Static and dynamic fault parameters of the Saitama earthquake of July 1, 1968

The source mechanism of the Saitama earthquake (36.07°N,139.40°E, M_s = 5.4) of July 1, 1968, is studied on the basis of P-wave first motion, aftershock, long-period surface-wave data and low-magnification long-period seismograms recorded in the nearfield. A precise location of the aftershocks is made using P and S—P time data obtained by a micro-earthquake observatory network. The synthetic near-field seismograms based on the Haskell model are directly compared with the observed near-field seismograms for wave form and amplitude to determine the dynamic fault parameters. The results obtained are as follows: source geometry, reverse dip slip with considerable right-lateral strike-slip component; dip direction, N6°E; dip angle 30°; fault dimension, 10 × 6 km²; rupture velocity, 3.4 km/sec in the direction S30°E; average dislocation, 92 cm; average dislocation velocity, 92 cm/sec; seismic moment, 1.9 · 10²⁵ dyn-cm; stress drop, 100 bar. The effective stress is about the same as the stress drop. For major earthquakes in the Japanese Islands, the dislocation velocity, .D, is found to be proportional to the stress drop, σ. This relation can be expressed by .D - (β/μ)σ, where β is the shear velocity and μ is the rigidity. This result has an importance in engineering seismology because the stress drop scales the seismic motion in the vicinity of an earthquake fault.     

Stress dependence of recrystallized-grain and subgrain size in olivine

New experiments on Mt. Burnet dunite have been carried out to evaluate the effects of important physical parameters on recrystallized-grain size and subgrain size in olivine deforming under steady-state conditions. The experiments, done under both wet and dry conditions in a Griggs solid-pressure-medium apparatus, were conducted in constant strain rate, constant stress and stress relaxation modes at 10 kbar confining pressure, temperatures from 1000°C to 1300°C, strain rates from 10⁻⁴ to 10⁻⁸/sec and stress differences of from 0.5 to 10 kbar. For dunite deformed under wet conditions, recrystallized-grain size is slightly temperature-dependent but under dry conditions it is only stress-dependent with D = 137 σ^−1.27 for D in μm and σ in kbar. Subgrain sizes also depend only on stress; for the dry experiments d = 28 σ^−0.62 and for the wet ones d = 15 σ^−0.69. Subgrain sizes decrease with increasing stress but do not increase with decreasing stress and hence record only maximum stress levels. Recrystallized-grain sizes adjust to both increasing and decreasing stress levels, at minimal strains and times, and thus record the stress history. Because of this and of the inherent stability of recrystallized grains, this technique is regarded as more reliable than the subgrain size and free dislocation density and curvature methods for estimating stress magnitudes in tectonites having deformed in the steadystate.     

Source parameters of the ML 4.1 earthquake of November 08, 2006, southeast Beni-Suef, northern Egypt

In the early morning hours on Wednesday November 08, 2006 at 04:32:10(GMT) a small earthquake of M_L 4.1 has occurred at southeast Beni-Suef, approximately 160 km SEE of Cairo, northern Egypt. The quake has been felt as far as Cairo and its surroundings while no casualties were reported. The instrumental epicentre is located at 28.57°N and 31.55°E. Seismic moment is 1.76 E14 Nm, corresponding to a moment magnitude M_w 3.5. Following a Brune model, the source radius is 0.3 km with an average dislocation of 1.8 cm and a 2.4 MPa stress drop. The source mechanism from a first motion fault plane solution shows a left-lateral strike-slip mechanism with a minor dip-slip component along fault NNW striking at 161°, dipping 52° to the west and rake −5°. Trend and plunging of the maximum and minimum principle axes P/T are 125°, 28°, 21°, and 23°, respectively. A comparison with the mechanism of the October, 1999 event shows similarities in faulting type and orientation of nodal planes.Eight small earthquakes (3.0  M_L < 5.0) were also recorded by the Egyptian National Seismological Network (ENSN) from the same region. We estimate the source parameters and fault mechanism solutions (FMS) for these earthquakes using displacement spectra and P-wave polarities, respectively. The obtained source parameters including seismic moments of 4.9 × 10¹²–5.04 × 10¹⁵ Nm, stress drops of 0.2–4.9 MPa and relative displacement of 0.1–9.1 cm. The azimuths of T-axes determined from FMS are oriented in NNE–SSW direction. This direction is consistent with the present-day stress field in Egypt and the last phase of stress field changes in the Late Pleistocene, as well as with recent GPS measurements.     

Recent plate tectonics of the Arctic Basin and of northeastern Asia

The recent tectonics of the Arctic Basin and northeastern Asia are considered as a result of interaction between three lithospheric plates: North-America, Eurasia and Spitsbergen. Seismic zones (coinciding in the Norway-Greenland basin with the Kolbeinsey, Mohns and Knipovich ridges, and in the Arctic Ocean with the Gakkel Ridge) clearly mark the boundaries between them. In southernmost Svalbard (Spitsbergen), the secondary seismic belt deviates from the major seismic zone. This belt continues into the seismic zone of the Franz Josef Land and then merges into the seismic zone of the Gakkel Ridge at 70°–90°E. The smaller Spitsbergen plate is located between the major seismic zone and its secondary branch.Within northeastern Asia, earthquake epicenters with magnitude over 4.5 are concentrated within a 300-km wide belt crossing the Eurasian continent over a distance of 3000 km from the Lena estuary to the Komandorskye Islands. A single seismic belt crosses the northern sections of the Verkhoyansky Ridge and runs along the Chersky Ridge to the Kolymo-Okhotsk Divide.To compute the poles of relative rotation of the Eurasian, North-American and Spitsbergen plates we use 23 new determinations of focal-mechanism solutions for earthquakes, and 38 azimuths of slip vectors obtained by matching of symmetric mountain pairs on both sides of the Knipovich and Gakkel ridges; we also use 14 azimuths of strike-slip faults within the Chersky Ridge determined by satellite images. The following parameters of plate displacement were obtained: Eurasia/North America: 62.2°N, 140.2°E (from the Knipovich Ridge section south of the triple junction); 61.9°N, 143.1°E (from fault strikes in the Chersky Ridge); 60.42°N, 141.56°C (from the Knipovich section and from fault strikes in the Chersky Ridge); 59.48°N, 140.83°E, α = 1.89 · 10⁻⁷ deg/year (from the Knipovich section, from fault strikes in the Chersky Ridge and from the Gakkel Ridge section east of the triple junction). The rate was calculated by fitting the 2′ magnetic lineations within the Gakkel Ridge).North-America/Spitsbergen: 70.96°N, 121.18°E, α = −2.7 · 10⁻⁷ deg/year from the Knipovich Ridge section north of the triple junction, from earthquakes in the Spitsbergen fracture zone and from the Gakkel Ridge section west of the triple junction). Eurasia/Spitsbergen: 70.7°N, 25.49°E, α = −0.99 · 10⁻⁷ deg/year (from closure of vector triangles).     

Some source and medium properties of the Vrancea seismic region, Romania

Simple spectral theory of seismic sources was used to determine source parameters directly related to medium properties (stress drop, seismic efficiency and fracture energy) and quality factors of the Vrancea (Romania) seismic region. The results show an increase in maximum static stress drop, maximum seismic efficiency and fracture energy with depth. The seismic efficiency is magnitude independent, but the stress drop is magnitude independent only for events with M_L > 3.8; below this value, the logarithm of stress drop increases quasi-linearly with magnitude. In the depth interval 50–160 km the stress drop increases with a slope of about 2–3 bar/km. The fracture energy per unit area of the fault has values of the order of 10⁵–10⁸ erg/cm².The frequency independent quality factors indicate that the attenuation of P waves is generally higher than that of S waves and that Q_p values are in agreement with recent tectonic models for the Vrancea region: total decoupling of the slab now sinking gravitationally is present only in the southwestern part of the Vrancea region, as suggested by the spatial position of intermediate depth hypocenters.     

Solubility and dissolution rate of silica in acid fluoride solutions

We performed 57 batch reactor experiments in acidic fluoride solutions to measure the dissolution rate of quartz. These rate data along with rate data from published studies were fit using multiple linear regression to produce the following non-unique rate law for quartz

where 10^−5.13 < a_HF < 10^1.60, −0.28 < pH < 7.18, and 298 < T < 373 K. Similarly, 97 amorphous silica dissolution rate data from published studies were fit by multiple linear regression to produce the following non-unique rate law for amorphous silica

where 10^−2.37 < a_HF < 10^1.61, −0.32 < pH < 4.76 and 296 < T < 343 K. Regression of the rates versus other combinations of solution species, e.g.  + H⁺, F⁻ + H⁺, HF + , HF + F⁻, or  + F⁻, produced equally good fits. Any of these rate laws can be interpreted to mean that the rate-determining step for silica dissolution in fluoride solutions involves a coordinated attack of a Lewis acid, on the bridging O atom and a Lewis base on the Si atom. This allows a redistribution of electrons from the Si–O bond to form a O–H group and a Si–FH group.     

Density distribution functions and correlations of earthquake parameters

The statistical analysis of the source parameters of 9 earthquake sets of different types (aftershocks, scattered events, swarms) and of different seismic regions shows that the density distribution function (ddf) of the linear dimension l of a fault is represented by a negative power law, as well as the ddf of the static stress drop σ and of the scalar seismic moment M_o. It is then suggested, and tentatively verified, that also the ddf of the root mean square ground acceleration, defined as a function of l and σ, may be represented by a negative power law and that, at least in the cases examined, it scales like the ddf of σ. It is seen that the variability of the static stress drop is significant from one region to another, as is well known, but it seems remarkable also in the same seismic region (in particular in California, σ varies by several orders of magnitude) and in the different sets of events of a given region (as observed again for California). It is hypothesized that a correlation, although weak, between the stress drop and the linear dimension of a fault exists and the analyses seem not to contradict that σ may be a decreasing function of l. Finally, it is suggested that the seismicity of a region may be represented two-dimensionally as a function of the ddf of the stress drop and of the linear dimension of a fault instead of the classic b and b_o values.     

A source study of the 6 July 2003 (Mw 5.7) earthquake sequence in the Gulf of Saros (Northern Aegean Sea): Seismological evidence for the western continuation of the Ganos fault

The July 2003 sequence in the Gulf of Saros (Northeastern Aegean Sea) is investigated, in terms of accurate event locations and source properties of the largest events. The distribution of epicenters shows the activation of a 25-km long zone, which extends in depth between 9 and 20 km. The major slip patch of the 6 July 2003 M_w 5.7 mainshock is confined in a small area (45 km²), which coincides with the deeper (12–20 km) part of the activated zone. The epicenters of the sequence follow the northern margin of the Saros depression. This observation supports recent studies, according to which the continuation of the Ganos fault in the Gulf of Saros does not coincide with the fault along the northern coast of the Gelibolu peninsula, but it is located at the northern boundary of the Saros depression. This is further supported by the fact that the focal mechanisms of the mainshock and of the largest aftershocks of the 2003 sequence imply almost pure dextral strike-slip faulting, whereas the fault bounding the Gulf of Saros to the south appears as a normal fault on seismic sections. Thus, we infer that the principle deformation zone consists of a major strike-slip fault, which lies close to the northern margin of the Saros depression and this fault could be regarded as the continuation of the northern branch of the North Anatolian Fault into the Saros Gulf and North Aegean Trough as suggested by regional tectonic models. The northeastern extent of the 2003 sequence marks the western termination (at 26.3° E) of a long-term seismic quiescence observed in the period following the 1912 Ganos earthquake, which may be associated with the extend of the rupture of the particular earthquake.     

CO2 and sulfuric acid controls of weathering and river water composition

Reactions of CO₂ with carbonate and silicate minerals in continental sediments and upper part of the crystalline crust produce HCO₃⁻ in river and ground waters. H₂SO₄ formed by the oxidation of pyrite and reacting with carbonates may produce CO₂ or HCO₃⁻. The ratio, ψ, of atmospheric or soil CO₂ consumed in weathering to HCO₃⁻ produced depends on the mix of CO₂ and H₂SO₄, and the proportions of the carbonates and silicates in the source rock. An average sediment has a CO₂ uptake potential of ψ = 0.61. The potential increases by inclusion of the crystalline crust in the weathering source rock. A mineral dissolution model for an average river gives ψ = 0.68 to 0.72 that is within the range of ψ = 0.63 to 0.75, reported by other investigators using other methods. These results translate into the CO₂ weathering flux of 20 to 24 × 10¹²mol/yr.     

Principal fault zone width and permeability of the active Neodani fault, Nobi fault system, Southwest Japan

The internal structure and permeability of the Neodani fault, which was last activated at the time of the 1891 Nobi earthquake (M8.0), were examined through field survey and experiments. A new exposure of the fault at a road construction site reveals a highly localized feature of the past fault deformation within a narrow fault core zone. The fault of the area consists of three zone units towards the fault core: (a) protolith rocks; (b) 15 to 30 m of fault breccia, and (c) 200 mm green to black fault gouge. Within the fault breccia zone, cataclastic foliation oblique to the fault has developed in a fine-grained 2-m-wide zone adjacent to the fault. Foliation is defined by subparallel alignment of intact lozenge shaped clasts, or by elongated aggregates of fine-grained chert fragments. The mean angle of 20°, between the foliation and the fault plane suggests that the foliated breccia accommodated a shear strain of γ<5 assuming simple shear for the rotation of the cataclastic foliation. Previous trench surveys have revealed that the fault has undergone at least 70 m of fault displacement within the last 20,000 years in this locality. The observed fault geometry suggests that past fault displacements have been localized into the 200-mm-wide gouge zone. Gas permeability analysis of the gouges gives low values of the order of 10⁻²⁰ m². Water permeability as low as 10⁻²⁰ m² is therefore expected for the fault gouge zone, which is two orders of magnitude lower than the critical permeability suggested for a fault to cause thermal pressurization during a fault slip.     

3-D velocity structure of the 2003 Bam earthquake area (SE Iran): Existence of a low-Poisson's ratio layer and its relation to heavy damage

To understand the generation mechanism of the Bam earthquake (Mw 6.6), we studied three-dimensional V_P, V_S and Poisson's ratio (σ) structures in the Bam area by using the seismic tomography method. We inverted accurate arrival times of 19490 P waves and 19015 S waves from 2396 aftershocks recorded by a temporal high-sensitivity seismic network. The 3-D velocity structure of the seismogenic region was well resolved to a depth of 14 km with significant velocity variations of up to 5%. The general pattern of aftershock distribution was relocated by using the 3-D structure to delineate a source fault for a length of approximately 20 km along a line 4.5 km west of the known geological Bam fault; this source fault dips steeply westward and strikes a nearly north–south line. The main shallow cluster of aftershocks south of the city of Bam is distributed just under the minor surface ruptures in the desert. The 3-D velocity structure shows a thick layer of high V_S and low σ (minimum: 0.20) at a depth range of 2–6 km. The deeper layer, with a thickness of about 2 km, appears to have a low V_S and high σ (maximum: 0.28) from 6 km depth beneath Bam to a depth of 9 km south of the city. The inferred increase of Poisson's ratio from 2 to 10 km in depth may be associated with a change from rigid and SiO₂-rich rock to more mafic rock, including the probable existence of fluids. The main seismic gap of aftershock distribution at the depth range of 2 to 7 km coincides well with the large slip zone in the shallow thick layer of high V_S and low σ. The large slip propagating mainly in the shallow rigid layer may be one of the main reasons why the Bam area suffered heavy damage.     

Isotopic fractionation associated with biosynthesis of fatty acids by a marine bacterium under oxic and anoxic conditions

Shewanella putrefaciens (Strain MR-4), a gram negative facultative marine bacterium, was grown to stationary phase under both aerobic and anaerobic conditions using lactate as the sole carbon source. Aerobically-produced cells were slightly enriched in ¹³C (+1.5‰) relative to the lactate carbon source, whereas those from anaerobic growth were depleted in ¹³C (−2.2‰). The distribution of fatty acids produced under aerobic conditions was similar to that resulting from anaerobic growth, being dominated by C_16:1 ω7 and C_16:0 fatty acids with a lesser amount of the C_18:1 ω7 component. Low concentrations of saturated even numbered normal fatty acids in the C₁₄ to C₁₈ range, and iso-C_15:0 were synthesized under both conditions. Fatty acids from anaerobic cultures (average δ¹³C=−37.8‰) were considerably depleted in ¹³C relative to their aerobically-synthesized counterparts (−28.8‰). The distinct differences in isotopic composition of both whole cells and individual fatty acid components result from differences in assimilation pathways. Under aerobic conditions, the primary route of assimilation involves the pyruvate dehydrogenase enzyme complex which produces acetyl-CoA, the precursor to lipid synthesis. In contrast, under anaerobic conditions formate, and not acetate, is the central intermediate in carbon assimilation with the precursors to fatty acid synthesis being produced via the serine pathway. Anaerobically-produced bacterial fatty acids were depleted by up to 12‰ relative to the carbon source. Therefore, detection of isotopically depleted fatty acids in sediments may be falsely attributed to a terrestrial origin, when in fact they are the result of bacterial resynthesis.     

Seismic velocities of granulite-facies xenoliths from Central Ireland: Implications for lower crustal composition and anisotropy

V_p and V_s values have been measured experimentally and calculated for granulite-facies lower crustal xenoliths from central Ireland close to the Caledonian Iapetus suture zone. The xenoliths are predominantly foliated and lineated metapelitic (garnet–sillimanite–K-feldspar) granulites. Their metapelitic composition is unusual compared with the mostly mafic composition of lower crustal xenoliths world-wide. Based on thermobarometry, the metapelitic xenoliths were entrained from depths of c. 20–25 ± 3.5 km and rare mafic granulites from depths of 31–33 ± 3.4 km. The xenoliths were emplaced during Lower Carboniferous volcanism and are considered to represent samples of the present day lower crust.V_p values for the metapelitic granulites range between 6.26 and 7.99 km s^{− 1} with a mean value of 7.09 ± 0.4 km s^{− 1}. Psammite and granitic orthogneiss samples have calculated V_p values of 6.51 and 6.23 km s^{− 1}, respectively. V_s values for the metapelites are between 3.86 and 4.34 km s^{− 1}, with a mean value of 4.1 ± 0.15 km s^{− 1}. The psammite and orthogneiss have calculated V_s values of 3.95 and 3.97 km s^{− 1}, respectively.The measured seismic velocities correlate with density and with modal mineralogy, especially the high content of sillimanite and garnet. V_p anisotropy is between 0.15% and 13.97%, and a clear compositional control is evident, mainly in relation to sillimanite abundance. Overall V_s anisotropy ranges from 1% to 11%. Poisson's ratio (σ) lies between 0.25 and 0.35 for the metapelitic granulites, mainly reflecting a high V_p value due to abundant sillimanite in the sample with the highest σ. Anisotropy is probably a function of deformation associated with the closure of the Iapetus ocean in the Silurian as well as later extension in the Devonian. The orientation of the bulk strain ellipsoid in the lower crust is difficult to constrain, but lineation is likely to be NE–SW, given the strike-slip nature of the late Caledonian and subsequent Acadian deformation.When corrected for present-day lower crustal temperature, the experimentally determined V_p values correspond well with velocities from the ICSSP, COOLE I and VARNET seismic refraction lines. Near the xenolith localities, the COOLE I line displays two lower crustal layers with in situ V_p values of 6.85–6.9 and 6.9–8.0 km s^{− 1}, respectively. The upper (lower velocity) layer corresponds well with the metapelitic granulite xenoliths while the lower (higher velocity) layer matches that of the basic granulite xenoliths, though their metamorphic pressures suggest derivation from depths corresponding to the present-day upper mantle.     

Isotopic and molecular composition of coal-bed gas in the Amasra region (Zonguldak basin—western Black Sea)

Previous studies on the coal-bed methane potential of the Zonguldak basin have indicated that the gases are thermogenic and sourced by the coal-bearing Carboniferous units. In this earlier work, the origin of coal-bed gas was only defined according to the molecular composition of gases and to organic geochemical properties of the respective source rocks, since data on isotopic composition of gases were not available. Furthermore, in the western Black Sea region there also exist other source rocks, which may have contributed to the coal-bed gas accumulations. The aim of this study is to determine the origin of coal-bed gas and to try a gas-source rock correlation. For this purpose, the molecular and isotopic compositions of 13 headspace gases from coals and adjacent sediments of two wells in the Amasra region have been analyzed. Total organic carbon (TOC) measurements and Rock-Eval pyrolysis were performed in order to characterize the respective source rocks. Coals and sediments are bearing humic type organic matter, which have hydrogen indices (HI) of up to 300 mgHC/gTOC, indicating a certain content of liptinitic material. The stable carbon isotope ratios (δ¹³C) of the kerogen vary from −23.1 to −27.7‰. Air-free calculated gases contain hydrocarbons up to C₅, carbon dioxide (<1%) and a considerable amount of nitrogen (up to 38%). The gaseous hydrocarbons are dominated by methane (>98%). The stable carbon isotope ratios of methane, ethane and propane are defined as δ¹³C₁: −51.1 to −48.3‰, δ¹³C₂: −37.9 to −25.3‰, δ¹³C₃: −26.0 to −19.2 ‰, respectively. The δD₁ values of methane range from −190 to −178‰. According to its isotopic composition, methane is a mixture, partly generated bacterially, partly thermogenic. Molecular and isotopic composition of the gases and organic geochemical properties of possible source rocks indicate that the thermogenic gas generation took place in coals and organic rich shales of the Westphalian-A Kozlu formation. The bacterial input can be related to a primary bacterial methane generation during Carboniferous and/or to a recent secondary bacterial methane generation. However, some peculiarities of respective isotope values of headspace gases can also be related to the desorption process, which took place by sampling.     

A note on fault reactivation

Reactivation of existing faults whose normal lies in the σ₁^′σ₃^′ plane of a stress field with effective principal compressive stresses σ₁^′ >σ₂^′ >σ₃^′ is considered for the simplest frictional failure criterion, τ = μσ_n^′ = μ(σ_n − P), where τ and σ_n are respectively the shear and normal stresses to the existing fault, P is the fluid pressure and μ is the static friction. For a plane oriented at θ to σ₁^′, the stress ratio for reactivation is (σ₁^′/σ₃^′) = (1 + μ cot θ)/(1 − μ tan θ). This ratio has a minimum positive value at the optimum angle for reactivation given by (1/μ) but reaches infinity when θ = 2θ^*, beyond which σ₃^′ < 0 is a necessary condition for reactivation. An important consequence is that for typical rock friction coefficients, it is unlikely that normal faults will be reactivated as high-angle reverse faults or thrusts as low-angle normal faults, unless the effective least principal stress is tensile.     

Stable carbon isotope compositions and source rock geochemistry of the giant gas accumulations in the Ordos Basin, China

Ordos Basin, the second largest sedimentary basin in China, contains enormous natural gas resources. Each of the four giant gas fields discovered so far in this basin (i.e., Sulige, Yulin, Wushenqi and Jingbian) has over 100 billion cubic meters (bcm) or 3.53 trillion cubic feet (tcf) of proven gas reserves. This study examines the stable carbon isotope data of 125 gas samples collected from the four giant gas fields in the Ordos Basin. Source rocks in the Upper Paleozoic coal measures are suggested by the generally high δ¹³C values of C₁–C₄ gaseous hydrocarbons in the gases from the Sulige, Yulin and Wushenqi gas fields. While the δ¹³C_iC4 value is higher than that of the δ¹³C_nC4, the dominant ranges for the δ¹³C₁, δ¹³C₂, and δ¹³C₃ values in these Upper Paleozoic reservoired gases are −34 to −32‰, −27 to −23‰, and −25 to −24‰, respectively. The δ¹³C values of methane, benzene and toluene in gases from the Lower Paleozoic reservoirs of the Jingbian field indicate a significant contribution from humic source rocks, as they are similar to those in the Upper Paleozoic reservoirs of the Sulige, Yulin and Wushenqi gas fields. However, the wide variation and reversal in the δ¹³C₁, δ¹³C₂ and δ¹³C₃ values in the Jinbian gases cannot be explained using a single source scenario, thus the gases were likely derived dominantly from the Carboniferous-Permian coal measures with some contribution from the carbonates in the Lower Permian Taiyuan Formation. The gas isotope data and extremely low total organic carbon contents (<0.2% TOC) suggest that the Ordovician Majiagou Formation carbonates are unlikely to be a significant gas source rock, thus almost all of the economic gas accumulations in the Ordos Basin were derived from Upper Paleozoic source rocks.     

The rupture process during the 1999 Düzce, Turkey, earthquake from joint inversion of teleseismic and strong-motion data

Teleseismic and strong-motion data are inverted to determine the rupture process during the November 1999 Düzce earthquake in NW Turkey. The fault geometry, rise time and rupture velocity are determined from the aftershock distribution and preliminary inversions of the teleseismic data. Joint inversion of the teleseismic and strong-motion data is then carried out for the slip distribution. We obtain the strike 264°, dip 64°, rake −172°, seismic moment 5.0×10¹⁹ N m (M_w 7.1), and average stress drop 7 MPa. This earthquake was characterized by bilateral fault rupture and asymmetric slip distribution. Two asperities (areas of large slip) are identified, the eastern one being 1.5 times larger than the western one. The derived slip distribution is consistent with the aftershock distribution, surface rupture and damage. The point of rupture initiation in this Düzce earthquake coincided with the eastern tip of the aftershock distribution of the August 1999 Izmit earthquake.     

Earthquake source nucleation as self organization process

The theory of an earthquake source nucleation is discussed. Based on the assumption that self organization of damage process takes place in the zone of an earthquake source nucleation the theory incorporates the damage rheology framework of Lyakhovsky et al. and the approach of phenomenology theory of second-order phase transition. Namely, the free energy governing the process of an earthquake source nucleation depends on two collective variables α and φ in addition to the strain tensor ε_ij. The variable α quantifies the fracture of the medium and the variable φ quantifies the interaction effect of cracks. The region Ω_S is associated with a potential source of an earthquake if the damage variable α exceeds the critical value α_cr⁽¹⁾ inside Ω_S. The important feature of a potential source is that interaction of fractures causes acceleration of damage process inside the region of potential source and the material should lose stability primarily in this region. Interaction of fractures results also in occurrence of a residual stress caused by nonuniformity of fracture density. The appearance and development of the potential source result in increase of intensity of damage process not only in the region of potential source but also in a certain neighborhood of the last. It is compatible with such observed effects as acceleration of seismic energy release and growth of correlation length of weak seismicity before large earthquake. Transition of potential source to the stage of avalanche-unstable fracturing is associated with instability generated by explosive increase of interaction of fractures when the damage variable α exceeds the second critical value α_cr⁽²⁾ inside Ω_S.     

Co-seismic displacements associated to the Molise (Southern Italy) earthquake sequence of October–November 2002 inferred from GPS measurements

The 2002 earthquake sequence of October 31 and November 1 (main shocks Mw = 5.7) struck an area of the Molise region in Southern Italy. In this paper we analyzed the co-seismic deformation related to the Molise seismic sequence, inferred from GPS data collected before and after the earthquake, that ruptured a rather deep portion of crust releasing a moderate amount of seismic energy with no surface rupture. The GPS data have been reduced using two different processing strategies and softwares (Bernese and GIPSY) to have an increased control over the result accuracy, since the expected surface displacements induced by the Molise earthquake are in the order of the GPS reliability. The surface deformations obtained from the two approaches are statistically equivalent and show a displacement field consistent with the expected deformation mechanism and with no rupture at the surface. In order to relate this observation with the seismic source, an elastic modeling of fault dislocation rupture has been performed using seismological parameters as constraints to the model input and comparing calculated surface displacements with the observed ones. The sum of the seismic moments (8.9 × 10¹⁷ Nm) of the two main events have been used as a constraint for the size and amount of slip on the model fault while its geometry has been constrained using the focal mechanisms and aftershocks locations. Since the two main shocks exhibit the same fault parameters (strike of the plane, dip and co-seismic slip), we modelled a single square fault, size of 15 km × 15 km, assumed to accommodate the whole rupture of both events of the seismic sequence. A vertical E–W trending fault (strike = 266°) has been modeled, with a horizontal slip of 120 mm. Sensitivity tests have been performed to infer the slip distribution at depth. The comparison between GPS observations and displacement vectors predicted by the dislocation model is consistent with a source fault placed between 5 and 20 km of depth with a constant pure right-lateral strike-slip in agreement with fault slip distribution analyses using seismological information. The GPS strain field obtained doesn't require a geodetic moment release larger than the one inferred from the seismological information ruling out significant post-seismic deformation or geodetic deformation released at frequencies not detectable by seismic instruments. The Molise sequence has a critical seismotectonic significance because it occurred in an area where no historical seismicity or seismogenic faults are reported. The focal location of the sequence and the strike-slip kinematics of main shocks allow to distinguish it from the shallower and extensional seismicity of the southern Apennines being more likely related to the decoupling of the southern Adriatic block from the northern one.