Geology and landforms
Multiple terranes
The area is well known worldwide for the complexity of its landforms, the region being composed of at least six terranes (continental, seabed, or island arc fragments with distinct characteristics) pushed together over millions of years by the forces of plate tectonics. As a consequence, many types of rock and soil are found in the region. Formations include the sedimentary rocks of sandstone, limestone, and shale in uplifted seabeds, metamorphic serpentine rock, coal deposits, and igneous forms as the basalt flows and ash deposits of extinct volcanos. Pleistocene-era fossils of mammals are abundantly present in some locations.
Vertical relief
The region has considerable vertical relief in its landscapes that are not in the alluvial plains leading to the bay or in inland valleys. In combination with the extensive water regions this has forced the fragmented development of urban and suburban regions and has led to extensive building on poor soils in the limited flatland areas and considerable expense in connecting the various subregions with roads, tunnels, and bridges.
Several mountains are associated with some of the many ridge and hill structures created by compressive forces between the Pacific Plate and the North American plate. These provide spectacular views (in appropriate weather) of large portions of the Bay Area and include Marin County's Mount Tamalpais at 2,571 feet (784 metres). Contra Costa County's Mount Diablo at 3,849 feet (1,173 metres), Alameda County's Mission Peak at 2,517 feet (767 m), and Santa Clara County's Mount Hamilton at 4,213 ft (1,284 m), the latter with significant astronomical studies performed at its crowning Lick Observatory.
The three major ridge structures (part of the Pacific Coast Range) which are all roughly parallel to the major faultlines:
Major Waterways
Earthquake faults
The region is also traversed by at least five major slip-strike fault systems with hundreds of related faults many of which are "sister faults" of the infamous San Andreas Fault, all of which are stressed by the relative motion between the Pacific Plate and the North American Plate or by compressive stresses between these plates. Significant blind thrust faults (faults with near vertical motion and no surface ruptures) are associated with portions of the Santa Cruz Mountains and the northern reaches of the Diablo Range and Mount Diablo.
Natural hazards
Earthquakes
The region is particularly exposed to hazards associated with large earthquakes,[14] owing to a combination of factors:
- Numerous major active faults in the region.
- A combined thirty year probability of a major earthquake in excess of seventy percent.
- Poorly responding native soil conditions in many places near the bay and in inland valleys, soils which amplify shaking as shown in the map to the right.
- Large areas of filled marshlands and bay muds that are significantly urbanized, with most subject to liquefaction, becoming unable to support structures.
- A large inventory of older buildings, many of which are expected to perform poorly in a major earthquake.
- Extensive building in areas subject to landslide, mudslide, and in some locations directly over active fault surface rubble zones.
- Most lowrise construction is not fireproof and water systems are likely to be extensively damaged and so large areas are subject to destruction by fire after a large earthquake.
- The coastal location makes the region vulnerable to Pacific Ocean tsunamis.[15]
Some of these hazards are being addressed by seismic retrofitting, education in household seismic safety, and even complete replacement of major structures such as the eastern span of the San Francisco-Oakland Bay Bridge.
For an article concerning a typical fault in the region and its associated hazards see Hayward Fault Zone.
Flooding
Some flooding occurs on local drainages under stustained wet conditions when the ground becomes saturated, more frequently in the North Bay area, which tends to receive substantially more rainfall than the South Bay. In one case, the Napa River drainage, floodplain developments are being purchased and removed and natural wetlands restored in the innovative Napa River Flood Project as the previous channelization of insufficient capacity around such developments was causing flooding problems upstream. Many of the local creeks have been chanelized, although modern practice and some restoration work includes returning the creeks to a natural state with dry stormwater bypasses constructed to handle flooding. While quite expensive, the restoration of a natural environment is of high priority in the intensively urbanized areas of the region.
Windstorms
A very strong Pacific storm can bring both substantial rainfall and strong wind gusts that can cause trees to fall on power lines. Owing to the wide area involved (sometimes hundreds of miles of coast), service can be interrupted for up to several days in some more remote localities, but service is usually restored quickly in urban areas.
In the spring and fall, strong offshore winds periodically develop. These winds are an especially dangerous fire hazard in the fall when vegetation is at its driest.
Firestorms
Firestorms are rare with large urban fires occurring a few times a century, but owing to wildland/urban interface areas can be a potential problem during some times of the year, particularly with a dry easterly wind in late Summer and early Autumn. This has proved to be a recurring problem in the hills above Berkeley and Oakland.
Mudslides and Landslides
Some areas which are geologically unstable have been extensively urbanized, and can become unstable due to changes in drainage patterns and grading created for development. These are usually confined to small areas, but there have been larger problems in the Santa Cruz Mountains. |
