@article {harmon_tilt_2022,
	title = {Tilt Corrections for Normal Mode Observations on Ocean Bottom Seismic Data, an example from the PI-LAB experiment},
	journal = {Seismica},
	volume = {1},
	number = {1},
	year = {2022},
	note = {Number: 1},
	month = {11/2022},
	abstract = {Earth{\textquoteright}s normal modes are fundamental observations used in global seismic tomography to understand Earth structure. Land seismic station coverage is sufficient to constrain the broadest scale Earth structures. However, 70\% of Earth{\textquoteright}s surface is covered by the oceans, hampering our ability to observe variations in local mode frequencies that contribute to imaging small-scale structures. Broadband ocean bottom seismometers can record spheroidal modes to fill in gaps in global data coverage. Ocean bottom recordings are contaminated by signals from complex interactions between ocean and solid Earth dynamics at normal mode frequencies. We present a method for correcting tilt on broadband ocean bottom seismometers by rotation. The correction improves the ability of some instruments to observe spheroidal modes down to 0S4. We demonstrate this method using 15 broadband ocean bottom seismometers from the PI-LAB array. We measure normal mode peak frequency shifts and compare with 1-D reference mode frequencies and predictions from 3-D global models. Our measurements agree with the 3-D models for modes between 0S14 - 0S37 with small but significant differences. These differences likely reflect real Earth structure. This suggests incorporating ocean bottom normal mode measurements into global inversions will improve models of global seismic velocity structure.},
	keywords = {marine geophysics, normal modes, ocean bottom seismometer},
	issn = {2816-9387},
	doi = {10.26443/seismica.v1i1.196},
	url = {https://seismica.library.mcgill.ca/article/view/196},
	author = {Harmon, Nicholas and Laske, Gabi and Crawford, Wayne and Rychert, Catherine}
}
@article {agius_thin_2021,
	title = {A thin mantle transition zone beneath the equatorial Mid-Atlantic Ridge},
	journal = {Nature},
	volume = {589},
	number = {7843},
	year = {2021},
	note = {Publisher: Nature Publishing Group},
	month = {01/2021},
	pages = {562{\textendash}566},
	abstract = {The location and degree of material transfer between the upper and lower mantle are key to the Earth{\textquoteright}s thermal and chemical evolution. Sinking slabs and rising plumes are generally accepted as locations of transfer1,2, whereas mid-ocean ridges are not typically assumed to have a role3. However, tight constraints from in situ measurements at ridges have proved to be challenging. Here we use receiver functions that reveal the conversion of primary to secondary seismic waves to image the discontinuities that bound the mantle transition zone, using ocean bottom seismic data from the equatorial Mid-Atlantic Ridge. Our images show that the seismic discontinuity at depths of~about 660~kilometres is broadly uplifted by 10~{\textpm}~4 kilometres over a swath about 600~kilometres wide and that the 410-kilometre discontinuity is depressed by 5~{\textpm}~4~kilometres. This thinning of the mantle transition zone is coincident with slow shear-wave velocities in the mantle, from global seismic tomography4{\textendash}7. In addition, seismic velocities in the mantle transition zone beneath the Mid-Atlantic Ridge are on average slower than those beneath older Atlantic Ocean seafloor. The observations imply material transfer from the lower to the upper mantle{\textemdash}either continuous or punctuated{\textemdash}that is linked to the Mid-Atlantic Ridge. Given the length and longevity of the mid-ocean ridge system, this implies that whole-mantle convection may be more prevalent than previously thought, with ridge upwellings having a role in counterbalancing slab downwellings.},
	keywords = {Geodynamics, Geophysics, seismology},
	issn = {1476-4687},
	doi = {10.1038/s41586-020-03139-x},
	url = {https://www.nature.com/articles/s41586-020-03139-x},
	author = {Agius, Matthew R. and Rychert, Catherine A. and Harmon, Nicholas and Tharimena, Saikiran and Kendall, J.-Michael}
}
