Gust Bambakidis

Modeling of seismo-electromagnetic phenomena

A model for seismo-electromagnetic (SEM) phenomena is described. The electromagnetic signals generated by mechanical disturbances in the earths crust have been calculated and compared with reported seismo-electromagnetic signals (SEMS). The major known SEM phenomena, namely, tectonomagnetic variations, electrotelluric anomalies, geomagnetic variations in the ultra-low frequency range and electromagnetic emission in the radio frequency range, have been considered. We have calculated the spectral densities associated with various types of sources. The set of formulas necessary to calculate the detected (filtered and averaged) electric and magnetic fields generated by mechanical disturbances for a wide range of frequencies and at various distances from the source are presented. Based on these formulas, we discuss the conditions under which electrokinetic, piezomagnetic and piezolectric eects could be responsible for SEMS. A comparison of estimated values of SEMS with reported field measurements leads to the conclusion that the sources of most anomalous SEMS are relatively close to the detector. In other words, the source of the signal is local, although the source of the mechanical disturbance which activates it, i.e. the epicenter of an earthquake, may be far away. Recommendations for field experiments (appropriate detector sitting, detector parameters and frequency range) following from the model developed here are presented.

Episodic tremors and slip in Cascadia in the framework of the Frenkel-Kontorova model

Received 27 August 2010; revised 20 October 2010; accepted 28 October 2010; published 13 January 2011. [1] The seismic moment for regular earthquakes is proportional to the cube of rupture time. A second class of phenomena, collectively called slow earthquakes, has very different scaling. We propose a model, inspired from the phenomenology of dislocation dynamics in crystals, that is consistent with the scaling relations observed in the Cascadia episodic tremor and slip (ETS) events. Two fundamental features of ETS are periodicity and migration. In the northern Cascadia subduction zone, ETS events appear every 14.5 months or so. During these events, tremors migrate along‐strike with a velocity of 10 km/day and simultaneously zip back and forth in the relative plate‐ motion direction with at ypical velocity of 50 km/h. Our model predicts the formation of a sequence of slip pulses on the boundary of the plates, which describes the major features of fault dynamics, including periodicity and the migration pattern of tremors. Citation: Gershenzon, N. I., G. Bambakidis, E. Hauser, A. Ghosh, and K. C. Creager (2011), Episodic tremors and slip in Cascadia in the framework of the Frenkel‐Kontorova model, Geophys. Res. Lett., 38, L01309,

Thermal stabilities of the crystalline and amorphous TiyCuHx systems

The irreversible exothermic transitions of the crystalline and amorphous TiyCuHx systems, where y = 1 or 2 and 0 < x < 2.71, were investigated using differential scanning calorimetry (DSC) at temperatures between 300 and 900 K. Room temperature X-ray diffraction measurements were made to identify the phases and unit cell parameters before and after the DSC runs. The transition temperatures, kinetic activation energies and heats of transition were found to be dependent on the hydrogen concentration. The transition temperature and kinetic activation energy are closely correlated with one another, but not with the heat of transition. The reduced thermal stability of a-TiCuH1.41 relative to that of both a-TiCu and c-TiCuH0.96 is attributed to greater atomic diffusivities in the amorphous hydride, with hydrogen diffusivity playing an important role. All hydrided samples underwent exothermic transitions in the range 500–700 K that produced γ-TiHx and copper metal. On further heating TiCu3 was formed. At temperatures above about 900 K γ-TiCuHx and δ-TiCu were produced.

Resistiometric determination of phase transformations in VHx and VDx

Abstract Phase transitions in the systems VH x and VD x for 0.443 ⩽ x ⩽ 0.815 were determined using a resistiometric technique supplemented by differential scanning calorimetry. The use of inflection points in the resistivity versus temperature heating curve to determine the phase boundaries was found to give results consistent with those obtained using differential scanning calorimetry. Comparisons with recently published VH x and VD x phase diagrams give generally good agreement and verify the unusually large isotopic differences observed by other techniques. No evidence is found for the η phase or for the β 1 + β 2 mixed phase in the VH x system. The concentration dependence of the resistivity in the a phase shows a small isotope effect. For both VH x and VD x the variation with x indicates the existence of local regions of β phase the extent of which are of the order of 25A or more. In the β phase of VH x the x dependence of the resistivity indicates partial ordering of hydrogen in the β 1 region and complete disorder in the β 2 region.

Theoretical intensity-dependent response of nonlinear periodic structures

We have modeled the response of a nonlinear periodic structure by means of the Abeles 2×2 matrix method. Our structure differs from the usual rejection‐band filter designs, in that we have chosen the filter elements to be index matched in the absence of radiation, providing a rejection band that both grows and shifts as a function of incident intensity. The intensity output function of the model not only directly demonstrates optical bistability, but also limiting, switching, self‐pulsing, and chaos.

Model of Deep Nonvolcanic Tremor Part II: Episodic Tremor and Slip

Abstract Bursts of tremor accompany a moving slip pulse in episodic tremor and slip (ETS) events. The sources of this nonvolcanic tremor (NVT) are largely unknown. We developed a model describing the mechanism of NVT generation. According to this model, NVT is a reflection of resonant‐type oscillations excited in a fault at certain depth ranges. From a mathematical viewpoint, tremor (phonons) and slip pulses (solitons) are two different solutions of the sine‐Gordon equation describing frictional processes inside a fault. In an ETS event, a moving slip pulse generates tremor due to interaction with structural heterogeneities in a fault and due to failures of small asperities. Observed tremor parameters, such as central frequency and frequency attenuation curve, are associated with fault parameters and conditions such as elastic modulus, effective normal stress, penetration hardness, and friction. Model prediction of NVT frequency content is consistent with observations. In the framework of this model, it is possible to explain the complicated pattern of tremor migration, including rapid tremor propagation and reverse tremor migration. Migration along the strike direction is associated with movement of the slip pulse. Rapid tremor propagation in the slip‐parallel direction is associated with movement of kinks along a 2D slip pulse. A slip pulse, pinned in some places, can fragment into several pulses, causing tremor associated with some of these pulse fragments to move opposite to the main propagation direction. The model predicts that the frequency content of tremor during an ETS event is slightly different from the frequency content of ambient tremor and tremor triggered by earthquakes.

Determination of shallow minority-acceptor concentration in multiply doped silicon

A method is presented, based on photothermal ionization spectroscopy (PTIS), for determining the shallow minority‐acceptor concentration in multiply doped silicon, over the concentration range 1013/cm3–1015/cm3. The method is an extension of a model developed previously for the PTIS response in a multiply doped semiconductor, which accounts for the experimentally observed change in signature, from negative to positive, of the lower‐energy lines of the deeper acceptor as the temperature is increased. It uses a calculated curve of the dependence on the shallow‐acceptor concentration of the temperature at which the change in signature occurs, for a given line, and compares it to a determination of this temperature from actual spectra for the sample. The method is applied to the determination of boron concentration in the Si(Ga,B) system.

Sensing network for electromagnetic fields generated by seismic activities

The sensors network is becoming prolific and play now increasingly more important role in acquiring and processing information. Cyber-Physical Systems are focusing on investigation of integrated systems that includes sensing, networking, and computations. The physics of the seismic measurement and electromagnetic field measurement requires special consideration how to design electromagnetic field measurement networks for both research and detection earthquakes and explosions along with the seismic measurement networks. In addition, the electromagnetic sensor network itself could be designed and deployed, as a research tool with great deal of flexibility, the placement of the measuring nodes must be design based on systematic analysis of the seismic-electromagnetic interaction. In this article, we review the observations of the co-seismic electromagnetic field generated by earthquakes and man-made sources such as vibrations and explosions. The theoretical investigation allows the distribution of sensor nodes to be optimized and could be used to support existing geological networks. The placement of sensor nodes have to be determined based on physics of electromagnetic field distribution above the ground level. The results of theoretical investigations of seismo-electromagnetic phenomena are considered in Section I. First, we compare the relative contribution of various types of mechano-electromagnetic mechanisms and then analyze in detail the calculation of electromagnetic fields generated by piezomagnetic and electrokinetic effects.

Thermal Stability of Hydrides of Disordered and Amorphous Alloys

Hydrides of amorphous/glassy alloys have received considerable attention recently because of their potential applications as energy carriers, chemical storage of hydrogen, heat pumps, fuel cells, and heat engines (1,2). In all of these applications the uncharged intermetallic compound and the corresponding ternary hydride are subjected to a large number of charging and decharging cycles. A major disadvantage to using ternary hydrides which results after a relatively large number of cycles is the decomposition or disproportionation of the material so that it no longer absorbs hydrogen gas in a reversible way. It has been shown that this decomposition is a reaction by part of the ternary hydride to form the corresponding binary hydride of the more stable (stronger hydrogen-attracting component) plus free metal of the less hydrogen-attracting component (1). It is not well understood why this disproportionation reaction is significant for some systems and almost insignificant for other systems. In some cases an intermetallic compound is formed along with the more stable binary hydride instead of the free metal. Buschow, Bouten and Miedema (1) list over 100 intermetalic hydrides that have been prepared and partially characterized. There are several times this many that are known, yet few are really satisfactory for the applications listed above. This paper is limited to transition metal-transition metal type alloys where one metal (A) is the stronger hydrogen-attracting and (B) is the weaker hydrogen-attracting. Examples of A-type metals are early (IIIb, IVb, Vb such as Sc, Y, La, Ti, Zr, Hf, V, Nb, etc.) and B-type (late) (VIIIb, Ib such as Fe, Co, Ni, Cu, Rh, Pd, Ir, Pt, etc.). The alloys may be intermetallic compounds but often are all compositions that may be prepared by the melt-spinning technique and are only limited in composition by the range of glass-stability. These alloys are designated by the atom percent composition, such as Ti45Cu55 for an alloy of 45 atom percent Ti and 55 atom percent Cu. Both the crystalline (c−) and amorphous/glass (a−) alloys will be discussed in this paper. The systems that are discussed here are: a-TiCuHx, c-TiCuHx, c-Ti2CuHx, a-Zr2PdHx, c-Zr2PdHx, a-Zr3RhHx, where x refers to noninteger values for the hydrogen composition. In addition, the intermetallic alloys will be included in the properties discussed wherever appropriate. The experimental methods used to study thermal stability are differential scanning calorimetry (DSC), isothermal annealing, and powder X-ray diffraction (XRD). All XRD data have been taken at room temperature following quenching of the DSC or annealing studies.

A simple model for impurity photoabsorption in silicon

A simple model for absorption of infrared radiation by impurity atoms in silicon crystals has been developed and applied to electronic excitations of the Group V donors Bi, Sb, As, and P, and the Group III acceptors B, Al, Ga, and In. The model is based on the quantum‐defect method for approximating bound donor or acceptor wave functions outside the core region of the impurity. For each donor species, the relative oscillator strengths have been calculated for the transitions from the ground state to the first four excited levels. For each acceptor species, the relative oscillator strengths were calculated for transitions from the P3/2 ground state to the first three P1/2 excited levels. Comparison with high‐resolution absorption spectra show qualitative agreement for the low‐lying transitions.