Fault System

Several geologic faults have been inferred within or immediately adjacent to Forest Park. The extent of these faults and timing of their activity is not entirely understood; however, they strongly suggest a significant past crustal movement. No evidence of recent fault movement has been observed along the traces of existing faults.

Earthquake History

Within recorded history, the Portland area has a significant record of seismic activity. Beginning in 1846, seventeen recorded earthquakes with a Richter magnitude in excess of 4.0 have occurred. Since 1961, there have been 7 events ranging in magnitude from 4.3 to 5.8. The recurrent rate in the Portland area for earthquakes of magnitude 5.5 and greater is 100 to 150 years.

Geologic Hazards

The following kinds of geologic hazards were inventoried in Forest Park: old landslides, areas of Portland Hills Silt, potential rockfall areas, and potential block landslides.

These areas are presenty stable, but could present some level of risk in the event of strong seismic activity or construction. Extremely heavy rainfall could also trigger movement of rock or soil, which was once again witnessed in the winter of 2006/7.