|What is your areas earthquake risk?
Although many earthquakes occur on previously unknown faults, you can find out if there are any faults mapped near you, and whether they are considered active (there are many known faults that geologists no longer expect to move). Different rock and soil types behave differently in earthquakes. Are you on bedrock? If so, earthquake waves may be dampened in your area. Are you on alluvium in a river valley? If so, waves may be amplified; unfortunately, this is the case for much of the Willamette Valley.
The Oregon Department of Geology and Mineral Industries (DOGAMI) has produced maps for many communities that specifically address earthquakes and a wealth of information can be found on our website: www.oregongeology.org, or by calling our Portland office at 971-673-1555.
Are there special geologic hazards in the area?
Steep slopes can be particularly hazardous during and after earthquakes. In the long rainy season of winter and spring, soils can become saturated and quakes can produce rapidly moving landslides. In dry areas, rock fall can be deadly; one death in the 1993 Klamath Falls earthquake was from rock fall.
Earthquakes can turn soil to quicksand, a process called liquefaction. This typically happens along river channels, or former river channels. Another special hazard of river soil, or alluvium, is amplification. In the Willamette Valley, the area of amplification is several miles wide in general. Small, deep earthquakes near Woodburn in the past several years have been felt in an anomalously large area; this is probably attributable to the amplification properties of the valley alluvium. DOGAMI earthquake hazard maps include outlines of specific areas that are susceptible to landslides, liquefaction, and amplification.
Along coastal areas, tsunamis can be the most devastating part of an earthquake. Computer models suggest a tsunami would follow a subduction zone earthquake in just a few minutes, and could produce waves 30-50 feet high. Research has shown that in events where the population did not know how to respond to a tsunami, up to 60 percent of the population died. In areas such as Japan where the populace knows what to do, fatalities were less than 20 percent.
DOGAMI has produced general tsunami hazard maps for the entire coastline; some areas have more detailed maps. These maps can be used in conjunction with other resources to produce safe evacuation routes and staging areas.
Oregon Department of Geology and Mineral Industries
800 NE Oregon Street, Suite 965, Portland, OR 97232-2162
971-673-1555, FAX 971-673-1562
email us at DOGAMI