As many Santa Clarita Valley residents were preparing to start their Friday, an earthquake measuring 3.5 on the Richter scale jolted those still in slumber out of their beds.
At 5:42 a.m., the quake struck with originating in Castaic west of The Old Road and south of Oak Valley Road, southwest of Northlake Hills Elementary.
It struck near a fault that stretches from Ventura County, northwest of Pyramid Lake, through Castaic southwest of the lake, along Newhall Ranch Road through Bridgeport to Bouquet Junction and along Golden Valley Road to the San Fernando Valley.
At a depth of 10.8 miles below the earth’s surface, the quake registered at IV on the Modified Mercalli Intensity (MMI) scale.
According to the USGS:
The effect of an earthquake on the Earth’s surface is called the intensity. The intensity scale consists of a series of certain key responses such as people awakening, movement of furniture, damage to chimneys, and finally – total destruction. Although numerous intensity scales have been developed over the last several hundred years to evaluate the effects of earthquakes, the one currently used in the United States is the Modified Mercalli (MM) Intensity Scale. It was developed in 1931 by the American seismologists Harry Wood and Frank Neumann. This scale, composed of increasing levels of intensity that range from imperceptible shaking to catastrophic destruction, is designated by Roman numerals. It does not have a mathematical basis; instead it is an arbitrary ranking based on observed effects.
The Modified Mercalli Intensity value assigned to a specific site after an earthquake has a more meaningful measure of severity to the nonscientist than the magnitude because intensity refers to the effects actually experienced at that place.
The lower numbers of the intensity scale generally deal with the manner in which the earthquake is felt by people. The higher numbers of the scale are based on observed structural damage. Structural engineers usually contribute information for assigning intensity values of VIII or above.
- Many of the mountains, and some of the valleys, in Southern California were formed by movement within the San Andreas fault system—the tectonic boundary between the Pacific and North American tectonic plates.
- The San Andreas fault is the primary feature of the system and the longest fault in California, slicing through Los Angeles County along the north side of the San Gabriel Mountains. It can cause powerful earthquakes—as big as magnitude 8.
- A study by the U.S. Geological Survey indicates that a portion of the San Andreas fault near Tejon Pass could be overdue for a major earthquake. Earthquakes occur about every hundred years on average, along this section of the fault, with the last major earthquake occurring in 1857: the magnitude 7.9 Fort Tejon quake.
- There are over a hundred smaller active faults in the region that can cause damaging earthquakes like the Northridge earthquake in 1994, such as the Raymond fault, the Santa Monica fault, the Hollywood fault, the Newport-Inglewood fault, and the San Jacinto and Elisnore faults.
- Soils in lowland areas away from major faults may be subject to liquefaction. Houses on liquefied soil may settle or even move laterally on gentle slopes. Landslides are possible on steep hillsides.