A next-generation network of seismic and wave sensors in the southwestern Pacific will warn coastal residents of an approaching tsunami before they see the wave.

<p><span>The occurrence of slow slip events (SSEs) in subduction zones has been proposed to be linked to the presence of, and fluctuations in near-lithostatic fluid pressures (P</span><sub><span>f</span></sub><span>) within the megathrust shear zone and subducting oceanic crust. In particular, the 'fault-valve' model is commonly used to describe occasional, repeated breaching of a low-permeability interface shear zone barrier, which caps an overpressured hydrothermal fluid reservoir. In this model, a precursory increase in fluid pressure may therefore be anticipated to precede megathrust rupture. Resulting activation of fractures during slip opens permeable pathways for fluid migration and fluid pressure decreases once more, until the system becomes sealed and overpressure can re-accumulate. While the priming conditions for cyclical valving behaviour have been observed at subduction zones globally, and evidence for post-megathrust rupture drainage exists, physical observations of precursory fluid pressure increases, and subsequent decreases, particularly within the subducting slab where hydrothermal fluids are sourced, remain elusive. </span></p><p><span>Here we use earthquake focal mechanisms recorded on an ocean-bottom seismic network to identify changes in the stress tensor within subducting oceanic crust during four SSEs in New Zealand’s Northern Hikurangi subduction zone. We show that the stress, or shape ratio, which describes the relative magnitudes of the principal compressive stress axes, shows repeated decreases prior to, and rapid increases during the occurrence of geodetically documented SSEs. We propose that these changes represent precursory accumulation and subsequent release of fluid pressure within overpressured subducting oceanic crust via a ‘valving’ model for megathrust slip behaviour. Our observations indicate that the timing of slow slip events on subduction megathrusts may be controlled by cyclical accumulation of fluid pressure within subducting oceanic crust.</span></p><p><span>Our model is further supported by observations of seismicity preceding a large SSE in the northern Hikurangi Margin in 2019, captured by ocean-bottom seismometer</span><span>s</span><span> and </span><span>absolute </span><span>pressure </span><span>recorders.</span> <span>O</span><span>bservations of microseismicity </span><span>during this period </span><span>indicate that a stress state conducive to vertical fluid flow was present in the downgoing plate prior to SSE initiation, before subsequently returning to a</span><span> down-dip</span><span> extensional state following the SSE. We propose this precursory seismicity is indicative of fluid migration towards the interface shear zone from the lower plate fluid reservoir, which may have helped triggering slip on the megathrust. </span></p><p><span>We also present preliminary results of a moment tensor study to investigate spatial and temporal patterns in earthquake source properties in SSE regions along the Hikurangi Margin. In particular, earthquakes near Porangahau – a region susceptible to dynamic triggering of tremor and where </span><span>shallow </span><span>SSEs occur every 5 years or so – exhibit distinctly lower double couple components than elsewhere along the margin. We </span><span>attribute this to elevated fluid pressures within the crust here, which is consistent with recent observations of high seismic reflectivity from an autocorrelation study. Such high fluid pressure may control the broad range of seismic and aseismic phenomena observed at Porangahau. </span></p>

<p>In recent years, Ocean Bottom Seismometers (OBSs) have become widely used to expand the coverage of seismic networks onto the ocean. This study takes advantage of offshore observations at the northern end of the Hikurangi margin and southwestern Okinawa Trough to study the tectonics in both regions.</p><p>In the Hikurangi subduction zone, slow slip events (SSEs) have been observed, which are caused by the subduction of the Pacific Plate under New Zealand. The behaviour of SSEs and how they influence the physical properties of Earth materials are open to question. From 2014 to 2015, 15 OBSs were deployed offshore Gisborne, New Zealand on the Hikurangi margin. Ambient noise data from the OBSs are used to study velocity changes related to SSEs. Single station cross-component correlations and auto-correlations are computed, from which coda waves are used to monitor the velocity changes before, during and after the SSEs in 2014 and 2015 to analyse the slow earthquake behaviour and its relation to stress changes. Different rotation on horizontal components is tested by rotating horizontal components to N-E direction and parallel-perpendicular to the coastline. The dv/v computed by different components or rotation show different changes. The averaged dv/v displays a 0.1% velocity decrease during the SSE in October 2014.</p><p>The southwestern Okinawa Trough tapers towards Taiwan. How the back-arc crust accommodates the narrowing processing remains to be understood. At various times between 2010 and 2017, 22 OBSs on a small scale (~0.2°×0.3°) were deployed in Southwestern Okinawa Trough offshore northeast Taiwan. Ambient noise recorded on vertical velocity and pressure sensors is used to retrieve Scholte waves for studying shear wave velocity structure. Phase velocities are forward-modeled according to a model proposed by Kuo et al. 2015 and shear strength and density results from ODP1202. Phase velocity dispersion curves are measured from cross-correlations and unwrapped according to the modeled phase velocities. The fundamental mode phase velocities averaged from different station pairs are 0.62 km/s at 3 s period and 1.56 km/s at 6.5 s period. A 3D inversion will be conducted for a shear wave velocity structure from the basin center to the edge.</p>

<p>Crustal earthquakes in low deform rate regions are rare in the human life span but bear heavy losses when occurring. Limited observations also hinter robust earthquake forecasts. In this study, we use a high-resolution catalog to investigate the triggering of the 2010-2011 Canterbury earthquake sequence, New Zealand. The seismic sequence occurred in the North Canterbury Plains, a low-stress, low-seismicity region relatively close to active plate boundaries where large earthquakes are frequent, such as the 2009 M<sub>W</sub> 7.8 Dusky Sound Earthquake. To map the post-seismic stress transfers of remote large events acting in the region, we calculate the temporal and spatial seismic rate changes in the crust from 2005 to the 2010 Mw 7.1 Darfield Earthquake, the first mainshock of the Canterbury sequence. We use template matching analysis to obtain a new high-resolution seismic catalog that includes events previously undetected by routine network monitoring. Detection quality is further established through the usage of a Support Vector Machine classifier. Using the new catalog, we observe a seismic quiescence on the North Canterbury Plain between Dusky Sound Earthquake and the Darfield Earthquake. The quiescence is accompanied by a reduced rate in micro-seismicity, suggesting a lowered b-value in the region primed for the Canterbury sequence. The lack of proof of dynamic or static triggering suggests that complex fault interactions lead to the onset of the Darfield Earthquake.</p>

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