Beamforming Eikonal Tomography of Long-Period Surface Waves from a Small-Aperture Dense Array: Active Fault Imaging in Southwest Taiwan

SUMMARY Small-aperture dense arrays provide dense spatial sampling of surface-wave wavefields, but conventional ambient-noise tomography using interstation dispersion measurements becomes difficult when surface-wave wavelengths approach or exceed the array aperture. We present Beamforming Eikonal Tomography (BET), which combines distant virtual sources, a beamforming-derived reference phase moveout, residual phase-delay measurements, and Eikonal analysis to reconstruct Rayleigh-wave phase-velocity tomography. Applied to a 96-station array spanning approximately 7 × 11 km2 in southwestern Taiwan, BET extends the usable period range beyond that achievable from internal station-pair measurements. Dense station spacing provides dense sampling of the phase-traveltime field, from which spatial gradients are estimated to recover lateral variations in Rayleigh-wave phase-velocity over periods of 3–8 s. Although the longer-period (7–8 s) maps are less well constrained because of the limited number and uneven back-azimuthal distribution of qualified virtual sources, and interpretation of fine-scale phase velocity anomalies should be treated with caution because of finite-frequency effects, the tomography reveals two broad low-phase-velocity belts spatially associated with mapped fault zones. These results demonstrate the potential of BET to image first-order lateral velocity heterogeneity across active fault systems using small-aperture dense arrays.