We welcome the opportunity to address the issues raised by Schweickert and Lahren in their Comment, which claims, among other charges, that we are guilty of flawed procedures and conceptual errors. It should be emphasized from the outset that we have employed a conservative approach in estimating slip rates—one that requires clear evidence for Holocene movement across features such as paleoshoreline terraces or slides that have been either directly sampled and dated, or based on an estimated date using a straightforward extrapolation of sedimentation rate. We recognize that it is possible to concoct scenarios to either increase or decrease slip rates across faults based on the geological evidence at hand. However, we have taken an Occam's Razor approach, making as few assumptions as possible in determining slip rate to ensure that our methodology produces a robust minimum slip rate. Below, we address the most significant issues raised by Schweickert and Lahren.
Tahoe-Sierra Frontal Fault Zone
Schweickert and Lahren are critical of our work, in part, because we did not address the presence of the Tahoe-Sierra frontal fault zone (Schweickert et al., 2004). In 2002, we collected a dense grid of over 25 km of seismic chirp sonar data to image any post-Tioga movement across the mapped Tahoe-Sierra frontal fault where it crosses Emerald Bay (Howle, 2000). The seismic data reveal no offset of post-Tioga sediments or offset bedrock scarps within the basin (Fig. 1), despite the fact that the slip rate estimate provided by Howle (2000) should produce 10–20 m of post-Tioga vertical slip across the Tahoe-Sierra frontal fault. The magnitude of slip rate estimated by Howle precludes the possibility that strain has accumulated during the Holocene, but has not ruptured or produced faulted sediments or bedrock, because the 10–20 m of vertical offset represents at least several earthquake cycles. The simplest explanation, requiring the fewest assumptions, is that the Tahoe-Sierra frontal fault has not been active in the Holocene (or potentially at any time in the past). Although the Tahoe-Sierra frontal fault zone was published on a preliminary fault map through the Nevada Bureau of Mines and Geology (Schweickert et al., 2000), a subsequent California Geological Survey map (Saucedo et al., 2005) based on community input and consensus eliminated the Tahoe-Sierra frontal fault from the updated geologic map of the region.
Issues of fault rotation may indeed affect the estimate of slip in cases where off-fault deformation is used to infer total vertical slip. Schweickert and Lahren's complaint is that basin asymmetry associated with tilt (e.g., listric geometry) would underestimate total displacement across the fault—which is true. The proper question, however, is how to best estimate basin asymmetry (if it does occur) at Lake Tahoe so that slip rate estimates derived from displaced shoreline terraces can be updated (and increased if necessary). There are three features within the bathymetric and seismic data that suggest basin asymmetry: 1) an offset delta near Sugar Pine Point; 2) asymmetry of lake floor bathymetry in the Rubicon–Cave Rock corridor; and 3) tilt of the catastrophic slide within this same corridor. Each of these features point to perhaps a doubling of slip rate across the West Tahoe fault if: 1) the isolated, faulted fan delta records only post-Tioga slip, due to negligible sediment input during the Holocene; 2) sediment deposition during glaciation in-filled fault-induced accommodation, and thus reset and flattened lake floor topography post-Tioga; and/or 3) the distal portions of the catastrophic slide within this corridor were laid down flat. Each of these ideas is testable, but require additional seismic imaging, coring, and dating to test the degree of asymmetry within this basin. The cautious approach, however, is to stick with minimum slip rates, only shifting them upwards when, and if, any of these assumptions are found to be true. Schweickert and Lahren's Comment also asserts that our west-east correlation of the paleoshoreline terraces is flawed and that we have misidentified the outer half of the eastern terrace surface as a lake-bottom multiple. Such an accusation is without merit. Down-to-the-east normal faulting offers the simplest explanation for the east-west asymmetry observed in the SHOALS lidar data.
We applaud Schweickert and Lahren's concern for the ambiguities of detrital charcoal 14C dating, and agree that large errors could arise if one simply assumed that detrital charcoal samples represented the ages of sediments. Fortunately, we did not exclusively rely on detrital charcoal dates as the Comment's authors assumed, but rather included a suite of short-lived macrofossils such as pine needles and insects. Short-lived macrofossils are not prone to reworking issues, because they are less chemically inert than charcoal. Furthermore, our deep-water cores included the Tsoyowata ash, and the corresponding bounding dates are consistent with the age determination of this ash layer. We found that dated samples from several identified turbidite layers did include outlier ages that were too old, and were excluded from our analysis. The age shift from dendrocalibration is insignificant compared to the age extrapolation that we used to estimate the age of the McKinney Bay slide. The age estimate of the paleoshoreline terrace used 14C dates only as supporting evidence and relied almost exclusively on the optically stimulated luminescence dating techniques. Presently, there are several piston cores located in the vicinity of the dated core presented in our Geology article, including a site location that is coincident with the profile and map shown in Kent et al. (2005; their Figures 1 and 3). The stratigraphy identified within several piston cores located adjacent to the fault scarp, on the hanging wall block, are nearly identical—including depths to the Tsoyowata ash and several turbidite layers. As such, we projected the dates from one core to the next, based on stratigraphic layering and their close proximity—a practice that is not uncommon in seismic stratigraphy.
In summary, we have provided the first quantitative slip rate estimates for the Tahoe basin, using a conservative methodology to ensure robust minimum slip rate estimates. This effort has now led to the first successful onshore paleoseismic investigation, providing a clear measure of earthquake magnitude within the basin (Seitz et al., 2005). The authors of the Comment believe that the normal faults within the Tahoe basin have significantly higher slip rates than presented in our article; however, they have not presented quantitative constraints to back this speculation.