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FUTURE DEVELOPMENTS IN

EARTHQUAKE-INDUCED

LANDSLIDES SCENARIOS

Roberto W. Romeo

University of Urbino, ITALY

roberto.romeo@uniurb.it

The Next Generation of Research on

Earthquake-Induced Landslides

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SCENARIO: from the Latin scǽna(a stage with a plot)

• A scenario is a sketch, outline or description of an imagined situation or sequence of events. (from the Oxford English Dictionary)

• Misuse and overuse of the term has attracted hostile attitudes by the people

• It doesn’t represent a prediction tool but rather a way to induce people to think about unthinkable even if unpredictable events

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Task of a Scenario

To provide planners and managers with a plot of a possible sequence of events so that they may test and plan strategies against uncertain future developments a story about what happened in the future (from The Forecasting Dictionary)

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Characteristics of a SLS• A SLS refers to a wide area, usually at

basin scale for the purpose of investigating uniform climatic and geomorphological features…

• …thus requiring a large number of landslides of different type (i.e., falls, slumps and flows) to be analyzed through an appropriate …

• tool for a complete DAD process, namely Draw-Analyze-Display, usually accomplished by GIS-tools

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The SLS in the scientific literature

In the scientific literature:

213,000 articles

refer directly or indirectly to:

GROUND MOTION SCENARIOS

21,400 to LANDSLIDE SCENARIOS

9,700 to SEISMIC landslide SCENARIOS

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Landslide inventories and triggering earthquakes

In the last decade of the 20th century only 4 to 12 earthquakes have well documented inventories (Keefer, SG 2002 and Rodriguez et al., SDEE 1999) of landslides triggered by earthquakes versus about 1,500 earthquakes above magnitude of 6 (USGS Earthquake Hazard Program) according to the minimum reported magnitude in the inventories

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Empirically-based SLSs: a mission impossible (?)

• Landslides triggered by earthquakes are RARE events in a RARE cause :

PSL = P[L|Y] P[Y]• Being P[Y] = seismic hazard• and P[L] = failure probability, that is f(type

of movement, geotechnical properties, morphology, hydraulic conditions)

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• … and here’s why most of the methods to

produce SLSs rely on theoretical models

(mainly geotechnical) rather than on

empirical ones (mainly statistical: look at

the next talk by C.T. Lee)

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The milestone of SLSs

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Slope performance instead of slope stability

• They replaced the approach based on the pseudo-static safety factor with the Newmark’s sliding displ. analysis as implemented by Wilson and Keefer (BSSA, 1983) introducing the concept of slope performance rather than that of stability

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Further developments

• Jibson-Harp-Michael (Eng.Geol., 2000) sketched the procedure for a GIS-based SLH map

SL susceptibility

SH

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Probability assessment

• … and developed a method to estimate failure probabilities from Newmark displacements

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Towards a Performance-based approachRomeo-Jibson-Pugliese (2007, 1st NALC, Vail-CO)

• They performed a landslide scenario based on the results of a formal seismic hazard analysis, and

• … they afforded the task of estimating the potential damage to exposed goods in the area

Ac IA = f (M,R,S)

Yearly Fexc(D>Dc)or mean T

D as a function of the

IA

values

Probablistic AND/OR Deterministic

Landslide scenarios

capacity demand

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Common features of SLSs

1) Simplified Newmark’s sliding block analysis to asses slope performance on the basis of computed displacements

2) Shallow landslide model (i.e., infinite slope analysis)

3) A deterministic approach to the seismic hazard

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Merits …

• They allow the integration of the whole analysis into a single computer program (GIS environment)

• They usually (but not necessarily always) provide conservative results

• They have the potential for a real-time upgrade of slope performance under a given (even observed) seismic shaking

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… and shortcomings

• They provide approximate results usually affected by large uncertainties

• They are limited to shallow landslides (Keefer’s type I) that although being the predominant ones during eqks. nevertheless bring other important slides such as slumps and flows to be neglected

• They refer to a very limited seismic and boundary conditions thus reducing their capability to effectively mitigate the risk

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1st key-Q: May simplified Newmark’s sliding block analysis be replaced by an improved one?

• A simplified analysis replaces a rigorous one (double integration of a time history) through a regression equation like this:

Log(Dn) = k1 + k2 Log(gm) + k3 Log(ac)

• It is easy to be implemented into a GIS-environment for rapid and extensive analyses

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0

500

1000

1500

2000

0 2 4 6 8 10

Arias intensity, m/s

Dis

pla

cem

ent,

cm

Jib07+sig

Jibson_2007

Jib07-sig

e = 0.656

Uncertainty of simplified methods

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The use of a rigorous displacement analysis (e.g., Miles & Ho, SDEE

1999) is limited by: • Computations become cumbersome for a

wide area and necessarily require external calls (offline software)

• A substantial lack of improvement in absence of a formal local seismic response analysis that would allow amplification phenomena due to site effects being taken into account

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1st key-A: simplified methods still represent the most suitable for SLSs, nevertheless

they may be improved provided that:

• they are computed on a regional basis due to the current availability of a huge amount of strong motion records

• Amplified motion due to site effects can be taken into account in the regression models or through site amplification factors such as those used in the seismic codes (e.g., Eurocode 8)

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Regionally-based simplified equations

0.00001

0.0001

0.001

0.01

0.1

1

10

100

1000

0.01 0.1 1 10Arias intensity, m/s

Dis

pla

ce

me

nt,

cm

California (RWJ93)Italy (RWR00)Worldwide (RWJ07)

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Influence of site effects on the seismic displacements

0

100

200

300

400

500

0 2 4 6 8 10

Arias intensity, m/s

Dis

pla

cem

en

t, cm

soilall conditionsrock

Jibson_2007

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2nd k-Q: may the infinite slope model be replaced by a less approximate one?

• This is the only one that can be implemented into a GIS without requiring external calls even if GIS is a 2-½ dimensional tool, it can analyze heights but not depths

• … and this rigorously would restrict the analysis to cohesionless soils in dry or wet conditions alone

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2nd key-A: the infinite slope model is still the only one directly applicable in

a GIS-environment, anyway: • Failure depths for cohesive soils and water

table depths for partially saturated soils should be assessed with their own reliability

• Multiple seasonal scenarios may substitute the probabilistic estimate of the hydraulic conditions

• Care shall be taken over the choice of the most appropriate geotechnical properties to differentiate ductile from brittle behaviors

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3rd key-Q: may we refer to a seismic scenario which is not deterministic?• A reference earthquake is usually selected

whose gm-values may induce the largest ground failures, anyway:

• Over a large area usually many sources are active and more than one earthquake is required to encompass all the possible causes of failure

• A deterministic eqk. can be effectively useful mainly in view of a rigorous displacement analysis and a LSRA which are anyway not feasible for the reasons cited above

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3rd key-A: a deterministic scenario may be well replaced by a probabilistic one, since:

• It is consistent with the simplified displacement analysis whose regression equations derive from a set of earthquakes

• It allows performing a full probabilistic analysis of slopes performance under seismic shaking

• It may be better implemented in a conventional risk assessment procedure

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Displacement hazard curves

figure from Rathje & Saygili, JGGE 2008

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A PB approach

Romeo (2008), technical report

0

0.2

0.4

0.6

0.8

1

1.2

1.E-09 1.E-06 1.E-03 1.E+00 1.E+03

intensità

CD

Fs

cap

acit

y

0

0.2

0.4

0.6

0.8

1

1.2C

CD

F d

eman

d

buildings ind.facilities lifelines roads hazard

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GM from earthquake

Far - Strong

Near - Moderate

All inclusive (PSHA)

Hydraulic conditions

Spring Sp/F-S Sp/N-M

Fu

ll

Pro

bab

ilistic*

Summer Su/F-S Su/N-M

Fall F/F-S F/N-M

Winter W/F-S W/N-M

SCENARIO

The SLS matrix

* with the possibility of hazard deaggregation

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THANK YOU VERY MUCH FOR

YOUR ATTENTION !