We live in earthquake country. Earthquakes happen when two large pieces of the Earth below us, called tectonic plates, move against each other. Plates can slide past one another or push one under the other. The sideways movement is called strike-slip, and the area where it occurs is a strike-slip fault. Vertical movement is called dip-slip, and the area where it occurs is a dip-slip fault.
The length of a fault determines how much energy can be released. Generally, a dip-slip earthquake produces more violent shaking than an equal-size strike-slip earthquake because the ground moves vertically rather than horizontally.
Many faults in our area can produce shaking we feel and sometimes minor damage. You may remember the rolling shaking from the Sylmar, Northridge, Landers, Big Bear, and Ridgecrest (Searles Valley) earthquakes. These quakes occurred some distance away and caused little local damage but did serious damage near their epicenters.
Three nearby faults are of primary concern:
- San Andreas Fault: Begins in northern Mexico and goes out to sea north of San Francisco. In Southern California, it runs along the east side of the Salton Sea, north of Palm Springs, along the south face of the San Bernardino Range, and north through Cajon Pass.
- San Jacinto Fault: Runs along the west side of the Salton Sea, along the base of the San Jacinto Mountains, just south of Redlands and San Bernardino, and joins the San Andreas at Cajon Pass. It passes near the Interstate 10–Interstate 215 interchange and next to the Southern California Edison substation that provides Riverside’s only connection to the statewide electric grid.
- Cucamonga Fault: Runs along the southern base of the San Gabriel Mountains.
The San Andreas and San Jacinto are strike-slip faults. The Cucamonga is a dip-slip fault. Each is capable of producing a damaging to catastrophic earthquake in Riverside. A large quake on one could trigger movement on another. Small to medium earthquakes are frequent, but none of these faults has had a major quake in a very long time. Based on historical patterns, all three are overdue for a significant event.
If a damaging quake occurs, expect:
- Collapse or damage to older buildings.
- Failure of some roads, bridges, and overpasses.
- Localized or regional failure of electric, water, telephone, gas, and sewer services.
Earthquakes cannot be predicted. Recent swarms in the Ontario, Rialto, and Lytle Creek area slightly increase the statistical likelihood of a larger earthquake—about a 5% rise—but the odds drop quickly once the swarm stops. Swarms are common and are not reliable predictors.
How to Prepare for an Earthquake
During a quake:
- Duck under a strong desk or table.
- Cover your head and hold on.
- If driving, pull over safely and wait for shaking to stop.
At home:
- First aid: Keep a well-stocked first aid kit.
- Home safety: If you smell gas, see ruptured water lines, or notice structural damage, evacuate. Expect aftershocks.
- Water: Store at least one gallon per person per day for drinking. Include extra for bathing, dishwashing, and pets. Water from toilet tanks (not bowls) and hot water heaters is drinkable. Do not use pool water.
- Food: Eat perishable items first if power is out. Stock nonperishable foods that do not require cooking. Include pet food.
- Medicine: Maintain a 30-day supply of essential medications for people and pets.
- Shelter: Plan alternative shelter if your home is unsafe. Keep tents and sleeping bags ready.
- Sanitation: If water service is out, place a plastic garbage bag in the toilet bowl, tie it shut after use and store it outside. You can also dig a trench and backfill it as needed. Do not flush toilets with pool or other water sources, since without service there is not enough flow to move solids through the sewer system, which can cause pipes to clog.
- Light: Use flashlights with extra batteries. Candles are fire hazards.
- Gas: Turn off the meter if you smell gas; otherwise, leave it on.
- Generator: Use generators to power individual appliances only. Do not connect a generator to your electrical panel. Portable solar panels can recharge phones and tablets.
At work or other locations:
- Know emergency procedures and supplies.
- After shaking stops, help others and assess safety.
- Evaluate the situation before attempting to go home.
When driving:
- Pull over and stop if possible. Plan where to go or wait for help.
- Keep a backpack with two to three days of food and water, a first aid kit, comfortable walking shoes, and warm clothes.
Be flexible and prepared. After a major earthquake, some people will need help, and others will be able to provide it.
Look for next month’s column for information on earthquake building retrofits.
