ACTIVE FAULTS DISCOVERED WITH RECENT LIDAR IMAGERY REQUIRE REASSESSMENT OF SEISMIC HAZARDS IN EAST-CENTRAL OREGON

Cascadia subduction zone earthquakes drive high earthquake hazards in Western Oregon but have little impact on hazard in eastern Oregon. Instead, active faults associated with the Cascade Range, the northern edge of the Basin and Range, and the Olympic Wallowa Lineament drive areas of higher hazard in Central, South-Central and Southeast, and Northeast Oregon, leaving a large area of East-Central Oregon as an anomalous block of inferred low hazard. Review of recently acquired lidar data covering parts of this latter region has identified at least five previously unknown surface-rupturing faults. The youthful appearance and relatively long traces of some of these faults suggests that they represent important earthquake sources. Little field work has been completed to assess the earthquake history for most of these faults but analysis of the lidar data can provide some constraints on age, geometry and the magnitude of prehistoric earthquakes. The five faults include the Strawberry Mountain segment of the John Day fault, which is a normal fault that cuts glacial deposits and colluvium, and extends for 21 km. The Summit Prairie fault is a normal fault that cuts colluvium and extends for 12 km. The Dry Mountain Fault is a normal fault that cuts alluvial fans and colluvium on steep slopes and extends at least 8.5 km. The Long Creek Fault is a normal fault that cuts colluvium and extends for 3 km to the edge of the available lidar. Finally, the Paulina Fault is a reverse fault that cuts terraces and colluvium and may extend for 16 km. Using the Wells and Coppersmith (1994) relationship between surface rupture length and magnitude, estimated minimum moment magnitudes for the observed ruptures are: Strawberry Mountain Mo=6.6, Summit Prairie Mo=6.3, Dry Mountain Mo=6.0, and Paulina M 6.5. The rupture length of the Long Creek fault is too poorly known to estimate a magnitude.

Lidar is available for about 25% of Oregon’s eastern counties and none has been targeted for locating active faults. The discovery of so many young faults in the small area covered implies that the seismic hazard of this area is significantly underestimated in the USGS hazard maps. A concerted effort is needed to evaluate the seismic potential of these five faults and to obtain lidar coverage of the remaining areas, to fully understand the seismic hazard of Eastern Oregon.