ABSTRACT:
Seismicity generated by rockfall was monitored intermittently over a 5 week period at the Helmet Mountain cirque in British Columbia, Canada. To understand the timing and frequency of rockfall, we installed a seismic monitoring system consisting of two single-component geophones emplaced on the headwall. Two types of rockfall events were observed: single block fall and surficial talus slide. The seismic response during single block fall is characterized by multiple bursts of high-frequency energy as the rock strikes the wall periodically during the fall. A talus slide, on the other hand, has an emergent response and a constant release of seismic energy as the slide progresses. The timing of seismicity indicated that rockfall activity peaks from 10:00h to 18:00h when melting ice destabilizes loose blocks. The rockfall frequency mimics diurnal air temperature fluctuations, although the direct cause of rockfall is related to warming of the cliff.
1 INTRODUCTION
Rockfall originating high on a mountain cliff generates a seismic disturbance as it strikes the wall or other debris en route to the base of the slope Tilling et al. (1975) provided a comprehensive overview of rockfall seismicity from their observations during a period of intense rockfall at the Kilauea volcano in Hawaii. They observed that the seismic response during a large rockfall event is emergent, meaning that the arrival of seismic energy is gradual, and that high-frequency components attenuated rapidly with distance. Other researchers have observed large rockfall events on regional seismic networks and were able to triangulate the epicentral location, finding good agreement with local ground observations (e.g. Norris & Lester 1996). Whereas most previous research focused on seismicity created by large rockfall, our study focuses on smaller, localized events. In this work we will distinguish between two types of rockfall: single block fall and surficial talus slide.
