Earth and Planetary Science Letters 449 (2016) 39–47 Contents lists available at ScienceDirect Earth and Planetary Science Letters www.elsevier.com/locate/epsl Source locations of teleseismic P, SV, and SH waves observed in microseisms recorded by a large aperture seismic array in China Qiaoxia Liu a,b,c , Keith D. Koper b,∗ , Relu Burlacu b , Sidao Ni d , Fuyun Wang c , Changqiao Zou c , Yunhao Wei c , Martin Gal e , Anya M. Reading e a School of Earth and Space Sciences, University of Science and Technology of China, Hefei, 230026, China b Dept. of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112, United States c Dept. of Integrated Geophysics, Geophysical Exploration Center of China Earthquake Administration, Zhengzhou, 450002, China d State Key Laboratory of Geodesy and Earth’s Dynamics, Institute of Geodesy and Geophysics, Chinese Academy of Sciences, Wuhan, 430077, China e School of Physical Sciences (Earth Sciences) and CODES Centre of Excellence in Ore Deposits, University of Tasmania, Hobart, Tasmania, Australia a r t i c l e i n f o a b s t r a c t Article history: Received 29 January 2016 Received in revised form 3 May 2016 Accepted 22 May 2016 Available online xxxx Editor: P. Shearer Keywords: body wave microseisms P SV SH seismic array northwest China Transversely polarized seismic waves are routinely observed in ambient seismic energy across a wide range of periods, however their origin is poorly understood because the corresponding source regions are either undefined or weakly constrained, and nearly all models of microseism generation incorporate a vertically oriented single force as the excitation mechanism. To better understand the origin of transversely polarized energy in the ambient seismic wavefield we make the first systematic attempt to locate the source regions of teleseismic SH waves observed in microseismic (2.5–20 s) noise. We focus on body waves instead of surface waves because the source regions can be constrained in both azimuth and distance using conventional array techniques. To locate microseismic sources of SH waves (as well as SV and P waves) we continuously backproject the vertical, radial, and transverse components of the ambient seismic wavefield recorded by a large-aperture array deployed in China during 2013–2014. As expected, persistent P wave sources are observed in the North Atlantic, North Pacific, and Indian Oceans, mainly at periods of 2.5–10 s, in regions with the strong ocean wave interactions needed to produce secondary microseisms. SV waves are commonly observed to originate from locations indistinguishable from the P wave sources, but with smaller signal-to-noise ratios. We also observe SH waves with about half or less the signal-to-noise ratio of SV waves. SH source regions are definitively located in deep water portions of the Pacific, away from the sloping continental shelves that are thought to be important for the generation of microseismic Love waves, but nearby regions that routinely generate teleseismic P waves. The excitation mechanism for the observed SH waves may therefore be related to the interaction of P waves with small-wavelength bathymetric features, such as seamounts and basins, through some sort of scattering process.  2016 Elsevier B.V. All rights reserved. 1. Introduction Models that successfully explain how ocean waves excite mi- croseismic Rayleigh waves in the solid Earth were developed over 65 years ago (Longuet-Higgins, 1950). Primary, or single frequency, microseisms at periods of ∼10–20 s are created by the direct in- teraction of swells with the sloping seafloor in shallow coastal regions (Hasselmann, 1963), while secondary, or double frequency, microseisms at periods of ∼2–10 s are created by the nonlinear in- teraction of nearly counter-propagating ocean waves in coastal and * Corresponding author. E-mail address: koper@seis.utah.edu (K.D. Koper). deep water regions (e.g., Kedar et al., 2008; Ardhuin et al., 2011). Recent theoretical work has extended these types of models to different mode types, such as microseismic body waves and micro- baroms (Ardhuin and Herbers, 2013). The early models have also been extended to periods of hundreds of seconds (Webb, 2008; Traer and Gerstoft, 2014; Ardhuin et al., 2015) and shown to match observations (Ardhuin et al., 2015) of the seismic hum—the con- tinuous low-level excitation of Earth’s normal modes (e.g., Suda et al., 1998; Ekström, 2001; Rhie and Romanowicz, 2004, 2006; Nishida, 2013, 2014). Almost all ocean wave excitation models in- corporate a near-vertical single force load on the solid Earth as the source mechanism. Therefore, while these models excel at ex- plaining the Rayleigh and P wave component of microseisms, they do not, by themselves, explain the existence of transversely polar- http://dx.doi.org/10.1016/j.epsl.2016.05.035 0012-821X/ 2016 Elsevier B.V. All rights reserved.