Seismic monitoring of small alpine rockfalls – validity, precision and limitations
Rockfall in deglaciated mountain valleys is perhaps the most important post-glacial geomorphic process for determining the rates and patterns of valley wall erosion. Furthermore, rockfall poses a significant hazard to inhabitants and motivates monitoring efforts in populated areas. Traditional r...
Main Authors: | , , , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2017-10-01
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Series: | Earth Surface Dynamics |
Online Access: | https://www.earth-surf-dynam.net/5/653/2017/esurf-5-653-2017.pdf |
Summary: | Rockfall in deglaciated mountain valleys is perhaps the most important
post-glacial geomorphic process for determining the rates and patterns of
valley wall erosion. Furthermore, rockfall poses a significant hazard to
inhabitants and motivates monitoring efforts in populated areas. Traditional
rockfall detection methods, such as aerial photography and terrestrial laser
scanning (TLS) data evaluation, provide constraints on the location and
released volume of rock but have limitations due to significant time lags or
integration times between surveys, and deliver limited information on
rockfall triggering mechanisms and the dynamics of individual events.
Environmental seismology, the study of seismic signals emitted by processes
at the Earth's surface, provides a complementary solution to these
shortcomings. However, this approach is predominantly limited by the strength
of the signals emitted by a source and their transformation and attenuation
towards receivers. To test the ability of seismic methods to identify and
locate small rockfalls, and to characterise their dynamics, we surveyed a
2.16 km<sup>2</sup> large, near-vertical cliff section of the Lauterbrunnen Valley
in the Swiss Alps with a TLS device and six broadband seismometers. During
37 days in autumn 2014, 10 TLS-detected rockfalls with volumes ranging from
0.053 ± 0.004 to 2.338 ± 0.085 m<sup>3</sup> were independently detected
and located by the seismic approach, with a deviation of 81<sub>−29</sub><sup>+59</sup> m
(about 7 % of the average inter-station distance of the seismometer
network). Further potential rockfalls were detected outside the TLS-surveyed
cliff area. The onset of individual events can be determined within a few
milliseconds, and their dynamics can be resolved into distinct phases, such
as detachment, free fall, intermittent impact, fragmentation, arrival at the
talus slope and subsequent slope activity. The small rockfall volumes in this
area require significant supervision during data processing: 2175 initially
picked potential events reduced to 511 potential events after applying
automatic rejection criteria. The 511 events needed to be inspected manually
to reveal 19 short earthquakes and 37 potential rockfalls, including the 10
TLS-detected events. Rockfall volume does not show a relationship with
released seismic energy or peak amplitude at this spatial scale due to the
dominance of other, process-inherent factors, such as fall height, degree of
fragmentation, and subsequent talus slope activity. The combination of TLS
and environmental seismology provides, despite the significant amount of
manual data processing, a detailed validation of seismic detection of small
volume rockfalls, and revealed unprecedented temporal, spatial and geometric
details about rockfalls in steep mountainous terrain. |
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ISSN: | 2196-6311 2196-632X |