California

Fundamental Geography

Fire Seismic hazard: Earthquakes Volcanoes Landsliding Flooding

Fire

Fires The climate in much of California is a Mediterranean type; Mild rainy winters and warm (or hot) dry summers.  Vegetation grows during the winter and spring, and dries out during the long dry summers.  In the parts of California covered by chaparral vegetation, fire is always a danger, because chaparral plants are often very flammable. There is a tendency to suppress fires and as a consequence there will be a build-up of fuel as plants grow larger, and dead wood and other plant material accumulate.  Eventually, perhaps at the end of a hot, dry summer, something will set off a fire, and if the weather conditions are right (or wrong), it can potentially be a very big fire because of the accumulated fuel load.  Fire weather in California is hot and dry and windy.   The right conditions for dangerous fires occur when hot dry winds blow towards the coast from inland areas.  The winds are the result of high pressure systems over the Great Basin region, pushing the air outwards. As the warm air flows down from higher elevations, it warms up and dries out still more. Following a fire, the burned areas often experience flooding, excessive soil erosion, and landslides, because the bare slopes cannot hold the soil as well as a vegetated slope would.

Volcanoes

Volcanos Northern part of the state has a number of volcanos associated with the Cascade range. Most notable are Lassen peak and Mt. Shasta Lassen Peak last erupted between 1914 and 1917.  Shasta has not erupted in historic times, but has probably erupted at least three times in the last 750 years, and may have erupted as recently as 1786. Mt Shasta in particular is potentially dangerous, because there is ample evidence that past eruptions have sent pyroclastic flows There have also been some very large volcanic mudflows that have surged down the slopes of Shasta.  In fact, several towns are built on the site of old flows, and could be in the way of danger if the volcano should erupt again. In addition to the northern cascade region, Long Valley in the Mammoth area is under a large plume and Long Valley itself is a large caldera. Eruptions in the area occurred about 400 to 500 years ago around Inyo Craters, and 250 years ago at Mono Lake, 20 kms to the north. Starting in 1980, swarms of earthquakes in the area suggested that magma was again moving up from below, and the USGS has established a volcano observatory to monitor conditions

Earthquakes

Earthquakes The western section of California, west of a line running from the Imperial Valley in the south to the San Francisco area in the north, is on the Pacific Plate, which is moving towards the northwest at an average rate of about 5.5 centimeters per year.  East of that boundary is the North American Plate, which is moving more slowly towards the west.   The San Andreas is not the only fault along which an earthquake can occur. It is just the largest fault of a large complex of more or less parallel faults making up the San Andreas Fault Zone.  Other well-known faults that are part of this complex include the Elsinor and San Jacinto Faults in Southern California, and the Hayward and Calaveras faults in the north.  At different times one or another of the faults in this zone may rupture and move. One problem is that the fault is not perfectly straight and has a bend in it making it very difficult for the plates to move past each other. The plates lock and stress easily builds up. Another source of earthquakes are the smaller plates subducting to the north of California (e.g. Gorda plate)

Tsunamis Since California is a seismically active region, prone to earthquakes, as well as to volcanic activity, tsunamis along the coast are a possiblity.  There is particular concern along the north coast, in the Cascadia subduction zone, since a subduction-related earthquake could generate a large tsunami Since the source of the waves would be close to the coast, such a tsunami would come onshore with little warning, and could be devastating.  One tsunami has caused deaths in Northern California in historic times: the Alaska earthquake of  1964 generated a tsunami that killed 12 people in Crescent City, California

Landsliding

Landsliding A huge problem in California. Associated with El Nino and heavy rainfall - $150mil damage in San Francisco Bay during the winter of 1998 (mainly debris flows). Also accelerated by wildfires. Many landslides (and flooding) occur directly after the fire season – reduces biomass, root stability and shear strength. E.g. a debris flow immediately followed the South Canyon Fire of 1994 and nearly dammed the Colorado River. Mankind also thought to accelerate the hazard risk – mainly through deforestation, urbansiation and irrigation. E.g. March, 1995 – 9 homes buried in La Conchita – owners of the home considered it directly related to increased irrigation on the ranch above. However, very difficult – El Nino, steep cliffs etc.

Flooding

Flooding Central valley though to be at high risk of flooding (particularly the Sacramento and San Joaquin rivers). Many factors: Large part due to poorly constructed levee systems. Increasingly intense Pacific storms (accelerated by El Nino – e.g. 1998) Increasing urbanisation and thus vulnerability. Minor part = landuse change. All increases the intensity of the rainfall and the inability of the land surface to aid infiltration. High rates of overland flow and greater people at risk.