Discussion and Review of “Seismology of the Oso-Steelhead landslide” by Hibert, Stark, and Ekstrom

Abstract

In this study, Hibert et al. use the seismic waves generated by the disastrous 22 March 2014 landslide near Oso, Washington to investigate the dynamics of the event by combining long-period source inversions and short period analysis. Overall the paper is concise and well written and takes an innovative approach by integrating both high- and low-frequency seismic data together in the interpretation. It also demonstrates the useful prospect of resolving a trajectory of the center of mass of the landslide from a few traces of noisy seismic data. However, the main point of discussion that I would like to address stems from differences between the findings by Hibert et al. and the findings of a recently published multidisciplinary paper on the Oso landslide (Iverson et al., 2015). I am a co-author on the paper and performed the seismic analysis, which was similar to that of Hibert et al., yet the interpretation of how the event unfolded differs significantly between the two papers. In my opinion, this stems largely from a difference in information, some of which Hibert et al. unfortunately did not have access to prior to submission because it was just published in Iverson et al. (2015). However, differences also seem to stem from technical details of the methods and data used. Typically, such details are only of interest to seismologists, but in this case they make a substantial difference in understanding how the event unfolded and how it became so disastrous. For that reason, they are worthy of discussion here. The crux of the difference lies in two points 1) Iverson et al. (2015) found a multi-stage initiation of the main event, while Hibert et al. did not and 2) Iverson et al. (2015) found the second high-frequency event to be from a significantly smaller debris fall whereas Hibert et al. estimate it to be a substantial mass. I detail these two points below, then follow with an additional comment regarding the trajectory estimation made by Hibert et al. and some minor comments.