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SASHA code for probabilistic hazard assessment: the new version 2.03

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Dipartimento di Scienze Fisiche, della Terra e dell’Ambiente

Universityof Siena, Italy

Vera D’Amico

Istituto Nazionale di Geofisica e Vulcanologia

Sez. di Milano, Italy

SASHA code for probabilistic hazard assessment: the new version 2.03

Earthquakes exhibit an a inherent irregularity in individual occurrences that does not depend on the quality of monitoring

In principle, seismic wavefield generated by any future earthquake could be perfectly predicted if information on faulting process and mechanical properties of subsoil were known in sufficient detail: but this is not the case

On the other hand, coping with future earthquake damages requires the definition of a reasonable upper bound (seismic hazard) for expected ground shaking at any site of interest for a relatively long exposure time (tens of years)

In this situation, each forecast is a bet about possible future occurrences

Seismological knowledge only allows the identification of a (large) number of possible future scenarios (without any fixed upper bound)

What we can do is to attribute to each of these scenarios a different degree of belief (probability distribution) on the basis of available knowledge

This makes mandatory the use of a probabilistic language when we deal with seismic hazard

Available PSHA procedures differs in that the relevant degrees of belief can be attributed by considering several pieces of information (statistics of past seismicity, geodynamic information, etc.) combined in some way

What is a mandatory in this context is that, whatever is the procedure adopted for likelihood estimates, internal coherency of the adopted procedure should be maintained along with the empirical testability of final outcomes

The SASHA (Site Approachto Seismic Hazard Assessment) approach is a PSHA procedure of this kind, entirely based on a statistical analysis of past seismicity, on purpose developed to manage intensity data

Because we are mainly interested in Risk assessment and Intensity is direct expression of the effects produced by earthquakes on the anthropic environment

Because most of information concerning past earthquakes are only expressed in the form of documentary data: this is particularly true where seismicity rates are relatively low and long lasting historical records are available (in Italy 70% of damaging earthquakes are only known from historical descriptions)

What is “peculiar” in a macroseismic approach to PSHA?

- Requires management of Intensity values that are ordinal, discrete and defined on a finite range of possible values, which prevents the use of formalizations currently used for PSHA
- Differently from the “source dependent” view of modern seismology, the macroseismic point of view focuses on local data (site seismic history)
- In many cases, available information does not allow an univocal attribution of local intensity: parameterization of such uncertainty has to be compatible with the peculiar character of macroseismicinformation

- Hazard Curve where, for each Intensity threshold Is , the probability is assessed that at least one event with Intensity not less than Is will occur in the exposure time at the site under study
- From this hazard curve any reference Intensity Iref can be determined by considering specific applications of concern
- Deaggregation analysis can be performed to identify magnitude/distance couples more representative for the reference ground motion. Furthermore, significant past events responsible for local hazard can be identified

Step 1

Local seismic history is reconstructed which is expressed in the form of a sequence of macroseismic intensities values at the sitedetermined from documentary information, epicentral data, ground motion simulations

Each intensity evaluation is considered as affected by a measurable uncertainty depending on the available information

Completeness and representativity of information locally available is evaluated along the relevant uncertainty (local completeness)

Step 2

Seismic Recurrence at the site for each Intensity threshold is parameterized by a fully distribution-free approach not requiring any pre-processing (aftershock removal, selection of mainshocks within a seismic sequence, ecc.). Deaggregation analysis is performed

- Local seismic history can be retrieved by combining four different sources:
- Macroseismic felt effects documented at the site (observed intensity)
- Epicentral information (distance from the source, orientation with respect to the fault, magnitude, epicentral intensity, etc.) by using empirical probabilistic attenuation relationships (expected intensity)
- Macroseismic effects documented nearby the site (neighbouring observations)
- Geophysical/Geological information by numerical/physical ground motion simulations (synthetic intensity)

- Uncertainty affecting Intensity values is the effect of the relevant lack of information
- To parameterize and manage this uncertainty, each intensity value is implemented in the form of a discrete probability distribution P(Is) representing for each value Is the degree of belief (probability) associated to the statement
- “During the i-th event, seismic effects at the site where not less than Is”

Parameterization of uncertainty on Intensity from documentary data

The interpreter (historian) is asked to evaluate, on the basis of available documentary sources and collateral information, the likelihood of a statement such as:

“During the earthquake the effects at the site were not less than I“

The likelihood is represented by a numerical value in the range [0,1] (probability) e.g. by using as a rule:

0.0 : False; 0.25: unlikely; 0.50: uncertain;

0.75: likely; 1.0true

No!

More severe

VI MCS

Possible

VII MCS

VIII MCS

Possible

Possible

IX MCS

- Possible alternative codings
- >VI MCS and < X MCS
- VII-IX MCS
- p(VII)=33%; p(VIII)=33%; p(IX)=33%
- P(≥VII)=100%; P(≥VIII)=66%; P(≥IX)=33%

No!

Less severe

X MCS

0.5

I II III IV V VI VII VIII IX X XI XII

P(≥I)

0.5

I MCS

I II III IV V VI VII VIII IX X XI XII

This implies that any documented intensity is expressed in the form of a probability distribution

1.0

p(i)=(0,0,0,0,0,0,0.33,0.33,0.33,0,0,0)

I MCS

P(i)=(1,1,1,1,1,1,1,1,0.66,0.33,0,0,0)

1.0

Site seismic histories (if available) are provided in the form of a list of macroseismic observations

NCPTI11 Year Mo Da NDBMI11 Istat01 LatIDPLonIDPIloc

1 1005 0 0 1 09051002 43.463 11.879 7.5

1 1005 0 0 2 09047014 43.932 10.913 5

2 1005 0 0 5 12060019 41.488 13.831 7

General information

Feltintensity(intermediate values indicate uncertaintybetween the twocontiguousvalues)

Site Code

Codingof the seismicevent

Site Coordinates

Codingof the individual information

Epicentral data and attenuation pattern to compute expected intensity (if required)

Epicentral information

Attenuation pattern

In the newversionof the SASHA code (2.03), attenuation pattern isattributedindividuallytoeachearthquake

Three possible attenuation patterns have been implemented so far each having the form of a probability distribution

1. Normal distribution N[m(I; Io, r), s]with variance s and average in the form

I=Io+c(D-h)+d[ln(D)-ln(h)]; D=(r2+h2)1/2

being c, d and h empirical parameters supplied by the user and Io is the epicentral intensity ad r is the distance from the source

2. Normal distribution N[m(I; Io, r), s]with variance s and average in the form

I=a+bD+cln(D)+dI0 ; D=(r2+h2)1/2

beinga, b, c, d and hparameterssuppliedby the user

A third probability distribution is the one provided by (Rotondiand Zonno, 2004; Azzaro et al., 2012), requires 8 parameters supplied by the user and accounts for anysotropic attenuation patterns

A third source of information are Synthetic” Intensities: seismic effects can also be computed from geological/seismological information via numerical simulations.

In this case, convolution of two kinds of uncertainty will be considered relative to source parameters and conversion from ground-motion parameters to intensity via empirical conversion relationships

Site coordinates

Probabilitydistributionassociatedtoeachsimualatedintensity (this can bealsousedtosupplyintensityobservationsaffectedbyuncertaintylargerthantwocontiguousdegrees

Occurrencetime (notused)

Locality code

Hazardcomputations

Hazard computation are performed by considering

A distribution free approach for time recurrence modelling

Varying levels of local completeness and representativity of the local seismic history

Two outcomes are provided

An hazard curve relative to a fixed exposure time

Iref

It is worth noting that Irefmust correspond to one value of the scale

No non-integer value (e.g.,VII-VIII) is possible by definition

In order to compare distribution-free SASHA outcomes (function of the exposure time) with results commonly provided by a Poissonian modelling (function of of the average return time or ART) suitable correspondence rules have to be accounted for

SASHA WORKSHOP

Urban Disaster Prevention Strategies Using MAcroseismic Fields and FAult Sources (UPStrat-MAFA)GRANT AGREEMENT n. 230301/2011/613486/SUB/A5

Deaggregation analysis

Hazard estimates provided in terms of Intensity can be very useful when risk assessment is of concern, but cannot be used for design purposes

However, by considering availability of an epicentral catalogue where magnitude estimates (macroseismic or instrumental) and epicentral locations are reported for each earthquake, Magnitude/distance couples representative for the local seismic hazard can be retrieved and use to evaluate expected ground motion spectra from empirical attenuation relationships

Data in the timeinterval 1000-2002

Iref= IX MCS

Imax= IX MCS

Hazard curve (50 y ofexposuretime)

YearMonthDay Lat LonMw R W

These probabilities can be summed up by computing the “expected” number of events in the magnitude/distance bin

The new version of the SASHA allows the coherent management of uncertainty (on local seismic history, catalogue completeness and representativity, earthquake occurrence) affecting hazard estimates in terms of macroseismic intensity

In particular, a very general parameterization of uncertainty affecting basic data is provided that can be fitted to specific situations

In this approach all information available at the site can be considered (including those in the “less complete” part of the catalogue) in the frame of a distribution-free coherent probabilistic approach

SASHA outcomes are in line with requirements of risk assessment procedures and also allow to gain a deeper insight into the role of single events in the local hazard that could require particular attention or modelling

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