Frequency‐Dependent P Wave Polarization and Its Subwavelength Near‐Surface Depth Sensitivity

Near‐surface structure is crucial to assessing seismic hazards and understanding earthquakes and surface processes yet is a major challenge to robustly image. Recently, an approach based on body‐wave polarization was introduced for constraining shallow seismic structure, but the depth sensitivity of the polarization measurement has remained unclear. Using waveform simulations based on a layer over a half space, we find that the depth sensitivity of P wave polarization peaks at the surface and decreases abruptly over a depth range shorter than its wavelength. A strong frequency dependence provides constraints on local 1‐D structure, with frequencies between 0.1 and 10 Hz illuminating structure at depths of 10 m to several kilometers. Applying these results to teleseismic recordings in Japan provides constraints on structure at about 120 to 750 m, including a distinctive weak zone along the Median Tectonic Line in the Kii peninsula and Awaji Island. Plain Language Summary Shallow structure controls the level of ground shaking and hence is critical in assessing seismic hazards. Conventionally, drilling or local seismic surveys are performed to image the near surface, but their high cost has resulted in extremely sparse coverage of studied sites worldwide. Here we demonstrate that the directions of P waves that are naturally generated by local or distant earthquakes constrain the structure immediately below the seismometer where the P waves are recorded. Our finding suggests that frequency‐dependent measurements of P wave directions is a promising tool for studying shallow structure that is noninvasive and cost effective. Application to data recorded in Japan reveals structure at about 120‐ to 750‐m depth, including a weak zone along a segment of the Median Tectonic Line. Key Points The depth sensitivity of P wave polarization peaks at the surface and becomes negligible at a depth that is shallower than its wavelength The frequency content and the near‐surface shear wave speeds are the main factors that determine the depth sensitivity P wave polarization is a useful tool for constraining shallow 1‐D structure from about 10 m to several kilometers at 0.1 to 10 Hz