comparative studyof the intraplateBasel - ETH E
Transcription
comparative studyof the intraplateBasel - ETH E
DISS. ETH NO. 15537 Adaptation of the paleoseismologicalapproach to local tectonic regime: comparative study of the intraplate BaselReinach fault, Switzerlandand the interplate North Anatolian fault, Turkey. A dissertation submitted to the SWISS FEDERAL INSTITUTEOF TECHNOLOGYZÜRICH For the Degree of Doctor of Sciences Presented by FERRY, MATTHIEU ALEXIS DEA IPGS EOST STRASBOURG Born March 23rd, 1973 Citizen of France - - Acceptedon the recommendationof Prof. Dr. D. Giardini, examiner Prof. Dr. M. Meghraoui, co-examiner Prof. Dr. A. Pfiffner, co-examiner Prof. Dr. R. Armijo, co-examiner 2004 Abstract The assessment of seismic risk in a given region relies on the proper estimate of related seismic hazard, i.e. the probability that an earthquake of said magnitude occurs within a said time window. Calculating that probability requires a precise knowledge of local seismotectonic parameters such as the frequency of damaging earthquakes, their magnitude and the annual deformation rate. These parameters may only be properly determined when observed over a representative time window. As return periods for strong earthquakes vary between a few decades and several millennia, instrumental and historical seismological datasets often prove to be insufficient. To improve the completeness of available seismicity catalogues, we applied the paleoseismologicalapproach to two different regions: the Basel region (southern Upper Rhine Graben) which was Struck by a destructive earthquake in 1356 and the Izmit region (east of the Marmara Sea), partly destroyed in August 1999 by a magnitude 7.4 tremor. A füll integrated study involving geomorphology, geophysics and trenching in the Basel region led to the identification of the seismogenic fault responsible for the last strong earthquake and to deciphering its seismic history for the Holocene. Thus, we demonstrate that the newly identified Basel-Reinach fault is the locus for a magnitude 6.4-6.7 earthquake every -2600 years, on average. Furthermore, we give strong evidence for the probable extension of the fault through the city of Basel to the north and the folded Jura to the south. These results have a direct impact on the level of seismic risk to the region as it was consequently lowered by one degree on the basis of our original work. We applied a similar, though adapted, approach to the North Anatolian fault around the Marmara Sea. An extensive trenching effort combined with ground-penetrating profiles carried out on the Ganos fault to the west and the Izmit-Sapanca segment to the east evidence several recent events and yield measurements of pertaining co-seismic offsets. They clearly show characteristic behavior on both sides of the Marmara Sea, however with varying offsets: 4 to 5 m for the Ganos fault and 2 to 2.5 m for the Izmit-Sapanca segment. This brings new constrains to better characterize faulting behavior in the region and consequent seismic risk to the city of radar Istanbul. 5- Resume risque sismique dans une region donnee passe par l'evaluation de l'aleas, c'est-ä-dire la probabilitequ'un seisme de magnitude definie se produise dans un intervalle de temps defini. Le calcul de cette probabilite repose sur certains parametres sismo-tectoniques locaux tels que la frequence des tremblements de terre destructeurs, leur magnitudeainsi que le taux de deformation annuel. Ces parametres ne seront convenablement determines qu'en disposant d'une fenetre d'observation repräsentative des phenomenes en jeu. Face ä des periodes de retour variant de quelques decennies a plusieurs millenaires, les La prise donnees en compte du sismologiques instrumentales et historiques se revelent souvent completer les catalogues de sismicite disponibles, nous avons appliques les methodes de la paleosismologie ä deux regions tres differentes: la region de Bäle (sud du Fosse Rhenan) frappee par un seisme destructeuren 1356 et la region d'lzmit (est de la Mer de Marmara) en grande partie detruite en Aoüt 1999 par un seisme de magnitude7.4. Une etude integree complete de la region de Bäle a permis d'identifier la faille responsable du dernier grand seisme puis de retracer son histoire sismique holocene. Nous avons ainsi demontre qu'elle est le lieu d'un evenement de magnitude 6.4-6.7 tous les 2600 ans, en moyenne. De plus, nous donnons des indications fortes sur son prolongement au travers de la viHe de Bäle, au nord, et du Jura Plisse, au sud. Ces resultats ont une incidence directe sur le niveau de risque sismique regnant dans la region. En effet, celui-ci a diminue d'un degre depuis la prise en compte de ces elements jusqu'ici inedits. Nous avons applique une approche similaire, quoique adaptee, ä la faille Nord insuffisantes.Afin de Anatolienne autour de la Mer de Marmara. Une campagne de tranchees menee sur la faille de Ganos ä l'ouest et sur le segment Izmit-Sapanca ä Test ainsi que des profils de radar geologique ont mis en evidence plusieurs evenements recents et permis de determiner l'amplitude des mouvementsco-sismiques produits lors de ces seismes. Ceux-ci montrent clairement un comportement caracteristique des deux cotes de la Mer de Marmara, avec toutefois des deplacements co-sismiques differents : 4 ä 5 m pour Ganos et 2 ä 2.5 m pour Izmit-Sapanca.