Evaluating earthquake-induced rockfall hazard near the Dead Sea Transform
In: Natural hazards and earth system sciences: NHESS, Band 19, Heft 4, S. 889-906
ISSN: 1684-9981
Abstract. We address an approach for rockfall hazard evaluation where the study area
resides below a cliff in an a priori exposure to rockfall hazard, but no
historical documentation of rockfall events is available and hence
important rockfall hazard parameters like triggering mechanism and
recurrence interval are unknown. We study the rockfall hazard for the town of Qiryat Shemona, northern Israel,
situated alongside the Dead Sea Transform, at the foot of the Ramim
escarpment. Numerous boulders are scattered on the slopes above the town,
while pre-town historical aerial photos reveal that boulders had reached
the location that is now within town limits. We use field observations and optically
stimulated luminescence dating of past rockfall events combined with computer
modeling to evaluate the rockfall hazard. For the analysis, we first mapped
the rockfall source and final downslope stop sites and compiled the boulder
size distribution. We then simulated the possible rockfall trajectories using
the field observed data to calibrate the simulation software by comparing
simulated and mapped boulder stop sites along selected slopes, while
adjusting model input parameters for best fit. The analysis reveals areas of
high rockfall hazard at the southwestern quarters of the town and also
indicates that in the studied slopes falling blocks would stop where the
slope angle decreases below 5–10∘. Age determination suggests that
the rockfalls were triggered by large (M>6) historical earthquakes.
Nevertheless, not all large historical earthquakes triggered rockfalls.
Considering the size distribution of the past rockfalls in the study area and
the recurrence time of large earthquakes in the region, we estimate a
probability of less than 5 % to be affected by a destructive rockfall
within a 50-year time window. Here we suggest a comprehensive method to evaluate rockfall hazard where
only past rockfall evidence exists in the field. We show the importance of
integrating spatial and temporal field observations to assess the extent of
rockfall hazard, the potential block size distribution and the rockfall
recurrence interval.