What causes the flux blob?

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DeepDyn doctoral student Harini presented her work in the Seismology Project at the biennial SEDI (Study of the Earth's deep interior) conference in Massachusetts (USA). She is working with other researchers from the University of Münster and the KIT in Karlsruhe such as Yvonne to investigate the influence of seismic structures at the core-mantle boundary on regions with intense magnetic flux.

High flux density of the Earth's magnetic field in Siberia - sunspots on Earth?

Harini is concentrating on the High Latitude Flux Lobe beneath Siberia. This flux lobe is part of the Siberian pair (Siberia - South Indian Ocean), and has been stable in this location for at least 250 years, despite a constantly changing magnetic field around these regions. Basically, you can imagine these flux lobes like sunspots on Earth, even though the underlying mechanisms of their formation are not completely similar.

But what causes the flux lobes on Earth? Obviously, the movements of the fluids in the outer core must be different in areas with exceptionally high flux density. These movements could be influenced by downwellings of colder material to the core-mantle boundary.

Searching for the relevant earthquake wave records

To find out more about the structure above the core-mantle boundary under Siberia, Harini is searching earthquake databases for recorded earthquake waves that have crossed the lower mantle under Siberia. For her analysis, she is also using data from eight so-called seismological arrays, a kind of network of many seismometers that, when combined, help to make weak earthquake waves more visible. The TA seismological network in Alaska was particularly valuable to her here, as it is located at high geographical latitudes. She has evaluated over 18,000 such paths of waves following strong earthquakes through the lower mantle to the receiver seismometers.

Harini also wants to make statements about the anisotropy, i.e. the direction dependence, in the lower mantle. This will then make it possible to draw conclusions about the possible mineral composition of the structures in the deep mantle. To do this, Harini is looking in particular for earthquake waves from different directions: e.g. earthquakes in Japan, which are recorded in German arrays, and earthquakes in the Hindu Kush, which were recorded in Alaska - their paths cross almost at right angles in the area of ​​the Siberian Flux Lobe.

Next steps

Harini will soon compare the recorded earthquake waves (seismograms) with synthetic seismograms, which are created by a model assuming a 300 km thick layer at the core-mantle boundary, and examines whether the real recordings match the theoretical ones.

As a preliminary result, she could confirm that a transition layer between the Earth's mantle and core, the so-called D'' layer, about 300 km thick, exists under Siberia. Following the conference, Harini wants to repeat the procedure for the Canada Flux Lobe and include the anisotropy measurements.

Abstract

Link zum Tagungsband

Testing the consistency of D" reflected waves beneath Siberia for different crossing paths

Harini Thiyagarajan, Christine Thomas Universität Münster, Institute of Geophysics, Germany

In recent geomagnetic field models, patches of intense magnetic flux can be identified. The north magnetic field is characterized by two such flux lobes, one underneath Canada and one underneath Siberia, known as High Latitude Flux Lobes (HLFL). A third HLFL is postulated underneath the Norther Atlantic but has not been observed. Studies show that the lower mantle influences the magnetic field through the control of the geodynamo. The aim of this study is to investigate how the underlying lower mantle structure and mineralogy may influence these regions of high magnetic signature. Using array methods, we search for D" reflected PdP and SdS waves which arrive as precursors to the core-reflected PcP and ScS waves and that sample the lowermost mantle beneath Siberia with a number of intersecting paths. Especially the new Alaskan station (TA) deployment allows for a better number of crossing paths that are needed to establish whether anisotropy is present. Vespagram and slowness-backazimuth analysis are carried out to detect the presence of lower mantle reflectors at the top of the D" and establish the wave's travel direction (in plane versus outof-plane). A comparison with synthetic seismograms establishes whether the observations can be explained by a previously suggested 300km thick D" layer. We present a number of observations in this region and a wider coverage than previously possible by showing the results for PdP and SdS waves, their travel time and polarity measurements for different crossing paths and focus on the consistency of the observations.