasting against the lesser stiffness of the deeply-weathered clay-rich soils found on terraces above the river, as on the campus.
To assess the relationship between our measured values of Vs30, and mapped geologic formation or soil type, we consulted geologic maps (Bonham and Bingler, 1973; and Bell and Bonham, 1987) and soil maps (NBMG 1973; USDA-SCS, 1978) along the transect. A geologic formation type and a soil type were assigned to each transect segment. The measured values of Vs30 from each segment are plotted for each geologic formation in fig. 6, and Table 1 describes the formations. The measured values of Vs30 for each soil type at each segment are plotted in fig. 7, and Table 2 describes the soil types.
|Fig. 6: Measured values of Vs30 for each geologic formation type sampled by our transect (Table 1). Geology was was drawn from the specific local classifications of Bell and Bonham (1987) and Bonham and Bingler (1973), which differ from the regional units broken out for all of Washoe county by the NBMG digital geologic map (2000). The velocity average and standard deviation for each formation are indicated. No deviation is shown for the formation sampled only once.|
|Fig.7: Measured values of Vs30 vs. soil-type designations (Table 2). Most individual units have just one Vs30 measurement and thus no deviation. The velocity average and standard deviation of the other units are indicated.|
Fifty-three of our locations were on the geologically youngest and most active fluvial units (in contrast to nearby older alluvium, and Tertiary sandstones and volcanics, fig. 1). However, these sites showed Vs30 values ranging from 286 m/s (NEHRP-D) to 849 m/s (NEHRP-B). This large range of our Vs30 measurements on these units suggests that the presence of young, active alluvium is not a predictor of low shear velocities (NEHRP classes C/D or D).
Following the suggestion in fig. 5 that the depth of the top of Tertiary sediments may be appearing in our velocity results, we developed a simple model using the thickness of Quaternary alluvium from fig. 2 to attempt an independent prediction of Vs30 along the transect. A good fit to our transect data was obtained by choosing a value of 400 m/s for the shear-wave velocity in Quaternary alluvium and 600 m/s for the underlying Tertiary sandstone and diatomite. The value of Vs30 was computed as follows, by arithmetic averaging of slownesses:
Vs30 = 30/(ta/400 + tr/600)where ta is the thickness of alluvium (<30 m) and tr = (30-ta). The results from this simple model are compared to measured Vs30 values in fig. 8.
|Fig.8: Vs30 according to an alluvium-depth model derived from a basin-gravity survey (Abbott and Louie, 2000) versus the measured values of Vs30 along the transect, with ±20% error bars on the measurements.|
In fig. 6, we could find no predictive correlation between geologic map unit and our Vs30 measurements. The sole exception was where the geology was Tertiary volcanics (Ta) at the east end of the transect, which yielded higher velocities. In fig. 7, we can similarly identify that the variance in Vs30 within each of the several soil groups along the transect is greater than the differences in the average Vs30 between soil groups. Thus, we cannot predict Vs30 from the Reno soil maps. Analysis of the correlations in fig. 6 and fig. 7 using the F statistic (Hoel, 1971) confirmed the lack of correlation.
Seeking any model with a better ability to predict Vs30 in this basin, we found that a simple model based on alluvium depth derived from gravity modeling yielded better predictions. Our assumed values of shear-wave velocity for Reno alluvium of 400 m/s and soft rock of 600 m/s are based on a fit to our transect measurements. All but four of 55 predicted values (93%) were within the nominal ±20% accuracy of the refraction microtremor measurements, as illustrated in fig. 8.
Louie (2001) reports the ±20% velocity accuracy for the refraction microtremor method. To assess the standard deviation due to noise source and dispersion-picking differences for the individual Vs30 values, we made repeat array recordings at three locations along the transect. We independently processed and modeled the two sets of recordings from each repeated segment. The results are shown in table 4. We concluded that the variability in the recorded sources of microtremor noise, and in the picking and modeling process could be expected to contribute perhaps less than ±10% error in Vs30.
We have also shown that gravity-derived alluvium-depth modeling can be combined with refraction microtremor Vs30 measurements to yield good shear-wave-velocity predictions for the Reno basin. Unlike the Los Angeles basin, the Reno basin's greatest depths have stiffer, Tertiary sediments underlying the surface. Weaker soils appear to occur east of downtown Reno in the floodplain of the Truckee River.
Although the majority of our locations were on the geologically youngest and most active fluvial units, these sites showed Vs30 values ranging from NEHRP hazard class D up to class B. Mapped geologic and soil units are not accurate predictors of Vs30 measurements in this urban area.
|Ta||Alta Formation: pyroxene, pyroxene hornblende, and hornblende andesite flows; debris flows; and pyroclastic flows.|
|Qfl||Flood plain deposits of the Truckee River: Light grey to dark grey-brown silt, sandy silt, and clayey silt with local lenses of well-rounded pebble to cobble gravel; derived from mainstream and overbank deposition of the Truckee River; includes oxbow lakes and old channels. Locally contains peat layers.|
|Qto||Tahoe Outwash: Gray, sandy, cobble to boulder gravel with lenses of light brown to light grey medium sand and light grey clayey silt. Gravel and sand are well stratified with fluvial crossbedding, and are moderately to poorly sorted. Gravel composed dominantly of well-rounded volcanic and granitic clasts; granitic boulders as much as 3 m in diameter occur in Truckee meadows and giant Mzv blocks as much as 10 m in diameter occur at Mustang. Deposits typically occur on terrace remnants and underlie much of Truckee meadows at shallow depths. Soil typically has strongly developed argillic B horizon 30-60 cm thick.|
|Qdo||Donner Lake Outwash: Grey to brown, sandy cobble to boulder gravel composed dominantly of well-rounded volcanic and granitic clasts. At Mustang deposit, is deeply channeled and contains giant Mzv boulders 3 m or more in diameter. Soil typically has a strongly developed argillic B horizon 60-100 cm thick underlain by a siliceous and calcareous duripan about 60 cm thick.|
|Qmg||Mainstream Gravel: Sandy cobble gravel confined to the present Truckee river floodplain.|
|450||Cave variant very cobbly, very fine sandy loam, 4 to 30% slopes|
|590||Springmeyer stony loam 0-2% slopes: very deep soils with moderately coarse to moderately fine textured subsoils on alluvial fans, terraces and scarps.|
|591||Springmeyer stony loam 2-4% slopes: very deep soils with moderately coarse to moderately fine textured subsoils on alluvial fans, terraces and scarps.|
|661||Qest bouldery sandy loam, 2-8% slopes: very deep soils with moderately coarse to moderately fine textured subsoils on alluvial fans, terraces and scarps.|
|669||Qest gravelly sandy loam, 0-2% slopes: very deep soils with moderately coarse to moderately fine textured subsoils on alluvial fans, terraces and scarps.|
|705||Poor and poorly drained soils on floodplains and low terraces: Jame Canyon very fine sandy loam, overwash, 0-2% slopes.|
|800||Truckee Loam: Poor and poorly drained soils on flood plains and low terraces.|
|802||Truckee Loam, strongly saline: Poor and poorly drained soils on flood plains and low terraces.|
|810||Rose Creek fine sandy loam: Poor and poorly drained soils on flood plains and low terraces.|
|812||Rose Creek loamy fine sand: Poor and poorly drained soils on flood plains and low terraces.|
|813||Rose Creek fine sand loam: Poor and poorly drained soils on flood plains and low terraces.|
|991||Madeland: Cut and fill areas of admixtures of soil and non-soil materials.|
|Walker Lake||1963||Antelope Valley|
|Distance||Vs30 Values||Vs30ave||sigma||(sigma/Vs30ave)x100 %||Approx.Difference, ±%|