The main cause of tsunamis is large subduction zone earthquakes with seismic magnitudes Mw > 7, but submarine volcanic processes can also generate tsunamis. At the submarine Sumisu caldera in the Izu–Bonin arc, moderate‐sized earthquakes with Mw < 6 occur almost once a decade and cause meter‐scale tsunamis. The source mechanism of the volcanic earthquakes is poorly understood. Here we use tsunami and seismic data from the recent 2015 event to show that abrupt uplift of the submarine caldera, with a large brittle rupture of the ring fault system due to overpressure in its magma reservoir, caused the earthquake and tsunami. This submarine trapdoor faulting mechanism can efficiently generate tsunamis due to large vertical seafloor displacements, but it inefficiently radiates long‐period seismic waves. Similar seismic radiation patterns and tsunami waveforms due to repeated earthquakes indicate that continuous magma supply into the caldera induces quasi‐regular trapdoor faulting. This mechanism of tsunami generation by submarine trapdoor faulting underscores the need to monitor submarine calderas for robust assessment of tsunami hazards.

A great earthquake struck the Semidi segment of the plate boundary along the Alaska Peninsula on 29 July 2021, re‐rupturing part of the 1938 rupture zone. The 2021 MW 8.2 Chignik earthquake occurred just northeast of the 22 July 2020 MW 7.8 Simeonof earthquake, with little slip overlap. Analysis of teleseismic P and SH waves, regional Global Navigation Satellite System (GNSS) displacements, and near‐field and far‐field tsunami observations provides a good resolution of the 2021 rupture process. During ∼60‐s long faulting, the slip was nonuniformly distributed along the megathrust over depths from 32 to 40 km, with up to ∼12.9‐m slip in an ∼170‐km‐long patch. The 40–45 km down‐dip limit of slip is well constrained by GNSS observations along the Alaska Peninsula. Tsunami observations preclude significant slip from extending to depths <25 km, confining all coseismic slip to beneath the shallow continental shelf. Most aftershocks locate seaward of the large‐slip zones, with a concentration of activity up‐dip of the deeper southwestern slip zone. Some localized aftershock patches locate beneath the continental slope. The surface‐wave magnitude MS of 8.1 for the 2021 earthquake is smaller than MS = 8.3–8.4 for the 1938 event. Seismic and tsunami data indicate that slip in 1938 was concentrated in the eastern region of its aftershock zone, extending beyond the Semidi Islands, where the 2021 event did not rupture.

Large megathrust earthquakes on the subduction interface extend from near-trench to depths and display very different depth-varying slip behaviors (Lay et al., 2012). Large earthquakes that rupture the shallowest portion of the subduction interface (<15 km) can generate devastating tsunamis, but they appear to rupture slowly with inefficient excitation of short-period seismic waves disproportionately to their seismic moment and tsunami. These earthquakes are "tsunami earthquakes" (Kanamori, 1972). At deeper depths (15-50 km), large thrust earthquakes have faster rupture velocities and stronger radiation of short-period seismic energy with inefficient tsunami generation. Their contrasting rupture characteristics are well interpreted by two distinct types of fault properties; the slow slip of shallow tsunami earthquakes is commonly attributed to weak sediments and low rigidity of the upper plate (Bilek & Lay, 1999;Prada et al., 2021;Sallarès & Ranero, 2019), while the brittle failures of unstable fault patches explain the fast deeper earthquakes.On 12 August 2021, a great earthquake (M w > 8) struck the South Sandwich Island region of the south Atlantic Ocean (Figure 1a). This event occurred close to the South Sandwich trench, where the South American plate subducts beneath the South Sandwich plate at a velocity of 7 cm/year (Pelayo & Wiens, 1989). A remarkable observation of this earthquake is its far reaching-tsunamis. The tsunamis spread to the north Atlantic, Pacific, and Indian Oceans, where tide gauges measured peak amplitudes of ∼20 cm at over 10,000 km distance from the source (Figure S1 in Supporting Information S1). Although modeling these tide gauge observations is challenging because of the lack of detailed bathymetry data between the source and gauges, the observed tsunamis at global distances appear to suggest that the South Sandwich Island earthquake could be categorized as a regular shallow tsunamigenic earthquake.However, the South Sandwich Island earthquake seems to have extended to large depths with a complex temporal history. The early report (PDE) from the National Earthquake Information Center (NEIC) of the US Geological Survey listed two events within 3 min: (a) NEIC1,

The 2021 MW 6.0 Yangbi, Yunnan strike‐slip earthquake occurred on an unmapped crustal fault near the Weixi‐Qiaoho‐Weishan Fault along the southeast margin of the Tibetan Plateau. Using near‐source broadband seismic data from ChinArray, we investigate the spatial and temporal rupture evolution of the mainshock using apparent moment‐rate functions (AMRFs) determined by the empirical Green's function (EGF) method. Assuming a 1D line source on the fault plane, the rupture propagated unilaterally southeastward (∼144°) over a rupture length of ∼8.0 km with an estimated rupture speed of 2.1 km/s to 2.4 km/s. A 2D coseismic slip distribution for an assumed maximum rupture propagation speed of 2.2 km/s indicates that the rupture propagated to the southeast ∼8.0 km along strike and ∼5.0 km downdip with a peak slip of ∼2.1 m before stopping near the largest foreshock, where three bifurcating subfaults intersect. Using the AMRFs, the radiated energy of the mainshock is estimated as ∼1.6×10130.25emnormalJ $1.6\times {10}^{13}\,\mathrm{J}$. The relatively low moment scaled radiated energy ER/M0 ${E}_{R}/{M}_{0}$ of 1.5 × 10−5 and intense foreshock and aftershock activity might indicate reactivation of an immature fault. The earthquake sequence is mainly distributed along a northwest‐southeast trend, and aftershocks and foreshocks are distributed near the periphery of the mainshock large‐slip area, suggesting that the stress in the mainshock slip zone is significantly reduced to below the level for more than a few overlapping aftershock to occur.

Large tsunamis are generated by moderate-sized volcanic earthquakes at a submarine caldera.• Tsunami and seismic data indicate that abrupt uplift of the submarine caldera by trapdoor faulting causes large tsunamis.• Continuous magma supply into the submarine caldera induces submarine trapdoor faulting on a decadal timescale.