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Structure of the inner core is often measured through traveltime differences between waves that enter the inner core (PKPdf) and waves that travel through the outer core only (PKPab and PKPbc). Here we extend the method to converted waves PKSdf and SKPdf and compare results to PKP wave measurements. PKSdf and SKPdf have a very similar path to PKPdf and if velocity variations are present in the inner core, all three wave types should experience them equally. Since traveltime deviations can be due to velocity changes (either isotropic or anisotropy) as well as wave path deviations born from heterogeneity, we simultaneously investigate wave path directions and traveltimes of PKP, SKP and PKS waves for several source-array combinations. One of the path geometries is the anomalous polar corridor from South Sandwich to Alaska, which has strong traveltimes anomalies for PKPdf relative to more normal equatorial path geometries. Here we use array methods and determine slowness, traveltime and backazimuth deviations and compare them to synthetic data. We find that path deviations from theoretical values are present in all wave types and paths, but also that large scatter exists. Although some of the path deviations can be explained by mislocation vectors and crustal variations, our measurements argue that mantle structure has to be considered when investigating inner core anisotropy. Our polar path data show similar traveltime residuals as previously published, but we also find slowness residuals for this path. Interestingly, SKPdf and PKSdf for the South Sandwich to Alaska path show traveltime residuals that are partly opposite to those for PKPdf, possibly due to an interaction with a localized ultra-low velocity zone where waves enter the core. [ABSTRACT FROM AUTHOR]

Rotational seismology opens a new avenue to study the deep interior of the Earth. Using data from the Wettzell Observatorium, Germany, where a ring laser gyroscope and a 3-component translational broadband seismometer are co-located, we report the presence of clear S, ScS and SdS signals on both rotational and translational seismograms. Using S wave arrivals, we propose a new methodology to extract information on velocity changes in the Earth mantle and we show that, by combining both translational and rotational data, we are able to solve the well known velocity-depth ambiguity inherent to classical inverse problems. The methodology is validated using ray theory and 2.5-D finite-difference synthetics. We provide a proof-of-concept showing that future studies of the Earth's deep interior can be improved by combining translational and rotational records. [ABSTRACT FROM AUTHOR]

Summary: We take advantage of the new large AlpArray Seismic Network (AASN) as part of the AlpArray research initiative (www.alparray.ethz.ch), to establish a consistent seismicity-catalogue for the greater Alpine region (GAR) for the time period 2016 January 1–2019 December 31. We use data from 1103 stations including the AASN backbone composed of 352 permanent and 276 (including 30 OBS) temporary broad-band stations (network code Z3). Although characterized by a moderate seismic hazard, the European Alps and surrounding regions have a higher seismic risk due to the higher concentration of values and people. For these reasons, the GAR seismicity is monitored and routinely reported in catalogues by a 11 national and 2 regional seismic observatories. The heterogeneity of these data set limits the possibility of extracting consistent information by simply merging to investigate the GAR's seismicity as a whole. The uniformly spaced and dense AASN provides, for the first time, a unique opportunity to calculate high-precision hypocentre locations and consistent magnitude estimation with uniformity and equal uncertainty across the GAR. We present a new, multistep, semi-automatic method to process ∼50 TB of seismic signals, combining three different software. We used the SeisComP3 for the initial earthquake detection, a newly developed Python library ADAPT for high-quality re-picking, and the well-established VELEST algorithm both for filtering and final location purposes. Moreover, we computed new local magnitudes based on the final high-precision hypocentre locations and re-evaluation of the amplitude observations. The final catalogue contains 3293 seismic events and is complete down to local magnitude 2.4 and regionally consistent with the magnitude 3+ of national catalogues for the same time period. Despite covering only 4 yr of seismicity, our catalogue evidences the main fault systems and orogens' front in the region, that are documented as seismically active by the EPOS-EMSC manually revised regional bulletin for the same time period. Additionally, we jointly inverted for a new regional minimum 1-D P -wave velocity model for the GAR and station delays for both permanent station networks and temporary arrays. These results provide the base for a future re-evaluation of the past decades of seismicity, and for the future seismicity, eventually improving seismic-hazard studies in the region. Moreover, we provide a unique, consistent seismic data set fundamental to further investigate this complex and seismically active area. The catalogue, the minimum 1-D P -wave velocity model, and station delays associated are openly shared and distributed with a permanent DOI listed in the data availability section. [ABSTRACT FROM AUTHOR]

There is mounting evidence for the presence of seismic reflectors in the mantle at a depth of around 1000 km, however, the cause for these reflectors is not yet well established and published observations show a range of depth estimates for these structures. Discussion is also still ongoing whether it is one global or several different reflectors. Here, we investigate the presence of seismic reflectors beneath the North Atlantic using a large number of PP and SS underside reflections. We analyse over 2600 earthquakes with M w ≥ 5.7 and use array seismic methods to improve the visibility of the small-amplitude reflected signals. The measured time lag between PP/SS arrivals and their corresponding precursors on robust stacks are used to estimate the depth of the reflector. Our results reveal the presence of mid-mantle structure beneath the North Atlantic in a depth range of ∼700–1300 km, consistent for both P- and S -wave observations. The reflector depth is shallower than 1000 km beneath the southern part of the investigation area and deepens seemingly abruptly towards the northern part of the North Atlantic. We find polarity variations in a region of strong depth change which we assume to be due to wave interference. Using 3-D waveform modelling, we implement models with strong topography as well as models with two overlapping reflectors and two separated reflectors and show that a large step in reflector depth over a short lateral distance, as imaged with our data set, can be resolved, while the discrimination between strong topography and two overlapping and separated reflectors, respectively, is also possible. The variations in precursor polarity can also be observed in our synthetic data in the region of strong topography confirming our assumption of wave interference for generating apparent precursor polarity and waveform changes. While the opposite polarity can be produced by the two overlapping reflectors as well, the strong topography model is preferred in this region, since we do detect only one reflected signal in our observations. [ABSTRACT FROM AUTHOR]

Maxwell's equations are valid regardless of the choice of the coordinate system. By this property a change of coordinates can be equivalently expressed as a change of the material parameters. This idea opens a new approach to the problem of accurate electromagnetic modelling in the vicinity of steep topography or bathymetry. Via a change of coordinates, any earth model with complicated layer interfaces can be represented by an equivalent model where those interfaces are flat, but with its materials correspondingly altered. This new model could then be discretized on a regular mesh and fields could be computed by an appropriate finite difference or integral equation code. Unfortunately, this is not straightforward because both the new electric and magnetic materials are fully anisotropic. By instead applying a finite element secondary field approach to the equivalent model, we can completely account for the topography interface in the planar layered background model. The only modification required to existing finite element formulations is a slightly more complicated right-hand side of the linear system of equations, whereas the system matrix is unchanged in any coordinate system. In a numerical modelling experiment we confirm that our technique gives increased accuracy when compared with a recently published technique for dealing with topography in a secondary field formulation for the case of a magnetotelluric source field. In turn, in the vicinity of conductivity anomalies, accuracy can also be negatively affected. [ABSTRACT FROM AUTHOR]

In this contribution we study elastic wave propagation via the introduction of the micropolar theory. As a generalization of a classical linear elastic medium, a micropolar medium allows each particle to have intrinsic rotational degrees of freedom (spin). We perform numerical experiments using the Pseudospectral method. We find analytical harmonic micropolar solutions for different problem configurations, which result in waveform differences between the classical linear elastic and micropolar media. In contrast to linear elastic media, wave propagation in micropolar media is dispersive. We study how the spin waveform depends on the micropolar elastic parameters and frequency content of the simulation. The micropolar effect on numerical seismograms has a direct implication on the phase, amplitude and arrival time. For frequencies lower than the cut-off frequency, the spin waveform has the same amplitude as the macrorotation field. For frequencies higher than the cut-off frequency, the amplitude of the spin waveform decreases with increasing frequency, so that then it is no longer comparable to the amplitude of macroscopic rotations. When both frequencies are equal there is no wave propagation. This work attempts to clarify the theory of micropolar media for its applications in seismology. We argue that micropolar theory should be further investigated for its potential uses in seismology to, for example, describe energy dissipation, seismograms recorded with rotational seismometers and rupture processes. [ABSTRACT FROM AUTHOR]

Differential traveltimes of P4KP and PKP waves are used in an attempt to constrain the topography of the core-mantle boundary (CMB) in localized areas. For epicentral distances of approximately 150?, PKP and P4KP follow the same path through the crust and mantle and differ only in the core, since P4KP reflects three times at the underside of the CMB. Hence traveltime differences should only be due to the path differences in the core. We use array techniques in order to enhance the signal to noise ratio and confirm the slowness and backazimuth of the P4KP waves. Data from the Gräfenberg Array in southern Germany yield a total of 27 events with observable P4KP waves. We find most traveltime residuals between theoretical and observed P4KP minus PKP arrival time generally within a range of ±7 s. A ray tracing method is used to model the deviations of traveltimes due to changes of CMB depth at the reflection points of the P4KP waves. It is not possible to explain the observed traveltime residuals with changes in depth of the CMB only; in several cases, the residuals are too large to be caused by CMB topography alone. Due to the expected coinciding ray paths of PKP and P4KP, the remaining variations are unlikely to be produced by heterogeneities in the lowermost mantle, and so remain for the moment unaccounted for. We find that the reflection coefficient of P4KP undergoes a polarity reversal for some scenarios of lower mantle velocities and densities and the observed P4KP polarity at this distance range can therefore help to investigate the lowermost mantle in addition to CMB topography. [ABSTRACT FROM AUTHOR]

We introduce the Complex-Step-Finite-Difference method (CSFDM) as a generalization of the well-known Finite-Difference method (FDM) for solving the acoustic and elastic wave equations. We have found a direct relationship between modelling the second-order wave equation by the FDM and the first-order wave equation by the CSFDM in 1-D, 2-D and 3- D acoustic media. We present the numerical methodology in order to apply the introduced CSFDM and show an example for wave propagation in simple homogeneous and heterogeneous models. The CSFDM may be implemented as an extension into pre-existing numerical techniques in order to obtain fourth- or sixth-order accurate results with compact three timelevel stencils. We compare advantages of imposing various types of initial motion conditions of the CSFDM and demonstrate its higher-order accuracy under the same computational cost and dispersion-dissipation properties. The introduced method can be naturally extended to solve different partial differential equations arising in other fields of science and engineering. [ABSTRACT FROM AUTHOR]