Earthquakes in Switzerland and surrounding regions during 2012

This report of the Swiss Seismological Service summarizes the seismic activity in Switzerland and surrounding regions during 2012. During this period, 497 earthquakes and 88 quarry blasts were detected and located in the region under consideration. With a total of only 13 events with M_L ≥ 2.5, the seismic activity in the year 2012 was far below the average over the previous 37 years. Most noteworthy were the earthquake sequence of Filisur (GR) in January with two events of M_L 3.3 and 3.5, the M_L 4.2 and M_L 3.5 earthquakes at a depth of 32 km below Zug in February and the M_L 3.6 event near Vallorcine in October. The epicentral intensity of the M_L 4.2 event close to Zug was IV, with a maximum intensity of V reached in a few areas, probably due to site amplification effects.     

Historical intensity VIII earthquakes along the Rhone valley (Valais, Switzerland): primary and secondary effects

In recent years the upper Rhone Valley has been one of the most intensively investigated regions by the Swiss Seismological Service. The high seismicity in the region encourages research in the seismological field and one main focus has been historical seismology. This report presents the state of the art of our historical investigations by giving an overview of the effects of four damaging earthquakes with intensity larger than VII, for which a fairly large number of documents could be found and analyzed. The overview includes the events of 1584 (Aigle, epicentral intensity VIII), 1755 (Brig, epicentral intensity VIII), 1855 (Visp, epicentral intensity VIII), and 1946 (Sierre, epicentral intensity VIII for the main shock and intensity VII for the largest aftershock). The paper focuses mainly on primary and secondary effects in the epicentral region, providing the key data and a general characterization of the event. Generally, primary effects such as the reaction of the population and impact on buildings took more focus in the past. Thus building damage is more frequently described in historic documents. However, we also found a number of sources describing secondary effects such as landslides, snow avalanches, and liquefaction. Since the sources may be useful, we include citations of these documents. The 1584 Aigle event, for example, produced exceptional movements in Lake Geneva, which can be explained by an expanded sub aquatic slide with resultant tsunami and seiche. The strongest of the aftershocks of the 1584 event triggered a destructive landslide covering the villages Corbeyrier and Yvorne, VD. All macroseismic data on the discussed events are accessible through the webpage of the Swiss Seismological Service (http://www.seismo.ethz.ch).     

Earthquakes in Switzerland and surrounding regions during 2009

This report of the Swiss Seismological Service summarizes the seismic activity in Switzerland and surrounding regions during 2009. During this period, 450 earthquakes and 68 quarry blasts were detected and located in the region under consideration. The three strongest events occurred about 15 km NW of Basel in southern Germany (M_L 4.2), near Wildhaus in the Toggenburg (M_L 4.0) and near Bivio in Graubünden (M_L 3.5). Although felt by the population, they were not reported to have caused any damage. With a total of 24 events with M_L ≥ 2.5, the seismic activity in the year 2009 was close to the average over the previous 34 years.     

A theory for the solar type-I radio continuum

The type-I radio continuum may arise from the combination of two electrostatic waves, both directed nearly normal to the magnetic field. One wave, near the upper-hybrid frequency, is generated by gyroresonance with superthermal electrons and comes into equilibrium with these electrons. The other wave, at the lower-hybrid frequency, is generated by the loss-cone instability of trapped superthermal protons in those wave directions for which the lower-hybrid frequency is an exact multiple of the proton gyrofrequency. The brightness temperature of the continuum indicates both the energy of the superthermal electrons and the existance of at least a small number of superthermal protons.     

Freshwater Phytoplankton Bloom in Visovac Lake - A Possible Cause of Benthic Mortality in Krka Estuary (Adriatic Sea, Croatia)

Abstract. In late summer and early autumn of 1989 benthic mortality in the upper part of Krka estuary extended from the Skradinski buk waterfalls to approximately 5 km downstream. Dead benthic organisms were found on the bottom at depths of up to 4.5 m. During August, a freshwater phytoplankton bloom was recorded in Visovac lake (which supplies the Skradinski buk waterfalls). The green tide in the lake was composed of up to 11.9 million cells. 1^--¹, predominantly of Synedra acus and Dinobtyon sertularia . At that time the maximal total inflow of phytoplankton cells through the waterfalls was approx. 0.41 S^-I (38000 1. d^-1). Cells of these two species died after reaching the salt wedge and the cummulative effect of their decomposition apparently caused an oxygen demand sufficient enough to trigger the benthic mortality. Due to the water mass dynamics, the spreading of mortality downstream was slow (from August to October, approx. 5 km). Certain benthic fishes (like Gobius sp.) were observed on the bottom of the affected area in mid-October 1989.     

The earthquake of 250 a.d. in Augusta Raurica, A real event with a 3D site-effect?

The Roman city Augusta Raurica is located East of Basel, Switzerland. One important topic of the city’s history concerns the hypothesis of an earthquake striking the city in the middle of the third century a.d. This idea had been formulated according to archaeological features and findings, but had not been tested so far. A selection of the archaeological features were reviewed and dated in order to test the hypothesis of a single event. However, archaeological investigations do not draw a conclusive picture; it could not be proven that all features of possible destruction date to the same event. Detailed seismological investigations were performed. These included geological and geotechnical mapping of the unconsolidated sediments. Important parameters such as the thickness and composition of the unconsolidated sediments, the terrain topography and the topography of the bedrock surface were mapped. Ambient vibration H/V measurements provided the fundamental frequency of resonance for the unconsolidated sediments. The velocity of shear waves traveling through sediments is the controlling parameter for amplification of seismic waves. This material property is estimated using the relation between the ellipticity of the fundamental mode Rayleigh wave and the H/V curve. From all information we compiled a three-dimensional model of the surface geology. This model is used to simulate earthquake ground motion and amplification effects in the city, and to map the variability of the amplification. In the part of the city where possible earthquake damage was recognized, amplification occurs in the frequency band of building resonance (2–8 Hz). In the other part of the city amplification occurs much above the building’s resonance. From 1D modelling we estimate a difference in spectral amplification of about a factor of 2.5 to 3 between the two parts of the city. This corresponds approximately to a difference in macroseismic intensity of one unit. 3D modelling showed a large variability of ground motion within very close distance in the part of the city where possible earthquake damage was recognized. The maximum amplification reaches values up to a factor of nine, which is due to 3D effects and the choice of using vertically incident waves. Finally, all paleoseismological findings in the area of Basel were reviewed in order to find indications of a large event in the time-period of interest. Paleoseismological findings provide no hints to a large earthquake in the third century. If we assume that an earthquake caused at least part of the identified damage in Augusta Raurica, we have to assign to this event a magnitude Mw of about 6.0 or even lower, that is much smaller than the value of 6.9 that is actually in the Swiss earthquake catalogue. The earthquake source of this event must then be very close to the site of Augusta Raurica and a strong site-effect occurred in one part of the city.     

3D Hybrid Ray-FD and DWN-FD Seismic Modeling for Simple Models Containing Complex Local Structures

Hybrid approaches find broad applications wherever all-in-one modelling of source, path, and site effects is too expensive. Our new 3D hybrid approach allows to compute the seismic wavefield in elastic isotropic models containing a complex local structure embedded in a large, but considerably simpler, regional structure. The hybrid modelling is realized in two successive steps.In the 1^st step, the ray or discrete wave number (DWN) method is used to compute the seismic wavefield due to the source and simple regional structure. The complex local structure is not present. Thus, the excitation contains the source and regional path effects. The time history of this wavefield (excitation), recorded at the points of so called excitation box, is stored on a disk. The excitation box envelopes a small portion of a computational domain.The 2^nd step of the hybrid method, now containing the complex local structure, is computed by finite differences (FD) inside the excitation box and its close vicinity. The excitation from the 1^st step is now used to inject the 1^st step wavefield into the 2^nd step computation. After that, the hybrid combination of the 1^st and 2^nd steps contains the source, regional path, and local structure effects at reasonably lower computational costs than in case of all-in-one modelling.The 3D ray-FD method is tested on models in which the locally complex structure is the well-known Volvi lake basin, embedded in various 1D structures. The wavefield is excited by the point source situated outside the basin. Although the structure outside the excitation box may be less dimensional (2D, 1D, homogeneous), the whole problem is actually 3D due to the 3D features of the structure inside the excitation box, 3D shape of the excitation box, and arbitrary source — excitation-box configuration. Simple (1D) structures outside the excitation box allow for comparison with the alternative hybrid DWN-FD results. However the ray method is suitable for computation of 3D regional structures outside the excitation box. The results from both approaches show a very good agreement for realistic crustal and local structural models.     

Simulated Ground Deposition Of Fine Airborne Particles In An Array Of Idealized Tree Crowns

Wind-tunnel experiments were used to investigate the ground deposition of fine airborne particles in an array of idealized tree crowns. The particle ground deposition was modelled with a gaseous tracer instead of solid particles, which is an approach for very fine particles. A chemical method based on the reaction of ammonia and manganese chloride was used to quantify the mass transfer from the simulated atmospheric boundary-layer flow to the surface. Using a tracer gas instead of solid particles can be considered only if turbulent diffusion is the decisive deposition mechanism and effects of sedimentation, impaction, interception or molecular diffusion can be approximately ignored. These constraints are necessary due to scaling problems concerning particle modelling in the small-scale experiment. The intention was to determine the obstacle arrangement density in which the mean ground deposition is maximized for a defined crown form. A deposition amplification factor was defined as the quotient of deposition efficiencies for an area with tree crowns and an open ground with identical similarity parameters. Based on this calculation an increase of the ground deposition by up to 60% should be realistic through a favourable arrangement of tree crowns and tree number density. An increase in turbulence intensity in the flow leads to a significant amplification of the mean ground deposition.     

Impact of fish farming on foraminiferal community, Drvenik Veliki Island, Adriatic Sea, Croatia

This study examined the impact of fish farming on foraminiferal communities in the Adriatic coastal zone. Samples were taken directly beneath the farm, near the edge of the farm, and at a reference station away from the farm. The foraminiferal community near the farm is characterized by Epistominella exigua, Globocassidulina subglobosa, Haynesina germanica and the genera Elphidium, Bulimina and Brizalina. These foraminiferal species are less abundant seaward. Asterigerinata mamilla, Neoconorbina terquemi and genus Cibicides are almost absent below the cages. Total phosphorus (TP) and total nitrogen (TN) in the sediments decrease with distance from the cages. The abundances of E. exigua, G. subglobosa, H. germanica and the genera Elphidium, Bulimina and Brizalina are correlated with TP and TN, indicating their dependence on nutrient input. The absence of A. mamilla, N. terquemi and the genus Cibicides below the cages is a due to a degraded Posidonia community. According to our study, foraminiferal community composition can be used as indicator of organic enrichment caused by fish farm activities.