Wave activity in the ionosphere during the geospace storm of November 7–10, 2004

This paper presents the results of studies of wave disturbances in the electron concentration N in the ionosphere during a prominent geospace storm, in the process of which the electron concentration decreased by a factor of 6–7, whereas the temperatures of ions and electrons at night increased up to 2000 and 3000 K, respectively. The height-time variations in the parameters of wave disturbances are also analyzed. It is shown that the geospace storm was accompanied by a substantial change in wave activity in the ionosphere. In the period of negative ionospheric storms, the amplitude Δ _N decreased by a factor of 2–10. At the same time, the relative amplitude δ _N = ΔN/N changed insignificantly and was within the limits 0.05–0.10 during day-time. At night, δ _N reached 0.25–0.30 and sometimes even 0.4–0.5. During both disturbed and undisturbed days, quasi-periodic processes with a period of 40–60 and 80–120 min prevailed. The maximum values of the absolute and relative amplitudes were achieved at a height of 200–270 km. A soliton-like disturbance formed near the main phase of the magnetic storm on November 10, 2004 was detected. Its appearance was related to the oblique coherent reflection of sounding signals.     

Solar eclipse of August 1, 2008, above Kharkov: 2. Observation results of wave disturbances in the ionosphere

Quasi-periodic variations in the power of incoherent scattered signals, caused by wave disturbances of the electron concentration in the ionosphere, are analyzed for the day of a partial solar eclipse and for a background day. The windowed and adaptive Fourier transforms and the wavelet transform are used for spectral analysis. The spectral parameters of the wave disturbances at altitudes of 100–500 km in the 10–120 min period range differed significantly on the day of the solar eclipse and on the background day. Variations in the spectrum began near the onset of the phase of maximum disk occultation and continued no less than 2 h. The amplitude of time variations N was 2 × 10⁹–4 × 10¹⁰ m^?3, and the relative amplitude was 0.10–0.15. Wave disturbances have been compared for five solar eclipses; the comparison shows a noticeable variation in the spectrum of the wave disturbances during these events.     

Wave response of the ionosphere to the partial solar eclipse of August 1, 2008

Quasi-periodic variations in the Doppler shift of the HF range frequency at a vertical path and critical frequency of the F ₂ layer caused by wave disturbances in the ionosphere on the day of the partial (the magnitude was about 0.42) solar eclipse and on background days are analyzed. For the spectral analysis, the window Fourier transform, adaptive Fourier transform, and wavelet analysis were jointly used. It is shown that on the day of the eclipse and the background day, spectral characteristics of wave disturbances within the 150–200 km height range differed substantially. The changes in the spectral composition began approximately 30–35 min after the solar eclipse beginning and lasted more than 1.5 h.     

A study on the focal parameters of Shidian M =5.9 earthquake sequences in 2001

On the basis of about 300 earthquake wave forms observed in the Shidian M _S=5.9 sequences on April 12, 2001 recorded in Kunming Digital Seismic Network, the spectra of shear wave have been used to estimate the focal parameters of these earthquake sequences. The results show that within the magnitude range of 1.5–5.3, the seismic moments are 10¹⁰–10¹⁶ N·m, the corner frequencies are 0.2–0.8 Hz, radii of the focal rupture are 200–2 500 m and the stress drops are 0.1×10⁵–20×10⁵Pa. Through the statistical analyses of variation of corner frequency f _c and stress drop Δσ with time, it is discovered that the average corner frequency of the foreshock sequences is obviously lower than that of the aftershock sequences. Contrarily, the average stress drops Δσ of the foreshock sequences are clearly higher than that of the aftershocks. It is considered that these variation characteristics of average corner frequency and stress drops before and after the main shock have index significance to the precursory information before a strong earthquake. The higher stress drops for the foreshock sequences show that the higher shear stresses have been stored in the area of main shock. After the main shock, most of the stresses have been released, so the aftershock sequences show a rupture process of lower stresses. Foundation item: Scientific and Technological Key Project of Yunnan Province (2001NG46)     

Parameters of traveling ionospheric disturbances estimated from satellite beacon observations in low-earth orbit

Based on beacon sounding the ionosphere using coherent signals of low-orbiting navigation satellites, the following parameters of medium-scale traveling ionospheric disturbances have been determined: the effective thickness of the atmospheric waveguide and the altitude of its longitudinal axis, the horizontal spatial period, the velocity, the maximum amplitudes of disturbances, and the inclination of the disturbance phase front. It has been found that the horizontal spatial period of studied disturbances increases with increasing distance to an initiating impact location and its delay. In this case the minimum value of the horizontal spatial period always exceeds ∼150 km, and the disturbance amplitude increases with increasing horizontal period and distance from the initiating disturbance location and with decreasing altitude of the main ionization maximum. All disturbance parameters are independent of the initiating impact nature. It has been found that disturbances with relative amplitudes of 0.1–0.7 are often observed. Disturbances that are registered more often travel southward, and their absolute velocities are 7–60 m/s.     

Source mechanism and source parameters of May 28, 1998 earthquake,Egypt

On May 28, 1998, a moderate size earthquake of mb 5.5 occurred offshore the northwestern part of Egypt (latitude 31.45°N and longitude 27.64°E). It was widely felt in the northern part of Egypt. Being the largest well-recorded event in the area for which seismic data from the global digital network are available, it provides an excellent opportunity to study the tectonic process and present day stress field occurring along the offshore Egyptian coast. The source parameters of this event are determined using three different techniques: modeling of surface wave spectral amplitudes, regional waveform inversion, and teleseismic body waveform inversion. The results show a high-angle reverse fault mechanism generally trending NNW–SSE. The P-axis trends ENE–WSW consistently with the prevailed compression stress along the southeastern Hellenic arc and southwestern part of the Cyprean arc. This unexpected mechanism is most probably related to a positive inversion of the NW trending offshore normal faults and confirms an extension of the back thrusting effects towards the African margin. The estimated focal depth ranges from 22 to 25 km, indicating a lower crustal origin earthquake owing to deep-seated tectonics. The source time function indicates a single source with rise time and total rupture duration of 2 and 5 s, respectively. The seismic moment (M _o) and the moment magnitude (M _w) determined by the three techniques are 1.03 × 10¹⁷ Nm, 5.28; 1.24 × 10¹⁷ Nm, 5.33; and 1.68 × 10¹⁷ Nm, 5.42; respectively. The calculated fault radius, stress drop, and the average dislocation assuming a circular fault model are 7.2 km, 0.63 Mpa, and 0.11 m, respectively.     

Study of calibration function for surface wave magnitude of DK1 seismographs

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Estimation of Source Parameters for the Aftershocks of the 2001 Mw 7.7 Bhuj Earthquake, India

The source parameters for 213 Bhuj aftershocks of moment magnitude varying from 2.16 to 5.74 have been estimated using the spectral analysis of the SH- waveform on the transverse component of the three-componnet digital seismograms as well as accelerograms. The estimated stress drop values for Bhuj aftershocks show more scatter (Mo^{0.5 to 1} ∞ Δσ) toward the larger seismic moment values (log Mo ≥ 10^14.5 N-m, larger aftershocks), whereas, they show a more systematic nature (Mo³ ∞ Δσ) for smaller seismic moment (log Mo < 10^14.5 N-m, smaller aftershocks) values. This size dependency of stress drop has also been seen from the relation between our estimated seismic moment and source radius, however, this size-dependent stress drop is not observed for the source parameter estimates for the other stable continental region earthquakes in India and around the world. The estimated seismic moment (Mo), source radius (r) and stress drop (Δσ) for aftershocks of moment magnitude 2.16 to 5.74 range from 1.95 × 10¹² to 4.5 × 10¹⁷ N-m, 239 to 2835 m and 0.63 to 20.7 MPa, respectively. The near-surface attenuation factor (k) is found to be large of the order of 0.03 for the Kachchh region, suggesting thick low velocity sediments beneath the region. The estimated stress drop values show an increasing trend with the depth indicating the base of seismogenic layer (as characterized by larger stress drop values (>15 MPa)) lying in 22–26km depth range beneath the region. We suggest that the concentration of large stress drop values at 10–36km depth may be related to the large stress/strain associvated with a brittle, competent intrusive body of mafic nature.     

The ~4-ka Rungwe Pumice (South-Western Tanzania): a wind-still Plinian eruption

The ~4-ka trachytic Rungwe Pumice (RP) deposit from Rungwe Volcano in South-Western Tanzania is the first Plinian-style deposit from an African volcano to be closely documented focusing on its physical characterization. The RP is a mostly massive fall deposit with an inversely graded base. Empirical models suggest a maximum eruption column height H _T of 30.5–35 km with an associated peak mass discharge rate of 2.8–4.8 × 10⁸ kg/s. Analytical calculations result in H _T values of 33 ± 4 km (inversion of TEPHRA2 model on grain size data) corresponding to mass discharge ranging from 2.3 to 6.0 × 10⁸ kg/s. Lake-core data allow extrapolation of the deposit thinning trend far beyond onland exposures. Empirical fitting of thickness data yields volume estimates between 3.2 and 5.8 km³ (corresponding to an erupted mass of 1.1–2.0 × 10¹² kg), whereas analytical derivation yields an erupted mass of 1.1 × 10¹² kg (inversion of TEPHRA2 model). Modelling and dispersal maps are consistent with nearly no-wind conditions during the eruption. The plume corner is estimated to have been ca. 11–12 km from the vent. After an opening phase with gradually increasing intensity, a high discharge rate was maintained throughout the eruption, without fountain collapse as is evidenced by a lack of pyroclastic density current deposits.     

Source parameters of the earthquakes of the Baikal rift system

The dynamic parameters of the earthquake source—the seismic moment, the moment magnitude, the source radius, the stress drop, and the amplitude of displacement—are determined by the amplitude Fourier spectra of the body shear waves (S-waves) for 62 earthquakes of the Baikal rift system with the energy class of K _P = 9.1–15.7. In the calculations I used the classical Brune model. The seismic moment of the earthquakes being investigated changes from 3.65 × 10¹¹ N m to 1.35 × 10¹⁸ N m, and the radii of earthquake sources vary from 390 m to 1.84 km. The values of the drop in stress Δσ grow with an increase in the seismic moment up to 1.7 × 10⁸ Pa. For the group of weak earthquakes (M _w = 1.7–3.3), extremely low values of the drop in stress 10³–10⁴ Pa are observed. The maximum amplitude of displacement in the source amounts to 5.95 m. The empirical equations between the seismic moment and the other dynamic parameters of the source are determined. The regional dependence of the seismic moment and energy class is obtained: log M ₀ ± 0.60 = 1.03K _P + 3.17. The character of the relationship between the seismic moment and the corner frequency indicates that the classical scaling law of the seismic spectrum for the earthquakes in question is not fulfilled. The obtained estimates of the dynamic parameters are in satisfactory agreement with the published data concerning the analogous parameters of the other rift zones, which reflects the general regular patterns of the destruction of the lithosphere and the seismicity in the extension zones of the lithosphere.     

Jadeitic rocks within high pressure metamorphic zone from northeastern Jiangxi Province and their forming and surviving conditions

There is a close relation in time, space and origin between the NEJXO, NEJXDFZ and HPMZ from NEJXP which are located in and constrained by both Yangtze Craton and Cathaysia. These HPMRs principally include (i) Jd-Q-Ab schist, (ii) Lw-Jd-Ab schist, (iii) Jd-Ab schist, and (iv) Gl-schist. The Jd is nearly pure (Jd91-98). Jd surrounded by Ab is separated from Q and survived due to the reaction between Jd and Q, while Lw encircled by Ab remains as a pseudomorph composed of CZo+An. The minerals associated with Jd are Lw, Gl, Ab, Q, CZo, An, Pa, II, Ru, Sp, Ap, and Zr. According to the textures, the mineral paragenesis is classified into four groups (i) PI-Am-Q, (ii) Jd-I,w-GI-Q, (iii) CZo-An-Ab-Q, (iv) Ab-Ana-Ser-Ur, corresponding to (i) 1.0–0.93 Ga, 250°C, 2–3 × 10⁸ Pa; (ii) 0.93–0.79 Ga, 300–350°C, 5–12 × 10⁸ Pa; (iii) 0.79–0.223 Ga, 350–400–120°C, 12–5–3 × 10⁸ Pa; and (iv) < 0. 223 Ga, < 120–50°C, < 3 × 10⁸ Pa, in time, temperature and pressure respectively. A clockwiseP-T-t path is isothermal increasing inP (up to 45 km)—isothermal decreasing inP (up to 5 km)— nearly isobar decreasing in T. Project supported by the National Natural Science Foundation of China.     

Studying large-scale traveling ionospheric disturbances according to the data of oblique-incidence sounding

The morphological features of wave-like ionospheric disturbances with periods of 1–2 h and the spatial extent exceeding 1000 km are studied. Oblique-incidence sounding data of the ionosphere, obtained in eastern Siberia during several continuous monthly experiments on three radio paths from 2006 to 2010, have been used. Large-scale traveling ionospheric disturbances generated during magnetic storms and large-scale wave-like ionospheric disturbances registered during geomagnetically quiet periods are considered. Small-scale ionospheric structures were also observed against a background of large-scale traveling iono-spheric disturbances considered in this study.     

Height profiles of the amplitudes of large-scale traveling ionospheric disturbances

Nighttime height profiles of the amplitudes of large-scale traveling ionospheric disturbances (LSTIDs) obtained from the data of vertical sounding in Almaty (76°55′ E, 43°15′ N) for the period 2000–2007 are analyzed. The height profiles are plotted using the time variations in electron density N _h (t) at a series of heights for the F region in the ionosphere with a height step of 10 km. In total, observations were conducted during 1166 nights, among which 581 nights are characterized by wave activity. Nights with the maximum amplitude of variations in N _h (t) exceeding 25% are selected for analysis. The total number of such nights is 63; LSTIDs have been recorded in both magnetically quiet and active periods. The regressive ratios between the height of the F-region maximum and the height that corresponds to the maximum absolute amplitude of a wave, as well as between the values of the maximum amplitude at a height profile and the value of the amplitude of variations in N _m F(t) at the layer maximum, are obtained.     

Variability of parameters of the <Emphasis Type="Italic">F</Emphasis>2-layer maximum in the quiet midlatitude ionosphere under low solar activity: 1. Statistical properties

Results of statistical analysis of the properties of variability of F2-layer maximum parameters (critical frequency foF2 and the height hmF2) in quiet midlatitude ionosphere under low solar activity in the daytime (1000–1500 LT) and nighttime (2200–0300 LT) hours are presented on the basis of Irkutsk station data for 2007–2008. It is found that the distribution density of δfoF2 could be presented as consisting of two distinctly different normal laws of this distribution, one of which corresponds to weak (|δfoF2| < 10%) fluctuations in foF2 and the other corresponds to strong (30% > |δfoF2| > 10%) fluctuations. Weak fluctuations in foF2 to a substantial degree are related to ionospheric variability at times less of than 1–3 h and determine the δfoF2 variability in the daytime hours. Strong fluctuations in foF2 are mainly related to day-to-day variability of the ionosphere at a fixed local time, the variability increasing by approximately a factor of 3 during the transition from day to night and determining the δfoF2 variability in the nighttime hours. The distribution density of ΔhmF2 is close to the normal distribution law. An interpretation of the different character of the distribution densities of δfoF2 and ΔhmF2 is given.     

Configuration of the upper boundary of the ionosphere

Variations of the upper boundary of the ionosphere (UBI) are investigated based on three sources of information: (i) ionosonde-derived parameters: critical frequency foF2, propagation factor M3000F2, and sub-peak thickness of the bottomside electron density profile; (ii) total electron content (TEC) observations from signals of the Global Positioning System (GPS) satellites; (iii) model electron densities of the International Reference Ionosphere (IRI^*) extended towards the plasmasphere. The ionospheric slab thickness is calculated as ratio of TEC to the F2 layer peak electron density, NmF2, representing a measure of thickness of electron density profile in the bottomside and topside ionosphere eliminating the plasmaspheric slab thickness of GPS-TEC with the IRI^* code. The ratio of slab thickness to the real thickness in the topside ionosphere is deduced making use of a similar ratio in the bottomside ionosphere with a weight Rw. Model weight Rw is represented as a superposition of the base-functions of local time, geomagnetic latitude, solar and magnetic activity. The time-space variations of domain of convergence of the ionosphere and plasmasphere differ from an average value of UBI at ∼1000 km over the earth. Analysis for quiet monthly average conditions and during the storms (September 2002, October–November 2003, November 2004) has shown shrinking UBI altitude at daytime to 400 km. The upper ionosphere height is increased by night with an ‘ionospheric tail’ which expands from 1000 km to more than 2000 km over the earth under quiet and disturbed space weather. These effects are interposed on a trend of increasing UBI height with solar activity when both the critical frequency foF2 and the peak height hmF2 are growing during the solar cycle.     

The 20-second regional magnitude <Emphasis Type="Italic">M</Emphasis><Subscript><Emphasis Type="Italic">S</Emphasis></Subscript>(20R) for the Russian Far East

A new modified magnitude scale M _S (20R) is elaborated. It permits us to extend the teleseismic magnitude scale M _S (20) to the regional epicenter distances. The data set used in this study contains digital records at 12 seismic stations of 392 earthquakes that occured in the northwest Pacific Ocean in the period of 1993–2008. The new scale is based on amplitudes of surface waves of a narrow range of the periods (16–25 s) close to the period of 20 s, for distances of 80–3000 km. The digital Butterworth filter is used for processing. On the basis of the found regional features concerning distance dependence for seismic wave attenuation, all the stations of the region have been subdivided into two groups, namely, “continental” and “island-arc.” For each group of stations, its own calibration function is proposed. Individual station corrections are used to compensate for the local features.     

Strain-geoelectric resistivity precursor and the virtual dislocation model to the Tangshan earthquake

The complete records of geoelectric resistivity before two earthquakes were analyzed, including 16 stations within 240 km around the Tangshan earthquake and 2 stations within 50–60 km from theM6.1 Datong earthquake. By eliminating various disturbances in the records and realizing the precursory anomalies to be reliable, the authors studied the distribution of the geoelectric precursor field, which proves to be physically related to the earthquake source stress field. Comparision of the sign distribution of coseismic resistivity changes with the solution of earthquake source mechanism indicates that, the coseismic resistivity changes are of opposite sign but similar spatial distribution with respect to the corresponding resistivity precursor changes. Therefore, from the resistivity observations we are of the opinion that the Tangshan earthquake is an elastic rebound process. A virtual dislocation model of geoelectric precursor for the Tangshan earthquake is proposed, in which the geoelectric precursors are supposed to be caused by the strain accumulation due to a virtual dislocation, which is opposite in sign to the actual slip taking place at the earthquake occurrence. Taking into account of the non-linear characteristics of the amplification factorK=(Δρ/ρ)/ɛ in a range of 10⁻⁷–10⁻⁵ strain changes, the theoretical distribution of geoelectric precursors for the Tangshan earthquake was calculated based on the theory of fracture mechanics and reasonably selected dislocation parameters. The results show that the semi-quantitative theoretical values are in good agreement with the observed, suggesting that the virtual dislocation model of the geoelectric precursor is appropriate to the Tangshan earthquake. Contribution No. 96A0023, Institute of Geophysics, SSB, China. This work was supported by the National Natural Science Foundation of China and the Science and Technology Department of SSB.     

Source parameters of bohai earthquake, July 18, 1969 and yongshan earthquake, May 11, 1974 determined by synthetic seismograms of teleseismic P waves

The source parameters of the Bohai Sea earthquake, July 18, 1969 and Yongshan, Yunnan earthquake, May 11, 1974 were determined by full — wave theory synthetic seismograms of teleseismic P waves. P+pP+sP wereform were calculated with WKBJ approximation and real integral paths. One — dimensional unilateral, finite propagation source was also considered. By trail — and — error in comparing the theoretical seismograms with the observational ones of WWSSN stations, the source parameters were obtained as follow: for Bohai earthquake, φ=195°, δ=85°, λ=65°,M _o=0.9×10¹⁹Nm,L=59.9km.V _R=3.5km/s, ∧ _R =160°; for Yongshan earthquake, φ=240°, δ=80°, ∧=150°,M _o=1.3×10¹⁸Nm,L=48.8km,V _R=3km/s, ∧ _R =−10°, where φ is strike, δ dip angle, λ slip angle,M _o seismic moment,L rupture length,V _R rupture propagation speed. As III type fractures the faulting propagated along the fault planes, and ∧ _R is the angle from the strike to the propagation direction. Yongshan earthquake showed complexity in its focal process, having four sub—ruptures during the first 60 seconds. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,13, 1–8, 1991.     

Experimental simulations of gas-driven eruptions: kinetics of bubble growth and effect of geometry

 Simulated gas-driven eruptions using CO₂–water-polymer systems are reported. Eruptions are initiated by rapidly decompressing CO₂–saturated water containing up to 1.0 wt.% CO₂. Both cylindrical test cells and a flask test cell were used to examine the effect of magma chamber/conduit geometry on eruption dynamics. Bubble-growth kinetics are examined quantitatively in experiments using cylindrical test cells. Uninhibited bubble growth can be roughly expressed as dr/dt≈λD(β-1)/(γt ^1/3) for a CO₂–water-polymer system at 0–22  °C and with viscosities up to 5 Pa·s, where r is the radius of bubbles, λ and D are the Ostwald solubility coefficient and diffusivity of the gas in the liquid, β is the degree of saturation (decompression ratio), and γ characterizes how the boundary layer thickness increases with time and is roughly 1.0×10^–5 m/s^1/3 in this system. Unlike the radius of cylindrical test cells, which does not affect the eruption threshold and dynamics, the shape of the test cells (flask vs cylindrical) affects the dynamics but not the threshold of eruptions. For cylindrical test cells, the front motion is characterized by constant acceleration with both Δh (the height increase) and ΔV (the volume increase) being proportional to t ²; for the flask test cell, however, neither Δh nor ΔV is proportional to t ² as the conduit radius varies. Test-cell geometry also affects foam stability. In the flask test cell, as it moves from the wider base chamber into the narrower conduit, the bubbly flow becomes fragmented, affecting the eruption dynamics. The fragmentation may be caused by a sudden increase in acceleration induced by conduit-shape change, or by the presence of obstacles to the bubbly flow. This result may help explain the range in vesicularities of pumice and reticulite. Received: 16 May 1997 / Accepted: 11 October 1997     

Receiver function study in northern Sumatra and the Malaysian peninsula

In this receiver function study, we investigate the structure of the crust beneath six seismic broadband stations close to the Sunda Arc formed by subduction of the Indo-Australian under the Sunda plate. We apply three different methods to analyse receiver functions at single stations. A recently developed algorithm determines absolute shear-wave velocities from observed frequency-dependent apparent incidence angles of P waves. Using waveform inversion of receiver functions and a modified Zhu and Kanamori algorithm, properties of discontinuities such as depth, velocity contrast, and sharpness are determined. The combination of the methods leads to robust results. The approach is validated by synthetic tests. Stations located on Malaysia show high-shear-wave velocities (V _S) near the surface in the range of 3.4–3.6 km s^− 1 attributed to crystalline rocks and 3.6–4.0 km s^− 1 in the lower crust. Upper and lower crust are clearly separated, the Moho is found at normal depths of 30–34 km where it forms a sharp discontinuity at station KUM or a gradient at stations IPM and KOM. For stations close to the subduction zone (BSI, GSI and PSI) complexity within the crust is high. Near the surface low V _S of 2.6–2.9 km s^− 1 indicate sediment layers. High V _S of 4.2 km s^− 1 are found at depth greater than 6 and 2 km at BSI and PSI, respectively. There, the Moho is located at 37 and 40 km depth. At station GSI, situated closest to the trench, the subducting slab is imaged as a north-east dipping structure separated from the sediment layer by a 10 km wide gradient in V _S between 10 and 20 km depth. Within the subducting slab V _S ≈ 4.7 km s^− 1. At station BSI, the subducting slab is found at depth between 90 and 110 km dipping 20° ± 8° in approximately N 60° E. A velocity increase in similar depth is indicated at station PSI, however no evidence for a dipping layer is found.