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Abstract The 1938M_S8.3 and 2021M_W8.2 earthquakes both ruptured within the Semidi segment of the Aleutian‐Alaska subduction zone. The large‐slip distribution of the 2021 event is well constrained within the depth range 25–45 km, with seaward tsunami observations excluding significant shallower coseismic slip. The 1938 event slip distribution is more uncertain. Regional and far‐field tide gauge observations for the 1938 event are modeled to constrain the location of large coseismic slip. The largest slip (2.0 m) is located below the continental shelf on a 180‐km‐long portion of the rupture extending further northeast than the 2021 rupture, to near Sitkinak Island. Minor slip (1.0 m) extends seaward under the continental slope to 8 km deep, where large slip may have occurred in 1788. The megathrust shallower than 25 km depth to the southwest experienced many small aftershocks and aseismic slip following the 2021 event, and has limited slip deficit. 

Abstract The subduction zone along Oaxaca, Mexico, has experienced multiple Mw ≥ 7 earthquakes that ruptured in close proximity several decades apart in at least three locations along the coast. Similarity of waveform recordings from a few long-period seismic stations at teleseismic distances has provided evidence for up to three repeated failures of the same slip patches, or persistent asperities, in the region. The evidence from prior single-station comparisons is bolstered by considering azimuthally distributed sets of body-wave recording pairs for the 1968 and 2018 Pinotepa Nacional (western Oaxaca), and 1965 and 2020 La Crucecita (eastern Oaxaca) earthquakes, as viewed in the long-period World-Wide Standardized Seismograph Network instrument passband (>5 s period). Drawing on detailed slip inversions for the most recent events and observations of their relationships with regional slow-slip events, we note features to be alert for in central Oaxaca where prior repeating events in 1928 and 1978 occurred and there is potential for a similar future event. 

Abstract The 2021 shallow plate‐boundary thrust‐faulting and 2023 outer rise normal‐faultingM_W7.7 earthquakes southeast of the Loyalty Islands produced significant, well‐recorded tsunamis around the North and South Fiji Basins. The two earthquakes occurred in close proximity on opposing sides of the Southern Vanuatu Trench with similar seismic moments and east‐west rupture lengths but different faulting mechanisms. This provides a basis to examine tsunami sensitivity to source geometry and location for paths in the complex southwest Pacific region. Finite‐fault models of the source processes for both events were inverted from teleseismic body wave data with constraints from forward, nonhydrostatic modeling of regional tide gauge and seafloor pressure sensor recordings. The wave motions are reversed in sign, with a leading crest generated by 1.31 m uplift on the upper plate slope for the 2021 tsunami and a leading trough from 2.37 m subsidence on the subducting plate near the trench for the 2023 tsunami. The more recent outer rise normal faulting produces narrower seafloor deformation beneath deeper water resulting in shorter period tsunami waves that shoal and refract more effectively along seamounts and island chains to produce a more elaborate radiation pattern. The source location relative to seamounts and small islands in the near field influences the energy lobes and directionality of the far‐field tsunami to the north. In contrast, both events have very similar radiation patterns to the south due to absence of major bathymetric features immediately southward of the sources. 

Abstract Most great earthquakes on subduction zone plate boundaries have large coseismic slip concentrated along the contact between the subducting slab and the upper plate crust. On 4 March 2021, a magnitude 7.4 foreshock struck 1 hr 47 min before a magnitude 8.1 earthquake along the northern Kermadec island arc. The mainshock is the largest well‐documented underthrusting event along the ∼2,500‐km long Tonga‐Kermadec subduction zone. Using teleseismic, geodetic, and tsunami data, we find that all substantial coseismic slip in the mainshock is located along the mantle/slab interface at depths from 20 to 55 km, with the large foreshock nucleating near the down‐dip edge. Smaller foreshocks and most aftershocks are located up‐dip of the mainshock, where substantial prior moderate thrust earthquake activity had occurred. The upper plate crust is ∼17 km thick in northern Kermadec with only moderate‐size events along the crust/slab interface. A 1976 sequence withM_Wvalues of 7.9, 7.8, 7.3, 7.0, and 7.0 that spanned the 2021 rupture zone also involved deep megathrust rupture along the mantle/slab contact, but distinct waveforms exclude repeating ruptures. Variable waveforms for eight deep M6.9+ thrusting earthquakes since 1990 suggest discrete slip patches distributed throughout the region. The ∼300‐km long plate boundary in northern Kermadec is the only documented subduction zone region where the largest modeled interplate earthquakes have ruptured along the mantle/slab interface, suggesting that local frictional properties of the putatively hydrated mantle wedge may involve a dense distribution of Antigorite‐rich patches with high slip rate velocity weakening behavior in this locale. 

Abstract On 18 November 2022, a large earthquake struck offshore southern Sumatra, generating a tsunami with 25 cm peak amplitude recorded at tide gauge station SBLT. OurW‐phase solution indicates a shallow dip of 6.2°, compatible with long‐period surface wave radiation patterns. Inversion of teleseismic body waves indicates a shallow slip distribution extending from about 10 km deep to near the trench with maximum slip of ∼4.1 m and seismic moment of  Nm (M_W7.3). Joint modeling of seismic and tsunami data indicates a shallow rigidity of ∼23 GPa. We find a low moment‐scaled radiated energy of , similar to that of the 2010M_W7.8 Mentawai event () and other tsunami earthquakes. These characteristics indicate that the 2022 event should be designated as a smaller moment magnitude tsunami earthquake compared to the other 12 well‐documented global occurrences since 1896. The 2022 event ruptured up‐dip of the 2007M_W8.4 Bengkulu earthquake, demonstrating shallow seismogenic capability of a megathrust that had experienced both a deeper seismic event and adjacent shallow aseismic afterslip. We consider seismogenic behavior of shallow megathrusts and concern for future tsunami earthquakes in subduction zones globally, noting a correlation between tsunami earthquake occurrence and subducting seafloor covered with siliceous pelagic sediments. We suggest that the combination of pelagic clay and siliceous sediments and rough seafloor topography near the trench play important roles in controlling the genesis of tsunami earthquakes along Sumatra and other regions, rather than the subduction tectonic framework of accretionary or erosive margin. 

Abstract Two major earthquakes (M_W7.8 and M_W7.7) ruptured left-lateral strike-slip faults of the East Anatolian Fault Zone (EAFZ) on February 6, 2023, causing >59,000 fatalities and ~$119B in damage in southeastern Türkiye and northwestern Syria. Here we derived kinematic rupture models for the two events by inverting extensive seismic and geodetic observations using complex 5-6 segment fault models constrained by satellite observations and relocated aftershocks. The larger event nucleated on a splay fault, and then propagated bilaterally ~350 km along the main EAFZ strand. The rupture speed varied from 2.5-4.5 km/s, and peak slip was ~8.1 m. 9-h later, the second event ruptured ~160 km along the curved northern EAFZ strand, with early bilateral supershear rupture velocity (>4 km/s) followed by a slower rupture speed (~3 km/s). Coulomb Failure stress increase imparted by the first event indicates plausible triggering of the doublet aftershock, along with loading of neighboring faults. 

Abstract Variations in fault zone maturity have intermittently been invoked to explain variations in some seismological observations for large earthquakes. However, the lack of a unified geological definition of fault maturity makes quantitative assessment of its importance difficult. We evaluate the degree of empirical correlation between geological and geometric measurements commonly invoked as indicative of fault zone maturity and remotely measured seismological source parameters of 34M_W ≥ 6.0 shallow strike‐slip events. Metrics based on surface rupture segmentation, such as number of segments and surface rupture azimuth changes, correlate best with seismic source attributes while the correlations with cumulative fault slip are weaker. Average rupture velocity shows the strongest correlation with metrics of maturity, followed by relative aftershock productivity. Mature faults have relatively lower aftershock productivity and higher rupture velocity. A more complex relation is found with moment‐scaled radiated energy. There appears to be distinct behavior of very immature events which radiate modest seismic energy, while intermediate mature faults have events with higher moment‐scaled radiated energy and very mature faults with increasing cumulative slip tend to have events with reduced moment‐scaled radiated energy. These empirical comparisons establish that there are relationships between remote seismological observations and fault system maturity that can help to understand variations in seismic hazard among different fault environments and to assess the relative maturity of inaccessible or blind fault systems for which direct observations of maturity are very limited. 

Abstract Foreshocks are the only currently widely identified precursory seismic behavior, yet their utility and even identifiability are problematic, in part because of extreme variation in behavior. Here, we establish some global trends that help identify the expected frequency of foreshocks as well the type of earthquake most prone to foreshocks. We establish these tendencies using the global earthquake catalog of the U.S. Geological Survey National Earthquake Information Center with a completeness level of magnitude 5 and mainshocks with Mw≥7.0. Foreshocks are identified using three clustering algorithms to address the challenge of distinguishing foreshocks from background activity. The methods give a range of 15%–43% of large mainshocks having at least one foreshock but a narrower range of 13%–26% having at least one foreshock with magnitude within two units of the mainshock magnitude. These observed global foreshock rates are similar to regional values for a completeness level of magnitude 3 using the same detection conditions. The foreshock sequences have distinctive characteristics with the global composite population b-values being lower for foreshocks than for aftershocks, an attribute that is also manifested in synthetic catalogs computed by epidemic-type aftershock sequences, which intrinsically involves only cascading processes. Focal mechanism similarity of foreshocks relative to mainshocks is more pronounced than for aftershocks. Despite these distinguishing characteristics of foreshock sequences, the conditions that promote high foreshock productivity are similar to those that promote high aftershock productivity. For instance, a modestly higher percentage of interplate mainshocks have foreshocks than intraplate mainshocks, and reverse faulting events slightly more commonly have foreshocks than normal or strike-slip-faulting mainshocks. The western circum-Pacific is prone to having slightly more foreshock activity than the eastern circum-Pacific. 

Abstract The 2021M_W6.0 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 ∼. The relatively low moment scaled radiated energyof 1.5 × 10⁻⁵and 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. 

Abstract On 29 July 2021, anM_W8.2 thrust‐faulting earthquake ruptured offshore of the Alaska Peninsula within the rupture zone of the 1938M_W8.2 earthquake. The spatiotemporal distribution of megathrust slip is resolved by jointly inverting regional and teleseismic broadband waveforms along with co‐seismic static and high‐rate GNSS displacements. The primarily unilateral rupture expanded northeastward, away from the rupture zone of the 22 July 2020M_W7.8 Shumagin earthquake. Large slip extends along approximately 175 km, spanning about two third of the estimated 1938 aftershock zone, with well‐bounded depth from 20 to 40 km, and up to 8.6 m slip near the hypocenter. The rupture terminated in the eastern portion of the 1938 aftershock zone in a region of very large geodetic slip deficit where peak slip appears to have occurred in the 1938 rupture. The 2021 and 1938 events do not have similar slip distributions and do not indicate persistent asperities.