TRIGS meeting April 7th 2009
The Team:
Jean-Robert Grasso
Researcher
Agnes Helmstetter
Researcher
David Amitrano
Researcher
Pascal Lacroix
Postdoc since
Nov. 2008
Lucile Tatard
PhD, 2007-2009
• Landslides :
seismic monitoring, triggering mechanisms, displacement
measurement… (JR Grasso. Tatard, A. Helmstetter, D. Amitrano, P. Lacroix)
• Volcanoes :
eruptions, seismicity, and forecasting(JR Grasso, L. Tatard)
• Earthquakes :
• statistical analysis of catalogs, triggering mechanisms and
forecasting(A. Helmstetter, D. Amitrano)
• Laboratory expermients :
Fracture and friction experiments(D Amitrano, JR Grasso)
Research Topics
Seismic monitoring of Séchilienne landslideWP5.1
Laffrey, 1999; 2005
Grenoble30 km
Séchilienne
Seveso plant
Séchilienne landslide and geodetic network
The landslide instrumented since 1988 by CETE Lyon, with extensometers, inclinometers, strainmeters, GPS and distancemeters (laser and radar).
Seismic network
• vertical seismometer∆ 3 component Seismometer
Stations THE and RUI installed may 2007GAL installed April 2008
Data, catalogs, … :Google OMIV -> Sechilienne-> seismology
Séchilienne Photo taken from just above the most active zone :
Seismic network : objectives
• detection and classification of seismic signals produced by the rockslide:
Micro-seismicity, distant EQs, rockfalls, natural or artificial tremors, …
• event location (work in progress)
• imaging the structure of the movement and its temporal evolution with
Seismic wave tomography with explosions or seismic noise
• site effects : amplification of ground motion due to the damaged zone
(project)
• comparison of micro-seismic activity with meteorological data,
displacement, PS, and external earthquakes
Micro-earthquake? Fracture? Crevasse opening?TH
E R
UI
G
AL
(int)
(ext)
Regional earthquakes
• Example of earthquake
located about 5km from the
landslide (p-s ≈ 0.7s)• Higher frequency than
local events !?
> Strong attenuation of
local events due to
damaged rocks?
Rockfall seismicsignal
Rockfall experiments
Rock boulder of ≈20x30x50cm pushed from the top of the ruins
Characteristics of events
energy (t)
duration (t)
amplitude (t) cdf(amplitude)
cdf(energy)
cdf(duration)
30s duration of rockfall propagation from the top of the ruins
T<5 secMicro EQs?
T>5, rockfalls?
0.05m3
≈100m3?
micro EQs? rockfalls?
Location of seismic signals using antenna methods
TH
E
RU
I
(in
t)
G
AL
(ext
)
• find seismic wave velocity V and
source location by maximizing the
average inter-traces correlation after
shifting the traces in time by the
travel time t=distance/V
1-40Hz
Map of average correlation
Location of seismic signals using antenna methods
Problems • We assume seismic wave velocity is constant in time and
space, but there is a mixture of waves of different type and V • V is highly heterogeneous in the rock mass • The types of seismic waves is unkown : P? S? surface waves?
(work in progress of waves polarization with 3C sensor)
Advantages: • Fully automatic • Works well with emergent signals or rockfall, and for looking at
the propagation of the source • Time accuracy potentially smaller than sampling rate
Propagation of rockfall avalanches
Location using a sliding time window
Location in the direction of propagation All 3 stations
Azimut relative to station THE, using only THE
Location of seismic signals using antenna methods
• events location using a single station : only the azimuth is constrained
• histogramm of the direction of natural local events recorded by stations RUI (207 events) and THE (349 events) from 10/4/2008 to 7/2008.
• most events located at top of the Ruins (grey area)
Influence of climate on displacement and seismicity
Displacement rate of target point #631 (CETE Lyon)
Chamrousse Weather station(Meteo France)
Natural local events recorded by station THE
Influence of climate forcing on displacement and seismicity
Influence of climate forcing on seismicity
Precipitation : • Peak correlation for zero time lag (<1hr)•+ rain increases on average 2 days before seismic events
Temperature: • seismic events associated with a temperature decrease (freezing?),
or just a result of the correlation between rain and temperature?
Correlation between climate and displacement rate
Precipitation : • Peak correlation for a time lag of 4days
+ rain increases on average 20 days before landslide acceleration
Temperature: • no significant correlation -> look at data since 1996?
Correlation between displacement and seismicity
Peak of correlation for zero time lag (<1 day),
But acceleration lasts longer than seismic crisis
Displacement since 1996 (Data CETE Lyon)
Influence of distant EQs on the landslide?
11/1/1999 m=3.5 1/10/2005 m=2.8
• 2 EQs occured about 4km from Séchilienne in 1999 and 2005, with m=3.5 and m=2.8• Only the 2nd smaller EQ produced an acceleration of the movement !?
Super Sauze displacement
Displacement amplitude measured by correlating pictures taken at one
month interval
Pictures from EOST Strasbourg (JP Mallet), with 1 month interval
Method developped and applied by P. Lacroix
1m0 60 m
• Same results than standard method, but 100 times faster
m
Displacement prior to the 2006 V>20000m3 rockfall
Displacement amplitude measured by correlating pictures taken one
day and a few hours before the rockfall, from the other side of the
valley
Pictures from CETE Lyon (Y. Kaspersky)
Method developped by P. Lacroix
0 15 m
Conclusion and perpectives for Séchilienne
• A few thousands seismic events recorded of varous types
• Intermittent activity, influenced by climate and distance EQs (?)
• Better location using a 3D velocity model (work in progress by
LGIT and SAGE), and seismometers in boreholes (project 2009
INERIS and CETE)?
• Video camera (installed summer 2009 by CETE) : calibration of
seismic netwok, estimation of rockfall volume and propagation
speed from seismic signals
• Quantify rockfall hazard and the influence of rain and distant
EQs, using numerous small events to estimate the probability
of a 106m3 event?
Project: seismic monitoring of snow avalanches
• P. Lacroix, A. Helmstetter, JR Grasso, and « Centre D’étude de la Neige » (Grenoble)• Col d’Ornon (~20km South of Grenoble), winter 2009-2010 • Comparison of seismic signals• Location and propagation of avalanches
OMIV: Muldisciplinary Observatory of Slope Instabilities
http://www-lgit.obs.ujf-grenoble.fr/observations/omiv/
Rockslides (Mica shists and gneiss)
Clay or soft rocks