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Key Points Periodic pore fluid pressure perturbations on a rate‐strengthening fault induce slow slip events (SSEs) Source properties of induced SSEs vary with perturbation characteristics (length scale, amplitude, period) Model reproduces source properties of shallow Hikurangi SSEs, and duration and magnitude of SSEs in different subduction zones 

Abstract Seamounts are found at many subduction zones and act as seafloor heterogeneities that affect slip behavior on megathrusts. At the Hikurangi subduction zone offshore the North Island, New Zealand, seamounts have been identified on the incoming Pacific plate and below the accretionary prism, but there is little concrete evidence for seamounts subducted beyond the present‐day coastline. Using a high‐resolution, adjoint tomography‐derived velocity model of the North Island, we identify two high‐velocity anomalies below the East Coast and an intraslab low‐velocity zone up‐dip of one of these anomalies. We interpret the high‐velocity anomalies as previously unidentified, deeply subducted seamounts, and the low‐velocity zone as fluid in the subducting slab. The seamounts are inferred to be 10–30 km wide and on the plate interface at 12–15 km depth. Resolution analysis using point spread functions confirms that these are well‐resolved features. The locations of the two seamounts coincide with bathymetric features whose geometries are consistent with those predicted from analog experiments and numerical simulations of seamount subduction. The spatial characteristics of seismicity and slow slip events near the inferred seamounts agree well with previous numerical modeling predictions of the effects of seamount subduction on megathrust stress and slip. Anomalous geophysical signatures, magnetic anomalies, and swarm seismicity have also been observed previously at one or both seamount locations. We propose that permanent fracturing of the northern Hikurangi upper plate by repeated seamount subduction may be responsible for the dichotomous slow slip behavior observed geodetically, and partly responsible for along‐strike variations in plate coupling on the Hikurangi subduction interface. 

Abstract We use earthquake‐based adjoint tomography to invert for three‐dimensional structure of the North Island, New Zealand, and the adjacent Hikurangi subduction zone. The study area, having a shallow depth to the plate interface below the North Island, offers a rare opportunity for imaging material properties at an active subduction zone using land‐based measurements. Starting from an initial model derived using ray tomography, we perform iterative model updates using spectral element and adjoint simulations to fit waveforms with periods ranging from 4–30 s. We perform 28 model updates using an L‐BFGS optimization algorithm, improving data fit and introducingP‐ andS‐wave velocity changes of up to ±30%. Resolution analysis using point spread functions show that our measurements are most sensitive to heterogeneities in the upper 30 km. The most striking velocity changes coincide with areas related to the active Hikurangi subduction zone. Lateral velocity structures in the upper 5 km correlate well with New Zealand geology. The inversion reveals increased along‐strike heterogeneity on the margin. In Cook Strait we observe a low‐velocity zone interpreted as deep sedimentary basins. In the central North Island, low‐velocity anomalies are linked to surface geology, and we relate velocity structures at depth to crustal magmatic activity below the Taupō Volcanic Zone. Our velocity model provides more accurate synthetic seismograms with respect to the initial model, better constrains small (50 km), shallow (15 km) and near‐offshore velocity structures, and improves our understanding of volcanic and tectonic structures related to the active Hikurangi subduction zone. 

ABSTRACT Numerical modeling of earthquake dynamics and derived insight for seismic hazard relies on credible, reproducible model results. The sequences of earthquakes and aseismic slip (SEAS) initiative has set out to facilitate community code comparisons, and verify and advance the next generation of physics-based earthquake models that reproduce all phases of the seismic cycle. With the goal of advancing SEAS models to robustly incorporate physical and geometrical complexities, here we present code comparison results from two new benchmark problems: BP1-FD considers full elastodynamic effects, and BP3-QD considers dipping fault geometries. Seven and eight modeling groups participated in BP1-FD and BP3-QD, respectively, allowing us to explore these physical ingredients across multiple codes and better understand associated numerical considerations. With new comparison metrics, we find that numerical resolution and computational domain size are critical parameters to obtain matching results. Codes for BP1-FD implement different criteria for switching between quasi-static and dynamic solvers, which require tuning to obtain matching results. In BP3-QD, proper remote boundary conditions consistent with specified rigid body translation are required to obtain matching surface displacements. With these numerical and mathematical issues resolved, we obtain excellent quantitative agreements among codes in earthquake interevent times, event moments, and coseismic slip, with reasonable agreements made in peak slip rates and rupture arrival time. We find that including full inertial effects generates events with larger slip rates and rupture speeds compared to the quasi-dynamic counterpart. For BP3-QD, both dip angle and sense of motion (thrust versus normal faulting) alter ground motion on the hanging and foot walls, and influence event patterns, with some sequences exhibiting similar-size characteristic earthquakes, and others exhibiting different-size events. These findings underscore the importance of considering full elastodynamics and nonvertical dip angles in SEAS models, as both influence short- and long-term earthquake behavior and are relevant to seismic hazard. 

Abstract Mature strike‐slip faults are usually surrounded by a narrow zone of damaged rocks characterized by low seismic wave velocities. Observations of earthquakes along such faults indicate that seismicity is highly concentrated within this fault damage zone. However, the long‐term influence of the fault damage zone on complete earthquake cycles, that is, years to centuries, is not well understood. We simulate aseismic slip and dynamic earthquake rupture on a vertical strike‐slip fault surrounded by a fault damage zone for a thousand‐year timescale using fault zone material properties and geometries motivated by observations along major strike‐slip faults. The fault damage zone is approximated asan elastic layer with lower shear wave velocity than the surrounding rock. We find that dynamic wave reflections, whose characteristics are strongly dependent on the width and the rigidity contrast of the fault damage zone, have a prominent effect on the stressing history of the fault. The presence of elastic damage can partially explain the variability in the earthquake sizes and hypocenter locations along a single fault, which vary with fault damage zone depth, width and rigidity contrast from the host rock. The depth extent of the fault damage zone has a pronounced effect on the earthquake hypocenter locations, and shallower fault damage zones favor shallower hypocenters with a bimodal distribution of seismicity along depth. Our findings also suggest significant effects on the hypocenter distribution when the fault damage zone penetrates to the nucleation sites of earthquakes, likely being influenced by both lithological (material) and rheological (frictional) boundaries.