Jhungli-Taiwan 2009

Testing earthquake triggered landslides against

earthquake aftershocks for space distributions

(implications for triggering processes)

l. Tatard, JR Grasso, S Garambois, A Helmstetter

LGIT, Observatoire de Grenoble, France

Tools - Cross analysis of space distribution of aftershocks and landslides; stack-averaged patterns (statistical physics)

Data: 5 sequences: Mw=7.6,7.6,7.2,6.6,5.4 (Thrusts + 1 normal faulting)

Results: landslide = aftershock

= > near field: aftershock <=> static stress changes: how to go beyond correlation?

= > landsliding: the ultimate site effect (need to work with the seismologist community)

=> landslide/aftershock: generic britlle crustal response to shocks (what we observe characterizes the medium)

Testing earthquake triggered landslide against

earthquake aftershocks for space distributions

Objectives - Constrains on processes that possibly drive slope failures

- Displacement (near-field, coseismic change in slope angle)?

- Static – dynamic stress changes?- Strain (PGV), strain rates (PGA)?

Jhungli-Taiwan 2009

Landslides and earthquake catalogues

NZ, M=7.2, 2003

NZ, Mw=5.4, 2004 x,y,z time size completenes

s

 aftershocks 1-10 km <sec M, Mo Mc

landslides few km >day-week m2- m3 Vc ?

NZ, 2001,2004 data from GNS>15% of data

NZ, Mw=7.2, 2003

Landslides and earthquake catalogues

NZ, M=7.2, 2003

.

NZ, Mw=7.2

Near field : ? (offshore)Middle field : Nldsl > Naft Far field : Nldsl > ? Naft

r-2

r-3

Gomberg and Fehlzer 2008

Landslides and earthquake catalogues

NZ, M=7.2, 2003

NZ, Mw=5.4, 2004

Numerous sources of possible bias (location, time, size)

r-2

r-3Near field : ? (offshore)Middle field : Nldsl > Naft Far field : Nldsl > ? Naft

Near field :Middle field : Nldsl = Naft Far field : Nldsl = Naft

r-1

Better fit between afts/landslide

….. when using the weakest dataset 2004

Landslides and earthquake catalogues

NZ, M=7.2, 2003California, Mw=6.6, 1994

Near field : Nldsl = Naft Middle field : Nldsl > Naft Far field : Nldsl > ? Naft

Landslides and earthquake catalogues

NZ, M=7.2, 2003Kashmir, Mw=7.6, 2005, 2004

Sato et al 2006

4 m

Kashmir, Mw=7.6, 2005, 2004 Pathier and Wright 2006

Grasso et al. 2006

Muzzaffarabad area

Kashmir, Mw=7.6, 2005

Surace fault trace and/or slide: peeling effect induce by change in slope value?

Landslides and earthquake catalogues

NZ, M=7.2, 2003Kashmir, Mw=7.6, 2005, 2004

Near field : Nldsl = Naft Middle field : Nldsl < Naft Far field : Nldsl < Naft

Landslides and earthquake catalogues

NZ, M=7.2, 2003Taiwan, Mw=7.6, 1999

Near field : Nldsl = Naft Middle field : Nldsl =/> Naft Far field : Nldsl = Naft

Landslides and earthquake catalogues

NZ, Mw=5.4 5.6, 2004

Near field : Nldsl = Naft Middle field : Nldsl =/> Naft Far field :

(Rotoehu aera: 223-312 mm rain within 3 days before the Mw5.6 event)

5 case-studies

- Landslide distribution in space mimics the one of aftershocks.- At a given distance Nldsl > Nafts (local data)

Eqs trigger

Mw Style Dist Nslide/

Dist Nafts

Eqs data

NZ 7.2 T-(SF) > local

Pakistan 7.6 T-SF < usgs

Chi-chi 7.6 T-SF = usgs

Northridge 6.6 T > local

NZ 5.4 N > local

Aftershock-Landslide response to quakes

NZ, M=7.2, 2003 NZ, Mw=5.4, 2004

Surface rupture rainrain (?)

Earthquake Triggered Landslides: Which candidates for driving processes?

NZ, M=7.2, 2003

Ground motion:surface rupture << burried fault

But how does it scale with distance (r) and earthquake size M=ln(L) ?

PGA r-2 x L-1.4

PGV r-1.8 x L-1.6

Displ. r-1.5 x L-1.8

Neqs r-2 x L-2

Gomberg and Fehlzer, 2008

Pitarka et al. 2009

Earthquake Triggered Landslides: Which candidates for driving processes?

NZ, M=7.2, 2003

PGA r-2 x L-1.4

PGV r-1.8 x L-1.6

Displ. r-1.5 x L-1.8

Neqs r-2 x L-2

Suggested Brodsky and Fehlzer, 2006Gomberg and Fehlzer, 2008

Earthquake Triggered Landslides: Which candidates for driving processes?

NZ, M=7.2, 2003

Earthquakes triggered slides: Saturation: 1-10 fault lengthSharp decrease: >20 D/L

Earthquake Triggered Landslides: Which candidates for driving processes?

PGA,PGV, Nldsl, Neqs : Northridge

PGV, PGA, Nldsl, Neqs : Chi-Chi

PGV-PGA Data from PEER web site

PGA, PGV # f(fHz, distance)

Note: Meunier et al. GRL 07 use (distance to epicenter + %(surface))

Wave trapping

Topographic& Landslide effects

Amplification frequency : f0=VS/4H

Frequency sensitivity : site effects

Exemple of the Utiku landslide (NZ)

collaboration LGIT (France) & GNS (NZ)

Frequency sensitivity of site effects(Utiku landslide, NZ)

Spectral ratio

On landslide

On bedrock

■ Utiku, UL1 site, H1=49 m, VS=400 m/s, fo=2.2 Hz =VS/4H)

■ PGA(fhz) observations differ from PGV (fhz) observationsparticularly in the [0.3-5 ] Hz frequency range where landslides are sensitive.

Earthquake Triggered Landslides: Which candidates for driving processes?

NZ, M=7.2, 2003

Landslide:- « simple » average pattern- f(distance)=aftershock- Tests for robustness (on going)

Processes: - Aftershocks: D/L=1-3, static stress; D/L>10 ?

Candidates for landslides ?- Is it definitively PGA for landslides?

- PGA-PGV # f(fHz, distance)=> PGA implies D>>, V>> ?

- Static-dynamics ?

or-Generic crustal response to shock: not driven by PGA-PGV-Disp-…

Huge landslide in the near field:

Meunier et al. GRL 07

warning:warningPGA threshold values for triggeringare derived from valley seismic stations(lack of instrumented slope)