U.S. Geological Survey Award Number G17AP00022 Final

U.S. Geological Survey Award Number G17AP00022

Final Technical Report

Site characterization in the Sacramento-San Joaquin Delta using seismic surface wave and reflection methods

Project Period: January 1st, 2017 – December 31st, 2017

Mitchell Craig Department of Earth and Environmental Sciences California State University, East Bay 25800 Carlos Bee Blvd. Hayward, CA 94542 Phone: 510-885-3425, 510-885-3486 [email protected]

Niket Kundariya Department of Earth and Environmental Sciences California State University, East Bay 25800 Carlos Bee Blvd. Hayward, CA 94542 [email protected]

Maximilian Burnham Department of Earth and Environmental Sciences California State University, East Bay 25800 Carlos Bee Blvd. Hayward, CA 94542 [email protected]

Koichi Hayashi Geometrics, Inc. 2190 Fortune Dr. San Jose, CA 95131 [email protected]

Key Words: near surface geophysics, surface wave methods, shear wave velocity, site characterization, San Francisco-San Joaquin Delta, levees

Site characterization in the Sacramento-San Joaquin Delta using seismic surface wave and reflection methods M. Craig, N. Kundariya, M. Burnham, and K. Hayashi

Abstract Seismic surface wave surveys were performed at nine new sites on three islands in the Sacramento-San Joaquin Delta to measure shear wave velocity (VS). The surveys provide improved VS models of the near surface. Eight surveys used small linear arrays to estimate VS in the upper 30 m, and two surveys used large triangular arrays to estimate VS to a depth of 400 m. The shallow surveys included both active and passive methods, normally using a linear array of 48 sensors with a 1 m sensor spacing. The MASW (multichannel analysis of surface waves) method was employed with a sledgehammer source and the MAM (Microtremor Array Measurement) method utilized ambient noise. Velocities of a peat layer in the upper 3 m to 7 m were as low as 40 m/s. Velocity of an underlying sandy unit generally reached 200-250 m/s at a depth of about 20 m. At open field sites away from levees, fundamental mode Rayleigh wave dispersion curves were clear and velocity could be estimated to depths of 20-30 m. At sites on or along levees, however, higher modes were dominant and depth penetration was limited to about 10 m. Deeper velocity measurements were performed at Twitchell and Sherman Islands using nodal seismographs to record ambient noise. Triangular arrays were used with a maximum radius of 231 meters, providing velocity estimates to a depth of 200 m at Sherman Island and 400 m at Twitchell. Two seismic reflection profiles, 193 m and 245 m long, were recorded on Bouldin Island with the aim of imaging deeper stratigraphy. Basemaps with line locations and sample field records are included. Introduction The Sacramento-San Joaquin Delta consists of a network of river channels that originate in the Sierra Nevada and drain through the Carquinez Strait to San Francisco Bay. Because its outlet is constricted, deposits of peat and mud up to 10 m thick accumulated in the central Delta during the Holocene. These estuarine and floodplain deposits are underlain by eolian sands and Pleistocene alluvial fan deposits (Atwater and Belknap, 1980). Delta islands have experienced 3 to 8 meters of subsidence during the past century due to oxidation and compaction of peat (Mount and Twiss, 2005). Projected sea level rise over the next century will contribute to an ongoing landward shift of the freshwater-saltwater interface, and increase the risk of flooding due to levee failure or overtopping. Ground motion due to an earthquake on a nearby fault could be sufficient to cause multiple levee failures (URS, 2009). Seismic shear wave velocity (VS) in the upper 30 m may be used to determine Uniform Building Code (UBC)/ National Earthquake Hazard Reduction Program (NEHRP) site class (BSSC, 2001). At a typical site, S waves from earthquakes experience a decrease in S wave velocity near the surface, which tends to cause amplification of ground motion. Knowledge of shear wave velocities is needed to accurately estimate site-specific variations in ground motion. The main goal of this study was to measure near surface (to 30 m depth) S wave velocities (VS) by conducting surface wave surveys (Figure 1). This was done at most sites using short linear arrays of sensors. Deeper (to 500 m depth) VS was measured at two sites using nodal seismographs to form larger areal arrays.

Figure 1. Schematic velocity model showing S-waves from earthquake and surface sources. Upgoing waves from the earthquake experience a velocity decrease near the surface and amplification of ground motion. S wave velocities of the near-surface layer may be measured from surface waves generated by either an artificial source (active method) or ambient noise (passive method). Method: A combination of active and passive seismic surface wave methods was used to determine shear wave velocity VS for site characterization at nine sites in the Sacramento Delta (Figures 1 and 2). Seismic reflection surveys Active surveys were conducted using the MASW (multichannel analysis of surface waves) method (Xia et al., 1999) with a linear array and sledgehammer source. Passive surveys were conducted using the MAM (microtremor array method) with ambient noise (Okada, 2003). Active and passive data were recorded at most sites using a linear array of 48 vertical-component 4.5 Hz geophones. Geophone spacing was 1 m and spread length was 47 m. The target depth of these surveys was 30 m. For active surveys, several sledgehammer blows were recorded at a few different offsets ranging from 1 m to 10 m from each end of the recording spread. Data were processed using SeisImager SW (Geometrics, 2009). Field records and frequency-velocity spectra were inspected and for each site the record with the most coherent dispersion curve was selected for further processing. For passive surveys, about 20 one- minute records were recorded at each site using the same linear array as was used for the active survey. Coherence was computed for each sensor spacing. The spatial autocorrelation (SPAC) method was used to prepare frequency-velocity spectra. Dispersion curves were picked from both active and passive data, combined, and edited. For additional discussion of the method and more data examples, see Craig and Hayashi (2016).

Figure 2. Basemap showing locations of velocity surveys in the Sacramento Delta. Yellow dots are 2017 sites (SHN, SHS, SHE = Sherman Island; TWW, TWC, TWN, TWS = Twitchell Island; BON, BOE = Bouldin Island). Red dots are 2015 sites (SIA = Sherman Island, WEB = Webb Tract, EMR = Empire Tract, SRB = Bethel Island, SMB = Sandmound Boulevard, HOL = Holland Tract, BAC = Bacon Island, CC = Clifton Court Forebay).

Velocity-depth profiles were determined from dispersion curves. An initial velocity model was prepared by mapping each frequency-velocity pick from the dispersion curve to a depth equal to one-third the wavelength. A non-linear inversion was then performed to obtain a final velocity model. The inversion seeks to minimize the RMS error between the observed dispersion curve and a theoretical fundamental mode dispersion curve (Hayashi and Craig, 2017). Frequency-velocity spectra and dispersion curves: Example frequency-velocity (f-v) spectra and dispersion curves from site TWN are discussed below. The spectrum from the active method (Figure 3a) displays energy, and the fundamental mode dispersion curve appears as a dark blue curved ridge of maximum energy, with red picks. The spectrum from the passive method (Figure 3b) was prepared using the spatial autocorrelation (SPAC) method, and shows the error between theoretical and observed coherence. Although the spectra from the active and passive methods appear similar, the dispersion curve of the passive spectrum corresponds to the zone of minimum error rather than maximum energy. Dispersion curves picked from the active and passive spectra were merged for the next step in data processing, the determination of a 1D velocity-depth model. In general, the dispersion curves from the two methods were in good agreement. As may be seen in the f-v spectra for site TWN velocities tend to be more tightly constrained at higher frequencies using the active method (Figure 3a) and at lower frequencies using the passive method (Figure 3b). a) b)

Figure 3. Velocity-frequency spectra for site TWN with picked dispersion curve. a) active method. b) passive method. Dark blue curved region with red picks is fundamental mode dispersion curve. F-v spectra for additional sites are shown below. All have clear fundamental mode dispersion curves that are seen in both active and passive data. These include TWE (Figure 4), BON (Figure 5), and BOE (Figure 6).

4 a) b)

Figure 4. Velocity-frequency spectra for site TWE with picked dispersion curve. a) active method. b) passive method. a) b)

Figure 5. Velocity-frequency spectra for site BON with picked dispersion curve. a) active method. b) passive method. Dark blue curved region with red picks is fundamental mode dispersion curve.

5 a) b)

Figure 6. Velocity-frequency spectra for site BOE with picked dispersion curve. a) active method. b) passive method.

Observed and theoretical dispersion curves Combined and edited dispersion curves from the active and passive methods are shown below in Figure 7, along with the theoretical dispersion curves corresponding to the best fit models. a) b)

c) d)

e) f)

Figure 7. Combined dispersion curves from active and passive methods (red) and theoretical dispersion curve (black). a) BOE, b) BON, c) SHN, d) SHS, e) TWN, and f (TWE). Minimum frequencies range from about 2 Hz to 5 Hz. Velocities range from about 35 m/s to 250 m/s.

Higher modes In some cases, f-v spectra contain appreciable higher modes in addition to the fundamental mode. This tends to be the case for surveys on or along engineered structures such as levees. An f-v spectrum from site SHS dominated by higher modes is shown in Figure 8a. Rather than attempting to pick only the fundamental mode dispersion curve, the overall trend of the combined fundamental and higher mode energy (effective mode) is picked, as shown with the red dots in Figure 8a. Velocity inversion is then performed by minimizing the difference between the observed dispersion curve and the theoretical effective mode curve (Hayashi and Craig, 2017). The theoretical fundamental mode and several higher mode curves are shown in Figure 8b, along with the picks from Figure 8a. The theoretical effective mode (yellow dots) is in reasonably good agreement with the observed data (red line). a) b)

Figure 8. f-v spectrum and dispersion curves showing higher modes. a) f-v spectrum, active method, site SHS, dominated by higher mode energy. The overall trend of the combined fundamental and higher mode energy (effective mode) is shown with the red dots. b) Dispersion curve for site SHS using combined data from active and passive methods (red curve and white circles). Black line is theoretical fundamental mode dispersion curve corresponding to solution, bold colored curves are higher modes, thin colored curves are their respective weights. Yellow circles indicate effective mode.

A survey was conducted at TWC along a small levee along the side of a shallow pond. The f-v spectrum and dispersion curve are shown in Figure 9 below. Surface waves contained significant higher modes. a) b)

Figure 9. f-v spectrum and dispersion curves showing higher modes. a) f-v spectrum, active method (MASW), site TWC, dominated by higher mode energy. The overall trend of the combined fundamental and higher mode energy (effective mode) is shown with the red dots. b) Observed dispersion curve (red curve and white circles). Black line is theoretical fundamental mode dispersion curve corresponding to solution, bold colored curves are higher modes, thin colored curves are their respective weights. Yellow circles indicate effective mode.

The survey at TWW was located at the extreme western tip of Twitchell Island on what appeared to be undeveloped ground. f-v spectra show appreciable higher modes (Figure 10).

a) b)

Figure 10. Velocity-frequency spectra for site TWW with picked dispersion curve. Dispersion curve contains significant higher modes. a) Active method, b) passive method, and c) combined dispersion curve.

Velocity models Velocity models were determined from dispersion curves through a nonlinear inversion as described in Hayashi and Craig (2017). VS30 (time-averaged velocity over the upper 30 m) is reported for models that extend deep enough to provide estimates with reasonable confidence. In general, if models extended to about 20 m depth, VS30 estimates were reported. These estimates do rely on extrapolation of velocities to greater depths, but since VS30 is a time average, it is weighted more heavily toward the lower velocities, from shallow depths, and is less sensitive to errors in velocity near the base of the model.

Twitchell Island velocity models At TWE (Figure 11), velocity is fairly constant in the upper 5 m at about 60 m/s, this apparently corresponds to the surficial peat layer. Velocity then steadily increases to the base of the model at 19 m. VS30 (time-averaged velocity over the upper 30 m) is 129 m/s. At TWN (Figure 12), velocity gradually increases over the entire depth range of the model, from 1 to 25 m. The depth to the base of peat is not apparent. VS30 is 181 m/s.

a) b)

Figure 11. Velocity models for Twitchell Island sites. a) TWE. Velocity is relatively constant at 60 m/s for the upper 5 m, then steadily increases with depth. b) TWN. Velocity increases gradually with depth over the entire profile.

a) b)

Figure 12. Velocity models for sites TWC and TWW on Twitchell Island. Both show a velocity reversal with higher velocity material at the surface overlying peat. a) TWC. A peat thickness of 5 meters was measured by hand auger at this site. Only data from the active method were used at this site, resulting in limited penetration. b) TWW. Both active and passive data were used at this site.

Bouldin Island velocity models Velocity models for Bouldin Island sites BOE and BON are shown in Figure 13. The velocity profile at BOE steadily increases from 1 to 8 m, the entire range of the model. The profile for BON shows extremely low VS (35 m/s) in the upper 2 meters. The jump in velocity near 2 m depth corresponds to base peat, confirmed by hand auger. Maximum depth of the velocity profile was 9 meters. The maximum depths of profiles BOE and BON were relatively small compared to other sites due to a combination of low velocities and limited low frequency content in the observed surface waves. a) b)

Figure 13. Velocity models for Bouldin Island sites. a) BOE. Velocities are 63 m/s near the surface. This site has organic-rich clay and silt rather than pure peat at the surface. b) BON. VS of a 2-meter thick pure peat layer is as low as 32 m/s, with silty clay layer beneath. Velocities of the two profiles reach a similar value, about 125 m/s, at a depth of 10 m. Maximum depth is limited at both sites due to very low velocity material at the surface, resulting in a small maximum wavelength.

Sherman Island Velocity Models Velocity profiles from Sherman Island are shown in Figure 14. The survey at SHS was recorded along the toe of a levee, parallel to the levee. The velocity profile shows a reversal in the upper 2 meters, evidently due to natural peat with an extremely low VS (40 m/s) overlain by faster fill material. This profile reached a depth of 17 m. The velocity profile for SHN has very low velocities (50 m/s) in the upper 2 meters, followed by gradually increasing velocities. The peat layer probably extends at least 4 meters deep, which corresponds to a velocity of 75 m/s. VS30 is 116 m/s.

Figure 14. Velocity models for Sherman Island sites. a) SHS shows a velocity reversal near the surface due to levee fill material over natural peat with a very low velocity of 40 m/s. b) SHN has a low velocity (50 m/s) peat layer in the upper 2 m, followed by a gradual increase to 200 m/s at a depth of 26 m.

Peat layer and borehole logs The presence of a surficial peat layer throughout the central Delta has been well documented through numerous geotechnical borings (Atwater and Belknap, 1980) and is responsible for extremely low shear wave velocities observed in the present study. On Bouldin Island, VS of the surficial peat layer was 32 m/s at a site with pure peat (BON) and 63 m/s at a site with peat with higher clay and silt content (Figure 13). The velocity of the underlying stiffer unit was 125 m/s

at a depth of 10 m at both Bouldin sites and continues to increase with depth. Nearby borehole logs confirm the presence of a peat layer about 10 ft (3 m) thick overlying sandy clay (Figure 15).

Figure 15. Borehole log showing peat layer at surface overlying sandy clay at a depth of 10 ft (3 m).

Table 1. Location, VS30 , and site class of sites where velocity surveys were conducted. All sites were

assigned to Class F due to the presence of peat and/or organic-rich clay. See Figure 2 for site map. VS30 is provided for sites where velocities could be extrapolated to 30 m depth.

Stn Lat Lon VS30 Class BOE 38.09927 -121.50045 113 F BON 38.10897 -121.53542 - F SHE 38.04859 -121.74154 - F SHN 38.04458 -121.76181 116 F SHS 38.03615 -121.75364 - F TWC 38.10748 -121.64677 - F TWE 38.09207 -121.64372 129 F TWN 38.11523 -121.64768 181 F TWW 38.08992 -121.68299 121 F

Deep Surveys Using Triangular Arrays One-dimensional microtremor array measurements using large triangular arrays were performed at site TWN on Twitchell Island and site SHE on Sherman Island to estimate velocities to greater depths. Seven sensors were used to form triangular arrays of three different sizes at each site (58, 115 and 231 m radius). An example array pattern is shown in Figure 16. Depth of

15 penetration was determined by minimum frequency or maximum wavelength. Minimum frequency was 1.0 Hz at Sherman Island and 0.7 Hz at Twitchell (Figure 17a). Maximum wavelength was 500 m at Sherman and 1000 m at Twitchell. VS was estimated to a depth of 200 m at Sherman Island and 400 m at Twitchell (Figure 17b). Velocities in the depth range 50-150 m are consistently 100 m/s lower at Sherman than at Twitchell.

Figure 16. Triangular array for deep passive surveys. Only the outer portion is shown. Additional arrays starting with a radius of 29 m were located in the center. a) b)

Figure 17. Dispersion curves and velocity profiles from deep velocity surveys using triangular arrays. a) Dispersion curves. b) Velocity profiles. Depth penetration is about 200 m at Twitchell Island and 400 m at Sherman. Velocities are clearly lower at Sherman at depths between 50 m and 150 m.

Seismic reflection surveys Seismic reflection surveys were carried out at two locations on Bouldin Island, near sites BON and BOE, with the aim of imaging deeper stratigraphy. Preliminary results are reported here. A basemap showing the line location of the site near BON and a sample field record is shown in Figure 18 below. The data quality is excellent, as indicated by several coherent reflected arrivals in the prestack field record. a) b)

Figure 18. Seismic reflection survey at Bouldin Island North (BON). a) Map showing location of seismic reflection profile. b) Sample field record showing several reflected arrivals. A basemap showing the line location at BOE and a sample field record is shown in Figure 19. The data quality is also good at this location, and reflections again appear to be present. Both surveys used 72 channels, 2 meter source spacing, 2 meter geophone spacing, 40 Hz geophones, a sledgehammer source, and 3 stacks per source location. The profile at BON was 193 meters long and the profile at BOE was 245 m long. Summary and conclusions: Active and passive seismic surface wave methods were used to estimate shear wave velocity (VS) and thickness of a surficial peat layer and underlying material. Surveys at eight sites utilizing a 47 m linear array of geophones with a 1 m spacing provided detailed velocity information over the upper 30 m. A surficial peat layer with VS less than 100 m/s is 3-7 m thick at most sites, with velocities as low as 40 m/s at some sites. The underlying stiffer material reaches velocities of 200-250 m/s by a depth of 20 m. Due to the presence of peat and/or highly organic clay at least 3 m thick at all eight of the sites where near-surface surveys were conducted, all were classified as site class F in accordance with NEHRP recommended provisions (BSSC, 2001, Section 4.1.2.1). Class F is reserved for sites that cannot be classified based on velocity alone, and require site-specific evaluation. Deeper surveys were performed at Twitchell and Sherman Islands using triangular arrays and provided VS estimates to depths of 200 m on Sherman Island and 400 m on Twitchell. Velocities are lower and depth to a stiff layer is greater at Sherman Island than at Twitchell.

a) b)

Figure 19. Seismic reflection survey at Bouldin Island East (BOE). a) Map showing location of seismic reflection profile. b) Sample field record showing several reflected arrivals.

Acknowledgements This study was funded by USGS Earthquake Hazards Program (NEHRP) award G17AP00022. Site access was provided by the California Department of Water Resources and Patty Oikawa of CSU East Bay. Patty Oikawa and Ankit Srinivas provided measurements of peat thickness at some of the sites.

References

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