Table 4

Correlation between Coseismic Subsidence Events and Beach Retreat Scarps in GPR Profiles from the Columbia River Littoral Cell

Subsidence * Event Designation Estimated Age ave. err. +-100 yr Subsidence Amount (m)   Beach Retreat** Event Designation Estimated Age Thousand years BP Size  
Y 1,700 AD ~1 A 0.3 ka Large
W 1,100 RCYBP ? B 1.1 ka Small
U 1,300 RCYBP <0.5 C 1.3 ka Very-small
S 1,700 RCYBP ~1 D 1.7 ka Large
N 2,500 RCYBP <0.5 E 2.5 ka Small
L 2,800 RCYBP ~1 F 2.8 ka Large
J 3,200 RCYBP ~1 G 3.2 ka Small
      H ~4.0 ka  
      I ~4.2 ka  
      J <5.0 ka  
* Subsidence event designation, approximate dates, and estimated amount of coseismic subsidence are from Atwater's compilation (2000).          
Age for event Y is known to the year (Satake, 1996). Event's W and U are known to nearest century due to tight C14 calibration curves.          
The older events S, N, L and J are less well constrained, but are estimated to the nearest 100 year interval for this report.          
Subsidence estimates are based on paleotidal-level indicators from adjacent estuarine wetlands (Shennan et al., 1996; Peterson et al., 2000)          
           
** Beach retreat event designations and relative size of scarp features in GPR profiles are from Table 3 in this Report.          
Ages of retreat events A-G are from correlation with reported subsidence events Y-J in this Report Table 4.          
Estimated ages of beach retreat events H and I are from correlation with dated beach ridge (Meyers et al., 1996; Table 2 of this Report).          
Limiting age estimate of event I is from basal shoreface deposit ages in nearby drill sites (Herb, 2000; Table 2 of this Report).