Johannes Maximilian Kemper


Last Name

Kemper

First Name

Johannes Maximilian

ORCID

0000-0003-1841-9590

Organisational unit

03476 - Giardini, Domenico (emeritus) / Giardini, Domenico (emeritus)

Filters

2023 - 20253
Reset filters

Search Results

Publications 1 - 3 of 3
  • Retracted: Self-consistent models of Earth’s mantle and core from long-period seismic and tidal constraints
    Item type: Journal Article
    Kemper, Johannes Maximilian; Khan, Amir; Helffrich, George; et al. (2023)
    Geophysical Journal International
    In this study, we inverted a large set of normal-mode centre-frequencies and quality (attenuation) factors, including astronomic-geodetic data (mass, moment of inertia and tidal response), using self-consistently built models of the radial elastic and anelastic seismic structure of the Earth. The mantle models are constructed using petrologic phase equilibria in combination with a laboratory-based viscoelastic model that connects dissipation from seismic to tidal periods, whereas seismic properties for a well-mixed and homogeneous core are computed using equations-of-state. Relative to the preliminary seismic reference model (PREM), we find that for the models to fit the observations, mantle P- and S-wave velocities have to be slightly faster and slower, respectively, while outer-core P-wave velocity is slower on account of a different velocity gradient, whereas inner-core velocity structure is similar, within the uncertainties of the inferred model parameters. In terms of density, we find that the lower mantle is less dense and the outer core more dense than PREM, while the inner core is similar to PREM. To study the impact of the inferred mantle seismic velocity structure, we computed P- and S-wave traveltimes and compared these to the observations of globally-averaged P- and S-wave traveltimes from the reprocessed ISC catalogue that resulted in an excellent match. In an attempt to further refine the seismic P-wave velocity structure of the outer core, we also considered multiple core–mantle-boundary underside-reflected body wave traveltime data. Although the match to the underside reflections clearly improves as a result of a steeper velocity gradient in the outer core relative to the normal-modes- and astronomic-geodetic-data-only case, subtle differences nevertheless persist that appear to support a change in velocity gradient in the outermost core, evocative of a stably-stratified layer. The laboratory-based viscoelastic model considered here resolves the anelastic response of Earth’s mantle from long-period seismic (∼100 s) to tidal (18.6 yr) periods, accounting for both normal-mode and tidal dissipation measurements. Finally, as a potential means of refining core composition, we considered the density contrast across the inner-core boundary (ICB) based on our inverted models. The most probable ICB density difference found here is 0.3–0.45 g cm⁻³, which is in the lower range of earlier body-wave- and normal-mode-based predictions. This suggests that the compositional heterogeneity associated with light-element partitioning, which is considered the principal driving mechanism for the compositional convection that powers the geodynamo, may be less effective than previously thought, calling for exsolution of solids from the liquid outer core as a possible additional source of energy. This would also help address the problem of a young inner core.
  • Searching the InSight Seismic Data for Mars’s Background-Free Oscillations
    Item type: Journal Article
    Duran, Andrea Cecilia; Khan, Amir; Kemper, Johannes Maximilian; et al. (2025)
    Seismological Research Letters
    Mars’s atmosphere has theoretically been predicted to be strong enough to continuously excite Mars’s background-free oscillations, potentially providing an independent means of verifying radial seismic body-wave models of Mars determined from marsquakes and meteorite impacts recorded during the Interior Exploration using Seismic Investigations, Geodesy, and Heat Transport (InSight) mission. To extract the background-free oscillations, we processed and analyzed the continuous seismic data, consisting of 966 Sols (a Sol is equivalent to a Martian day), collected by the Mars InSight mission using both automated and manual deglitching schemes to remove nonseismic disturbances. We then computed 1-Sol-long autocorrelations for the entire data set and stacked these to enhance any normal-mode peaks present in the spectrum. We find that while peaks in the stacked spectrum in the 2–4 mHz frequency band align with predictions based on seismic body-wave models and appear to be consistent across the different processing and stacking methods applied, unambiguous detection of atmosphere-induced free oscillations in the Martian seismic data nevertheless remains difficult. This possibly relates to the limited number of Sols of data that stack coherently and the continued presence of glitch-related signal that affects the seismic data across the normal-mode frequency range (∼1–10 mHz). Improved deglitching schemes may allow for clearer detection and identification in the future.
  • Modern computational methods applied to classical long-period seismology
    Item type: Doctoral Thesis
    Kemper, Johannes Maximilian (2023)
    Determining the structure of planets is paramount to the understanding of the origin and evolution of planetary systems. As seismic waves propagate freely through planets and interact with discontinuities and material structure, their study, seismology is the predominant discipline to resolve planetary interiors. Density, defined as mass per unit volume, is important in that it is the primary seismic parameter that informs us about the material composition of a planet. In addition, gaining insight into a planet's density structure has many implications for geodynamic, geomagnetic and seismic studies. Earth's mantle flow models are governed by these density differences and thus provide a connection between traditional seismic tomographic models and geodynamics. Moreover, the inner core boundary density contrast drives the convection in the outer core, generating the magnetic field through the geodynamo process. Combining density information with tomographic velocities allows the inference of material properties, such as shear and bulk modulus, that consequently give constrains on the geochemistry. For these reasons, the overarching goal of this thesis is to increase our understanding of density variations. To achieve this, we use long period seismic signals as they are affected by self-gravitation. This form of gravitation affects waves that, while propagating, change the density structure of their carrier material. This process gives these signals a direct feedback to the gravity potential, which originates from the density distribution. Their detection thus makes it possible to map the long wavelength and therefore large scale density structure. Generally, there are two types of long period signals: tides with a periods of multiple hours, and normal modes or free oscillations with periods from seconds up to one hour for the Earth. To excite free oscillations in planets a powerful seismic energy source is needed. Although the amount of high-quality long-period data has increased significantly in recent decades due to the occurrence of several very large quakes and high precision satellite measurements, there is still little agreement between different density models. The numerical description of normal modes started together with the emergence of the computer. Several methods based on different forms of numerical integration have been proposed to solve the physically motivated system of differential equations. To resolve density, accuracy of the solutions is of foremost importance as the magnitude of variations is assumed to be in the same order of the error of commonly employed numerical approximations. The present work describes our advances in long-period seismology with a strong emphasis on our numerical developments which culminated in the open-source publication of a new spectral element based normal mode code called specnm. This highly accurate numerical forward code enables us to invert the most recent normal mode measurements to obtain new one-dimensional Earth structure models with higher resolution in density than ever before. To improve on former modelling approaches, we create self-consistently built models of the radial anelastic seismic structure of the Earth. For this, we construct a petrologically and thermodynamically consistent mantle structure which we unite together with a laboratory-based visco-elastic attenuation model that connects dissipation from seismic to tidal periods, whereas core properties are computed using equations-of-state. Lastly, we improve the efficiency of the calculation for three-dimensional simulations in time of gravity affected waves by, among other things, using the crucial fact that the gravity potential outside of the planet has a simple 1/r dependence. Together, these advances may lead to a better understanding of the density variations in planets.
Publications 1 - 3 of 3