Yen Joe Tan

Compared to continental strike‐slip faults, oceanic transform faults (OTFs) are thought to mainly slip aseismically and host significantly more foreshocks triggered by precursory aseismic slip which enhance the mainshocks' short‐term predictability. However, long‐term high‐resolution observational constraints remain limited. In December 2024, one of the largest ever OTF earthquakes occurred offshore California on the Mendocino OTF. Here we show that foreshock activity is very limited within the rupture zone of this moment magnitude ( M _w ) 7.0 earthquake and does not indicate accelerating aseismic slip in the preceding month. The 2016 M _w 6.6 and 1994 M _w 7.0 Mendocino OTF earthquakes share similar characteristics. The 15 historical M _w ≥ 5.5 mainshocks also have significantly fewer foreshocks on average compared to continental strike‐slip earthquakes. Therefore, there is no clear evidence of aseismic‐nucleation‐related seismicity preceding large Mendocino OTF earthquakes. Our results further demonstrate that enhanced foreshock activity is not a general characteristic of OTFs despite prevalent aseismic slip.

The traditional view that volcanoes are underlain by a single, melt-dominated magma chamber has recently evolved into the idea that subsurface melt is heterogeneously distributed in a larger magma domain. While seismic imaging of small melt bodies beneath subaerial volcanoes remains difficult, melt bodies located several kilometers from mid-ocean ridge axes and submarine volcanic edifices have been identified. Nevertheless, the connectivity of these melt bodies within the magma domain and their role during eruptions remain unclear. We analyzed continuous ocean-bottom seismic data recorded in 2015–2025 CE that detected 415 volcanic tremor episodes that emerged ~60 h before, and continued during, the 2015 Axial Seamount eruption. The volcanic tremors initiated ~19 km southeast of the caldera before migrating into it and showed spatial overlap with funnel-shaped axial melt lenses and/or lithosphere-asthenosphere boundaries. Our results suggest that these volcanic tremors reflect the activation of or transport through distributed melt bodies over 10 km from the caldera, which provided enhanced magma supply for Axial Seamount's eruption and highlight a broader interconnected active magma plumbing system where melt can be mobilized within a matter of days for an eruption.

Due to the collision between the Indian and Eurasian plates and the southeastward compression of the Tibetan Plateau, western Yunnan is one of the regions in China with the most active seismic and volcanic activities as well as the most complex fault structures. We first built a deep-learning-based high-precision earthquake catalog for the Tengchong volcanic field over the past decade and found that 1) ∼59% of the seismicity occurred as swarms but on faults aligned with the regional tectonic stress field; 2) all swarms contained fluid-diffusion-like migration fronts; and 3) a year-long swarm, including two ML 5.2 earthquakes within two months, revealed complex fluid-fault interaction. Combined with the historical occurrences of M >6 earthquake swarms around the Tengchong volcanic field, our observations suggest potential increased likelihood of swarms with large-magnitude earthquakes where large tectonic faults and magmatic systems intersect.With the aid of machine-learning-based detection, we then outlined a complex 3D fault zone accommodating a small earthquake swarm near Yunlong city, western Yunnan, China from February to May 2013. Our results showed that the swarm initiated from a compressive stepover zone and subsequently activated a complex fault zone including six planar fault segments. The migration front of the swarm can be well-modeled by fluid diffusion, indicating the swarm was primarily driven by pressurized fluid. Within the stepover zone, complex and dense fractures act as conduits connecting the reservoir and fault zone, facilitating fluid flow. Meanwhile, the stress in the stepover zone tends to increase in response to the compression of the two boundary faults, which not only makes the stepover zone more susceptible to be triggered by those transient stresses but also forms a fluid pumping mechanism that drives fluids from the stepover zone into the complex fault zone.By integrating a deep-learning-based phase picker and an improved match-and-locate algorithm, we constructed a high -precision foreshock catalogue for the 2021 Yangbi earthquake. Meanwhile, a high-resolution earthquake source region velocity structure for the 2021 Yangbi sequence was also inverted in our study. Our results suggest that natural fluid diffusion is likely a driver of the Yangbi foreshock sequence based on three lines of evidence: 1) regions with low Vs and relatively high Vp/Vs are widespread within the fault system; 2) earliest foreshocks exhibit diffusion-like migration front, and 3) foreshock evolution coincides with typical fault valving behavior, and a few low-frequency signals whose distribution coincides with high Vp/Vs patches was clearly identified, strongly suggesting that fluid diffusion influenced nucleation.Therefore, we propose that the interaction between the widely distributed fluids and faults in western Yunnan may play a crucial role in the seismic activity.

Spatiotemporal correlations between moderate-to-large earthquakes and volcanic eruptions indicate that earthquake-induced stress changes can trigger eruptions. Triggering is rarely instantaneous, with time-dependent processes inferred to mediate magma ascent to the surface, such as fracture propagation or fluid pressurization. While various processes have been proposed, observational constraints on the specific triggering mechanisms remain limited. Here we integrate an earthquake catalogue, generated by machine-learning and template-matching techniques to enhance microseismicity detections, with geodetic data to show that seismic triggering of the 2018 eruption of Sierra Negra Volcano, Galápagos Islands, was facilitated by a cryptic phase of fluid activity. Following 13 years of near-continuous magmatic inflation, totalling > 6.5 m, stress changes induced by a moment magnitude 5.4 intra-caldera earthquake on 26 June 2018 did not immediately trigger magma release from the pressurized magma reservoir. Instead, long-period earthquake swarms revealed post-seismic fluid activity along reservoir-bounding faults in the northwestern caldera, locally promoting edifice failure and facilitating magma intrusion that initiated 8 hours later. These observations demonstrate that even when a pressurized reservoir experiences significant stress perturbation, cascading processes could potentially be essential to initiate volcanic eruptions.

Most of Earth’s volcanic eruptions are hidden beneath the ocean in complete darkness. Recent studies suggested that a type of impulsive event can track submarine lava flows, but their source mechanism remains uncertain. We analyze >20,000 impulsive events from the 2015 Axial Seamount eruption and find that their seismo-acoustic waveform characteristics suggest an implosive source mechanism. Integrating constraints from their spatiotemporal evolution with heat transfer estimates and geological observations, we propose that while the largest events might be related to volatiles degassed from magma, most events are generated by the implosion of bubbles formed from the vaporization of entrapped seawater by hot erupted lava. Similar events have been detected at other seamounts and slow to fast-spreading mid-ocean ridges, although eruptions at >3000-meter depth have proportionately fewer events because seawater vaporization is inhibited. Therefore, these impulsive seafloor events can be leveraged to remotely characterize eruption dynamics in most submarine volcanic settings.

At some submarine volcanoes, the influx and output of magma vary over time producing years‐to‐decades‐long cycles of inflation and deflation, which in turn cause pronounced physical changes in the overlying oceanic crust and the hydrothermal circulation hosted within. Permeability within the oceanic crust exerts primary control on seafloor fluid circulation and hence has important influences on the heat and chemical exchange between the earth's lithosphere and oceanic hydrosphere, as well as surface and subsurface biological communities. Despite its importance, permeability is one of the most poorly constrained hydrologic properties for most of the mid‐ocean ridge system. In this study, harmonic analysis of a high‐resolution, long‐term time series of effluent temperature measured at a high‐temperature hydrothermal vent on Axial Seamount yields time‐varying estimates of the effective permeability within the hydrothermal upflow zone. Comparing the records of permeability and volcanic deformation during and after the April–May 2015 eruption at Axial suggests a decrease in upflow‐zone permeability during co‐eruptive deflation and an increase in permeability during post‐eruption re‐inflation from July 2015 to June 2019. Modeling of the three‐dimensional strain field suggests that the temporal variations in effective upflow‐zone permeability can be explained by narrowing and expanding of hydrothermal pathways that accompany crustal compression and extension in relation to the volcano's deformation cycle.

Deep long‐period earthquakes (DLPs) are often detected near volcanoes from the crust down to the upper mantle. Exhibiting coincidence with volcanic eruptions, DLPs are recognized as potential precursors to volcanic activities yet their detection remains challenging. Meanwhile, their relation to volcanic activities and specific source mechanisms remains uncertain. In this study, we first classify earthquakes into volcano‐tectonic (VTs) and long‐period (LPs) earthquakes at 10 Alaska volcanoes automatically by the consistent differences in frequency index. The co‐location of both VTs and DLPs implies that the differences in their frequency content mainly arise from source effects. We then analyze 12 years of continuous waveforms by template matching and detect 5,421 new DLPs. Few potential repeating DLPs are identified, indicating their source processes are primarily non‐repetitive. Detections of mantle DLPs and positive correlations between DLPs and volume change rate in the magma reservoir support the involvement of magmatic activities in some DLP sources, though there is limited correlation in DLP depth distribution with magmatic water content and kinematic parameters of plate motion. In addition, since only ∼17% of DLP bursts correlate with eruptions, with ∼8% occurring simultaneously with VT bursts, this suggests that most DLP bursts at Alaska volcanoes are not directly linked to shallow volcanic unrest. Therefore, their significance as potential precursors requires cautious evaluation before operational use in eruption forecasting.

Climate change and urbanisation increase the risk of landslides, particularly mass movements with high mobility that can travel long distances and cause significant casualties and economic losses. Despite their impact, the mechanisms controlling long-runout landslides remain poorly understood due to their complexity. This study investigates the catastrophic 2000 Yigong landslide on the Tibetan plateau, using seismic signals to invert for the force–time function during its movement and evaluate changes in the basal friction coefficient. We identified five distinct stages in the event’s dynamics, beginning with two rock collapses in the source zone during the acceleration phase. In the entrainment phase, the mass attained its maximum velocity and forces when it impacted the Zhamu Creek. Notably, the equivalent friction coefficient dropped from 0.58 to 0.06, indicating a transition from rock collapse to debris flow. Our velocity-friction analysis aligns with flash heating theory, suggesting that high-speed sliding between rough surfaces generated heat, which partially melted the glacier and introduced fluid into the sliding process, resulting in enhanced mobility and facilitated long-runout distances down the Zhamu Creek. This study highlights the value of seismic signals in understanding the physical mechanisms behind catastrophic landslides and addressing the challenges of monitoring these geohazards in remote regions.