In terms of size and magnitude, Thursday's 3.9 earthquake, centered just off Kelley Point in North Portland, was small.
But it is believed to be the closest quake to hit downtown Ñ and the quake-vulnerable West Hills Ñ since a 1962 quake that registered 5.2 and was centered 9.3 miles northeast of the city.
Thursday's quake hit at 12:26 p.m. about 9.5 miles northwest of downtown Portland. Its magnitude, which registered 3.9 on the Richter scale, was considered very slight and apparently caused no damage.
An aftershock, registering 1.6, occurred at 12:34 p.m.
The activity hints at potential trouble for West Hills residents.
James Roddey, a spokesman for the Oregon Department of Geology, said that the Portland Hills faults run up and down both sides of the West Hills.
'They were formed because little quakes kept pushing them up and up and up,' Roddey said. Thursday's quake 'goes to show that it's business as usual for the Portland metro area as far as earthquakes go. We live in earthquake country and can expect little earthquakes like this all the time.'
Roddey said the North Portland area where Thursday's quake was centered has experienced several earthquakes in recent years, including two in March. One of those, which registered a magnitude of 2.1, took place on March 30; another, registering 2.2, occurred March 31.
Scott Burns, a Portland State University geology professor who monitored Thursday's event at the school's seismic measurement center, said: 'We really don't get any damage until it's around 4.5 and above. There's generally no significant damage until it's 5.5 and above.'
'I'm surprised we all felt this one.'