• Active normal faulting (0.1-0.5 mm/yr) on west side, less active normal on east ( dePolo et al., 1997).
• 50-100 km long Sierra Nevada range-front fault capable of M7+, extends into downtown area.
• Verdi basin synform on west, with W-E axis, has Pliocene lacustrine sequence (orange) of sand and diatomite.
• Thought Truckee River active floodplain (Jan. 1997) near airport on east side would be deepest basin, most hazardous.
• Despite 1994 M6 Double Spring Flat event 60 km south, very few strong-motion records exist.

Summer 1997 gravity campaign tripled coverage of the Reno area with 200 new stations. 5 mGal contours; terrain corrections to 167 km.

• Sparse previous measurements; not even along railroad or Interstate - mostly at geothermal prospects.
• Campaign followed a web of transects for efficiency; 100-200 m spacing to find gradients.
• 30 mGal low on west side of basin was missed completely. 6 km west of sub-basin at airport.
• Freeway interchange apparently on shallow part of basin.
• Casino towers on edge of western sub-basin.

We follow Blakely's USGS method to estimate basin gravity, by first estimating ``bedrock'' gravity. 5 mGal contours.

1. Quaternary and Recent alluvium, and Pliocene lacustrine sequence (with some tephras) are ``basin.''
2. Miocene-Pliocene andesites and basalts, and all older rocks, are ``bedrock.''
3. Select all stations within 100 of ``bedrock'' outcrop.
4. Surfer and ERMapper create kriged surface with kriging from bedrock terrain-corrected Bouguer anomalies.
5. Bedrock anomaly depends mostly on previous coverage, poor on some basin margins.

The ``basin'' anomaly estimate should allow modeling of basin geometry while ignoring the bedrock. 2 mGal contours; orange is zero anomaly.

• West and east sub-basins are prominent, with significant gradients.
• Shallow Verdi basin extends west along synform.
• Moderate basin in rapidly-developing south suburbs.
• Positive (red) basin anomalies are erroneous: result of poor bedrock coverage and/or bedrock density variations hidden by basins.
• Kriging stretches basin anomalies into known bedrock where there is no coverage.

Simple 1-d infinite-slab basin thickness estimates made at each measurement. 100 m thickness contours; light blue is zero thickness.

• Mostly a scaling of the previous basin gravity anomaly map.
• Uses stacks of slabs with basin density increasing down, as measured regionally in Nevada - increases thicknesses over constant density.
• West sub-basin >550 m deep, below suburbs and lifelines.
• East sub-basin >350 m deep, below rail yards.
• This geometry is not deep enough to produce gradients of west sub-basin.

A 2.5-d inversion fits the extra depth due to the gradients not modeled by the 1-d scaling.

• With constraints from water and geothermal wells, we found densities higher on average in the Reno basin than in other Nevada basins.

• Western sub-basin could be over 1 km deep.
• Eastern sub-basin not more than 0.6 km.
• Airport is over a ridge.

Well Control

• Deepest basin at 576 m oil prospect drilled in 1908 - entirely in Pliocene lacustrine sequence, and right in western sub-basin.
• Surface of west sub-basin is 100 m above floodplain, so deep basin was a surprise, since we had ignored the oil prospect.
• Other >100 m deep wells mostly in ``Moana Hot Springs'' for domestic heating.
• A casino drilled 1008 m injection well, hit andesite bedrock at 344 m.
• Moana H.S. wells are 30% deeper than 1-d thickness given by regional basin-density model:
  • Still poor gravity coverage in that area.
  • 1-d method underestimates depth near gradients.
  • Regional basin densities too low for Pliocene diatomites and Quaternary alluvium.
• No density logs, one shallow acoustic log.
• Below ~150 m of Quaternary alluvium the few logged wells suggest entire Reno basin underlain by the Pliocene lacustrine sequence.

Map of well correlations to 1-d basin gravity thickness estimates, with generalized basin geology. 200 m thickness contour interval. Filled circles show logged bedrock depths; open circles are TD of wells not hitting bedrock. Red is bedrock, orange are the Pliocene lacustrines, white is Quaternary alluvium, and yellow is Recent alluvium. Smaller circles show previous gravity coverage; small triangles show our new stations.

• The western sub-basin can be entirely filled by Miocene-Pliocene lacustrines (orange), to the >1 km depth.
• Well correlations show sediment column averages 2.2-2.35 g/cc.
• Drilling rate changes show buried diatomites still have much lower velocity than bedrock (red).
• Asymetric Miocene-Pliocene subsidence along a west-dipping normal fault.
• Recent subsidence probably along east-dipping normal faults.
• No west sub-basin subsidence needed in Quaternary; now uplifted 200-300 m.

Photo looking south over Truckee River and west sub-basin. Sierra Nevada mountain front on right. Truckee River runs right to left. Oil well was in center on bluff.

With new knowledge of the deep basin here, we can now begin an intelligent appraisal of whether previous seismic mitigation efforts have been adequate. Once we have modeled the basin-trapping and amplification effects, some questions might be:

• The largest structure in the photo is a 40 m high 5-lane highway bridge over the Truckee river. Were its notable engineering design features adequate?
• If shaking can be intense, is there greater landslip hazard to a concentration of trans-continental lifelines?
• Casino towers to the left, out of the photo, are on the edge of the sub-basin. Since Kim Olsen modeled the largest basin-trapping amplifications in Salt Lake City just inside basin edges, is there any similar increase in hazard here?

• Next step is to estimate amplification from shallow S-wave velocities.