H. Korhonen

Crustal structure of the baltic shield based on off-FENNOLORA refraction data

Abstract Since the early 1960s, deep seismic sounding experiments have been carried out on the Baltic Shield. In this study, we will mainly concentrate on the results obtained from two international profiles. Sveka and Baltic, carried out in Finland in 1981 and 1982. Results from these profiles are shown and discussed, and compared with those obtained from the FENNOLORA and from the other recent refraction profiles of the Baltic Shield in Fennoscandia. According to the results from Sveka and Baltic, and average crustal velocity is 6.6–6.7 km/s, which is rather high. Several distinct reflection boundaries have been found within the crust. In the lower part of the crust, a high-velocity layer with a P-wave velocity of 7.0–7.5 km/s has been found in some cases. In addition, the results indicate that the crustal structure has a clear block-like character, different blocks being separated from each other by deep fractures. The crustal thickness in the Baltic Shield is about 45 km on average, whereas around the Ladoga-Bothnian Bay zone in Central Finland, it is about 10 km thicker than this. Thus, there is a large-scale depression in the Moho boundary in the central part of the Baltic Shield.

Fennoscandian dynamics: Present-day earthquake activity

Abstract The Fennoscandian earthquake catalogue is constantly updated and revised, and we present the best available map of earthquake activity. It exhibits significant differences between areas of high and low seismicity. The focal mechanisms have been determined for many of the earthquakes. The total set of these focal mechanisms shows that the stress pattern is very consistent. The horizontal deviatoric stresses are characterized by a grouping in the NW-SE directions for the maximum compression. This stress field is not distorted even by the significant crustal inhomogeneity in the Fennoscandian Border Zone. This orientation of the maximum horizontal compression is precisely that expected from ridge push forces from the North Atlantic Ridge. It is not consistent with the stress field expected from postglacial uplift after the last Ice Age. However, recently, large neotectonic faults have been identified and studied in northern Scandinavia, and these are taken as indicators of large-scale displacements shortly after the end of the last Ice Age. Seen in parallel with similar studies in Scotland, this shows that the stress field has changed drastically with time. The present-day uniform stress field and the relatively scattered distribution of observed earthquakes lead to the conclusion that the earthquakes concentrate in specific zones of brittle, weak crust. We claim that arguments based on tectonics must be dependent on the specific tectonic characteristics of each area separately, and this remains the outstanding problem of Fennoscandian earthquake activity as observed presently.

Earthquakes and seismic hazard in Finland

Abstract In this paper the seismicity of Finland has been re-evaluated on the basis of a new regional magnitude scale, which gives somewhat lower magnitude values than the previously used macroseismic scale. Macroseismic magnitudes for Finnish earthquakes have been calculated by using the value 4.0 for the coefficient of attenuation, and the value 2.0 for the lower limit of intensity at the boundary of the felt area. Using these parameters, we obtained macroseismic magnitudes comparable to those of the instrumental ML-scale. The distribution of seismicity in Finland is presented in an epicentral map and maximum-intensity maps (1880–1980). These maps indicate some regional variation of seismic activity. The seismicity was also considered as a function of time using strain release curves. The b-value 0.59 was evaluated from the log N(M) relationship for the last hundred years. The time variation of b-values was also studied. Estimation of magnitude and intensity extremes by Gumbels method yielded the value 5.0 for the upper limit of magnitude and VI-VII for the upper limit of intensity. The return periods for different magnitudes and intensities were calculated as well as the probabilities that they are exceeded.

Observations of Fennolora Shots on Additional Lines in Finnish Lapland

ABSTRACT The additional field work in Finnish Lapland in connection with the Fennoscandian Long-Range Project is reported and some record sections made by computer for the profiles are presented. According to a preliminary model derived the average velocity in the crust is about 6.6 km/s, in the uppermost mantle about 8.4 km/s, and the thickness of the crust is 49 km.

The structure of the Baltic Shield region on the basis of DSS and earthquake data

The lithospheric models obtained for the Baltic Shield by using deep seismic soundings are discussed and results from different parts of the shield are compared with models achieved by the investigation of surface waves and of P to S converted waves. The results are found to agree rather well with each other particularly with regard to the first interface (at a depth of 10–15 km) and the whole thickness of the crust (c. 40 km). The macroseismic focal depth determinations of regional earthquakes are correlated with lithospheric structure. The main maximum in the focal depth distributions of Finnish earthquakes occur at a depth of 10–12 km. The geographical distribution of the earthquake epicentres suggests several seismo-active zones in the northern Baltic Shield. One new finding in this field concerns the Lapland zone, which runs in a north-westerly direction.

Tomographic study of crustal velocity structure in southern Finland

Abstract An inverse tomographic problem of the construction of a 3-D velocity structure has been solved for depths of 10, 20, 30, and 40 km using the P-wave arrivals registered by the seismic array in southern Finland and on the DSS lines Sveka (1981) and Baltic (1982). The interpreted results verify and enlarge DSS data obtained earlier for a high-velocity structure of the crust and explain some small lateral velocity variations. The concordant tomographic and DSS data specify a 3-D model for a complex geological structure in the region.

Earthquakes in Finland During the Last Hundred Years

ABSTRACT In the present seismicity and seismic risk studies only the events for the last hundred years were used because of the incompleteness of earlier data. The geographical distribution of earthquakes is presented in the epicentral and maximum intensity maps and indicates some seismically active areas. In addition, seismicity is considered as a function of time on the basis of a strain release curve. For the magnitude-frequency relationship the parameter b was computed, the mean value being 0.52 for the whole data and its variation in time and space 0.4 – 0.7. Estimates for return periods of magnitudes and upper magnitude limit yielded the value 5.7 for maximum magnitude and a return period of 70 years for an earthquake of M = 5.5.

Some Aspects of the Seismicity in the North Anatolian Fault Zone

A data file is compiled for studies of seismicity, seismic risk and earthquake prediction in the North Anatolian fault zone region. It contains 522 earthquakes occurred in and near the Anatolian fault zone in Turkey during the last 60 years. This file is used for statistical estimation of magnitudefrequency relationship, the upper magnitude limit and return periods for various magnitudes. The cumulative strain release pattern based on the file is also presented in time and space. In addition, the intensity distribution of earthquakes felt in the Anatolian fault zone and adjacent areas is considered. Maximum intensity map is computed on the basis of observed and estimated intensities.