Landslide inventory maps are generally prepared by interpreting the geomorphic expression of landsliding on aerial photos, topographic maps, or on the ground. Distinctive landslide geomorphology allows the recognition and mapping of landslides, although there are always landslides that have very subtle expression and are not identified. The difficulties of mapping landslides based on their geomorphic expression are amplified in heavily forested terrain. The ground surface is obscured by tree cover on aerial photographs, and landsliderelated features are often hidden. This limitation affects not only aerial photo interpretation, but also interpretation of topographic maps, which are based on aerial photographs.We compared five maps showing landslides in the Laurel Quadrangle in the Santa Cruz Mountains, California. These include a geologic map, a map prepared for the county based on interpretation of aerial photographs, a map prepared by us based on aerial photographs and compilation of previous work, a map of features interpreted from the U.S. Geological Survey 7.5-minute topographic map, and a detailed field-based landslide map.Comparison of these maps shows that the geologic map identifies few landslides, but most landslides on the geologic map are also shown on the other maps. The two maps based mainly on aerial photo interpretation tend to show the larger slides, but there is only about 60 percent correspondence of landslide areas between the two. Comparing the reconnaissance techniques with the much more detailed field mapping shows that the reconnaissance maps emphasize the large slides of bedrock and identify a lower percentage of shallow debris slides and debris flows.
Afterslip hazard map of the Browns Valley neighborhood and surrounding area. A detailed map explanation is presented on the following page. vi Caption for map on previous page: Levels of Afterslip Hazard for the Browns Valley Neighborhood, City of Napa, California: All fault traces shown on this map face potential future earthquake fault surface rupture hazard and other earthquake-related hazards such as shaking, liquefaction, and landslides; these hazards are treated separately in other publications and maps from CGS and USGS (with preliminary updates provided in this report). For all levels of afterslip hazard, the afterslip amount that is measured 90 days after the earthquake can be expected to as much as double by 10 years after the earthquake (less than double is also possible). Red Fault Trace-High level of afterslip hazard; very likely to experience more than 15 cm of afterslip during the 3 years after the earthquake. (Red is intentionally included, even though none is indicated on this map.) Yellow Fault Trace-Moderate level of afterslip hazard; likely to experience less than 15 cm, but more than 5 cm, of afterslip during the 3 years after the earthquake. (Additional afterslip accumulation is likely to gradually accumulate an additional 5 cm during the 10 years after the earthquake and an additional 5 cm 30 years after the earthquake.) Green Fault Trace-Low level of afterslip hazard; very unlikely to experience more than 5 cm of afterslip during the 3 years after the earthquake. (Faults that experienced <10 cm of coseismic offset and <5 cm of afterslip within the 3 months after the earthquake are included in this category. Some faults or lineaments shown as green had no measurable coseismic slip or afterslip associated with the August 24, 2014, earthquake. Faults and lineaments of several categories are shown for completeness. Some are previously mapped strands (U.S. Geological Survey and California Geological Survey, 2006); others represent preliminary mapping based on a combination of imagery interpretation and field mapping that has taken place since the August 24, 2014, earthquake. All of the faults and/or imagery lineaments shown as heavy green lines on this map may be considered to have a low level of afterslip hazard. Subsequent ongoing mapping, that is, work still in progress, may reveal that certain lineaments shown here are not actually faults.) Map orientation: North direction is toward top of map.
A statistical evaluation of shaking damage to wood-framed houses caused by the 2003 M6.5 San Simeon earthquake indicates that both the rate and severity of damage, independent of structure type, are significantly greater on hilltops compared to hill slopes when underlain by Cretaceous or Tertiary sedimentary rocks. This increase in damage is interpreted to be the result of topographic amplification. An increase in the damage rate is found for all structures built on Plio-Pleistocene rocks independent of topographic position, and this is interpreted to be the result of amplified shaking caused by geologic site response. Damage rate and severity to houses built on Tertiary rocks suggest that amplification due to both topographic position and geologic site response may be occurring in these rocks, but effects from other topographic parameters cannot be ruled out. For all geologic and topographic conditions, houses with raised foundations are more frequently damaged than those with slab foundations. However, the severity of damage to houses on raised foundations is only significantly greater for those on hill slopes underlain by Tertiary rocks. Structures with some damage-resistant characteristics experienced greater damage severity on hilltops, suggesting a spectral response to topographic amplification.
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