Institute
for Regional Studies of the CaliforniasInstitute
for Regional Studies of the CaliforniasThe San Diego part of the International Border Planning Area has a number of hazardous areas related to steep slopes, geologic faults, landslides, and floods. These areas are identified according to definitions established by U.S. federal agencies, including the Federal Emergency Management Agency and the Federal Insurance Administration and are reflected in local planning and zoning maps. The City of San Diego has incorporated federal, state, and local geologic information onto city-wide geologic hazard maps that identify specific hazards affecting development. Generally, construction is restricted on slopes greater than 25 percent and these slopes are indicated on Map 11. Also indicated is the 100-year flood plain, the area that is subject to innundation by the waters of a 100-year frequency flood. Map 11 also includes the 500-year flood plain. Known major faults are indicated on Map 11 and these, in combination with the geologic structure of the region, suggest areas of seismic risk.
Most of the Tijuana portion of the International Border Planning Area is heavily urbanized. The topography is characterized by a number of river valleys subject to flooding and steep canyons that cut into the hills and mesas. The soils and geologic formations underlying the city are often unstable or poorly engineered during development, subjecting areas of Tijuana to erosion and mud slides during the winter storms that are typical of the region. Due to the spread of spontaneous settlements and informal housing in canyons and on hillsides of Tijuana and insufficient storm sewers and other works, landslides and mud slides are a continuing concern. Map 11 shows these risk areas for Tijuana. Slopes greater than 35 percent are considered risk areas in Tijuana.
The Office of Civil Protection in Tijuana has identified a number of hazard zones for flooding in the planning area that were impacted during the torrential rains of 1993. These include the river bed and adjacent slopes of the Río Alamar, the canyons of the Colonia Nido de las Águilas in the easternmost part of the planning area, Pasteje-Aviación Canyon located to the southeast of Colonia Libertad and draining into the Tijuana River, and the canyons of the hills east of Playas de Tijuana, especially Los Laureles canyon that drains northward across the border. Los Laureles and Pasteje-Aviación canyons were declared disaster areas by the federal government as a result of the 1993 storms. Some of these areas were also affected by the storms of the winter of 1998. Although other areas of Tijuana's border planning are on steep slopes and canyons, paved streets, storm drains, and other civil works offer protection from heavy runoff during storm events.
The International Border Planning Area is susceptible to seismic activity. Geologic structures such as faults, ancient landslides, and unstable soils are present in this area. The faulting in the area between San Diego and Tijuana is complex. Several major potentially active faults pass through or close to the International Border Planning Area. In San Diego, these include the Rose Canyon fault zone and the La Nación fault zone. In Mexico, these include the Coronado-Agua Blanca fault zone and the Vallecitos-San Miguel fault zone. Of these, Rose Canyon and Vallecitos-San Miguel are considered the most hazardous on-shore fault zones, followed by the off-shore Coronado-Agua Blanca fault.
The major fault zone in the San Diego area is the Rose Canyon fault. It is generally accepted that the Rose Canyon fault has been active in recent geologic times (Holocene). The onshore portion of the Rose Canyon fault zone extends from La Jolla to San Diego Bay. The southern extension of the Rose Canyon fault zone from the City of San Diego to the Tijuana area has been suggested along three trends. One trend is along the coast of Baja California either parallel to or merging with the Agua Blanca fault zone. Another projection would join with the Los Buenos fault and related faults in the Tijuana area. A third projection would extend southeast along the Tijuana River Valley to possibly join with the Calabasas or Vallecitos-San Miguel faults.
The major faults in the northwestern part of Baja California are considered to be the Agua Blanca fault and the San Miguel-Vallecitos fault. The Agua Blanca fault is mapped south of Tijuana. The San Miguel fault is mapped to the southeast of Tijuana, with a trend toward the Tijuana area. The San Miguel fault has been the most seismically active fault near the Tijuana area.
The alluvial and sedimentary soils of some of areas of Tijuana and adjacent San Diego are subject to liquefaction, or reduction in the strength and stiffness of soils during the shaking of an earthquake. These areas include the river valleys of the Río Tijuana and the Río Alamar as well as the terraces of the Playas de Tijuana zone. Landslides are also a problem in Tijuana and parts of San Diego due to the nature of the soils, rainfall patterns, poor drainage, poor construction, and housing constructed on steep slopes. Areas of risk due to slope are identified on Map 11.
 Photo |
| Housing on the steep slopes in Cañón de Los Laurels is susceptible to flooding and landslides during storm events. 2000. |
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