November 19, 2012
by Bill Steele
From PNSN staff seismologists, 11/19/2012
Given the earthquake history of the Portland region, today’s earthquake represents no great surprise and is a useful reminder that like most people in the Pacific Northwest, Portlanders live in earthquake country. The comments below places today’s earthquake in a historical context.
Between 1877 and 1961 four earthquakes with modified Mercalli intensities of VI or VII (in 1877, 1892, 1941 and 1953 occurred near Portland, OR. Please remember that earthquake intensities are not the same as magnitudes. Intensities are a complicated function of earthquake magnitude, distance from the earthquake and local geologic conditions.
On 6 November 1962 a moment magnitude 5.2 earthquake occurred about a mile southwest of what is now the junction of I-205 and Washington Route 500.
The uncertainty of the location for the 1962 earthquake is approximately plus or minus three miles in the east-west direction and about half that in the north-south direction. Today’s earthquake lies approximately 8 miles west-northwest of the estimated location of the 1962 earthquake.
In the time period 1980-2011 fourteen earthquakes with magnitudes between 3.0 and 3.9 occurred within a radius of about 35 miles from downtown Portland.
Over this 32 year period there has been a persistent zone of earthquake activity extending about 1.5-2.0 miles northwest and southeast from the epicenter of today’s (19 November 2012) earthquake. Since 1999 this zone has been the source of four earthquakes with magnitudes of 3.0, 3.0. 3.1 and 3.9, and now a fifth, with magnitude 3.2. For convenience, we refer to this zone as the Sauvie Island zone, though it extends southeast beyond the end of Sauvie Island.
This zone of earthquakes lies some 3 miles northeast of the base of the Portland Hills. Many geologists believe that the Portland Hills are a surficial expression of a fault. Geologic, magnetic and gravity studies in the Portland area indicate there are probably other faults approximately parallel to the Portland Hills, and also at substantial angles relative to the Portland Hills. The relation of the Sauvie Island zone to the proposed Portland Hills fault is uncertain.
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