Adequate supplies of ground water for municipal use generally are not available within a 3-mile radius of Pescadero, San Mateo County, California. The required quantity of 100 gallons per minute probably could be obtained from one or more wells in the alluvium along Pescadero Creek; however, the quality of the water probably would deteriorate with time and might not be suitable for public supply for more than 20 or 30 years. Sand and gravel beds below 60 feet in the alluvium near the junction of Honsinger and Pescadero Creeks offer the best potential for developing domestic water supplies.

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USGS1978

Trenches across the 1906 trace of the San Andreas Fault in northern San Mateo County, California

Two trenches were excavated across the 1906 trace of the San Andreas fault near fences that were displaced by the 1906 faulting. About 18 displacements equal to the 1906 displacement would account for the offset of a stream adjacent to one of the trenches. Review of divergent reports on the amount of fault displacement in 1906 at 7 localities indicates that right slip ranged from about 2 in (meters) to about 3 m at the main trace and was about 0.6 m on a subsidiary trace near one of the trench sites. Materials exposed in the trenches included Franciscan bedrock, Merced (?) sandstone, alluvial deposits, and modern soils. Visibility of the evidence for faulting covered a wide range. Faulting was expressed in the trenches as conspicuous color differences, abrupt termination of geologic units, fractures, gouge, granulation of rock, streaked contacts, rounding of rock fragments, and slickensides. Irregularly shaped masses of gouge and other materials found in both trenches may also be a manifestation of faulting. Neither caving of the walls nor the inflow of ground water were reliable indicators of the locations of faults in these trenches. The features exposed in the trenches clearly indicated the presence of an important zone of faults with geologically young displacements, but they were inadequate to identify with certainty the 1906 break or the probable 1838 break. For example, evidence of the 1906 rupture could not be seen in a unit 200 mm (millimeters) thick that is believed to predate 1906. The approximate location in the trenches of the 1906 zone of ruptures could, however, be inferred from surface morphology, historic records, and evidence in the trenches. A moderately sharp and steep contact between two surface soils, in itself suggestive of very young faulting, did not have a fault below it. Brass monuments were installed at the trench sites so that future surface ruptures can be related to the faults and other structures mapped in the trenches.

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USGS1977

Map showing distribution of granular sediments above, within, and beneath Holocene estuarine deposits, San Mateo County, California

No abstract available.

California↗

USGS1974

Preparation and use of isopleth maps of landslide deposits

Isopleth maps derived from landslide inventory maps generalize and quantify the areal distribution of landslide deposits in contour form and may be uniquely useful for some types of regional planning. The isopleth format permits incorporation of landslide information with other quantified map data in the preparation of slope stability maps. The isopleth map of landslide deposits in San Mateo County, California (scale 1:300,000), serves as an example of this technique.

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USGS1972

Active faults, probable active faults, and associated fracture zones, San Mateo County, California

As used on this map, the term active fault is defined as a fracture or fracture zone several miles long along which the two sides either are moving relative to one another and parallel to the fracture or are judged likely to move in such fashion within the useful life of existing man-made structures. The judgment that a fault is active may be based on: 1) historic records of surface faulting and earthquake activity, 2) topographic evidence that demonstrates repeated and systematic displacements along the fault trace, 3) systematic displacement of Holocene or Pleistocene geologic units along the fault trace, 4) a linear pattern of small-earthquake activity either along the surface trace of the fault or parallel to it and within 2 miles of the surface trace, or 5) current and measurable systematic displacement across the surface trace of the fault. Most active faults shown on the map are recognized as active on the basis of two or more of these criteria.

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USGS1972

Landslide susceptibility in San Mateo County, California

Information on areas susceptible to failure by landsliding is available in many general geologic maps and reports. However, it is commonly presented in complex, technical language, together with data not connected with slope stability. This derivative map is intended to convey such information in nontechnical form so that it will be intelligible to the non-geologist. It portrays only those geologic factors that bear directly on the susceptibility of specific areas to landsliding.

California↗

USGS1971