THE SIMI-SANTA ROSA FAULT

The Simi-Santa Rosa fault extends from the Oxnard Plain through Camarillo and the northern side of the Tierra Rejada Valley, south of the railroad at Madera Road, and marking the base of the Santa Susana Mountains as far east as Las Llajas Canyon. The fault may connect with and become the Northridge fault. The fault is a relic of the collision of the North Pacific Plate and North American Plate. The San Andreas fault marks the boundary between those two great plates. The angular collision of those plates, starting in the Palms Spring area and extending into northwestern Ventura County, has resulted in the North Pacific Plate having to alter its course to the west northwest near Palm Springs and then again to the north northwest east of Cuyama Valley. The net result is a shortening, i.e., thickening, of the Earth's crust in the form of folding and faulting. The Transverse Ranges Physiographic Province, consisting of the San Bernardino Mountains, the San Gabriel Mountains and the Santa Monica Mountains, including the mountains of Ventura County and the Simi-Santa Rosa fault, are part of the complicated topography that has resulted. The fault is a high angle thrust fault with the hanging wall, i.e., the up thrust portion, on the north northwest side of the fault.

The fault is classified by the California Division of Mines and Geology as being "active" (as far east as Las Llajas Canyon), which means that it has experienced surface rupture during the last 10,000-11,000 years. Recent work on the fault in the area just north of the site of the former Simi Valley Drive-in Theater, has led to the conclusion that the fault has experienced surface rupture during the last 1,500 to 6,000 years. With further study, it is hoped that more precise information on the activity status of the fault in the Simi Valley area and just how often surface rupture has occurred can be determined.

What geologists are now finding is that in some discrete areas the hanging wall has been thrown over the fault to the south southeast, creating a shingling effect. For example, on the western side of the 23 Freeway on the northern side of the Tierra Rejada Valley is a vernal pool that historically accumulated water during the winter and spring. The depression rests on an upland above the Tierra Rejada Valley. The actual trace of the fault runs well north of the depression. The uplands on which the depression is formed constitutes geological material that has been thrust up and over the fault in that shingling effect. (The area has now been developed for housing. Since that development was approved after the fault was classified as active, can be assured that development has occurred in compliance with the requirements of the Alquist-Priolo Earthquake Hazard Act.) Apparently, the same thing has occurred on the old drivein theater site and the subdivision to the west. The main trace of the fault runs near the top of the ridge to the northwest. However, faulting continues to within about 100 feet northwest of where the old concession building was located on the drive-in site. The developer of the old drive-in site was forced to exclude residential development on the northwestern approximately 40 percent of the property because of the presence of faulting. The same shingling effect has been identified north of the old Texas Tract. As a result, the layout of the new subdivision of the area has had to comply with the requirements of Alquist-Priolo. The requirement of the Alquist-Priolo Earthquake Hazard Act are intending to preclude construction of homes and most other structures directly over a fault that may result in surface rupture with a minimum margin of safety.

Does the Simi-Santa Rosa fault represent an earthquake hazard in Simi Valley? Certainly. The threat must be taken seriously, although there is no reason for panic. If an earthquake were to occur, it would likely be in the 6.4 to 6.7 range. If the entire fault system were to rupture - an unlikely event - and if the Northridge fault is part of that system, a moment magnitude 7.0 earthquake could occur. At this point it is impossible to say if and when whether either one of these events will occur. Either event could occur tomorrow, ten years from now or 4,000 years from now.

In any case, that, in a nutshell, is what we know about the fault.

                                                                                    Mike Kuhn

                                                                                    11-30-04