The Laalam (Algeria) damaging landslide triggered by a moderate earthquake (M<Subscript>w</Subscript> = 5.2)

On March 20th, 2006, a moderate earthquake (M_w = 5.2) struck the Laalam (NE-Algeria). The damage was centred in the Laalam village where four deaths, 68 injured and more than 40 housing units destroyed making dozens of peoples homeless. Damage and casualties are directly related to an earthquake-induced landslide occurred at 10 km far from the epicenter (ISC relocation). The landslide rupture, mapped by using a ground based survey, is striking NE-SW along a 1-km length rupture with a 45 cm of lateral displacement. The rupture plan is dipping 40° at the surface and showing horizontal striations. A 75 cm of pre-2006 lateral displacement is also observed indicating a repetitive behaviour of the landslide. The last time where the landslide was activated extends back to 1974 following another moderate earthquake. This landslide reminds us the high level hazard of induced ground failures due slope instabilities in mountainous zones of seismically active areas.     

Numerical evidence of site effects contributing to triggering the Las Colinas landslide during the 2001 <Emphasis Type="Italic">M</Emphasis><Subscript>w</Subscript> = 7.7 El Salvador earthquake

The Las Colinas landslide, which was triggered by the 13 January 2001, M_w?=?7.7 El Salvador earthquake, was highly destructive. The local site is composed of pyroclastic flows, brown cinders, soft pyroclastic fall deposits and a thin palaeosol and is characterised by steep slopes. The extremely high ground motions recorded near the landslide location are assumed to be both produced by site effects and responsible for the landslide. To characterise the ground motion amplifications due to site effects in terms of the variation in geometrical and geological settings, parametric studies were conducted with a linear elastic slope model, which was vertically subjected to the scattering SV wave of the Gabor wavelet. The results show that a maximum amplification is obtained when the model slope angle is approximately 30° (similar to the actual slope angle), and the maximum amplification is located approximately 20 m behind the crest, where the actual movement was initiated. Additionally, a slope with a height of approximately 160 m enhances ground motion amplification. The subsurface geology is found to induce a greater effect on amplification than that of the slope topography. In particular, a soft pyroclastic fall deposit is observed to contribute most of the ground motion amplification. According to the numerical results, it can be concluded that the local site conditions induced extremely high ground motions that then contributed to the slope movement. Although the thin, buried layer of palaeosol did not cause any significant amplification, its weak cohesion enhanced movement.     

Seismogenesis of the uninterrupted occurrence of the aftershock activity in the 2001 Bhuj earthquake zone, Gujarat, India, during 2001�C2010

Seismogenesis of aftershocks occurring in the Kachchh seismic zone for more than last 10?years is investigated through modeling of fractal dimensions, b-value, seismic velocities, stress inversion, and Coulomb failure stresses, using aftershock data of the 2001 Bhuj earthquake. Three-dimensional mapping of b-values, fractal dimensions, and seismic velocities clearly delineate an area of high b-, D-, and Vp/Vs ratio values at 15?C35?km depth below the main rupture zone (MRZ) of the 2001 mainshock, which is attributed to higher material heterogeneities in the vicinity of the MRZ or deep fluid enrichment due to the release of aqueous fluid/volatile CO₂ from the eclogitisation of the olivine-rich lower crustal rocks. We notice that several aftershocks are occurred near the contacts between high (mafic brittle rocks) and low velocity regions while many of the aftershocks including the 2001 Bhuj mainshock are occurred in the zones of low velocity (low dVp, low dVs and large Vp/Vs) in the 15?C35?km depth range, which are inferred to be the fractured rock matrixes filled with aqueous fluid or volatiles containing CO₂. Further support for this model comes from the presence of hydrous eclogitic layer at sub-lithospheric depths (34?C42?km). The depth-wise stress inversions using the P- and T-axes data of the focal mechanisms reveal an increase in heterogeneity (i.e., misfit) with an almost N?CS ??₁ orientation up to 30?km depth. Then, the misfit decreases to a minimum value in the 30?C40?km depth range, where a 60^o rotation in the ??₁ orientation is also noticed that can be explained in terms of the fluid enrichment in that particular layer. The modeling of Coulomb failure stress changes (??CFS) considering three tectonic faults [i.e., NWF, GF, and Allah bund fault (ABF)] and the slip distribution of the 2001 mainshock on NWF could successfully explain the occurrences of moderate size events (during 2006?C2008) in terms of increase in positive ??CFS on GF and ABF. In a nutshell, we propose that the fluid-filled mafic intrusives are acting as stress accentuators below the Kachchh seismic zone, which generate crustal earthquakes while the uninterrupted occurrence of aftershocks is triggered by stress transfer and aqueous fluid or volatile CO₂ flow mechanisms. Further, our results on the 3-D crustal seismic velocity structure, focal mechanisms, and b-value mapping will form key inputs for understanding wave propagation and earthquake hazard-related risk associated with the Kachchh basin.     

On a damaging earthquake-induced landslide in the Algerian Alps: the March 20, 2006 Laâlam landslide (Babors chain,northeast Algeria), triggered by the Kherrata earthquake (<Emphasis Type="Italic">M</Emphasis><Subscript>w</Subscript> = 5.3)

On March 20, 2006, an earthquake (M _w = 5.3; SED) struck the mountainous region of the Babors chain (Wilaya of Bejaia, northeast Algeria). The seismic epicenter was located near the Kherrata village. This earthquake was felt on a large area of the northeastern part of Algeria. It reached an intensity of VII (EMS scale) at the Laalam village, situated at about 20 km northeast of Kherrata. Here, many old and recent houses were damaged or collapsed totally, four people died and 68 were injured. Field investigations revealed that these casualties were caused by a landslide triggered by the earthquake. Many fissures were visible on ground throughout the site. They were generated by both sliding and settling phenomena. The Laalam site is prone to landslide, as revealed by some evidences on old instabilities. This is due to two main factors: local geomorphology and geology. These factors intervene synchronously for reducing the slope instability at the Laalam village. The March 20, 2006 Kherrata earthquake was the trigger that released the Laalam landslide.     

Divergent T–ƒO<Subscript>2</Subscript> paths during crystallisation of H<Subscript>2</Subscript>O-rich and H<Subscript>2</Subscript>O-poor magmas as recorded by Ce and U in zircon,with implications for TitaniQ and TitaniZ geothermometry

During solidification of magma chambers as systems closed to chemical exchange with environs, the residual siliceous melt may follow a trend of rising, constant, or decreasing oxidation state, relative to reference buffers such as nickel?+?nickel oxide (NNO) or fayalite?+?magnetite?+?quartz. Titanomagnetite–hemoilmenite thermometry and oxybarometry on quenched volcanic suites yield temperature versus oxygen fugacity arrays of varied positive and negative slopes, the validity of which has been disputed for several years. We resolve the controversy by introducing a new recorder of magmatic redox evolution employing temperature- and redox-sensitive trace-element abundances in zircon. The zircon/melt partition coefficients of cerium and uranium vary oppositely in response to variation of magma redox state, but vary in tandem as temperature varies. Plots of U/Pr versus Ce⁴⁺/Ce³⁺ in zircon provide a robust test for change in oxidation state of the melt during zircon crystallisation from cooling magma, and the plots discriminate thermally induced from redox-induced variation of Ce⁴⁺/Ce³⁺ in zircon. Temperature-dependent lattice strain causes Ce⁴⁺/Ce³⁺ in zircon to increase strongly as zircon crystallises from cooling magma at constant Ce⁴⁺/Ce³⁺ ratio in the melt. We examine 19 zircon populations from igneous complexes in varied tectonic settings. Variation of zircon Ce⁴⁺/Ce³⁺ due to minor variation in melt oxidation state during crystallisation is resolvable in 11 cases but very subordinate to temperature dependence. In many zircon populations described in published literature, there is no resolvable change in redox state of the melt during tenfold variation of Ce⁴⁺/Ce³⁺ in zircons. Varied magmatic redox trends indicated by different slopes on plots of zircon U/Pr versus Ce⁴⁺/Ce³⁺ are corroborated by Fe–Ti-oxide-based T–?O₂ trends of correspondingly varied slopes. Zircon and Fe–Ti-oxide compositions agree that exceptionally, H₂O-rich arc magmas tend to follow a trend of rising oxidation state of the melt during late stages of fluid-saturated magmatic differentiation at upper-crustal pressures. We suggest that H₂ and/or SO₃ and/or Fe²⁺ loss from the melt to segregating fluid is largely responsible. Conversely, zircon and Fe–Ti-oxide compositions agree in indicating that H₂O-poor magmas tend to follow a T–?O₂ trend of decreasing oxidation state of the melt during late stages of magmatic differentiation at upper-crustal pressures, because the precipitating mineral assemblage has higher Fe³⁺/Fe²⁺ than coexisting rhyolitic melt. We present new evidence showing that the Fe–Ti-oxide oxybarometer calibration by Ghiorso and Evans (Am J Sci 308(9):957–1039, 2008) retrieves experimentally imposed values of ?O₂ in laboratory syntheses of Fe–Ti-oxide pairs to a precision of ±?0.2 log unit, over a large experimental temperature range, without systematic bias up to at least log ?O₂?≈?NNO?+?4.4. Their titanomagnetite–hemoilmenite geothermometer calibration has large systematic errors in application to Ti-poor oxides that precipitate from very oxidised magmas. A key outcome is validation of Fe–Ti-oxide-based values of melt TiO₂ activity for use in Ti-in-zircon thermometry and Ti-in-quartz thermobarometry.     

The Longmala and Mengya’a skarn Pb–Zn deposits,Gangdese region,Tibet: evidence from U–Pb and Re–Os geochronology for formation during early India–Asia collision

The Longmala and Mengya’a deposits are two representative skarn Pb–Zn deposits of the Nyainqêntanglha Pb–Zn–(Cu–Mo–Ag) polymetallic belt in the Gangdese region, Tibet, China. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb dating of the mineralization-related biotite monzogranite from the Longmala deposit yielded a weighted mean age of 55.7 Ma, which can be interpreted as the emplacement age of the pluton. Re–Os dating of three molybdenite samples from the Longmala deposit yielded model ages of 51.8–54.3 Ma, with a weighted mean age of 53.3 Ma, which is interpreted as the mineralization age of the deposit and overlaps the age of the causative intrusion. The Re–Os dating of four molybdenite samples from the Mengya’a deposit yielded model ages of 60.4–65.8 Ma, with a weighted mean age of 63.6 Ma, which represents the mineralization age of this deposit. Our new precise age data for these two deposits are consistent with the existing ages of ca. 65–51 Ma for other skarn polymetallic deposits in the Nyainqêntanglha metallogenic belt. In addition, these new age data, combined with existing information on the geological evolution history of the Lhasa terrane, indicate that the belt of skarn deposits is closely related to initial collision between India and the Asian continents.