Evidence of palaeoseismicity in a flowstone of the Observatoire cave (Monaco)

Abstract

Monaco is a medium seismicity zone. The Observatoire cave, a well decorated show cave, is a good place for palaeoseismicity studies. On the floor of the cave it is possible to observe a great number of collapsed sodastraws. The breakages are attributed to the 1887 Ligurian earthquake. A borehole in a flowstone shows several levels of collapses that may indicate ancient earthquakes. © Elsevier, Paris     

A major seismogenic fault in a 'silent area': the Castrovillari fault (southern Apennines, Italy)

Large historical earthquakes in Italy define a prominent gap in the Pollino region of the southern Apennines. Geomorphic and palaeoseismological investigations in this region show that the Castrovillari fault (CF) is a major seismogenic source that could potentially fill the southern part of this gap. The surface expression of the CF is a complex, 10–13 km long set of prominent scarps. Trenches across one scarp indicate that at least four surface-faulting earthquakes have occurred along the CF since Late Pleistocene time, each producing at least 1 m of vertical displacement. The length of the fault and the slip per event suggest M =6.5-7.0 for the palaeoearthquakes. Preliminary radiocarbon dating coupled with historical considerations imply that the most recent of these earthquakes occurred between 380 BC and 1200 AD, and probably soon after 760 AD; no evidence for this event has been found in the historical record. We estimate a minimum recurrence interval of 1170 years and a vertical slip rate of 0.2-0.5 mm yr^-1 for the CF, which indicates that the seismic behaviour of this fault is comparable to other major seismogenic faults of the central-southern Apennines. The lack of mention or the mislocation of the most recent event in the historical seismic memory of the Pollino region clearly shows that even in Italy, which has one of the longest historical records of seismicity, a seismic hazard assessment based solely on the historical record may not be completely reliable, and shows that geological investigations are critical for filling possible information gaps.     

Postglacial fault movement and palaeoseismicity in western Scotland: A reappraisal of the Kinloch Hourn fault, Kintail

The Kinloch Hourn fault is the most prominent of a number of suspectedpostglacial faults in the western Scottish Highlands. These faults areinterpreted to have been reactivated by repeated large (M > 6)palaeoseismic events following deglaciation 10,000–13,000 years ago.Based on inferred deflections of drainage courses, previous studies of thefault have estimated 160 ± 40 m cumulative left-lateral displacementalong a 14 km long active segment during postglacial times. Reportedsoft-sediment deformation phenomena imply that activity on the KinlochHourn fault has persisted into the late Holocene, with the most recentmovement having been associated with a magnitude 5.5–6.0 surface-faultingevent between 3500 and 2400 years ago. The marked contrast betweensuch palaeoseismic activity and the present-day seismic quiescence ofwestern Scotland has stimulated this critical reappraisal of the KinlochHourn fault.This paper reassesses the key lines of evidence for postglacial fault activityand palaeoseismicty on the Kinloch Hourn fault, combining the analysis of1:15,000-scale air photos, field-based geomorphic mapping andpalaeoenvironmental investigations. Our reappraisal of inferred drainagedeflections across the fault contends that previous reports of significant(10² m) left-lateral slip on the fault during the Holocene arespurious. Instead, incidences of Holocene channel abandonment along thefault line are non-synchronous and probably reflect non-tectonic drainagechanges. The timing of soft-sediment deformation in the vicinity of the faultis revised to an early Holocene date (8990–8580 calendar years BP), whichis in accord with both the palaeoenvironmental history of the site andconsistent with published ages of earthquake-induced liquefactionphenomena documented elsewhere in western Scotland. An alleged recent(post-2400 radiocarbon years BP) ground rupture on the fault isquestioned in the light of uncertainty about both the nature of the faultedsoil deposit and the late Holocene age attributed to it.The study concludes that there is no convincing evidence for postglacialsurface rupture on the Kinloch Hourn fault and speculates that the casefor significant (10¹–10² m) postglacial movement on otherfaults in western Scotland may be similarly `unproven'.     

Soft-sediment deformation structures in late Albian to Cenomanian deposits, São Luís Basin, northern Brazil: evidence for palaeoseismicity

Soft-sediment deformation structures from the Alcântara Formation (late Albian to Cenomanian), São Luís Basin, northern Brazil, consist of (1) contorted structures, which include convolute folds, ball-and-pillow structures, concave-up paths with consolidation lamination, recumbently folded cross-stratification and irregular convolute stratification that grades into massive beds; (2) intruded structures, which include pillars, dykes, cusps and subsidence lobes; and (3) brittle structures, represented by fractures and faults displaying planes with a delicate, ragged morphology and sharp peaks. These structures result from a complex combination of processes, mostly including reverse density gradients, fluidization and liquefaction. Reverse density gradients, promoted by differential liquefaction associated with different degrees of sediment compaction, led to the genesis of convolute folds. More intense deformation promoted the development of ball-and-pillow structures, subsidence lobes and sand rolls, which are attributed to denser, and thus more compacted (less liquefied), portions that sank down into less dense, more liquefied sediments. Irregular convolute stratification that grades into massive beds would have formed at periods of maximum deformation. The subsidence of beds was accompanied by lateral current drag and fluid escape from water-saturated sands. In addition, the fractures and faults record brittle deformation penecontemporaneous with sediment deposition. All these mechanisms were triggered by a seismic agent, as suggested by a combination of criteria, including (1) the position of the study area at the edge of a major strike-slip fault zone that was reactivated several times from the Albian to the Holocene; (2) a relative increase in the degree of deformation in sites located closer to the fault zone; (3) continuity of the deformed beds over large distances (several kilometres); (4) restriction of soft-sediment deformation structures to single stratigraphic intervals bounded by entirely undeformed strata; (5) recurrence through time; and (6) similarities to many other earthquake-induced deformational structures.     

Enigma variations: the stratigraphy,provenance, palaeoseismicity and depositional history of the Lower Old Red Sandstone Cosheston Group,south Pembrokeshire,Wales

The Lower Devonian (Lochkovian‐Emsian) Cosheston Group of south Pembrokeshire is one of the most enigmatic units of the Old Red Sandstone of Wales. It consists of a predominantly green, exceptionally thick succession (up to 1.8 km) within the red c. 3 km‐thick fill of the Anglo‐Welsh Basin, but occupies a very small area (27 km²). Four formations—Llanstadwell (LLF), Mill Bay (MBF), Lawrenny Cliff (LCF) and New Shipping (NSF)—group into lower (LLF + MBF) and upper (LCF + NSF) units on stratigraphical and sedimentological criteria. Two palynostratigraphic associations (Hobbs Point and Burton Cliff) are recognised in the LLF. Overall, the Cosheston succession comprises a fluvial, coarsening‐upward megasequence, mostly arranged in fining‐upward rhythms. It is interpreted as the fill of an east‐west graben bounded by faults to the north and south of the Benton and Ritec faults, respectively. Both ‘lower Cosheston’ formations were deposited by east‐flowing, axial river systems draining a southern Irish Sea landmass. Drainage reversal, early in the deposition of the LCF, resulted in ‘upper Cosheston’ lateral, SW‐flowing rivers which carried predominantly second‐ and multi‐cycle detritus. The ‘lower Cosheston’ is characterized by an abundance of soft‐sediment deformation structures, probably seismically triggered by movements along the graben's northern bounding fault. A minimum average (≥ mesoseismic) earthquake recurrence interval of c. 4000 yr is estimated for the MBF. This and the correlative Senni Formation of south‐central Wales form a regionally extensive green‐bed development that represents a pluvial climatic interval. Copyright © 2006 John Wiley & Sons, Ltd.     

Palaeoseismicity of the Oquirrh fault, Utah from shallow seismic tomography

The magnitude and frequency of normal-fault palaeoearthquakes are usually determined by trenching studies that ascertain the size and number of colluvial wedges along the fault. Such information can be invaluable in predicting the seismic hazard and potential for a future earthquake in that region. Digging trenches across normal faults, however, is environmentally intrusive, expensive and limited in the penetration depth. To overcome these problems we propose the use of 3-D seismic tomography as a means to identify the shapes and sizes of colluvial wedges along normal faults. As an example,2-D and 3-D seismic surveys were conducted across the Oquirrh fault, Utah with the purpose of imaging the normal-fault structure to a depth of about 10  m. Results show that the 3-D tomogram clearly delineates the fault zone and a colluvial wedge, both of which correlate extremely well with the geological cross-section interpreted from an adjacent trench. The thickness of the colluvial wedge image is used in conjunction with a seismic section to compute an estimate of a 6.8 moment magnitude earthquake for the most recent event on this fault, which is in close agreement with the 7.0 estimate based on a nearby trenching study. These tomographic results demonstrate, for the first time, that seismic imaging methods can be used in some cases to estimate unambiguously the shapes of colluvial wedges and the sizes of prehistoric earthquakes. Thus, seismic tomography has the possibility of providing cheaper, deeper and wider, but less resolved, images of fault systems than the intrusive excavation of trenches across faults.     

Seismogenically induced fluidization of Jurassic erg sands, south-central Utah

Large bodies of fluidized sandstone occur in the Jurassic Entrada, Carmel, Page and Navajo Formations at several locations in south‐central Utah. They are most abundant in the Entrada Sandstone, where they commonly occur in clusters, have a cylindrical form and have a sharp contact with their cross‐bedded host rock. These clastic pipes are as wide as 75 m and have exposed heights of as much as 100 m. Some of the Entrada pipes extend well into the underlying Carmel redbeds. Other clastic pipes in the Entrada Sandstone are less deformed and display various degrees of brittle‐to‐hydroplastic deformation and liquefaction. Clastic pipes in the Page and Navajo Sandstones are less common, but are similar in size and form to those in the Entrada and Carmel, and probably have a similar origin. Some massive sandstone bodies are irregular in form and have tongue‐like projections into the host rock, implying forcible injection of fluidized sand. Several pipe–host contacts in the Entrada Sandstone display small‐scale ring faults. Where relative displacement can be clearly demonstrated, pipe sandstones are invariably down‐faulted, locally as much as 5 m. At two sites, Carmel host rock is upwarped around the Entrada pipes. Stratified and cross‐bedded breccia blocks occur in many Entrada pipes, and preliminary petrographic analysis indicates that at least some of these breccia blocks are derived from the host rock. Homogeneous pipe sandstones are also petrographically similar to their Entrada host rock, suggesting that some pipes originate through fluidization of the fine‐grained Entrada. Fluidization of the Entrada must have occurred in a water‐saturated environment during early diagenesis but before complete lithification, most probably under considerable porewater pressure. Although there are no known modern analogues to these huge masses of structureless sandstone, they may have a small‐scale modern counterpart in earthquake‐induced sandblows. These features were most probably caused by large‐magnitude seismic events during the Middle Jurassic, although other possibilities cannot be ruled out at this point.