Conductivity structure of the upper mantle in an active subduction zone

The results of deep electromagnetic soundings for the active transition ocean-continent zone at Sakhalin Island are presented. After an averaging procedure of the magnetotelluric response functions, the period range was extended up to 500 days by using the geomagnetic soundings of the Yuzhno-Sakhalinsk, Kakioka and Memambetsu observatory data. The existence of the asthenosphere and a high conductivity layer located at the base of the upper mantle was established by one-dimensional inverse methods. High resistivity revealed at depths of 250–450 km appears to be connected with the penetration of the cold slab into the mantle. The possible nature of a mid-mantle conductive layer and the relation of its conductances with the tectonic history are discussed.     

Intraplate Seamounts of the Northwest Sector of the Pacific Ocean

A method is proposed for identifying seamounts in the northwest sector of the Pacific based on the following criteria: a closed, close to isometric contour of the isobaths at the base of a structure, its quasi-conical shape, and angles of slope exceeding 5° within the limits of the closed contour. A catalog of the mountains has been compiled, consisting of 1995 objects and their quantitative characteristics. The catalog data were statistically processed, and the following was calculated: the correlation between the number of seamounts from the radius of their base and volume, the distribution of the number of mountains, and the total volume within the study area. It is shown that seamounts are characterized as multiscale, and they are located very unevenly: areas with and without their accumulation are distinguished, and the composition of volcanic rocks is typically isotopically and geochemically heterogeneous. It is concluded that currently there is no single geodynamic model that can explain the revealed phenomena in their entirety. At the same time, the data suggest that the nature of some magma chambers that feed intraplate volcanoes is caused by transformation of energy in the lithosphere as a nonlinear open system.     

Comprehensive paleoseismic geological studies in a key site in southwestern Kola Peninsula (Northeast of the Fennoscandian Shield)

This paper considers the results of detailed paleoseismic and geological investigations in a key site in the wall of the Imandra Lake depression (Kola Peninsula Region, Northeast of the Fennoscandian Shield). Study of different groups of paleoseismic deformations developed in the fault zone and the application of new methods and techniques made it possible to identify a large seismotectonic zone characterized by great earthquakes at the end of the Late Glacial and in the Holocene. The investigation data are indicative of the necessity to estimate the seismic potential in the Kola Atomic plant area in a different way.     

Specific features of the seismic regime in the lithosphere: Manifestations of the deep aqueous fluid action

We considered the seismic regime in the upper 70–100 km of the lithosphere and identified the layers (at depths of about 10, 20–30, and 60–80 km) characterized by relatively reduced effective strength and increased seismicity. The existence of such layers is related to changes in the regime of fluid-rock interaction, namely, to the characteristic depths of a jump-like decrease in the effective permeability of rocks and an increase in the spatial homogeneity of a fluid-rock system.     

Structure of the back-arc spreading basins and the fluid regime in the mantle

Seismic, geothermal, petrological and other data collected during the joint Soviet-Chinese-Japanese Project “Geotraverse: Pacific-China plain” are highly contradictory concerning their information on back-arc basins. The routine interpretation of the geothermal data leads, e.g. to the conclusion that the temperatures at depth are much higher than can be derived from other data. The discrepancies can be resolved by the back-arc spreading basins origin because of secondary mantle bulk or fluid convection. The inversion of the sign of the seismic velocity anomalies in the Pacific region at a depth of about 300 km can also be explained if active deep fluid regime is proposed. A new geotherm below the Mariana back-arc basin is proposed, and the velocity of the ascending mantle flow is estimated for this region.     

Megaearthquake of March 11, 2011, in Japan: The event magnitude and the character of the aftershock sequence

The great Japanese earthquake (GJE) of March 11, 2011, was a megaevent. The conditions under which such seismic catastrophes occurred are discussed. The regime of the aftershocks of this megaevent is compared with the data on the aftershock sequences which accompanied the Simushir earthquakes (2006 and 2007) and the Andaman earthquake (2004) and with the seismicity behavior in the generalized vicinity of a strong earthquake. The aftershock sequences of the abovementioned strong earthquakes are shown to represent the sets of trend changes in the postshock activity and specific outbursts of seismic activity. Activity outbursts are characterized not only by an increase in the number and energy of events, but also by a decrease in the recurrence plot slope (b value) and the average earthquake depth. Some such outbursts correspond to the occurrence of strong repeated shocks. A possible mechanism for outbursts of seismic activity is proposed. The possibility of a stronger repeated shock in the vicinity of the megaearthquake of March 11, 2011, is discussed.     

New Approach to the Characterization of M max and of the Tail of the Distribution of Earthquake Magnitudes

We develop a new method for the statistical estimation of the tail of the distribution of earthquake sizes recorded in the Harvard catalog of seismic moments converted to m _W -magnitudes (1977–2004 and 1977–2006). For this, we suggest a new parametric model for the distribution of main-shock magnitudes, which is composed of two branches, the pure Gutenberg-Richter distribution up to an upper magnitude threshold m ₁, followed by another branch with a maximum upper magnitude bound M _max, which we refer to as the two-branch model. We find that the number of main events in the catalog (N = 3975 for 1977–2004 and N = 4193 for 1977–2006) is insufficient for a direct estimation of the parameters of this model, due to the inherent instability of the estimation problem. This problem is likely to be the same for any other two-branch model. This inherent limitation can be explained by the fact that only a small fraction of the empirical data populates the second branch. We then show that using the set of maximum magnitudes (the set of T-maxima) in windows of duration T days provides a significant improvement, in particular (i) by minimizing the negative impact of time-clustering of foreshock/main shock/aftershock sequences in the estimation of the tail of magnitude distribution, and (ii) by providing via a simulation method reliable estimates of the biases in the Moment estimation procedure (which turns out to be more efficient than the Maximum Likelihood estimation). We propose a method for the determination of the optimal choice of the T value minimizing the mean-squares-error of the estimation of the form parameter of the GEV distribution approximating the sample distribution of T-maxima, which yields T _optimal = 500 days. We have estimated the following quantiles of the distribution of T-maxima for the whole period 1977–2006: Q _16%(M _max) = 9.3, Q _50%(M _max) = 9.7 and Q _84%(M _max) = 10.3. Finally, we suggest two more stable statistical characteristics of the tail of the distribution of earthquake magnitudes: The quantile Q _T (q) of a high probability level q for the T-maxima, and the probability of exceedance of a high threshold magnitude ρ _T (m*)  = P{m _k  ≥ m*}. We obtained the following sample estimates for the global Harvard catalog and The comparison between our estimates for the two periods 1977–2004 and 1977–2006, where the latter period included the great Sumatra earthquake 24.12.2004, m _W  = 9.0 confirms the instability of the estimation of the parameter M _max and the stability of Q _T (q) and ρ _T (m*) = P{m _k  ≥ m*}.     

Alternative to SOC concept-model of seismic regime as a set of episodes of random avalanche-like releases occurring on a set of metastable subsystems

The seismic process is usually treated as en example of development of self-organized criticality (SOC). However, implementation mechanisms of SOC in the case of a seismic process have not been proposed. In the present work, the seismicity is modeled as a sequence of a variety of stochastic episodes of avalanche-like relaxation of metastable subsystems. The model reproduces all known empirical features inherent in the seismic process. The simulated pattern of behavior was revealed to agree with the empirical regularities, which promotes an understanding of the physics of the seismic process.     

Seismicity in the generalized vicinity of large earthquakes

Earthquake catalogs are used to construct a generalized space-time vicinity of large earthquakes and to investigate the seismicity behavior in such areas. We investigate the character of the inverse cascade (seismicity rate increasing toward the time of the main shock) and of the aftershocks, as well as the lower seismicity increase relevant to the larger vicinity of the main shock. It is shown that the inverse and aftershock cascades are accompanied by several anomalies indicating the development of a precursory strength decrease in this space-time volume; this consists in decreased apparent stresses, an increased relative contribution of low frequency oscillations into the earthquake spectrum, and an increased correlation (homogeneity) of stress and strain. We emphasize the special significance of anomalies that also characterize the physical nature of the ongoing process, in addition to the existence of a strength decrease. An example of such an anomaly is provided     

Possible influence of polymorphic transitions in minerals (according to the quartz example) on seismotectonic processes in the lithosphere

The thermally activated physical processes essential for earthquake source physics and the formation of mylonite layers and deep shear zones are examined. The possibility of the influence of polymorphic (structural) phase transitions in the rock-forming minerals on the geodynamic effects is discussed. On the basis of experimental data, the deformation-strength and elastic properties of stratum containing quartz in the region of the α-β transition in quartz are studied as an example. It is shown that the α-β transition in quartz corresponds in a number of ways to the observed features specific to the physical properties of substance in connection with seismic processes.