probabilistic-deterministic hazard scenario...
TRANSCRIPT
Department of PhysicsFaculty of Sciences
Federico II UniversityNaples - Italy
[email protected]@na.infn.it
PROBABILISTIC-DETERMINISTIC HAZARD SCENARIO
FOR THE 1980 IRPINIA EARTHQUAKE, M=6.9, SOUTHERN ITALY
VINCENZO CONVERTITO, ANTONIO EMOLO, FRANTISEK GALLOVIC, AND ALDO ZOLLO
Department of GeophysicsFaculty of Mathematics and Physics
Charles UniversityPrague - Czech Republic
Theory framework
“Controlling earthquake”: the case of the 1980 Irpinia seismic event
Convertito et al. (2005), in a recent work, have developed a technique aimed at extending the Probabilistic Seismic Hazard Analysis (PSHA) to the case of a single causative fault. Their approach is based on a modification of the classical hazard integral (Cornell, 1968). In particular, a site-dependent evaluation of the exceeding probability for the chosen strong ground motion parameter is performed from a statistical analysis of the synthetic waveform data-base produced for a large number of possible rupture histories occurring on the causative fault. They reformulated the classical hazard integral as follows:
where r* represents, for each site of interest, the minimum distance from the surface projection of the rupturing fault. The probability density function f (m) describes the probability of occurrence of each earthquake having magnitude in the range of interest and, classically, it has a M
truncated exponential shape. Due to the selected source-geometry and the adopted distance definition, the new formulation of the hazard integral does not contain any PDF on the distance that describes the probability of occurrence of any earthquake at a given range of distances from the source. The a parameter in the equation (1) represents the seismic activity rate that, in the technique proposed by Convertito et al. (2005) for a single causative fault, is computed assuming the “characteristic earthquake” model (Youngs and Coppersmith, 1985). This model assumes that there is a range of magnitude (m , m +Dm ) having a constant probability of occurrence, characterized by a seismic activity rate a along with the C C 2 C
classical truncated exponential PDF characterized by a seismic activity rate a for lower magnitude (see panel 2). Thus, the use of the NC
characteristic earthquake model (Youngs and Coppersmith, 1985) allows to formulate an ad-hoc f (m) PDF to cmpute the appropriate seismic M
activity rate for the selected “controlling earthquake” and to solve the integral in the equation (1) providing a hybrid probabilistic-deterministic scenario-like description of the hazard.
(1)( ) ( ) ( )[ ]òD+
³a=³2
00
mm
maM
c
c
dm*r,mA*r,mApmfAAE
The technique developed by Convertito et al. (2005) has been applied to the case of the 1980 Irpinia seismic event (November 23, 1980, M =6.9). During this earthquake three normal fault segments were S
activated, the second and the third delayed, approximately, 20s and 40s after the first one. The source characteristics used for the waveform simulation were determined by Bernard and Zollo (1989).
We chose to perform two different simulation: the first involving only the 0s fault and the second all the three activated fault segments.For each fault segment we distributed the rupture nucleation points every 5km in the strike direction. Their position in the dip direction was fixed according to the depth of hypocentres determined by Bernard and Zollo (1989). This means that, according to the faults dimensions, for the 0s fault we had 8 rupture nucleation points, while we had 5 rupture nucleation points for both the 20s fault and 40s fault. For each nucleation point position there were 5 different heterogeneous final slip distributions. Each slip distribution is characterized by an average value corresponding to the seismic moment for the selected fault segment. Summarizing, for the 0s event a total of 40 different fault rupture scenarios were simulated while, when we consider the whole earthquake, the number of different source rupture scenarios were 25,000.
For each source rupture scenario, the synthetic ground acceleration was simulated at a dense regional grid of virtual receivers. Ultimately, we had a family of 40 synthetic accelerograms for each receiver in the case of the 0s event simulation and a family of 25,000 synthetic accelerograms for each receiver in the case of the whole earthquake. Ground motion is simulated by a hybrid source model based on the combination of both the integral (according to the representation integral) and composite (summation of subsources’ contributions) approaches. Details about the ground motion simulation technique can be found in the poster by A. Emolo and F. Gallovic in this same session (poster #12, abstract #944).The Peak Ground Acceleration (PGA) was the selected ground motion parameter for hazard analysis even if, due to the availability of waveforms, any other parameter of engineering interest could be selected. At each receiver, we characterized the simulated PGA family by the mean value with the associated statistical Coefficient of Variation (CoV). Thus, for each site, the probability of exceedance p in the equation (1) a
in the panel 1 could be finally computed using the mean PGA and the related uncertainty.
The seismic activity rate for the characteristic part of the magnitude PDF (equation 2 in the panel 1) is the last needed parameter to be computed in order to evaluate the hazard integral. We retrieved from the NT4.1 catalogue (Camassi and Stucchi, 1996) the values of the parameters b, m and a that characterize the seismic source zone in which the considered fault system is 0 cat
embedded and the magnitude range for which the characteristic behavior of the selected faults can be hypothesized. The parameter m was selected for each fault according to the Wells and max
Coppersmith (1994) scaling laws. Moreover, we adopted a value of 0.285 mm/year for the slip rate that is the average value estimated for the seismogenetic faults located in the Italian Southern Apennines region (Valensise et al., 2001). The hazard maps for both considered seismic events can now be computed. The selected return periods for our analysis were T =10,000 years, 1
T =20,000 years, and T =50,000 years.2 3
Parameters of the three fault segments activated during the 1980 Irpinia earthquake (after Bernard and Zollo, 1989).
Left column. Maps of simulated mean PGA values (top) and associated CoV (bottom) for the 0s event. Right column. Maps of simulated mean PGA values (top) and associated CoV (bottom) for the whole seismic event. In all figures the rectangles represent the surface projection of faults.
Magnitude PDF for the truncated exponential recurrence model (dashed line) and for the characteristic earthquake recurrence model for the three fault segments (solid lines).
3.22 10-4
40s
5.02 10-3
40s
5.02 10-3
40s
3.42 10-45.92 10-3
20s20s
5.92 10-3
20s20s
6.7
0s0s0s6.13 10-20.51.040s0s
4.00.4217Whole6.6 2.13 10-41.71 10-35.02 10-30.51.07.0
2.13 101.71 10-35.02 10-36.13 10-20.51.07.04.00.42170s
aCaNCaexp-1
(years ) (Y&C, 1985)acat-1
(years )Dm2Dm1mmaxmob -1(years ) (Y&C, 1985)
-1(years ) (Y&C, 1985)
-4
Parameters needed for the computation of the hazard integral. Legend. b - b-value of the Gutenberg and Richter’s law. m - minimum magnitude of interest. m - maximum magnitude of interest. Dm - 0 max 1
magnitude interval below the magnitude level considered as the characteristic earthquake magnitude. Dm - magnitude interval above the magnitude level considered as the characteristic earthquake 2
magnitude. a - seismic activity rate retrieved from the NT4.1 seismic catalogue (Camassi and Stucchi, 1996). a - seismic activity rate computed for the exponential model of earthquake occurrence cat exp
using the relation provided by Youngs and Coppersmith (1985). a - seismic activity rate for the non-characteristic part of the PDF f (m) computed using the relation provided by Youngs and Coppersmith NC M
(1985). a - seismic activity rate for the characteristic part of the PDF f (m) computed using the relation provided by Youngs and Coppersmith (1985).NC M
10 kyrs return period
0s e
ve
nt
wh
ole
fa
ult
syste
m
20 kyrs return period 50 kyrs return period
References! Bernard P., and A. Zollo (1989). The Irpinia (Italy) 1980 earthquake: detailed analysis of a complex normal faulting, J. Geophys. Res. 94, 1631-1647.! Camassi R. and M. Stucchi (1996): NT4.1: un catalogo parametrico di terremoti di area italiana al di sopra della soglia del danno (CNR-GNDT, Milano), 1-86.! Convertito V., A. Emolo, and A.Zollo (2005). Seismic hazard assessment for a characteristic earthquake scenario: an integrated probabilistic-deterministic method, accepted for publication on Bull. Seism. Soc. Am.! Cornell C.A. (1968). Engineering seismic risk analysis, Bull. Seism. Soc. Am. 58, 1583-1606.! Reiter L. (1990). Earthquake hazard analysis. Columbia University Press, New York, 254 pp.! Sabetta F., and A. Pugliese (1987). Attenuation of peak horizontal acceleration and velocity from Italian strong-motion records, Bull. Seism. Soc. Am. 77, 1491-1513.! Valensise G., R.Basili, M.Mucciarelli, and D.Pantosti (Editors) (2001). Database of potential sources for earthquakes larger than M 5.5 in Europe, a compilation of data collected by partners of EU project FAUST. Distributed through the Internet: URL http://www.ingv.it/! Wells D.L., and K.J. Coppersmith (1994)). New empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement, Bull. Seism. Soc. Am. 84, 974-1002.! Youngs R.R., and K.J. Coppersmith (1985). Implications of fault slip rate and earthquakes recurrence models to probabilistic seismic hazard estimates, Bull. Seism. Soc. Am. 58, 939,964.
~roma/banche/catalogo_europeo.
Santiago de Chile 2-8 October 2005
SS03: Strong ground motion, hazard and riskPoster #2
Abstract # 486
Abstract
Seismic hazard scenario analyses are of main concern when the target is represented by strategical structures such as nuclear power plants, waste deposits or civil structures that during their exposure time can suffer from the effect of one or more destructive earthquakes. Scenario analyses are performed using deterministic and/or probabilistic approaches. The former uses a “controlling earthquake” (Reiter, 1990) with its source characteristics (source geometry, magnitude, slip distribution,…) and provides, for each site, the expected ground motion level whose detail depends on the adopted ground motion simulation method. However, it does not take into account for the probability of occurrence of the earthquake and its return period. On the other side, the use of a Probabilistic Seismic Hazard Analysis (PSHA) for a scenario-like description of the hazard (Cornell, 1968) prevents to take into account for the details of single causative faults when the ground motion level have to be estimated at a given site. This is due to the empirical nature of the attenuation relationships used in the probabilistic approach to estimate the effect of the selected earthquake. In fact, they do not account for details of the rupture process at the source which can strongly affect the seismic radiation both at local and regional scale. In this work we propose the application of a probabilistic-deterministic technique for hazard scenario analysis, developed by Convertito et al. (2005) to the 1980 Irpinia earthquake, M=6.9, Southern Italy. In this application it is shown how the extended source, the associated radiation pattern and the directivity effect can be introduced in the frame of the classical PSHA approach by the hybrid technique of Convertito et al. (2005). In this way, we overcome the limitation of the deterministic analyses that, for their inner nature, provide “static” scenarios.
Comments
! For each return period, hazard maps obtained using the exponential model provide PGA values larger than those obtained using the characteristic earthquake model due to the difference between the computed and retrieved from catalogue activity rates.! Classical hazard maps for the case of the single event, look too simple to capture the real pattern of the expected ground motion shaking for a complex multiple event such as the one considered in the present analysis. More reliable hazard maps are obtained when the complex event is taken into account also in the case of the classical approach.! The hazard maps obtained using the hybrid technique show the effect of the extended source, of the associated source radiation pattern, and of the directivity, that can not be introduced in the classical PSHA with the same detail.! PGA distributions and values change when the return period increases due to a larger number of earthquakes occurrence.! The availability of synthetic waveforms can allow to introduce a more realistic site effect in comparison with the simple constant amplification factor used in the classical PSHA. Moreover, the hazard analysis can be easily extended to any ground motion parameter that can be retrieved from seismograms, both in time and in frequency domains.
Hazard maps