ALBERT

Description: Norway experiences low to intermediate seismicity, mainly caused by ridge push of the Mid-Atlantic ridge. To gain a better understanding of local tectonics and fundamental physics, we used the empirical Green’s function (EGF) method to obtain earthquake source parameters in Norway. We validated our findings against earthquake source parameters obtained from spectral analysis. Between January 1990 and May 2018, we found 263 earthquake pairs eligible for the EGF method. The local magnitudes of the 107 master events range between 1.3 and 3.4. Based on the Brune (J Geophys Res 75(26):4997–5009, 1970) source model, we obtained stress drops between 0.4 and 355 bar. We observed an increase of stress drop with earthquake size in southern Norway and the Svalbard archipelago. Calculated fault radii between 80 and 320 m suggest that larger magnitudes are caused more by increase in slip than by increase in fault dimension. Hence, our results indicate that smaller earthquakes in southern Norway and the Svalbard archipelago deviate from the assumption of self-similarity. Stress drops obtained for northern Norway show only weak correlation with seismic moment and therefore fit into the theory of earthquake self-similarity. © 2019, Springer Nature B.V.

Description: Ambient seismic noise is caused by a number of sources in specific frequency bands. The quantification of ambient noise makes it possible to evaluate station and network performance. We evaluate noise levels in Norway from the 2013 data set of the Norwegian National Seismic Network as well as two temporary deployments. Apart from the station performance, we studied the geographical and temporal variations, and developed a local noise model for Norway. The microseism peaks related to the ocean are significant in Norway. We, therefore, investigated the relationship between oceanic weather conditions and noise levels. We find a correlation of low-frequency noise (0.125–0.25 Hz) with wave heights up to 900 km offshore. High (2–10 Hz) and intermediate (0.5–5 Hz) frequency noise correlates only up to 450 km offshore with wave heights. From a geographic perspective, stations in southern Norway show lower noise levels for low frequencies due to a larger distance to the dominant noise sources in the North Atlantic. Finally, we studied the influence of high-frequency noise levels on earthquake detectability and found that a noise level increase of 10 dB decreases the detectability by 0.5 magnitude units. This method provides a practical way to consider noise variations in detection maps. © 2016, The Author(s).

Description: The propagation of seismic waves is influenced by changes in crustal structure as for example the transition from continental to oceanic crust along the Norwegian margin. We analyzed Lg wave propagation to map lateral crustal changes in Norway and adjacent areas. We used 1369 observations from 279 earthquakes recorded mostly by the Norwegian National Seismic Network between 1990 and 2017. First, we classified Lg wave propagation in terms of efficiency through Lg/Pn ratios and found significant changes between ray paths crossing offshore and onshore areas. Then we derived an average Q Lg (f) = 529 f 0.42 model for Norway, which is in the expected range for a stable tectonic environment. This was used as starting model for a tomographic inversion. We present tomographic models of Lg wave attenuation at frequencies 2 Hz, 4 Hz, and 6 Hz, respectively. We observed the most significant variation between offshore and onshore regions. This can be explained by changes in crustal structure and the occurrence of unconsolidated sediments in the offshore areas. © 2018, Springer Nature B.V.

Description: The Arctic mid-ocean ridge system constitutes the most active source of earthquakes in the north polar region. However, the characteristics of its earthquake activity at teleseismic and local scales are not well studied because of the remote location of the ridge. We present here a comprehensive seismicity analysis that compares the teleseismic earthquake record of 35 years drawn from the catalogue of the International Seismological Centre with reconnaissance-style local earthquake records at six locations along the ridge that were instrumented either with ocean bottom seismometers or with seismometers on drifting ice floes. The teleseismic earthquake activity varies along the ridge and reflects ultraslow spreading processes with more and larger earthquakes produced in magma-rich regions than in magma-starved areas. Large magnitude earthquakes M〉5.5 are common along this ultraslow spreading ridge. Locally recorded earthquakes are of small magnitude (M〈2) and probably reflect the formation of the pronounced topographic relief. Their size and event rate is not as variable along the ridge as that of teleseismic events. Locally recorded earthquakes in the upper mantle are generated at several locations. Their focal depths do not depend on spreading rate but reflect the thermal state of the lithosphere with very deep earthquakes indicating an exceptionally cold lithosphere.

Description: Ultraslow spreadingmid-ocean ridges have a low magma budget and melt is distributed unevenly along the ridge axis. There is little or no basaltic crust between isolated magmatic centers. The processes that focus melts to segments of robust magmatism are not yet understood. During a seismic survey of the ultraslow spreading Knipovich Ridge in the Norwegian-Greenland Sea with ocean bottom seismometers, we discovered a seismic gap in the upper mantle beneath Logachev Seamount, where micro-earthquakes clearly delineate a shallowing of the maximum depth of faulting. A topography of the lithosphere that allows melts to travel laterally along its base and rise in areas of thin lithosphere has been proposed as a possible mechanism to explain the focusing of melts at volcanic centers, but has never been confirmed observationally. Our results are the first geophysical evidence for an along-axis variation of the lithospheric thickness at an ultraslow spreading ridge.

Description: At mid-ocean ridges, the lithospheric plates drift apart, magma fills the gap to form new crust. This engine splutters at ultraslow speeds of less than 20 mm/y: Isolated volcanoes pierce the seafloor of ultraslow spreading ridges; between the volcanoes, there are long stretches without volcanism. The morphology and the mode of seafloor production at ultraslow spreading ridges differ fundamentally from all faster spreading ridges. The reasons for the uneven distribution of melts at ultraslow plate boundaries are still poorly understood as their main representatives, the Arctic ridge system and the Southwest Indian ridge, are difficult to access. We analysed the teleseismically recorded seismicity in 11 sections of ultraslow spreading ridges spanning altogether 7200 km. Epicentres located within 30–35 km of the rift axis were extracted from the catalogue of the International Seismological Centre for a time period of 35 years. The typical stripe-and-gap seismicity pattern of slow spreading ridges is not discernible at ultraslow spreading rates. Here, volcanic centres often have an increased seismicity rate relative to the background rather than appearing as seismic gap. In contrast, amagmatic segments of ultraslow spreading ridges are seismically only weakly active. Asymmetric accumulations of earthquakes at segment ends that are related to detachment faulting at slow spreading ridges do not exist at ultraslow spreading ridges. Local and teleseismic earthquakes at ultraslow spreading ridges occur down to depths of 20 km below seafloor, confirming the existence of a cold and brittle lithosphere. Underneath Logachev Seamount at Knipovich ridge, microearthquake hypocentre depths from a 10 days seismicity study with ocean bottom seismometers yield for the first time direct evidence for an undulating lithosphere-asthenosphere boundary which has often been postulated as one possible way to channel melts towards the centres of focussed magmatism.

Description: Mid-ocean rift systems opening at full spreading rates 〈 20 mm/y differ in their structure from all faster spreading ridges. One of the key differences is that the small amount of available melt is not distributed evenly along the ridge axis. Instead, rift sections of robust magmatism form with a crustal thickness of the order of 3-4 km. At more magma-starved locations, a magmatic crust may be entirely absent apart from pronounced volcanic centres where up to 9 km of crustal thickness can be achieved. Gakkel ridge and Knipovich Ridge in the Arctic show several of these volcanic centres, each of them can be traced off-axis by a bathymetric ridge in spreading direction that indicates increased crustal production. We collected and compiled for several years local, regional and global seismicity data of the ultraslow spreading ridges, the Arctic Ridge System and the Southwest Indian Ridge, to examine the structure and the still poorly known spreading processes of this class of mid-ocean ridge. At Knipovich Ridge, the maximum depth of faulting of local earthquakes marks the 600°C isotherm, which rises underneath Logachev Seamount. An undulating lithosphere-asthenosphere boundary has been postulated by many authors to explain the uneven melt distribution. They assume that melts travel horizontally along this sloping boundary towards the volcanic centres. Our observation of an undulating 600°C isotherm may thus provide the first geophysical evidence for this hypothesis. Extraordinarily deep local earthquakes (16 km - 20 km below sea floor) at several sites of ultraslow spreading ridges indicate that the lithosphere is colder than expected by thermal models. Confusingly, we observed such deep earthquakes also at the 85°E volcanic complex at Gakkel Ridge, 6 years after a major eruption when the lithosphere should still be warm. Magma-starved ridge portions of ultraslow spreading ridges show only few and weak teleseismically recorded earthquakes, while the volcanic centres may emit large numbers of strong earthquakes often in swarms. This contradicts the stripe-and-gap pattern of seismicity observed for example along the entire Mid-Atlantic ridge, where volcanic ridges are connected with reduced seismicity. In order to better understand the different spreading processes in magmatic and amagmatic rift sections of ultraslow spreading ridges, we performed the first comparative local seismicity study of an ultraslow spreading ridge. From Nov 2012- Nov 2013, two similarly designed networks of ocean bottom seismometers were recording local earthquakes at two geologically contrasting sites of the Southwest Indian Ridge. We will present a short impression of the recent recovery of the instruments during Polarstern cruise ANT-XXIX/8 and Meteor Cruise M101.