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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 M7.8 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 km2) 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 M7.8 Kaikoura earthquakeWaveform modeling indicates the presence of a thick, unusually low seismic‐velocity wedge of material in the northeastern North Island of NZLong‐duration ground motions caused by this wedge would enhance dynamic triggering of slow slip as well as hazard posed by ground shaking [ABSTRACT FROM AUTHOR]

A slow slip event (SSE) and tectonic tremors occurred approximately 1.5 month prior to the occurrence of the 2011 Tohoku‐Oki earthquake near the Japan Trench. Although we cannot completely rule out other sources of noise near the observation site, slow earthquakes would demonstrate the nucleation of a fast slip event from prior slow earthquake activity. Here we report on tectonic tremor activity detected from short‐period ocean‐bottom seismometer (OBS) data with a modified frequency scanning method that allows robust tectonic tremor detection by incorporating information from multiple frequency passbands in order to suppress false detections. We have identified at least five tectonic tremor sequences on the OBS records from the end of January 2011 to just before the occurrence of the Tohoku‐Oki earthquake in March 2011. To reveal the spatial distributions of tectonic tremors before the Tohoku‐Oki earthquake, we then calculated tremor energy, with corrected site amplification, from the identified tectonic tremor time window at each OBS that detected tremors. Possible sources of tremors were distributed in trenchward side of the SSE fault prior to the main shock. These tremors could be modulated by SSE to take into account consistent timing between tremor and SSE term, and consistency of spatial locations between estimated tremor sources and SSE faults. This spatiotemporal pattern in tectonic tremor activity demonstrates a significant separation of occurrence sources between tectonic tremors and regular earthquakes. Key Points: A modified frequency scanning method allowed investigation of tectonic tremors occurring before the largest Tohoku‐Oki foreshockThis method revealed details of tremor activity in the rupture area of Tohoku‐Oki earthquake, especially close to the Japan TrenchTemporal and spatial changes in tectonic tremor activity reflected slow slip event expansion processes that occur before large earthquakes [ABSTRACT FROM AUTHOR]

The Te Araroa earthquake occurred on 2 September 2016 (local time) offshore of the northeastern coast of New Zealand's North Island (Mw 7.1). When this event occurred, ocean bottom pressure gauges (OBPs), installed ~170 km south of the source area, clearly recorded direct tsunami from the source to OBPs (~ −1.5 cm), and tsunami from coastal reflections (~2 cm). We estimate the centroid location that best reproduces the OBP waveforms. When using the direct wave alone, the centroid location is poorly constrained, with a horizontal uncertainty of ~100 km. By combining both direct and reflected tsunami waveforms, we obtain a centroid location near the Global Centroid Moment Tensor centroid (~80 km northeast from the coast) with smaller uncertainty (~40 km). We also estimate the earthquake source dimension (length and width) and found that the models using coastal reflections require a source dimension larger than ~30 km long. Based on the slip distribution obtained by the finite fault inversion, we obtain an energy‐based stress drop ΔσE of 1.0 MPa, consistent with typical earthquake stress drop values. This study shows that the information added by coastal reflected tsunami provides much tighter constraints on the centroid location, source dimension, and stress drop of offshore earthquakes, which is difficult to obtain from the onshore seismic data alone. Future studies should utilize the information provided by coastal reflected waves to improve earthquake source modeling using ocean bottom pressure data. Key Points: Tsunami from coastal reflected waves associated with the 2016 Te Araroa EQ are clearly observed by offshore ocean bottom pressure gaugesWe greatly reduce uncertainties in the centroid location and fault dimensions of the Te Araroa EQ using tsunami reflected from the coastLater tsunami arrivals should be used more widely to extract earthquake source information in the future [ABSTRACT FROM AUTHOR]