Southern California

Natural Hazard Assessment


Team California

Aaron Mielke, Emily Oleksiuk, Tonia Roddick, Allison Smith




To determine high-risk areas due to wildfire, earthquake, and landslides in southern California.




Natural disasters in California have evoked concern among residents and insurance policy makers. Wildfires, earthquakes, and landslides have generated considerable monetary and physical damage throughout the state. Identifying high-risk areas for these natural hazards would be of substantial benefit to residents and insurance policy makers. Traditionally risk areas from specific natural hazards have been identified and mapped individually.  We will integrate wildfires, earthquakes, and landslides into one data layer depicting the aggregate risk from the combination of these natural hazards.


Study area:


Ten counties that comprise southern California are included in our study area: Santa Barbara, San Luis Obispo, Ventura, Kern, Los Angeles, Orange, San Bernardino, Riverside, San Diego, Imperial.  This area covers varying terrain, from the coast to the mountains, as well as both urban and rural landscapes. Southern California experiences earthquakes, landslides, and wildfires due to its geology, climate, and human land use. The area experiences frequent earthquakes due to its location on the convergence of two tectonic plates (Pacific and North American Plates), and approximately 300 associated faults. Wildfires are a natural and an important component of the Southern California ecosystem. Mild, rainy winters and hot, dry summers characterize the climate of Southern California. Vegetation grows well during the winter, providing an abundance of fuels that dry out during summer, setting the stage for wildfires to occur. Wildfires and earthquakes also increase landslide potential. Burned bare slopes lack the vegetation to hold the soil in place, and earthquake activity can potentially shift the underlying bedrock Several Southern California counties have been identified as federal disaster areas.


Proposed Tasks:


Define areas of high risk for earthquakes, wildfires, and landslides at the township level. Each risk area and its intensity will be determined separately using the procedures described below. To determine areas of high risk for wildfires we will calculate a fire threat index based on the combination of fuel rank and fire rotation (fire frequency). We will use a fuel rank assessment developed by the California Department of Forestry and Fire Protection. The fuel rank assigns rankings based on expected fire behavior derived from combinations of topography and fuels. We will also use fire rotation class intervals developed by the California Department of Forestry and Fire Protection, which are essentially fire frequency classes. The more frequent fire areas have a higher danger ranking that reflects a greater risk. We will union the fuel rank layer and the fire rotation layer in order to sum the rankings to develop a threat index. A final value of between zero and four will be assigned to each township with zero being a low risk for wildfire and four being high risk.


To calculate hazard areas from earthquakes we will use four factors (relative seismicity, ground shaking, past earth quakes, and fault lines).  For each of these factors, a value of zero to four will be assigned to the township.  A value of zero would mean that the factor is not contributing to the overall hazard and a value of four would mean the factor is contributing greatly to the overall hazard. The relative seismicity value is a value between one and seven and is assigned at the county level by the NEHRP. Each township will be given a value based on the relative seismicity value.  The ground-shaking factor is a value that describes the grounds response to seismic activity.  Each township will be given a value that corresponds to the average value acquired from ground shaking layer of the soil within that township.  The fault layer consists of all the faults in California.  A buffer will be placed at 1km, 5km, 10km and 15km around each fault.  A value that corresponds to the distance the township is from a fault will be given to each township.  Finally each township will be given a value from zero and four based on the past earthquake factor.  A final value for earthquake hazard will be the average of the values for each of the earthquake factors.


A similar method will be used to assign a value to each township for landslide hazard areas using four factors (land cover, slope, geologic bedrock, and soil type). Landslides will be categorized into five levels. Falls, topples, sags, slides and flows; with flows being the highest hazard area and flows being the lowest hazard area. Each level will incorporate the land cover, slope, geology and soil type for that township to show the potential hazard of landslide activity.


A value will be assigned to each township corresponding to the risk from each natural hazard, and then the numbers will be summed.  This number will represent the overall risk from any of the three natural disasters.  The value will be between zero and twelve; a value from zero to one will be very low risk, two to four low risk, five to seven medium risk, eight to ten high risk, eleven to twelve very high risk.


We will also generate an additional map showing natural hazard threat to people.  We will combine urban areas with the aggregate natural hazard risk map described in the paragraph above, in order to produce a natural hazard threat to people.