Paleoseismic History of the Dead Sea Fault Zone

International audienceThe aim of this entry is to describe the DSF as a transform plate boundary pointing out the rate of activedeformation, fault segmentation, and geometrical complexities as a control of earthquake ruptures. Thedistribution of large historical earthquakes from a revisited seismicity catalogue using detailedmacroseismic maps allows the correlation between the location of past earthquakes and fault segments.The recent results of paleoearthquake investigations (paleoseismic and archeoseismic) with a recurrenceinterval of large events and long-term slip rate are presented and discussed along with the identification ofseismic gaps along the fault. Finally, the implications for the seismic hazard assessment are also discussed

Arabidopsis mutants impaired in cosuppression.

Post-transcriptional gene silencing (cosuppression) results in the degradation of RNA after transcription. A transgenic Arabidopsis line showing post-transcriptional silencing of a 35S-uidA transgene and uidA-specific methylation was mutagenized using ethyl methanesulfonate. Six independent plants were isolated in which uidA mRNA accumulation and beta-glucuronidase activity were increased up to 3500-fold, whereas the transcription rate of the 35S-uidA transgene was increased only up to threefold. These plants each carried a recessive monogenic mutation that is responsible for the release of silencing. These mutations defined two genetic loci, called sgs1 and sgs2 (for suppressor of gene silencing). Transgene methylation was distinctly modified in sgs1 and sgs2 mutants. However, methylation of centromeric repeats was not affected, indicating that sgs mutants differ from ddm (for decrease in DNA methylation) and som (for somniferous) mutants. Indeed, unlike ddm and som mutations, sgs mutations were not able to release transcriptional silencing of a 35S-hpt transgene. Conversely, both sgs1 and sgs2 mutations were able to release cosuppression of host Nia genes and 35S-Nia2 transgenes. These results therefore indicate that sgs mutations act in trans to impede specifically transgene-induced post-transcriptional gene silencing

Structure and Deformation History of the Rapidly Growing Tainan Anticline at the Deformation Front of the Taiwan Mountain Belt

International audienceThis study aims at further documenting the mechanisms of shortening at the front of fold-and-thrust belts. We focus on an actively growing anticline located at the deformation front of the Taiwan fold-and-thrust belt. Based on a multidisciplinary approach combining mainly subsurface data and geodetic techniques, we show that the Tainan anticline is a pure-shear fault-bend fold growing above a 38-45° west dipping back thrust, the Houchiali fault, rooted on a 3.8-km-deep detachment. The cumulative shortening is estimated at 2-3 km since 310 ± 50 ka, including 30-50% of horizontal compaction shortening. The significance of the fold is little in terms of total shortening at the scale of the mountain piedmont, yet the Holocene shortening rate of 10.3 ± 1.0 mm/a accounts for 25% of the present-day shortening rate across the piedmont. Earthquake scaling relationships applied to the Houchiali fault predict Mw 6 earthquakes that would occur a lot more frequently than indicated from historical earthquake catalogs. Hence, the aseismic slip behavior observed from geodetic measurements since two decades is a representative behavior of the fault at least at the scale of a few centuries. Our results bear out the dominance of pure-shear folding at the front of fold-and-thrust belts and support horizontal compaction as a significant shortening mechanism. In contrast, the back thrust wedge structure and the aseismic slip are peculiar characteristics that likely arise from the combination of low friction and high-pore pressure related to the thick mudstone formation hosting the wedge and of high syntectonic sedimentation rates

Slip deficit and temporal clustering along the Dead Sea fault from paleoseismological investigations

International audienceTemporal distribution of earthquakes is key to seismic hazard assessment. However, for most fault systems shortness of large earthquake catalogues makes this assessment difficult. Its unique long earthquake record makes the Dead Sea fault (DSF) exceptional to test earthquake behaviour models. A paleoseismological trench along the southern section of the DSF, revealed twelve surface-rupturing earthquakes during the last 8000 years, of which many correlate with past earthquakes reported in historical chronicles. These data allowed us building a rupture scenario for this area, which includes timing and rupture length for all significant earthquakes during the last two millenaries. Extending this rupture scenario to the entire DSF south of Lebanon, we were able to confirm the temporal-clustering hypothesis. Using rupture length and scaling laws, we have estimated average co-seismic slip for each past earthquake. The cumulated slip was then balanced with long-term tectonic loading to estimate the slip deficit for this part of DSF over the last 1600 years. The seismic-slip budget shows that the slip deficit is similarly high along the fault with a minimum of 2 meters, which suggests that an earthquake cluster might happen over the entire region in the near future. Successions of intense periods of seismic activity rupturing significant length of a fault followed by longer periods of seismic quiescence have been documented along several strike-slip faults 1-6 , suggesting that temporal clustering of earthquakes might be a common behaviour for major strike-slip faults. Testing this assumption, however, has long been hampered by the lack of consistent earthquake time series for long-enough fault sections. Moreover, temporal clustering remains a critical issue for seismic hazard models that only started to be addressed in the most recent modelling attempts