Klaus Meyer

Anomalous electrotelluric residuals prior to a large imminent earthquake in Greece

Abstract Periodic anomalies, period 24 h, have been observed in the electrotelluric field prior to a large imminent earthquake in Greece. The periodic anomaly is modulated by an exponential term which implies that successive peaks increase toward the time of the impending earthquake. It is shown that the amplitudes of the periodic variations are much bigger than those accompanying changes of the geomagnetic field.

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.

Efficiency test of earthquake prediction around Thessaloniki from electrotelluric precursors

Abstract Since the completion of the network in January 1983, the electric field of the earth has been continuously monitored at four sites near Thessaloniki, the capital of northern Greece. From the present study and from previous investigations by similar measurements in Greece, it is evident that transient changes of the electrotelluric field occur prior to earthquakes. The analysis of these electric forerunners leads in many cases to a successful prediction of the epicentral area, the magnitude and the time of the impending event. Predictions prior to regional earthquakes are issued and documented with telegrams. From November 1983 until the end of May 1984 twelve earthquakes ( M L > 3.5 ) occurred in the vicinity of Thessaloniki. Ten of these were predicted and warnings given by telegram, whereas two smaller seismic events were missed. Two additional predictions were unsuccessful. Independent of their magnitudes, predicted events took place within a time window of 6 hrs to 6 days after the observations of the electrotelluric anomalies. The accuracy of the predicted epicenters in eight cases is better than 100 km, which corresponds roughly to the mean distance between the electric stations. Magnitude estimates deviate by less than 0.5 magnitude units from the seismically observed ones. Considering the two largest earthquakes, it is shown that the probability of making each of these predictions by chance is of the order of 10−2.

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.

Stress migration in the north atlantic and intraplate seismicity in Scandinavia—a proposal

Abstract Recent discussions about the causes of intraplate seismicity in Scandinavia suggest that remnant stresses in the lithosphere due to stress accumulation over geological times may have minor impact on the seismicity in Fennoscandia. An alternative explanation is preferred, implying that the driving forces of plate motions are responsible for the intraplate seismicity in Scandinavia. It is suggested that stresses at the nearest plate margin (North Atlantic Ridge) are connected to intraplate stresses (Scandinavia, North America) by stress migration (diffusion), following model of stress continuation in the lithospheric plates. Thus, it may be assumed that seismic energy release along the plate margin is also correlated with intraplate seismicity taking place at intraplate boundaries, regional fracture zones or rheological zones of weakness in general.

Electrotelluric forerunners to earthquakes in Kamchatka

Abstract Electrotelluric field (ETF) data observed at Shipunski-station (SPN), Kamchatka, were studied in detail for a period of 10 months, from August 1982 to June 1983. During this time interval seven large earthquakes, M ≥ 5.5, took place in the vicinity of Cape Shipunski, within 180 km of the SPN-station. A large earthquake, M = 5.7, took place on November 14, 1982. This event was preceded by a period of seismic quiescence of about 3 months and it took place at an epicentral distance of 74 km from SPN, in the oceanic part of the lithosphere. A striking change in the electrotelluric potential (ETP) data for November 8 is observed, 6 days before the occurrence of the earthquake. The change in the ETP is about 200 mV/km at a signal-to-noise ratio of roughly 4. The independent parallel line at SPN reveals the same anomalous ETP change, thus excluding instrumental and/or electrode effect. The possibility of a local source can be excluded after consideration of electric power sources in the area. On the other hand the ETF anomaly is rather similar to anomalies observed in Greece prior to the Kefallina earthquake of January 17, 1983. The ETF data are correlated with geomagnetic recordings from a station at Petropavlovsk (PRT), located about 90 km from Cape Shipunski. From the correlation of the two fields we conclude that the changes in the geomagnetic field cannot produce the sudden changes in the electric field if the electric conductivity is constant. Spectral analysis of the ETF data shows that the anomalies contain predominant periods of 24 h, 12 h, 8 h and 6 h, thus resembling the typical main spectral components of geomagnetic field changes. In spite of the apparent independent behaviour of the electric and geomagnetic fields during the anomalous period, we propose that electromagnetic induction is the primary cause of the electric anomaly, accompanied by rapid changes in the transfer functions between the two fields. This means changes in the electromagnetic induction after large, rapid changes of the resistivity in the fractured zone of the forthcoming earthquakes, initiated by the percolation of an interstitial fluid at a critical state of the fracturing process.

Observation and qualitative modelling of some electrotelluric earthquake precursors

Abstract The appearance of strong periodic anomalies, with a period of 24 h, in the electrotelluric field before two large imminent earthquakes is discussed. The precursor occurs as a periodic component superimposed on the large variation of potential difference which starts a couple of days before the earthquake. An attempt is made to find an appropriate geophysical model for the anomaly. It is suggested that changes in electromagnetic induction may take place, as a result of resistivity changes in the earthquake fault area. We account also for possible anisotropy effects of the resistivity response to a stress system. Anisotropy effects related to the T-axis of the fault-plane solution are expected, as resistivity changes are related to opening and closing of cracks and to water transport between cracks. These phenomena are highly sensitive to stress orientation. Consequently, any model of resistivity variations can be modified according to anisotropy of a cracked medium. Thus, we consider sectors of dilatancy and compression; dilatancy in dry or partly saturated rocks leads to a gradual resistivity increase which is followed by a sharp decrease (Fig. 1). This decrease is due to a percolation threshold which can be related to formation of a fracture plane. In saturated rocks the situation may be apparently different. In that case we can expect water to be squeezed out from a sector of compression, and the resistivity of a rock complex would consequently also change before an earthquake. Resistivity anisotropy effects are also expected and are related to the P-axis direction. Thus, changes in the electromagnetic induction are related to the orientation of the fault plane, and corresponding changes (anomalies) in the telluric field have preferred geometrical directions. The apparent resistivity response to resistivity changes in a narrow fault zone demonstrates local extreme values along the zone (Fig. 2). Extreme changes in the electromagnetic induction (and hence in the telluric field) are strongly dependent upon the location of the recording site (compare, for instance, the situation at recording sites A and B in Fig. 2).

High-velocity migration of large earthquakes along the Azores-Iran plate boundary segment

Between 1 January 1980 and 28 July 1981, a series of large earthquakes with body-wave magnitudes around 7, took place along the western segment of the Alpide belt. The sequence started in the Azores and migrated eastward along the belt at a rate of about 4400 km/yr with consecutive large events in northern Algeria, southern Italy, southern Greece and Iran. Two different methods are employed to identify similar series and corresponding migration velocities during earlier time periods of this century. The data set used contains all earthquakes with body-wave magnitudes larger than 6.3 and covers the time interval 1901–81. The concept of linear migration is tested for eastward and/or westward propagation, considering high migration velocities from 1600 to 11 000 km/yr. Results obtained are not homogeneous with respect to the two opposite migration directions, west-east and east-west, and we interpret this as a net drift of earthquake activity from the west to the east. Our efforts here are concentrated on analysis of observational data and on estabilishing the uniqueness of migration patterns. Because of the complexity of the tectonic system in question, we did not attempt to establish a mechanism explaining the migration of the observed earthquake sequences.

The Pn velocity in a dipping Moho

Abstract A method of computing the upper-mantle and the lower-crust P- wave velocities from the apparent P n velocities in two opposite directions is presented and exemplified.

Source processes of the 1981 Gulf of Corinth earthquake sequence from body-wave analysis

On 24 February 1981 (20 h 53 m), an offshore earthquake of magnitude M, = 6.7 occurred in the eastern part of the Gulf of Corinth, Greece. So far, it is the largest earthquake that has taken place in the area during this century. The shock caused considerable damage in coastal population centers on both sides of the Gulf. Sixteen people lost their lives and several hundred were injured, mostly by falling objects. The main shock was followed by two large aftershocks of magnitude M, = 6.4 on 2.5 February (02 h 35 m) and 4 March (21 h 58 m). These aftershocks caused additional severe damage in areas to the east of the main shock epicenter and were accompanied by surface faulting. Hypocenter parameters of the three shocks are listed in Table 1. Teleseismic long-period body-waves from the main shock on 24 February and its two principal aftershocks are studied to determine source characteristics. The stereographic projections of P-wave first motions for the three shocks are shown in Fig. 1. It is obvious that most of the available teleseismic observations are dilatational motions, indicating that the focal mechanisms are dominated by a 45” dip-slip