Focal mechanisms of aftershocks of the 1979 Carlisle earthquake: a re-interpretation

Summary. A composite fault plane solution of aftershocks of the 4.8 ML Carlisle earthquake of 1979 December 26, and the geographic distribution of their epicentres, indicate the predominance of right-lateral strike-slip focal mechanisms, with a significant component of dip-slip faulting, on a NW–SE trending fault zone. Data presented here, combined with an alternative interpretation of the published results of King, closely constrain the possible fit of nodal planes. The strike of the NW–SE nodal plane is in excellent agreement with a trend in aftershock epicentres. The aftershock zone is close to the edge of a long positive NW–SE trending gravity anomaly.     

Anomalous variations of the geomagnetic field in East Fergana — magnetic precursor of the May earthquake with M= 7.0 (1978 November 2)

Summary. A secular variation anomaly has been discovered at the north-east part of the Fergana vdey by repeated measurements every year or less. The change of total field Δ F at the 'magnetic epicentre' was 9 nT in 1977 and 16 nT in 1978 relative to the level of 1973. In 1977 an anomalous region was recognized, where according to the data from 25 observation points Δ F increased in the northern part up to 5.2 nT, and decreased by 4.7nTin the southern part according to a further 22 points. Permanent observations were begun at the epicentre in 1978 October. We normally observed variations of Δ F differences with magnitude ± 2–3 nT, which were not correlated with worldwide magnetic activity. Anomalous variations appeared on October 26 and rose to a maximum value of + 23 nT on October 30. The decrease of this anomalous field began on October 31. This made it possible to predict a potential earthquake. The Alay earthquake with M = 7.0 occurred on November 2 six hours after the prediction was issued; Δ F then returned to the initial level. Thus, using the geomagnetic field variations in the Fergana region, geophysicists were able to predict the moment of a strong earthquake.     

A study of geomagnetic storms

Summary. An attempt is made to find interplanetary magnetic field and solar-wind parameters which control the development of geomagnetic storms. For this purpose, the interplanetary energy flux is estimated in terms of the Poynting flux ( E × B /4π), and its time variations are compared with the rate of energy dissipation in terms of the ring-current particle injection u _i( t ), Joule dissipation in the ionosphere u_j ( t ) and auroral particle injection u_p ( t ) for 15 major geomagnetic storms.
It is shown that the growth of geomagnetic storms, namely the time variations of the rate of the total energy dissipation, u ( t ) = u _i( t ) + u _j( t ) + u _p( t ), is closely related to the Poynting flux by the following relation:
where l ₀≅ 7 R _E and θ' is a measure of the angle between the interplanetary magnetic field vector and the magnetospheric field vector at the front of the magnetosphere in the equatorial plane. Further, it is shown that within a factor of 2 for each storm period.
A large increase of u ( t ) is associated with substorm activity. Thus, the energy flux ɛ( t ) entering the magnetosphere is dissipated through magneto-spheric substorm processes within the magnetosphere, and their accumulated effects can be understood as geomagnetic storm phenomena.     

A geomagnetic study of the Great Glen Fault

Summary. An attempt is made to determine the range of two-dimensional current models consistent with the measured magnetovariational response, for periods from 5–30 min, near the Great Glen Fault in northern Scotland. All current models must be symmetric about the fault line but, because of uncertainty about the magnitude of the ocean effect, models ranging from a line current at 80 km depth to a uniform current sheet, 60 km wide, at 10 km depth are equally acceptable. Comparison with other geophysical studies of the same area suggests that a suitable conducting zone is unlikely to be present at shallow depths and interpretation in terms of a conducting zone in the 20–80 km depth range is favoured, although no such zone has been resolved by the other studies.     

A geomagnetic variation study of Scotland

Summary. Data from eighteen Gough—Reitzel magnetometers and four flux-gate magnetometers, which were operated in North Scotland, are presented and discussed. The coverage given by this set of instruments was not dense enough to resolve satisfactorily the complex induction anomalies in this area but some of the major features seen are described. The features observed cannot be accounted for either by oceanic induction effects or by source field effects. The Great Glen shows up as a major conductivity feature. Other effects are also observed, some apparently associated with the highly resistive granites found in this area.     

Centennial to millennial geomagnetic secular variation

A time-varying spherical harmonic model of the palaeomagnetic field for 0–7 ka is used to investigate large-scale global geomagnetic secular variation on centennial to millennial scales. We study dipole moment evolution over the past 7 kyr, and estimate its rate of change using the Gauss coefficients of degree 1 (dipole coefficients) from the CALS7K.2 field model and by two alternative methods that confirm the robustness of the predicted variations. All methods show substantial dipole moment variation on timescales ranging from centennial to millennial. The dipole moment from CALS7K.2 has the best resolution and is able to resolve the general decrease in dipole moment seen in historical observations since about 1830. The currently observed rate of dipole decay is underestimated by CALS7K.2, but is still not extraordinarily strong in comparison to the rates of change shown by the model over the whole 7 kyr interval. Truly continuous phases of dipole decrease or increase are decadal to centennial in length rather than longer-term features. The general large-scale secular variation shows substantial changes in power in higher spherical harmonic degrees on similar timescales to the dipole. Comparisons are made between statistical variations calculated directly from CALS7K.2 and longer-term palaeosecular variation models: CALS7K.2 has lower overall variance in the dipole and quadrupole terms, but exhibits an imbalance between dispersion in   g ¹₂  and   h ¹₂  , suggestive of long-term non-zonal structure in the secular variations.     

A model for the Am (Km) planetary geomagnetic activity index and application to prediction

We studied the existence of dynamical stochastic relations in the evolution of the am index. A first analysis of the autocorrelation functions showed evidence of several seasonalities. We first used linear (ARMA) models, and it was found that these do not account for the whole internal dynamics of the data series. We then used various non-linear models to provide a better fit to reality. The forecast performances of the non-linear models are not significantly different from those of the linear model. We give a tentative explanation for the failure of the non-linear predictions. Finally, ARCH models were used in order to take into account the fact that the confidence interval for the predicted value depends on past observations.     

A successfully stress-forecast earthquake

A M = 5 earthquake in Iceland has been successfully 'stress forecast' by using variations in time delays of seismic shear wave splitting to assess the time and magnitude at which stress-modified microcracking reaches fracture criticality within the stressed volume where strain is released. Local investigations suggested the approximate location of the forecast earthquake. We report the criteria on which this stress forecast was based.     

A geomagnetic scattering theory for evaluation of earth structure

Summary. Structural features of the Earth's lower crust and upper mantle can be mapped by the analysis of temporal geomagnetic fluctuations using the electromagnetic scattering theory developed in this paper. Decomposing geomagnetic field fluctuations at the Earth's surface into an excitation part and a scattered part forms the basis of a power series development. The vertical field component is interpreted as a scattering of the excitation field. The horizontal gradient and geomagnetic depth sounding methods are special cases of the theory developed. The horizontal gradient sounding method has a tensorial aspect which has not been recognized before; it should be included to obtain correct penetration depth parameter evaluations from field data.