Seismic visibility of melt at the core-mantle boundary from PKKP diffracted waves

The core-mantle boundary (CMB) is a complex and poorly understood region of the Earth. The existence of melt or partial melt at the CMB is strongly debated with evidence coming from a range of sources. While partial melt has been inferred in specific locations using seismology, often as an explanation for ultra-low velocity zones (ULVZs), no global layer of melt has been observed despite evidence that such melt should accumulate at the CMB. Using high frequency synthetic seismograms we have systematically examined the effect of a one-dimensional melt layer at the CMB on the global seismic wavefield. We find that PKKP diffracted waves are an extremely sensitive, robust and previously underutilised seismic phase for studying the CMB and, in synthetic testing, could resolve very thin melt layers. We have constructed a global data set of PKKP diffracted observations to attempt to observe or exclude melt at the CMB. We find that within the bounds of our data, we are unable to robustly exclude or observe a melt layer at the CMB on the order of single kilometres thick. The presence of a thin layer of unobservable melt at the CMB would have profound impacts for the internal dynamics of the Earth

Normal mode coupling due to hemispherical anisotropic structure in Earth's inner core

We present illustrative calculations of the effect of hemispherical variation in inner core anisotropy on Earth's normal modes of oscillation. Body wave studies show that the anisotropy in the inner core is not simple cylindrical anisotropy, which is often portrayed in models derived from normal mode data, but varies with longitude. 'Hemispherical' or odd degree, structure has to be studied by cross-coupling normal modes, as the self-coupling technique is sensitive only to even degree Earth structure. A completely general definition of inner core anisotropy would require a prohibitive number of degrees of freedom; however, we show that any existing cylindrical anisotropy model can be confined to only one part of the inner core. Using our new theory, we find that hemispherical anisotropy causes significant changes in the frequency and quality factor of several inner core sensitive normal modes. The effect of hemispherical inner core anisotropy can also be seen in synthetic seismograms. Radial, PKIKP and PKJKP modes all respond to the presence of hemispherical variation in inner core anisotropy. If the variations in inner core anisotropy seen in body wave data are part of a gross, large-scale pattern, then this structure should also affect normal mode data on an observable scale. © Journal compilation © 2009 RAS

A re-examination of ellipticity corrections for seismic phases

SUMMARY The Earth’s ellipticity of figure has an effect on the traveltimes of seismic waves over teleseismic distances. Tables of ellipticity corrections and coefficients have been used by seismologists for several decades; however, due to the increasing variety and complexity of seismic phases in use, current tables of ellipticity coefficients are now outmoded and incomplete. We present a Python package, EllipticiPy, for the calculation of ellipticity corrections, which removes the dependence on pre-calculated coefficients at discrete source depths and epicentral distances. EllipticiPy also facilitates the calculation of ellipticity corrections on other planetary bodies. When applied to both Earth and Mars, the magnitudes of ellipticity corrections are of the order of single seconds and are significant for some seismic studies on Earth but remain negligible on Mars due to other greater sources of uncertainty.</jats:p

Focal Mechanism Determination of Event S1222a and Implications for Tectonics Near the Dichotomy Boundary in Southern Elysium Planitia, Mars

Abstract: On 4 May 2022 the InSight seismometer SEIS‐VBB recorded the largest marsquake ever observed, S1222a, with an initial magnitude estimate of 4.6. Understanding the depth and source properties of this event has important implications for the nature of tectonic activity on Mars. Located ∼37° to the southeast of InSight, S1222a is one of the few non‐impact marsquakes that exhibits prominent surface waves. We use waveform modeling of body waves (P and S) and surface waves (Rayleigh and Love) to constrain the focal mechanism, assuming a double‐couple source, and find that S1222a likely resulted from reverse faulting in the crust (source depth near 22&nbsp;km). We estimate the scalar moment to be 2.5&nbsp;×&nbsp;1015–3.5&nbsp;×&nbsp;1015 Nm (magnitude MW 4.2–4.3). Our results suggest active compressional tectonics near the dichotomy boundary on Mars, likely due to thermal contraction from planetary cooling