GEOPHYSICAL INSIGHT INTO THE DOTY FAULT ZONE: POTENTIAL FIELD MODELING AND CONSTRAINTS ON ACTIVITY

We use new geologic mapping along with geophysical data and modeling to refine the neotectonic framework of southwest Washington by describing the length, geometry and activity level of the Doty fault and related structures. We develop 12 geologic and geophysical cross sections with a total combine length of 325 km, and image the subsurface with electrical resistivity, ground-penetrating radar, and active-source seismic surveys. We interpret the Doty fault zone (DFZ) as a west-striking 72-km-long steeply to moderately north-dipping fault zone. The newly identified eastern Doty fault (EDF) appears to connect the western Doty fault (WDF) with the Saint Helens seismic zone (SHSZ). The WDF zone is 32 km long, has ~2.5 km of total reverse offset and dips 50–60 degrees north. To the west, the WDF zone ends near the town of Doty where it connects to a newly named north-trending Brooklyn fault zone. Decreasing amplitude of the gravity gradients is associated with the EDF zone and mod­eling suggests lower total offset towards the east. Fault strands of the EDF have ~1.6–2.3 km reverse-slip offset, with a dip of 60–75 degrees. The DFZ accommodates shortening on a vector trending ~N60°E. Attempts to confirm Quaternary to Holocene activity on the DFZ through near surface geophysical techniques were inconclusive. Evaluation of the geologic and faulting history—from the Eocene to the present—suggests activity on the DFZ and other faults in the system, but slip rates have been low (~0.08–0.09 mm/yr for the DFZ).

In addition to the DFZ we characterize several other faults. The newly-identified Rainbow Falls fault is a 6-km-long, north-dipping, low-angle thrust fault, which could be genetically related to the DFZ. The >24-km-long, steeply west-dipping (60–75°) Scammon Creek fault has ~1.3-2.4 km of reverse offset and bounds the western edge of the Rochester basin. The Kopiah fault is a northwest-striking 23-km-long steeply east-dipping (70°) fault with <1 km total vertical offset bounding the east side of the Rochester basin. The northwest-trending, steeply east-dipping (70°) Cedar Creek fault in the Chehalis basin has 2 km of reverse slip. The 29-km-long west-striking Rochester fault dips north and has a reverse offset of ~1.5 km. New geophysical insight provides updated understanding of the fault networks in southwestern Washington.