Facts and Figures for Seismic and Earthquake Retrofitting in Portland, Oregon.
FACT: Frequency of Earthquakes in Cascadia Subduction Zone Occur Every 300 to 350 Years
Scientists are now in wide agreement that a powerful earthquake and tsunami will strike the Oregon coast (part of the Cascadia Subduction Zone) within the next 50 years. The latest findings come from Chris Goldfinger, Director of the Active Tectonics and Seafloor Mapping Lab at Oregon State University. Instead of thinking giant quakes occur every 500 years, Goldfinger’s work puts the cycle at 300 to 350 years.
The Cascadia Subduction Zone runs from Mid-Vancouver Island in British Columbia, Canada, south along the Pacific Northwest coast and into northern California. The last megathrust earthquake (with a magnitude of 8.7 to 9.2) within this zone struck on January 26, 1700. The length of the fault rupture was about 620 miles, with an average slip of 22 yards. The earthquake triggered a tsunami that struck the coast of Japan. The most important clue linking the tsunami in Japan and the earthquake in the Pacific Northwest comes from studies of tree rings (dendrochronology), which show that red cedar trees killed by lowering of coastal forests into the tidal zone by the earthquake have outermost growth rings that formed in 1699, the last growing season before the tsunami. Previous earthquakes within the Cascadia Subduction Zone are estimated to have occurred in 1310, 810, 400, 170 BC and 600 BC. An earthquake similar to the 2011 event in Japan will occur again along the Cascadia Subduction Zone-but when?
FACT: The Earth’s Plates are in Constant Motion
The Earth’s plates are in constant motion. The US Geological Survey (USGS) estimates that several million earthquakes occur throughout the world each year. Many go undetected because they hit remote areas or have very small magnitudes. The National Earthquake Information Center (NEIC) now locates about 50 earthquakes each day, or about 20,000 per year. According to long-term records compiled since 1900, about 17 major earthquakes (magnitudes of 7.0 to 8.9), and one great earthquake (with a magnitude of 9.0 and above) occur worldwide in any given year.
Megathrust earthquakes occur at subduction zones at destructive plate boundaries (convergent boundaries), where one tectonic plate is forced under (subducted) another. Due to the shallow dip of the plate boundary, which causes large sections to get stuck, these earthquakes are among the world’s largest, with moment magnitudes that can exceed 9.0. Since 1900, all six earthquakes of magnitude 9.0 or greater have been megathrust earthquakes. No other type of known tectonic activity can produce earthquakes of this.
(For a map of earthquakes occurring over the last two weeks in our area, click on this link: www.pnsn.org/earthquakes/recent)
FACT: It’s a Lot Easier to Prepare Ahead of Time than to Pick Up the Broken Pieces Afterward
The single most important safety improvement you can make to your home is to ensure the foundation itself is adequate and that the house is securely attached to it. In 1984, Michael O’Hearn, an architect in Santa Cruz, California, bought two identical Victorian style homes, situated side by side. The homes were built around the late 1880’s. O’Hearn’s intention was to seismically retrofit both homes. He began retrofitting the first house by installing plywood shear panels on the cripple walls and bolting the mudsill to the foundation. O’Hearn completed the retrofit on the first home, but before he could begin the second home, the 7.1 Loma Prieta earthquake hit on October 17, 1989. The first home that had been retrofitted, with its plywood shear panels and bolted foundation, suffered only minor damage. States O’Hearn, “The one we had retrofitted cost us $5,000. to repair. The other one cost us $260,000. to repair. The whole building had to be jacked up, repaired and slid back on a new foundation. O’Hearn offers this advice: “For homes more than 20 years old located in areas of seismic activity, I strongly urge owners to consider seismic retrofit. It’s a lot cheaper to retrofit a house now than to repair it after an earthquake.” And if the house does slide off its foundation, authorities will likely classify it as an uninhabitable home that in most cases still has a monthly mortgage payment due.(“Retrofit Case Study,” from page 7 of APA’s publication, Homeowner’s Guide: Earthquake Safeguards. Reprinted with permission from APA – The Engineered Wood Association.)
FACT: It Costs Less than You Might Think to Prepare Your Home
Fault Line Constructors offers a comprehensive whole-house Home Inspection for about $600., depending on the complexity of your home’s design, and a Foundation only inspection starting at about $250. Employing FEMA guidelines, a whole-house inspection consists of a complete walk thru of your home. You receive a detailed engineering report identifying those areas that do not meet current seismic recommendations and a cost to upgrade each area. For instance, properly securing a house to its foundation may range from $3,500. to $8,500. Engineered whole house retrofits may range from $20,000. to $45,000.
Fault Line Constructors will also provide suggestions for making simple, low-cost improvements such as securing water heaters, attaching large pieces of furniture (entertainment centers, bookshelves) to the wall, adding devices to prevent cupboard doors from flying open, etc.
When you give serious consideration to preparing and protecting your home and family, what is their safety really worth to you?
Earthquake Insurance Coverage
Most of us are now aware that earthquake insurance is available. Although each insurance company has their own set of requirements for obtaining coverage, here are a few standard items they will request:
- Houses that were built prior to 1980 will need to have the water heater double strapped at the top, around the belly and at the bottom. The water heater is required to be bolted to the wall studs.
- The sill plate must be properly bolted to the foundation. A structural engineer’s report or contractor’s receipt for retrofitting is considered acceptable verification of this requirement.
- Homes with cripple wall construction are required to have the walls properly reinforced with plywood or oriented strand board sheathing.
Our partner, Pac West Insurance Agency, has been underwriting earthquake insurance coverage for more than 25 years. Rates are based on several factors, including location of your home, year of construction, construction and foundation type, and the number of stories. Please call for additional contact information.