Spatial Patterns of Megathrust Seismogenic Behavior Modulated by a Subducting Seamount

Subducting bathymetric reliefs, such as seamounts, modify the slip behavior of megathrusts, thereby potentially dictating seismic segmentation, rupture dynamics, and the structural evolution of the subduction channel and upper plate. While geodetic data often suggest that the megathrust near subducting seamounts is weakly coupled and dominated by aseismic creep or microseismicity, several "seamount earthquakes" have been documented. The role of subducting topography in governing fault coupling, rupture dynamics, and the spatial distribution of rupture remains poorly understood.

Laboratory seismotectonic experiments provide an effective means of simulating earthquake cycles and observing fault slip behaviour with high spatiotemporal precision, thereby overcoming the limitations of sparse onshore and missing offshore geodetic networks, as well as short historical records. In our experiments, a topographic high with seamount geometry was subducted along a 15° dipping, velocity-weakening seismogenic zone accompanied by hundreds of analogue earthquake cycles. The model upper plate is a wedge composed of an elastoplastic granular material that can respond to seismic cycles and seamount-induced stresses. We constrained the interface slip distribution by combining analogue geodetic slip inversion of surface displacement with direct monitoring of the interface via side-view imaging.

The results reveal that during the early stages of seamount subduction, when the seamount has partially subducted beneath the upper plate, along-strike rupture propagation is arrested at the seamount, which acts as a barrier, producing partial ruptures. Progressively, as the main portion or the entire seamount becomes subducted, another consistent spatial pattern emerges: coseismic slip concentrates at the leading downdip edge of the seamount, while the center and updip regions remain largely aseismic, with minor shallow slip reflecting slope instabilities triggered by upper-plate extensional structures. This pattern aligns well with interseismic high-coupling patches, which can also extend to the deep flank of the seamount.

Our findings indicate that, while subducting seamounts inhibit earthquake nucleation and broadly arrest rupture propagation, they still allow slip to extend onto the seamount-bearing interface. This explains why the deeper flank of a subducting seamount or ridge remains seismically active. A series of earthquakes (1996 Mw 6.7 and Mw 6.8; 2024 Mw 7.1; 2025 Mw 6.8) systematically occurred around the downdip edge of a Kyushu-Palau Ridge. Similar rupture behavior has been documented for a series of Mw ~ 7 events in the southern Japan Trench and for the two Mw > 8 events in central Nankai. This spatial pattern is further supported by geological evidence of pseudotachylytes, which are only localized on the downdip side of the exhumed fossil seamount.

Beyond slip kinematics, our experiments demonstrate that subducting seamounts perturb the megathrust stress field, leading to heterogeneous stress accumulation along dip, consistent with previous numerical mechanical-hydrological modeling studies. This suggests that seamount-induced coupling enhances upper-plate deformation and long-term structural features, including forearc uplift, fault reactivation, and localized fracturing. The short- and long-term upper-plate deformation patterns provide a key means of identifying subducted topographic features and assessing their impact on earthquake and tsunami hazards.