1.
Hussein, H. M. and K. M. Abou Elenean (2008):
Source parameters of the significant earthquakes
in Egypt, 1992–1998 inferred from the P-waves magnitude spectra of teleseismic
seismograms.
Geofizika, 25, 1-26.
ABSTRACT:
Using the P-wave magnitude spectra of the vertical component of teleseismic broadband seismograms, average source parameters have been retrieved for five significant earthquakes of Mw ≥ 5.7 occurring in Egypt namely, 1992 Cairo earthquake, 1998 Alexandria earthquake and three events which occurred in the Gulf of Aqaba region between 1993 and 1995. The magnitude spectrum represents the velocity amplitude density spectrum at the earthquake source, scaled in magnitude units. The maximum of the magnitude spectrum along with the period at which the maximum occurs are used to estimate the source parameters. For precise determination of the source parameters, two different methods for deriving the corner periods are applied. The obtained source parameters were compared with those derived in previous studies. The results show that within the moment magnitude range 5.5 ~ 7.2, the corner periods are 1.29 ~ 11.6 s, length of the fault ruptures are 5.5 ~ 50 km and the stress drops are 0.5 ~ 4.8 MPa. The derived stress drop shows an increasing trend with the seismic moment for the three Gulf of Aqaba earthquakes. The 1995 Gulf of Aqaba earthquake of Mw = 7.2, the largest earthquake to have occurred in Egypt in the last century is characterized by a higher complexity compared to the other events, that are much simpler. The values of the corner periods for this earthquake are azimuth dependent due to complexity and strong directivity of its rupture. For a detailed description of the complexity of 1995 earthquake additional source parameters are also estimated in terms of an inhomogeneous source model. These parameters are the asperity radius, displacement across the asperity, localized stress drop and ambient faulting stress. The average stress drop of Cairo, 1992 and Alexandria, 1998 intraplate earthquakes shows larger values compared with the interplate 1993 Gulf of Aqaba earthquakes. Generally, the estimated seismic moment using the magnitude spectra reflect good agreement with the estimates made from the other techniques for simple source while the complex source yields smaller values.
Keywords: magnitude spectra, source parameters, source complexity.
[PDF]
2.
Verbanac, G.,
M. Korte and M. Mandea
(2008): On minimizing the external field contributions in annual means of the
European geomagnetic observatories.
Geofizika,
25,
27-39.
ABSTRACT:
The aim of this study is to provide a method for minimizing the external field contributions in the observatory annual means and try to separate, interpret and explain all the different external field signals present in these data. Investigating the European geomagnetic observatory biases over 42 years, considered as contributions of the crustal field, and generally assumed to be constant in time, we noticed the link to the solar cycle, with short period variations in the order of ±10 nT. Developing an empirical method for minimizing the external fields presented in the observatory annual means, we were able to reduce them better than the used of the external field description included in the global Comprehensive Model, CM4 (Sabaka et al., 2004) was. The external field module provided by the POtsdam Magnetic Model of the Earth, POMME-2.5, (Maus et al., 2005) was considered for investigating the sources and characteristics of various external field contributions. We have been able to separate magnetospheric and ionospheric fields, as well as to remove a still significant variation with a function properly scaled by Ap and Dst indices. With this procedure the external field contributions in the annual means are minimized to an uncertainty level of ±2 nT. Our study shows a way to obtain corrected observatory annual means, in order to be used for different studies of the internal geomagnetic field.
Keywords: observatory annual means, external field, ring current, magnetospheric field, ionospheric currents, magnetic field models.
[PDF]
3. Nath, S., A. K. Mitra and S. K. Roy Bhomwik (2008): Improving the quality of INSAT derived quantitative precipitation estimates using an neural network method. Geofizika, 25, 41-51.
ABSTRACT:
In this paper an Artificial Neural Network (NN) approach has been applied to improve the quality of the INSAT derived sub-division quantitative precipitation estimates (IMD-QPE) over the Indian region for the summer monsoon season. Data for the years 2001, 2003 and 2004 have been used as the training sample. The method is tested with independent sample data for the year 2005. For the subdivisions over the domains of high orographic and monsoon low pressure system, where very rainfall occasionally occurs, different network architectures are applied to minimize the IMD-QPE errors. An inter-comparison between NNQPE (NN model output IMD-QPE), IMD-QPE and actual rainfall indicates that the pattern of NNQPE is closer to the observed rainfall distribution. The weekly mean absolute error of IMD-QPE with respect to observed rainfall, which ranges between 10–99 mm, becomes 4–70 mm in case of NNQPE. The performance statistics shows that the proposed NN model is able to produce better IMD-QPE with higher skill score and correlation co-efficient with respect to observation in most of the sub-divisions. The method is found to be promising for operational application.
Keywords:
artificial neural network, INSAT derived
QPE, weekly subdivision rainfall, orographic, skill score.
4. Ould Cherif Ahmed, A., H. Yasuda, K. Hattori and R. Nagasawa (2008): Analysis of rainfall records (1923–2004) in Atar-Mauritania. Geofizika, 25, No1, 53-64.
ABSTRACT:
The shortage of water resources in Adrar region, especially in Atar (Capital city of Adrar prefecture) and the surrounding villages along Seguelli watershed has been taken the concern of the policy maker and populations of this region since long time. This area had been suffering from recurrent droughts and faced water crises many times. Several attempts have been made to overcome these problems. Meanwhile the risk is still endangering the life and agricultural activities in one of the most important oasis areas. This study analyzed Atar rainfall time series periodicity, trends, and its relationship with Sea Surface Temperature (SST). The nonparametric Spearman test was used for trend analysis and the serial autocorrelation for persistency. Also, power spectrum and Fourier fit were deployed for analysis of frequency and periodicity. The tendency of Atar rainfall time series shows rainy periods in 1920’s and 1950’s and decreased rainfall since the late of 1950s. On the other hand, the prolonged drought periods appeared during 1970’s in contemporaneous with the Sahelian drought. The persistency analysis indicated the presence of biennial components in the annual and bimonthly rainfall in last three decades. SSTs of Atlantic, Indian and Pacific Ocean were modulating Atar rainfall during 1923–1992 period.
Keywords: Atar-Mauritania, frequency, persistence, rainfall, SST, trend.
Varia
5. Skoko, D. (2008): Adam Kugler, geolphysicist and astronomer - life and work. Geofizika, 25, 65-79 (in Croatian).
[PDF]
6.
Paviæ M. and B. Grisogono
(2008):
Letter to the editor - comment on "Highlights of the year of
physics 2005 at the
Department of Geophysics in
[PDF]