Ronald Arvidsson

Fennoscandian Earthquakes: Whole Crustal Rupturing Related to Postglacial Rebound

Local and regional earthquake locations provide seismic evidence that large shield earthquakes have occurred in northern Fennoscandia. These paleoearthquakes, with fault lengths of up to 160 kilometers and average displacements of up to 15 meters, were triggered by nonisostatic compressive stresses caused by the removal of the ice at the end of the last deglaciation. The Fennoscandian faults were probably formed by single events that ruptured through most of the crust. The largest event, moment magnitude MW ≈ 8.2, was larger than other known stable continent earthquakes outside failed rifts or extended crust.

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 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.

Plate tectonic framework for the October 9, 1996, Cyprus Earthquake

We determine the centroid depths for the Mw=6.8, October 9, 1996, Cyprus earthquake and its largest after-shock to 32 and 27 km, respectively, by modeling P and SH waveforms. These depths are consistent with shallow subduction as the African plate, west of Cyprus, penetrates beneath the Anatolian plate. We calculate the pole of rotation between Anatolia and Africa and determine their relative motion. The result indicates that, along the Cyprean Arc, convergence between the African and Anatolian plate, in agreement with the observed seismicity, decreases from west to east. The northeasterly striking nodal plane is suggested as the plane on which the Cyprus earthquake ruptured since its strike agrees well with the derived plate motion vector. The faulting geometry suggests the emergence of a strike-slip tear fault within the African plate separating the continental Eratosthenes seamount south of Cyprus from the oceanic African lithosphere that subducts west of Cyprus.

Fault mechanisms and tectonic implication of the 1985–1987 earthquake sequence in south-western Ethiopia

Integrated inversions of short-period P, broadband P, and long-period P &s waves are done for fault mechanisms, focal depths, seismic moments, and source-time functions from the largest four earthquakes of the 1985 and 1987 earthquake sequence in south-western Ethiopia. These earthquakes had similar normal-faulting mechanisms. The general trends of the fault planes follow the Main Ethiopian Rift which is in agreement with foreshock-aftershock distribution, surface breaks and geology. Despite the morphological discontinuity of the Main Ethiopian Rift at its southern tip, the mode of deformation of the continental crust under study shows its extension southward. There are no significant strike-slip components trending NW–SE in all the mechanisms which would have been associated with the Aswa Fault Zone in southern Sudan or Anza Rift in northern Kenya. We also infer that the relatively broad fracture zone at the southern extreme of the Main Ethiopian Rift demonstrates the early stage of the break-up between the Nubia and Somalia plates in comparison with the Main Ethiopian Rift proper and the Afar Depression. The main shock of the sequence (Mw = 6.3) ruptured at a depth of 6.8 km, shallower than expected since the depth of earthquakes generally increase southward from the Afar Depression. The shallow depth of earthquake occurrence is supported by surface deformations with an overall trend in the direction of the Main Ethiopian Rift.

The 1992 Cairo earthquake: A case study of a small destructive event

On 12 October 1992, an earthquake, magnitude mb = 5.9 and M s = 5.2, hit the City of Cairo, Egypt. It was this centurys largest earthquake in northern Egypt with related destruction in the City of Cairo, the Nile Valley and the Nile Delta areas. Our source parameter determinations show that the 1992 earthquake had a normal faulting mechanism, seismic moment M o = 5.2 × 1017 Nm, centroid depth of 23 km and a source time function duration of 3 seconds. The mechanism is compared with those corresponding to two other events that occurred in the northern Red Sea. The similarity between the mechanisms as well as the spatial distribution of the geological faults around Cairo suggest seismic activity along the extension of the stress field of the Red Sea rift system to the area around the City of Cairo. This situation affects the level of seismic hazard in the Cairo area. The 1992 earthquake belongs to an unusual class of relatively small, M w > 6.0, yet destructive earthquakes. The damage caused by these events is usually attributed to their shallow focal depth, ≤ 5 km, and to amplification of seismic waves in the local soil beneath the damaged structures. However, the Cairo earthquake deviates from other events of this class since the focal depth was determined to be 23 km. We calculated synthetic accelerograms for the 1992 earthquake with the loose sediments observed in the Nile Valley, and show that this enhanced the amplitude of the acceleration by a factor of two. However, the determined accelerations, about 0.5 m s-2, cannot alone explain the relatively large amount of damaged structures. Hence, a major cause to the destruction is likely the poor state of construction of the Cairo buildings.

Enhancement of seismic electric signals using magnetotellurics

Abstract During two field studies carried out in Greece in 1986 and 1987–1988, electric precursors, superimposed on the telluric field, and the time dependence of the magnetotelluric apparent resistivity, were studied using the magnetotelluric method applied in the frequency band of 0.00028–0.5 Hz. The recognition of any transient electric signal in the telluric field was improved by reducing the main part of the magnetotelluric induction by making use of a residual electric field. The residual field is defined as the difference between the measured electric field and the field which is calculated by multiplying the measured magnetic field with the magnetotelluric impedance tensor. In concordance with a previous study, this method is shown to be rather effective. One conclusion from our study is that it is practical to employ the residual field method in a continuous daily analysis within an earthquake prediction research experiment. We have demonstrated that suspected electric transients are revealed as anomalies of non-magnetic origin. After a careful analysis, in three cases significant transient electric signals were classified as suspected seismic electric signals. They were clearly resolved on the residual field and all correlated well in time with three earthquakes occurring close to the stations during the time of recording.

Foreshocks and aftershocks of the MW = 7.1, 1992, earthquake in the Atrato region, Colombia

We study the October 18, MW = 7.1, 1992 Atrato earthquake, and its foreshocks and aftershocks, which occurred in the Atrato valley, northwestern Colombia. The main shock was preceded by several foreshocksof which the MW = 6.6, October 17 earthquacke was the largest. Inparticular, we examine foreshocks and aftershocks performing joint-hypocenter relocations using high quality Pn and Sn wave readingsfrom permanent regional networks. We observed a few hours prior to the main shock a sudden increase of foreshocks. Maybe this could be used as a predictor since foreshocks have been known for other major events in the region. Our locations align for 90 km with a trend of 5° ±4° in agreement with the Harvard CMT solution showing the faultplane trending 9° to be the plane of rupture. In relation to theepicenter of the main shock, maximum intensities were located to thesouth, consistent with a rupture that traveled from north to south witha larger energy release in the south as suggested by an empirical Greensfunction study (Li and Toksöz, 1993; Ammon et al., 1994). The boundarybetween the Panama and North Andes blocks has been placed close to thePanama-Colombia border as either a sharp boundary or a diffuse zone. TheAtrato earthquake, however, shows that the plate boundary between thePanama and North Andes microblocks is a diffuse deformation zone. Thiszone has a width of at least 2° stretching from 78°W to 76°W. Quantification of earthquake moment release (during the past30 years) in this zone shows a similar amount of moment release in thewestern and eastern parts of this zone.