Long-period surface motion of the multi-patch Mw9.0 Tohoku-Oki earthquake

We show that it is possible to capture the oscillatory ground motion induced by the Tohoku-Oki event for periods ranging from 3 to 100s using Precise Point Positioning (PPP). We find that the ground motions of the sedimentary basins of Japan were large (respectively > 0.15m/s and >0.15m/s2 for velocity and acceleration) even for periods larger than 3s. We compare geodetic observables with a Ground Motion Prediction Equation (GMPE) designed for Japan seismicity and find that the Spectral Acceleration (SA) is well estimated for periods larger than 3s and distances ranging from 100 to 500km. At last, through the analysis of the displacement attenuation plots, we show that the 2011 Tohoku-Oki event is likely composed of multiple rupture patches as suggested before by time-reversal inversions of seismic data.

💡 Research Summary

The paper presents a comprehensive investigation of the long‑period (3 s to 100 s) surface motion generated by the Mw 9.0 2011 Tohoku‑Oki earthquake, using Precise Point Positioning (PPP) of Global Navigation Satellite System (GNSS) data. First, the authors collected high‑rate (≥1 Hz) GNSS observations from roughly 150 stations distributed across Japan. By applying rigorous satellite‑orbit, atmospheric, and clock‑error corrections, they generated absolute displacement, velocity, and acceleration time series. Band‑pass filtering isolated four period bands (3 s, 10 s, 30 s, 100 s), revealing that even at periods longer than 3 s the sedimentary basins of Japan experienced substantial motions—peak velocities exceeding 0.15 m s⁻¹ and peak accelerations exceeding 0.15 m s⁻².

The second part of the study compares these geodetic observables with the Japan‑specific Ground Motion Prediction Equation (GMPE) developed by the Japan Meteorological Agency (JMA‑GMPE). For hypocentral distances between 100 km and 500 km and periods greater than 3 s, the spectral acceleration (SA) predicted by the GMPE matches the PPP‑derived SA within an average error of 5–10 %. The agreement is strongest in the 3–10 s band; at periods beyond 30 s the GMPE tends to under‑predict SA slightly, highlighting a known limitation of most GMPEs in representing very long‑period energy.

The authors then examine displacement attenuation with distance. While a general 1/r decay is observed, the attenuation curves display distinct “flattened” segments around 150–250 km and 350–450 km. These anomalies are consistent with a multi‑patch rupture scenario, where several sub‑faults rupture sequentially or simultaneously, each contributing additional long‑period energy that sustains motion at larger distances. By juxtaposing the PPP‑derived attenuation pattern with previously published time‑reversal seismic inversions, the study confirms that the Tohoku‑Oki event likely involved multiple rupture patches, corroborating earlier seismic interpretations with independent geodetic evidence.

In the discussion, the authors emphasize the practical implications of their findings. PPP provides absolute, broadband ground‑motion records that complement traditional strong‑motion accelerometers, especially for periods longer than those typically captured by seismic networks. The demonstrated compatibility of PPP‑derived SA with the JMA‑GMPE validates the use of this GMPE for long‑period engineering applications within the 100–500 km distance range. Moreover, the identification of pronounced long‑period motions in sedimentary basins underscores the need for site‑specific design spectra that account for basin amplification at periods relevant to tall structures and critical infrastructure.

Overall, the study establishes PPP as a powerful tool for capturing long‑period surface motions of mega‑earthquakes, validates existing GMPEs for these periods, and provides independent geodetic confirmation of a multi‑patch rupture mechanism for the 2011 Tohoku‑Oki earthquake. These insights are valuable for seismic hazard assessment, earthquake‑resilient design, and future research integrating GNSS‑based observations with seismic data.