Ota Kulhánek

An array study of lithospheric structure across the Protogine zone, Värmland, south-central Sweden — signs of a paleocontinental collision

Abstract A small seismological array was installed on both sides of the Protogine Zone (PZ) in Varmland, south-central Sweden, to study the structure of the mantle lithosphere and lateral variations of its anisotropy. No distinct isotropic velocity anomalies were detected by tomography in the upper mantle around the PZ. The observed velocity variations depending on the direction of propagation can be explained by anisotropy within the subcrustal lithosphere on both sides of the suture. The best solution of a joint analysis of anisotropic inferences from teleseismic P-residual spheres and an inversion of shear-wave splitting parameters, resulted in 3D self-consistent anisotropic models of blocks of the subcrustal lithosphere. The anisotropic structures within the lithosphere are approximated by hexagonal models ( k P =5%) with low-velocity symmetry axes. The high-velocity planes dip to the E in a region westward of the PZ and to the NW eastward of the PZ. The PZ can be interpreted as a steep and narrow suture cutting the whole lithosphere and separating the two anisotropic blocks of different origin.

Recent seismicity of the East African Rift system and its implications

Abstract The seismicity of the East African Rift system and southern Red Sea is studied here. Location of earthquake epicenters in East Africa shows that there is a seismicity gap in space and time between the Main Ethiopian Rift system and the eastern rift. However, distribution of earthquake epicenters together with the energy mapping suggest a continuity of seismic activity or stress field from the Main Ethiopian Rift system to the western rift system via the southernmost rifts of Ethiopia. In general (except for some earthquakes which occurred at different complex tectonics regions) mechanisms of earthquakes studied here show dominantly normal faulting suggesting that the rift system is an extensional zone on the continent. The presence of greater focal depth earthquakes to the southern part of the rift system may indicate that softer materials at a shallower depth are present in Afar and neighboring regions than in the remaining part of the East African Rift system. This interpretation is supported by other geophysical studies (low electrical resistivity and gravity data) performed in Afar. It is also supported by low and high stress drops found for the northern part (Afar depression) and southern part of the East African Rift system, respectively.

Spatial and temporal variations of b-values along the East African rift system and the southern Red Sea

Abstract Spatial and temporal variation of b -values for the East African rift system and the southern Red Sea is studied. In general, average b -values obtained for the whole East African rift system range from 0.5 to 1.5. Mapping of b -value shows that relatively low average b -values (0.5–0.8) are observed in certain segments of the southern Red Sea, Afar Depression and north of the Western rift. If experimental laboratory works are correct then, this observation may imply that during the time interval studied here (1963–1990), parts of the northern East African rift system have been experiencing relatively high stress levels when compared with the southern part of the region. Plots of temporal variation of b -values for different segments of the rift show different behavior. These observations suggest that different rift segments are under different stress conditions which in turn may reflect the status of the ongoing rifting processes in each segment. In southern Africa, a sudden significant increase in b -value followed by a gradual decrease is observed prior to the occurrence of an intermediate-size earthquake with normal faulting mechanism. A thorough investigation of the temporal change of b -values in the southern Red Sea prior and after the occurrence of an intermediate-size earthquake with a strike-slip mechanism could not reveal such a precursory phenomena. Observations are consistent with those made by other researchers in other regions.

Causality between interplate (North Atlantic) and intraplate (Fennoscandia) seismicities

Abstract Annual seismic energy release patterns have been calculated, for the time period 1917–1987, for Fennoscandia and the North Atlantic Ridge (NAR) between 40° N and 80° N. A brief visual comparison shows the great similarity between the energy release patterns in Fennoscandia and individual segments along the NAR. To express the observed resemblance quantitatively and to provide an objective test of the phenomenon, systematic cross-correlations between seismicities along various NAR segments and in Fennoscandia were carried out. The cross-correlations obtained are statistically significant and show maximum values for time lags mostly between 0 and −3 years. Negative time lags mean that the energy release in the NAR precedes that in Fennoscandia. The closest similarity is found between Fennoscandia and the ridge segment between Iceland and Jan Mayen (66°–71° N). Periodicities of 8, 13 and 38 years are seen in the energy release from both regions. The significant cross-correlations and consistent periodicities in the temporal patterns of seismic energy release along the NAR and in Fennoscandia serve as an indication of a tectonic connection between the interplate (NAR) and intraplate (Fennoscandia) seismicities.

Recent Kattegat earthquakes — evidence of active intraplate tectonics in southern Scandinavia

Abstract On June 15, 1985, an earthquake with a local magnitude M L (UPP) value of 4.6 occurred in the Kattegat area close to the Swedish-Danish border. It was one of the largest earthquakes in Sweden and Denmark during this century. Two more events occurred in the same area: on April 1, 1986 ( M L (UPP) = 4.2), and May 24, 1990 ( M L (UPP) = 3.3). The derived focal mechanisms have north-south trending P -axes which deviate by 45° from the NW-trending compressive stress field postulated by the ridge-push theory. The mechanisms can, however, be explained by local neotectonism. Both the locations and focal mechanisms, strike-slip faulting on NW striking planes, correlate well with the dominant neotectonic feature of the region, the Skalderviken depression. Seismic moments of the 1985, 1986 and 1990 events were 3.6 × 10 14 Nm, 1.4 × 10 14 Nm and 6.0 × 10 12 Nm, respectively. The 1985 earthquake had an estimated maximum intensity of VII (modified Mercalli scale) and was felt over an area with a mean radius of 180 km. The 1986 earthquake had a maximum estimated intensity of VI and a radius of perceptibility of 100 km. Despite the recent low seismicity of the area, the earthquakes studied here indicate the potential for the occurrence of major events. This is supported by the historical seismicity.

P-wave amplitude spectra of Nevada underground nuclear explosions

SummaryShort-period vertical-componentP-wave spectra have been determined for twelve Nevada underground nuclear explosions recorded by the Swedish seismic station network. Selected events cover the magnitude range frommb=5.9 tomb=7.0 and the shot-depth range from 460 m to 1400 m. All explosion spectra show pronounced minima near 1 sec period. Within individual test areas the period at which the minimum occurs increases systematically with increasing shot-depth. This dependence makes it possible to interpret the observed minima in terms of interference betweenP- andpP-waves. One atmospheric explosion from Novaya Zemlya is analyzed to verify the inferred minima intrpretation.The minimum-period shows also a systematic linear dependence upon the magnitude. However, the physical explanation for this dependence is not evident and it may just be a consequence of a magnitude-depth relation.

Seismic Hazard of Egypt

Earthquake hazard parameters such as maximum expected magnitude,Mmax, annual activity rate,λ, andb value of the Gutenberg-Richter relation have been evaluated for two regions of Egypt. The applied maximum likelihood method permits the combination of both historical and instrumental data. The catalogue used covers earthquakes with magnitude ≥3 from the time interval 320–1987. The uncertainties in magnitude estimates and threshold of completeness were taken into account. The hazard parameter determination is performed for two study areas. The first area, Gulf of Suez, has higher seismicity level than the second, all other active zones in Egypt.b-values of 1.2 ± 0.1 and 1.0 ± 0.1 are obtained for the two areas, respectively. The number of annually expected earthquakes with magnitude ≥3 is much larger in the Gulf of Suez, 39 ± 2 than in the other areas, 6.1 ± 0.5. The maximum expected magnitude is calculated to be 6.5 ± 0.4 for a time span of 209 years for the Gulf of Suez and 6.1 ± 0.3 for a time span of 1667 years for the remaining active areas in Egypt. Respective periods of 10 and 20 years were reported for earthquakes of magnitude 5.0 for the two subareas.

Spectra of the earthquake sequence February-March, 1981, in south-central Sweden

Kulhanek, O., Van Eck, T., John, N., Meyer, K. and Wahlstrom, R., 1983. Spectra of the earthquake sequence February-March, 1981, in south-central Sweden. In: J. Duda and K. Aki (Editors), Quantification of Earthquakes. Tectonophysics, 93: 337-350. On February 13, 1981 a relatively strong earthquake occurred in the Lake V&tern region in south-central Sweden. The shock had a magnitude of M, = 3.3 and was followed within three weeks by three aftershocks, with magnitudes 0.5 i M, I 1.0. The focal mechanism solution of the main shock indicates reverse faulting with a strike in the N-S or NE-SW direction and a nearly horizontal compressional stress. The aftershocks were too small to yield data for a full mechanism solution, but first motions of P-waves, recorded at two stations, are consistent for the aftershocks. Dynamic source parameters, derived from Pg- and Sg-wave spectra, show similar stress drops for the main shock (2 bar) and the aftershocks (1 bar), while the differences in seismic moment (1.5.1020 resp. 4.10’* dyne cm), fault length (0.7 resp. 0.2 km) and relative displacement (0.15 resp. 0.03 cm) are significant.

P-wave velocity anomalies in the earth's mantle from the Uppsala array observations

SummaryDistribution of compressional-wave velocities in the mantle is determined fromdT/dΔ measurements using the Uppsala seismograph array station (UPSAS). Short-period vertical-component seismograms from 181 events in the epicentral distance range 16°–100° have been used. The velocity distribution shows anomalous variations at depths of 750, 1500, 1800, 2300 and 2550 km. Evidence of lateral heterogeneity beneath the northern part of the Asian continent, in the depth range 1700–2300 km, is discussed. Computed travel times, based on this velocity-depth relation, are tested by an examination of travel-time residuals from the Long Shot and Milrow explosions on Amchitka, Aleutian Islands.

Source parameters of earthquakes in and around the Korean Peninsula deduced from spectral analysis

Abstract Spectral analysis of > 200 seismograms from seven moderate-magnitude earthquakes in the southwestern Sea of Japan, the Korean Peninsula and the Yellow Sea is performed to determine their source parameters and to discuss their tectonic implications. Short- and long-period records of body and Rayleigh waves are employed. The deduced seismic moments vary from 5.9 × 1023 to 2.9 × 1025 dyn cm, and the stress drops vary from 8 to 56 bar. Generally, rather low stress drop values are obtained, which suggests that materials beneath the studied region are highly fractured and softened by the high heat flow related to the opening of the Sea of Japan and the rise of the North China Basin.