Flooding resulting from the bursting of dams formed by landquake events such as rock avalanches, landslides and debris flows can lead to serious bank erosion and inundation of populated areas near rivers. Seismic waves can be generated by landquake events which can be described as time-dependent forces (unloading/reloading cycles) acting on the Earth. In this study, we conduct inversions of long-period (LP, period ≥20 s) waveforms for the landquake force histories (LFHs) of ten events, which provide quantitative characterization of the initiation, propagation and termination stages of the slope failures. When the results obtained from LP waveforms are analyzed together with high-frequency (HF, 1–3 Hz) seismic signals, we find a relatively strong late-arriving seismic phase (dubbed Dam-forming phase or D-phase) recorded clearly in the HF waveforms at the closest stations, which potentially marks the time when the collapsed masses sliding into river and perhaps even impacting the topographic barrier on the opposite bank. Consequently, our approach to analyzing the LP and HF waveforms developed in this study has a high potential for identifying five dam-forming landquake events (DFLEs) in near real-time using broadband seismic records, which can provide timely warnings of the impending floods to downstream residents.
Abstract. Landslide hazard motivates the need for a deeper understanding of the events
that occur before, during, and after catastrophic slope failures. Due to the
destructive nature of such events, in situ observation is often difficult or
impossible. Here, we use data from a network of 58 seismic stations to
characterise a large landslide at the Askja caldera, Iceland, on 21 July
2014. High data quality and extensive network coverage allow us to analyse
both long- and short-period signals associated with the landslide, and
thereby obtain information about its triggering, initiation, timing, and
propagation. At long periods, a landslide force history inversion shows that
the Askja landslide was a single, large event starting at the SE corner of
the caldera lake at 23:24:05 UTC and propagating to the NW in the following
2 min. The bulk sliding mass was 7–16 × 1010 kg, equivalent
to a collapsed volume of 35–80 × 106 m3. The sliding
mass was displaced downslope by 1260 ± 250 m. At short periods, a
seismic tremor was observed for 30 min before the landslide. The tremor is
approximately harmonic with a fundamental frequency of 2.3 Hz and shows
time-dependent changes of its frequency content. We attribute the seismic
tremor to stick-slip motion along the landslide failure plane. Accelerating
motion leading up to the catastrophic slope failure culminated in an aseismic
quiescent period for 2 min before the landslide. We propose that precursory
seismic signals may be useful in landslide early-warning systems. The 8 h
after the main landslide failure are characterised by smaller slope failures
originating from the destabilised caldera wall decaying in frequency and
magnitude. We introduce the term “afterslides” for this subsequent,
declining slope activity after a large landslide.
Continuous seismic records near river channels can be used to quantify the energy induced by river sediment transport. During the 2011 typhoon season, we deployed a seismic array along the Chishan River in the mountain area of southern Taiwan, where there is strong variability in water discharge and high sedimentation rates. We observe hysteresis in the high-frequency (5–15 Hz) seismic noise level relative to the associated hydrological parameters. In addition, our seismic noise analysis reveals an asymmetry and a high coherence in noise cross-correlation functions for several station pairs during the typhoon passage, which corresponds to sediment particles and turbulent flows impacting along the riverbed where the river bends sharply. Based on spectral characteristics of the seismic records, we also detected 20 landslide/debris flow events, which we use to estimate the sediment supply. Comparison of sediment flux between seismologically determined bedload and derived suspended load indicates temporal changes in the sediment flux ratio, which imply a complex transition process from the bedload regime to the suspension regime between typhoon passage and off-typhoon periods. Our study demonstrates the possibility of seismologically monitoring river bedload transport, thus providing valuable additional information for studying fluvial bedrock erosion and mountain landscape evolution.
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