Paleoseismic Evidence for Holocene Surface Rupturing Earthquakes on the Gales Creek Fault, Northwest Oregon & New Insights into Paleoseismic Age Models — Detrital Charcoal Inbuilt Ages

Little is known about the Holocene activity of northwest-striking faults in and around the Portland Basin. Recent paleoseismic studies of the Gales Creek fault (GCF) have yielded new earthquake-timing constraints for the 73-km-long, northwest-trending fault with youthful geomorphic expression. We excavated a paleoseismic trench across the GCF and document evidence of three surface rupturing earthquakes from stratigraphic and structural relationships. Radiocarbon samples from offset stratigraphy constrain these earthquakes to have occurred ~1,000, ~4,200 and ~8,800 calibrated years before present. Our results suggest the GCF has a recurrence interval of ~4,000 years and if the full 73-km length were to rupture it would result in a Mw 7.1 to 7.4 earthquake, providing a significant seismic hazard for the greater Portland Metropolitan area. 

Earthquake age models determined from paleoseismic data often depend on 14C ages from large numbers of detrital charcoal samples collected from offset stratigraphy. Charcoal is commonly used to develop these models but can be hundreds of years older than the stratigraphic layer that contains it. Dendrochronological age constraints at the Hazel Dell paleoseismic site provide a means to explore the potential for systematic bias in age estimates of earthquakes in forested settings including the Santa Cruz Mountains section of the San Andreas Fault (SAF), California. Age constraints developed from detrital charcoal are compared to absolute dates from dendrochronology. We develop a new method for estimating inbuilt layer ages that makes use of more than just the youngest radiocarbon dates in a given layer. The improved age model relates likely layer deposition dates with observed 14C ages. We find that for Hazel Dell the most likely charcoal sample (mean) is ~ 322 years older than the actual age of the deposit that contains it. In this portion of the talk I’ll introduce the methodology we’ve proposed and explore how inbuilt age for detrital charcoal impacts age models for paleoseismic earthquake histories.

Ashley Streig, Ph.D., Paleoseismologist, Assistant Professor, Portland State University

Dr. Streig has twenty years of experience in active tectonics, paleoseismology, structural geology and tectonic geomorphology. Her varied research experiences range from investigations of strike-slip, reverse and normal fault studies. Her research and geologic consulting experience include; the study of active faults and folds, earthquakes and associated hazards, earthquake recurrence, fault behavior and rupture characteristics, estimating paleo-earthquake magnitude by relating observed deformation and earthquake timing between paleoseismic sites along a fault, fault reactivation, earthquake triggering.  Dr. Streig has worked both nationally and internationally on seismic source characterizations and paleoseismic investigations on the Gales Creek fault, Mt. Hood fault, and Cascadia Subduction zone in Oregon; the San Andreas, Hayward, Calaveras, Mesquite Lake, Kern Canyon and Breckenridge faults, in California; un-named normal faults in the Snake River Plain, Idaho; the Meers Fault, Oklahoma; the Mae Chan fault, Thailand, and the Chelungpu fault, Taiwan. She has conducted field reconnaissance and mapping after surface rupturing earthquakes following the 2011 Mw 6.6 Iwaki earthquake, Japan, 2010 Mw 7.2 El Mayor-Cucapah earthquake, Mexico, and the 1999 Mw 7.6 Chi Chi, Taiwan earthquake.  Dr. Streig has extensive experience with C14 and dendrochronologic dating techniques, and Bayesian statistical analysis using OxCal calibration software for earthquake age determination models.