EGU2020-19434
https://doi.org/10.5194/egusphere-egu2020-19434
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

On the importance of fault modelling for seismic risk estimate

Oona Scotti1, Francesco Visini2, Lucilla Benedetti3, Paolo Boncio4, Joanna Faure Wlaker5, Bruno Pace4, Laura Peruzza6, and Gerald Roberts5
Oona Scotti et al.
  • 1IRSN, Bureau Evaluation Risque Sismique Installations Nucleaires, Fontenay aux Roses, France (oona.scotti@irsn.fr)
  • 2INGV, Istituto Nazionale di Geofisica e Vulcanologia
  • 3CEREGE, Aix- Marseille Université
  • 4DiSPUTer, Università degli studi di Chieti-Pescara
  • 5IR and DR , University College of London
  • 6OGS, Istituto Nazionale di Oceanografia e Geofisica Sperimentale

In Central Italy more than 393 thousands people live in villages and towns located at less than 5 km distance from a known, mapped, active fault, capable of generating Mw>6 earthquake. Improving seismic risk estimates in such places requires the use of (i) informative databases of active faults and (ii) the implementation of appropriate building-codes. 
The current level of knowledge regarding activity of active faults in Central Italy has been stored in a recently compiled database (160 slip rates estimates for 88 faults). Given the complex nature of fault ruptures, we adopted a multi-fault rupture approach (SHERIFS) that accounts for both individual ruptures and multi-fault complex ruptures, involving more than one seismogenic fault section. Our earthquake rupture forecast model includes 1249 possible combinations of fault ruptures with lengths ranging from 7 to 42 km. Slip rates and associated errors are used to estimate recurrences of the ruptures assuming a  Gutenberg-Richter frequency-magnitudedistribution. The computed distribution is validated against the CPTI15 catalogue.
The multi-fault model approach and a seismogenic area approach are used to estimate damages based on published typological fragility curves for typical building classes derived from 30 years of data in Italy (Rota et al., 2006) assuming earthquake occurrence for the faults follows a Poisson time-independent process. Two fragility curves are considered here: one for reinforced concrete designed according to seismic regulations and one for masonry with irregular layout and without tie rods and tie beams, a typical typology for the region. Expected levels of damage for 150 villages and towns in Central Italy are computed for all damage states considering a 50 years risk target period. 
Results obtained with the fault approach show a much higher variability of the estimated risk depending on the location of the village/town w.r.t. the fault system and the hanging-wall/footwall location. The probability of collapse in 50 years for a typical masonry building ranges between 0.01 and 0.07 in the fault approach and 0.01 and 0.04 for the area approach. For both approaches, the probability of collapse for reinforced concrete buildings is ~90 % less than that for typical masonry structures. Even if this can be considered obvious, it must be underlined that most buildings in Italy were built before 1975 (before the first applicative decree of the seismic Italian law No. 64 of 1974). Thanks to the availability of the detailed database of active faults a strategy to prioritize resources for seismic risk reduction could be adopted.
 
Rota, M., Penna, A. & Strobbia, C. (2006). Typological fragility curves from Italian earthquake damage data. First European Conference on Earthquake Engineering and Seismology Geneva, Switzerland, 3-8 September 2006 Paper Number: 386

How to cite: Scotti, O., Visini, F., Benedetti, L., Boncio, P., Faure Wlaker, J., Pace, B., Peruzza, L., and Roberts, G.: On the importance of fault modelling for seismic risk estimate , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19434, https://doi.org/10.5194/egusphere-egu2020-19434, 2020

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  • CC1: Questions from the chat, Esther Hintersberger, 08 May 2020

    You talk about "known faults". But what is the impact of "unkonwn faults", that could be numerous in various parts of Italy (and eslewhere)?

    • AC3: Reply to CC1, Oona Scotti, 09 May 2020

      Dear Luca,

      our work is not focused on Italy but on the Central Apennines, where there is a tremendous amount of data on active faults. However, were we to consider this excercise in other regions of Italy, we could still apply the methodology. We simply distribute the total estimated seismic moment rate in the region (deduced from either geological or geodetical data) between the documented faults and the background: the more your fault database is incomplete, the more seismic moment-rate you may want to spend on background seismic sources. 

      We have been applying the SHERIF approach to many regions of the world, even in France, where knowledge of active faults is at best poor and highly uncertain. Through SHERIFS we can explore different possible rupture scenarios and see which model best reproduces the data at hand and explore the associated uncertainties. A step forward from end-less expert-opinon driven discussions on Mmax.

      I hope this helps you appreciate the usefullness of our approach, especially for site-specific hazard purposes and critical infrastructures but also for regional studies such as the Central Apennines where so many people live at less than 5 km from mapped active faults.

      All the best from Paris

      Oona

  • CC2: Questions from the chat, Esther Hintersberger, 08 May 2020

    in your multifaults rupture examples you show scenarios rupturing at the center of you area. Could there be any border effect for multifaults ruptures occuring at the limit of your area (eg involving Mt Vetorre fault)?

  • CC3: Questions from the chat, Esther Hintersberger, 08 May 2020

    Hi Oona, great work and results. I really like the approach. Do you think the 20 % reduction is a kind of aseismic slip tax :)

    • AC2: Reply to CC3, Oona Scotti, 09 May 2020

      Dear Pierre,

      thank you for asking this question. The short answer is yes, you can interpret this as an aseismicity factor. In the UCERF3 approach a 10% aseismicity factor is applied to all the faults. In mypresentation I applied a 20% factor. But bare in mind that these are preliminary results, considering all the faults presently documented in the database. We are in the process of finilizing the database and this value may change, depending on the fault model explored (which faults are considered active) and the assumed deformation model (uniform slip profile or bell-shaped).

      I hope this answers your question

      All the best

      Oona

  • CC4: Questions from the chat, Esther Hintersberger, 08 May 2020

    Can you at some point take into account other types of interaction (e.g. stress changes) to incorporate longer-term sequences of earthquakes that are spatially co-located?

  • CC5: Comment on EGU2020-19434, Oksana Lunina, 09 May 2020

    Dear Oona,

    What database of faults do you use in calculation of seismic hazards in your work? As I understand you don't use DISS of INGV (Database of seismogenic sources). Why?  

    • AC4: Reply to CC5, Oona Scotti, 11 May 2020

      Dear Oksana,

      We use a database that we are in the process of publishing. This database contains published fault trace locations, activity, fault slip-rate data, and fault geometry for active faults in the central Apennines. In our database, seismic sources are displayed at the individual fault trace level, which allows constructing numerous rupture scenarios, and point data of individual site measurements, which allows constructing long-term slip rate profiles. These are two key elements that are not available in DISS and that make all the difference when it comes to seismic hazard and fault-displacement hazard assessment. We also document the uncertainties that come along with this type of data. 

      The DISS database, at least in the Central Apennines, is a model and not a database of active faults. The resolution of the parameters provided in DISS does not reflect the richness of data provided in the literature.

      Based on the range of detailed available published information, we show that we can improve seismic hazard assessments, especially for the case of high-resolution calculations required for critical infrastructure but also for regional assessments.

      I hope this answers your questions.

      Looking forward to seeing you soon in person,

      All the best from the FAULT2SHA Central Apennines ESC WG

      • CC6: Reply to AC4, Oksana Lunina, 12 May 2020

        Dear Oona,

        Thank you very much for your reply. Indeed important for me. I also created a database. One of them is on .  I also quite well know DISS. One of my post-graduate students defended his thesis. We follow the DISS concept but parallel did the Pliocene-Quaternary fault database. Your reply is indeed substantiated. I am not able to do seismic hazard analysis myself because I am just a structural geologist who gathers information about faults and coseismic effects and analyzes to get new irregularities but I understood that DISS could be used for only one scenario. Thank you again and good luck with your work!

         

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