Testing Potential Triggering Mechanisms of Long-runout Catastrophic Rock Avalanches in the Nooksack River Basin, Whatcom County…

The Nooksack basin contains five large Holocene long-runout rock avalanche deposits. Similarities and spatial proximity of the deposits suggest that they may share a common triggering mechanism. Two of the deposits, Van Zandt Landslide Complex (VZLC) and Church Mountain Sturzstrom (CMS) have published ages that overlap with prehistoric earthquakes, suggesting seismicity may be the cause. To further test this idea, I mapped and dated the remaining three deposits, Racehorse Creek (RHC), Middle Fork (MFN), and Maple Falls (MF). To establish ages for the slides, I combined radiocarbon dating of organics in sediment cores from bogs in the debris fields and cosmogenic radionuclide exposure dating of boulders in the debris fields. Despite large uncertainties in the dating, all landslides overlap with at least one local (Boulder Creek Fault) or regional (Cascadia Subduction Zone) earthquake, suggesting that earthquakes are likely triggers for at least some of these failures: MFN-1 (10170-9690 cal. yr B.P.); MFN-2 (4510-2700); RHC (4420-3990 cal. yr B.P.; Pringle et al., 1998); MF-1 (4110-2600 yr B.P.); CMS (2700-2150 cal. yr B.P.; Pringle et al., 1998); VZLC (1330-1270 cal. yr B.P.; Malick, 2018); MF-2 (1030-230 yr B.P.). Heavy precipitation events, however, remain as a viable alternative trigger for any individual slide. In addition, my results indicate that exposure dating of large boulders in debris fields of such landslides can be a viable method of determining their emplacement timing. Apparent cosmogenic inheritance in some boulders, however, indicates that such analyses may provide insights into emplacement dynamics of these landslides.