Toward a Better Understanding of Spatiotemporal Rupture Evolution Through a Concrete Accounting of Site Effects: The Iwate–Miyagi 2008 Case

ABSTRACT Local soil conditions significantly influence the characteristics of observed seismic waves, often obscuring or distorting the original source signature and modifying recorded waveforms. Local backprojection (BP) imaging, a common technique for identifying the spatiotemporal high-frequency energy release during seismic ruptures, typically relies on strong motion data that is frequently not installed on bedrock. We evaluate the impact of soil effects on local BP images by analyzing data obtained from the 2008 Iwate–Miyagi Nairiku earthquake and recordings from both surface and downhole stations of the Kiban–Kyoshin network. Initially, we conducted a detailed study of sites near the seismic source. Surface station data were corrected for site effects using horizontal-to-vertical spectral ratios and established correction methods that account for nonlinearity effects, providing accurate estimations of horizontal amplification, which we then eliminate from the seismic signals. Extensive comparisons between corrected surface records, uncorrected surface records, and borehole records reveal that uncorrected surface records can distort BP coherence, leading to inaccurate results. However, applying site effect corrections significantly enhances BP image quality, aligning outcomes more closely with those from borehole records. In addition, we examine the site independence of borehole data concerning site effects. This study highlights the importance of incorporating site effect corrections in BP analyses to improve seismic data accuracy. Finally, we propose a comprehensive workflow for integrating these corrections into future BP studies.