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Statistical analysis on yearly seismic moment release data to demarcate the source zone for an impending earthquake in the Himalaya

Mukhopadhyay B., Acharyya A., Dasgupta S.

Acta Geophysica

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2009

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Vol. 57, no. 2

387-399

EN

Tectonism in the Himalayan fold-thrust belt had generated great earthquakes in the past and will spawn more in the future. Sequential cumulative moment release data of macroearthquakes (Mb ≥ 4.5) over the years 1964–2006 in four zones of the Himalaya was analysed by nonparametric RUD method. The Z values of RUD analysis had neither rejected nor supported the null hypothesis of randomness. However, the Hurst analysis and plot, a statistical procedure to identify clustering of low and high values in a time series, brought out a pattern for earthquake prognostication. The pattern was a negative sloping segment representing a sluggish moment release over years, followed by a positive sloping segment indicating a sudden high moment release with occurrence of medium/large size earthquake(s). In recent past, such a negative sloping has been found in Zones B (1992–2006) and D (1998–2006), indicating an impending moderate/mega earthquake event in near future.

2

Source parameters of the 2004 Kaliningrad earthquakes

Domański B.M.

Acta Geophysica

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2007

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Vol. 55, no. 3

267-287

EN

An analysis of source parameters of the two unexpected earthquakes from the Kaliningrad (Russia) area is presented. The earthquakes occurred on 21 September 2004 at 11:05:01 and 13:32:31 UT, respectively. The first event was located at the latitude φ= 54.924°N and the longitude λ= 20.120°E, with a depth h = 16 km, and the second event at φ= 54.876°N, λ= 20.120°E and h = 20 km. Magnitudes Mw of the two events were very similar: 5.1 and 5.2. The magnitude values reported by various international data centers have been meaningfully different. The reason is the presence of high-frequency components in Z velocity component of the S wavefield. They were observed along the direction defined by two stations, BLEU in Sweden and SUW in Poland, located in opposite sides of the source. Along the direction perpendicular to it, the effects are relatively very small. The high-frequency waves are understood to mean components in the 6-8 Hz band for event 1 and 2-4 Hz for event 2. The effects in question are also clearly visible on displacement spectrograms. The magnitude values calculated at such stations from S-wave amplitudes or from seismic spectra are clearly overestimated and are close to 6. Therefore, we made a careful selection of channels in order to determine the spectral parameters and, on this basis, the source parameters. The size of the source is relatively small, of about 2 km. The closest seismic station is at 100 source radii from the source. One can clearly see the effect of the TT zone which markedly reduces the seismic moment value for seismic stations laying on the opposite sides of the source. Both events have very similar spatial dis-tributions of the source parameters: magnitude, seismic moment and radius.

3

Stress release during earthquake sequences

Gibowicz S.

Acta Geophysica Polonica

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2004

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Vol. 52, nr 3

271-299

EN

The average stress drop and apparent stress released during 65 complex earthquake sequences from all over the world are estimated separately for foreshocks, main shocks and aftershocks. All of them but one occurred between 1977 and 1997. The Harvard CMT solutions are available for 60 main shocks, 38 foreshocks and 218 aftershocks. The stress drop is estimated using the values of seismic moment and source half duration and the scaling relations between various source parameters: seismic moment, source half duration, fault surface and fault length, taken from Harvard catalogues and other publications. The average stress drop is the highest during foreshocks (1.8 MPa), middle during main shocks (1.1 MPa) and the lowest during aftershocks (0.4 MPa). Similarly, the apparent stress, estimated from seismic moment and radiated energy taken from PDE-NEIC bulletins and other publications, is also the highest for foreshocks (0.6 MPa), middle for main shocks (0.4 MPa) and the lowest for aftershocks (0.2 MPa).

4

Scaling and statistical trends of the apparent stress for heterogeneous seismic sources

Senatorski P.

Acta Geophysica Polonica

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2003

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Vol. 51, nr 2

125-134

EN

The apparent stress can be understood as the correlation integral of the slip velocity field over an earthquake rupture area (Senatorski, 2003). This means that the apparent stress is a macroscopic function, which can be expressed as an average of microscopic quantities describing details of the rupture process. Here it is shown, using this statistical mechanics approach and a view of the slip velocity pulse-like rupture propagation, that the apparent stress can be formulated as a function of three other macroscopic parameters - the seismic moment, the rupture area and the slip acceleration 0 treated as independent variables. Moreover, the scaling relationship for these quantities is derived. This relationship is used to explain statistical trends of the apparent stress and other macroscopic earthquake parameters observed in earthquake populations.

5

Random fluctuations of the apparent stress among heterogeneous seismic sources

Senatorski P.

Acta Geophysica Polonica

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2003

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Vol. 51, nr 1

23-36

EN

The apparent stress, (tau)a, defined as the ration of seismic energy, Es, and seismic moment, M0, has been formulated as the average stress associated with radiation resistance of a sliding fault during as earthquake. The over damped dynamics approximation of a seismic source implies that the seismic energy rate at a given time is proportional to the square of the slip velocity integrates over the rupture area. This result allows us to interpret the apparent stress as a correlation integral of the slip velocity field over space and time. Consequently, other macroscopic parameters, such as momentary and local apparent stress, (tau)m and (tau)l, are proposed to characterize spatial and temporal heterogeneity of complex seismic sources. This approach is used to understand fluctuations of the apparent stress, and other macroscopic parameters in earthquake populations, in terms of their microscopic representation.

6

Radiated energy scaling for seismic events induced by mining

Gibowicz S.

Acta Geophysica Polonica

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2001

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Vol. 49, nr 1

95-111

EN

Seven sets of the values of radiated energy from 987 seismic events from two South African gold mines, four Polish coal and copper mines, and from the Underground Research Laboratory (URL) in Canada are used to examine their scaling with seismic moment, apparent stress and average displacement. Moment magnitude of these events ranges from -3.6 to 3.8, the seismic energy ranges from 1x10(-3) to 1x10(11) J, and the apparent stress is between 0.0025 and 5 MPa. The slope of regression straight line between the logarithm of radiated energy and that of seismic moment is very close to 1.5 in all cases, which seems to have theoretical justification. The average values of seismic energy, however, referred to the same range of seismic moment, are about 30 times higher for seismic events from South African hard-rock mines that those observed from events in Polish mines. Similarly, the slope of a linear regression between the logarithm of apparent stress and that of energy is fairly close to 1/3 for the events from South African and Polish mines. But the average apparent stress, referred to the same range of seismic energy, is about 10 times higher for the events in South Africa than for those in Poland. The relations between the average slip and the energy are on a logarithmic scale surprisingly consistent and more regular than similar relations based on seismic moment. The events from the URL follow the same trend as the events from South Africa, whereas for the events from Poland the average slip is about 5 times lower than that for the other events.