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