Papers by Stefanie Hempel
EGU General Assembly Conference Abstracts, Apr 1, 2015
EGU General Assembly Conference Abstracts, Apr 1, 2017
There is a long-term history of complexity at the core-mantle boundary. On the mantle side, ultra... more There is a long-term history of complexity at the core-mantle boundary. On the mantle side, ultra-low velocity zones, the post-perovskite phase transition and its spatially variable manifestations, complex anisotropic properties, and interpretations of subduction-related structures in this region have been discussed over the past 40 years (Wright, 1973, Lay and Helmberger, 1983, Wookey et al., 2005). The literature on each of these features is extensive, and frequently contributions are oriented towards characterizations and interpretations of single features.
Planetary and Space Science, 2017
Understanding the internal structure of an asteroid has important implications for interpreting i... more Understanding the internal structure of an asteroid has important implications for interpreting its evolutionary history, for understanding its continuing geological evolution, and also for asteroid deflection and in-situ space resource utilisation. Given the strong evidence that asteroids are seismically active, an in-situ passive seismic experiment could provide information about the asteroid surface and interior properties. Here, we discuss the natural seismic activity that may be present on Didymoon, the secondary component of asteroid (65803) Didymos. Our analysis of the tidal stresses in Didymoon shows that tidal quakes are likely to occur if the secondary has an eccentric orbit. Failure occurs most easily at the asteroid poles and close to the surface for both homogeneous and layered internal structures. Simulations of seismic wave propagation in Didymoon show that the seismic moment of even small meteoroid impacts can generate clearly observable body and surface waves if the asteroid's internal structure is homogeneous. The presence of a regolith layer over a consolidated core can result in the seismic energy becoming trapped in the regolith due to the strong impedance contrast at the regolith-core boundary. The inclusion of macroporosity (voids) further complexifies the wavefield due to increased scattering. The most prominent seismic waves are always found to be those traveling along the surface of the asteroid and those focusing in the antipodal point of the seismic source. We find also that the waveforms and ground acceleration spectra allow discrimination between the different internal structure models. Although the science return of a passive seismic experiment would be enhanced by having multiple seismic stations, one single seismic station can already vastly improve our knowledge about the seismic environment and sub-surface structure of an asteroid. We describe several seismic measurement techniques that could be applied in order to study the asteroid internal structure with one three-component seismic station.
Space Science Reviews, 2016
Panning et al.
Solid Earth Discussions, 2014
We present a methodology to compute 3-D global seismic wavefields for realistic earthquake source... more We present a methodology to compute 3-D global seismic wavefields for realistic earthquake sources in viscoelastic anisotropic media, covering applications across the observable seismic frequency band with moderate computational resources. This is accommodated by mandating axisymmetric background models that allow for a multipole expansion such that only a 2-D computational domain is needed, whereas the azimuthal third dimension is computed analytically on the fly. This dimensional collapse opens doors for storing space-time wavefields on disk that can be used to compute Fréchet sensitivity kernels for waveform tomography. We use the corresponding publicly available AxiSEM (www.axisem.info) open-source spectral-element code, demonstrate its excellent scalability on supercomputers, a diverse range of applications ranging from normal modes to small-scale lowermost mantle structures, tomographic models, and comparison with observed data, and discuss further avenues to pursue with this methodology. 1. 3-D wavefields for realistic sources and structures, 2. across the observable frequency band, 3. at a reasonable computational cost for tomography.
The Moon's interior was seismically investigated using a four station seismic network in cont... more The Moon's interior was seismically investigated using a four station seismic network in context of the US Apollo missions, which recorded artificial and natural impacts on the surface as well as moonquakes in two different depth ranges: 50-300km and 700-1300km. The Apollo seismic stations were deployed on the lunar near side within a circle of 40 degrees diameter and operated
Based on the registration of more than 12000 seismic events on the Moon during the Apollo mission... more Based on the registration of more than 12000 seismic events on the Moon during the Apollo missions from 1969 to 1977, moonquake classifications and locations of half the deep moonquakes clusters as well as structural models of crust and mantle have been derived. Most Moonquakes were located on the lunar near side (the hemisphere which is always facing the Earth).
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Papers by Stefanie Hempel