Dynamics of the Oso-Steelhead landslide from broadband seismic analysis

G Ekström,C Hibert,C P Stark

doi:10.5194/nhess-15-1265-2015

G Ekström, C Hibert + Show 1 more

Open Access

https://doi.org/10.5194/nhess-15-1265-2015

Copy DOI

Abstract

Abstract. We carry out a combined analysis of the short- and long-period seismic signals generated by the devastating Oso-Steelhead landslide that occurred on 22 March 2014. The seismic records show that the Oso-Steelhead landslide was not a single slope failure, but a succession of multiple failures distinguished by two major collapses that occurred approximately 3 min apart. The first generated long-period surface waves that were recorded at several proximal stations. We invert these long-period signals for the forces acting at the source, and obtain estimates of the first failure runout and kinematics, as well as its mass after calibration against the mass-centre displacement estimated from remote-sensing imagery. Short-period analysis of both events suggests that the source dynamics of the second event is more complex than the first. No distinct long-period surface waves were recorded for the second failure, which prevents inversion for its source parameters. However, by comparing the seismic energy of the short-period waves generated by both events we are able to estimate the volume of the second. Our analysis suggests that the volume of the second failure is about 15–30% of the total landslide volume, giving a total volume mobilized by the two events between 7 × 106 and 10 × 106 m3, in agreement with estimates from ground observations and lidar mapping.

Highlights

On 22 March 2014, a catastrophic landslide occurred 6.4 km east of Oso (Washington, USA), in the North Fork Stillaguamish River valley, destroying the neighbourhood known as “Steelhead Haven” and causing 43 fatalities (Keaton et al, 2014; Iverson et al, 2015)
We present the results of the inversion of the landslide force history (LFH) of the long-period signals generated by the first landslide, and provide an estimate of its mass, peak velocity and acceleration
Several differences between the seismic signals of the two events are identified: (1) the seismic signal of the second event has a more impulsive onset than the first (Fig. 2a and b); (2) several distinct amplitude peaks are observed in the signal of the second event filtered in the 1–3 Hz frequency and not for the first event (Fig. 2a); (3) the seismic signal of the second event has less energy in the frequency band above 5 Hz compared to the first (Fig. 2c); (4) a long-period signal was generated by the first event, and absent for the second

Summary

Introduction

On 22 March 2014, a catastrophic landslide occurred 6.4 km east of Oso (Washington, USA), in the North Fork Stillaguamish River valley, destroying the neighbourhood known as “Steelhead Haven” and causing 43 fatalities (Keaton et al, 2014; Iverson et al, 2015). In this study we present a joint interpretation of the longperiod force history of the Oso-Steelhead landslide and the associated short-period seismic signals. We note that our seismological analysis is different from that of Iverson et al (2015), and that our results differ in important ways from those obtained in that study. We discuss these differences and their likely explanation in some detail in the Appendix. We discuss, based on a comparison of the seismic records and with reference to the ground observations, possible source characteristics of the second event

Seismic observations

Force history of the first Oso-Steelhead landslide

Findings

Dynamics of the first event from comparison of the LFH and short-period data

Conclusions