Ultra‐long Duration of Seismic Ground Motion Arising From a Thick, Low‐Velocity Sedimentary Wedge. Issue 10 (17th October 2019) - Kaneko, Yoshihiro
Abstract: Sedimentary basins are known to amplify and increase the duration of ground motions that accompany earthquakes. A similar phenomenon is expected, but not as well documented, in low seismic‐velocity accretionary prisms along subduction margins. In this study, we report anomalously long duration of long‐period ground motions observed in the North Island of New Zealand during seismic wave propagation from the M 7.8 Kaikoura earthquake ∼600 km away. Unique waveform data captured by strong‐motion, high‐rate GPS and ocean bottom pressure sensors reveal that long‐period ground motions lasted longer than 450 s in the northeastern North Island. These waveforms indicate one of the longest durations of long‐period (>10 s) ground motions ever recorded at similar epicentral distances for comparable, large earthquakes. To understand the underlying mechanism, we use numerical simulations of seismic wave propagation. We find that a velocity model that includes an accretionary prism, modeled as a large‐scale (∼30, 000 km 2 ) wedge characterized by extremely low seismic wave speeds, can explain the observed long durations of long‐period ground motions as the reverberations of seismic waves within the low‐velocity wedge. We show that the long duration of long‐period ground motions leads to prolonged dynamic stressing on the plate interface, likely accentuating the triggering of slow slip that occurred following the Kaikoura earthquake. Accretionary prisms characterized by extremely low seismic velocities may enhance the generation of tsunami earthquakes and dynamic triggering of slow slip events observed in the northern Hikurangi and other subduction margins. Plain Language Summary: In this study, we discovered an unusually long duration (up to 8 minutes) of ground shaking near Gisborne, on the east coast of the North Island of New Zealand during the magnitude 7.8 Kaikoura earthquake in 2016, using data from accelerometer, GPS and ocean bottom pressure sensors. This is arguably the longest duration of long‐period ground shaking ever documented from similar magnitude 7–8 earthquakes worldwide. To resolve why this occurred, we use numerical simulations of seismic wave propagation during the Kaikoura earthquake to show that the unusually long ground shaking near Gisborne was caused by the presence of large‐scale (approximately 150, 000 cubic kilometers) body of sediments offshore and beneath the northeastern North Island. This long‐duration shaking was also responsible for triggering a large slow slip event on the Hikurangi subduction zone offshore Gisborne, over 600 km away from the Kaikoura earthquake. Our result has important local and global implications for ground shaking hazard in areas with similar geological characteristics to those in the northeastern North Island, and improves understanding of processes that can trigger slow slip events and earthquakes on subduction zones. Key Points: Anomalously long duration of long‐period ground motions was observed during seismic wave propagation from the 2016 M 7.8 Kaikoura earthquake Waveform modeling indicates the presence of a thick, unusually low seismic‐velocity wedge of material in the northeastern North Island of NZ Long‐duration ground motions caused by this wedge would enhance dynamic triggering of slow slip as well as hazard posed by ground shaking