RES 94118B - Laserfiche WebLink

2.2 Project No, 5541 1 June 1993 are relatively small features only a few tens of feet across. These possess sharp, well- defined outlines and appear to have moved within the last 10 years. The second type, termed "recent" landslides, are medium-sized features ranging from a few tens to a few hundreds of feet across. These possess vague outlines that have been subjected m a long period of erosion, and they appear to have moved within the last 1000 years. The third ~'pe is termed "old" landslides. These comprise the matrix on which the othe~ two ~'pes are superposed, and they underlie nearly the entire hillslope and extend hundreds of feet upslope above the subject property. These landslides have been extensively modified by erosion and can be defined as having moved in the last 100,000 years. They are considered to be currently stable. Rainfall and/or seismically induced movement of some of the active and recently active landslides can be anticipated in the future. There is no historical evidence of debris flow activity; however, soil deposits on slopes steeper than about 2:1, which occur in localized areas on the hillside, may be subject to debris flows. The Calaveras Fault Zone is located I200 to 1500 feet offsite 1o the southwest. There are no known faults within the Geologic HaT~rd Abatement Dislrict. Soil creep may occur in the smficial soils on all sloping areas. Creep is the very slow, more or less continuous, deformation in a downslope direction sustained by the upper few feet of soil. Several springs are located at the heads of swales in the upper portion of the hillside area, Springs may adversely effect the stability of subsurface materials in adjoining and downslope areas. The most significant geologic hazard in the flat lying area of the site is the seismic baTa~d of ground shaking. Because the District is located in the seismically active Bay Area. significant ground shaking due to earthquake activity can be anticipated during the design life of the project. Structures built on alluvial soils are generally more severely stressed by seismic motion than those in bedrock areas because of the longer period and larger amplitude of the seismic waves, Ground shaking may be generated from the nearby Calaveras fault or other active faults in the region. A low risk of secondary ground cracking due to earthquake activity exists along the toe of the hillslope in areas where subsurface materials transition from deep alluvium to bedrock. Active and recently active landslides in the hillside area could affect portions of the flat lying area at the toe of the hillslope. 3