Geobulletin alpha

Source info:

Author:	Stéphane Baize
Date:	2017-04-02 14:02:00
Blog:	Stephane on Blogger Earthquakes, geology and related topics... but not only
URL:	http://stephaneonblogger.blogspot.com/2017/04/active-faults-at-front-of-french-alps.html

Summary:

Metropolitan France is not famous for its earthquake activity: recordings and felt reports of modern earthquakes are sparse, popular memory of past ones is almost null and very few faults have been studied to figure out their prehistoric activity. However, when paying attention to the available datasets, one can realize that the seismic hazard is far from negligible:- Instrumental seismicity: Bureau Central Sismicité en France- Historical seismicity: SISFRANCE and European Archive of Historical EArthquake Data (AHEAD)- Areal seismogenic sources (Baize et al., 2013) and potential active faults (Jomard et al., submitted).In this post, I focus on a specific case. Twenty years ago (13/07/1996), the Annecy area was struck by a very shallow M5 earthquake. The source has been identified to be the Vuache fault (Thouvenot et al., 1998), a left-lateral NW-SE and 35-km long fault. The 15/07/1996 Epagny earthquake. Left: Mainshock focal mechanism and location of the monitored aftershocks in the uppermost crust (above 3.5 km), above the décollement level between basement (crosses) and sedimentary cover (limestones). Open and black circles represent two different strands of aligned aftershocks. Right: slight damages due to the mainshock in the epicentral area (Epagny church) This NW-SE fault disrupts the Molasse basin from the Alps (to the SE) to the Internal Jura Mts (to the NW). It can be described as a "lateral ramp" fault more or less in the transport direction (NW-SE to WNW-ESE in the area) of the nappes of the Jura fold-and-thrust belt. In 2011, we compiled a series of geophysical, geomorphological annd geological data to draw some highlights on this Fault (Baize et al., 2011). Location of the Vuache fault (VF) and the Culoz fault (CF), which show evidences of active faulting at the ground surface (from Baize et al 2011) Its imprint in the landscape suggests a long-term activity (0.15-0.4 mm/y), for instance with systematic left-lateral deflections of creeks incised in Miocene Molasse, just above the fault. Quaternary deposits are also locally faulted. Left-lateral deflections of the drainage network (incised within the Molasse deposits), across the Vuache fault (to the left); to the right, clear evidences of left-lateral shearing in the Miocene molasse, at one of the drainage deflection (frop Baize et al., 2011). The commonly admitted structural model includes a décollement level at shallow depth (Triassic evaporites at ~3 km), limiting the width of shallow earthquake faults (like during the 1996 event): however, the industrial seismic lines that we could grab suggest that the surface fault are located above basement fault. If a linkage could occur, the surface faults could indeed represent a severely increased hazard. Recently, De La Taille et al. (2015) used high-resolution seismic lines in the Bourget Lake, several tens km south of the Vuache area in Savoy. They could evidence that the post-Late Glacial deposits of the lake are deformed by the Culoz fault (CF on the map above), another "lateral ramp" fault associated with the fold-and-thrust belt. High-resolution seismic lines in the northern part of Bourget Lake, depicting several strands of the Culoz Fault offsetting the Holocene lacustrine deposits. Unit 1: basement rocks; Unit 4 and 5: Holocene deposits. From De La Taille et al. (2015)

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Keywords:

active faults