Large earthquakes rupture faults over hundreds of kilometers within minutes. Finite-fault models elucidate these processes and provide observational constraints for understanding earthquake physics. However, finite-fault inversions are subject to non-uniqueness and substantial uncertainties. The diverse range of published models for the well-recorded 2011 M_w 9.0 Tohoku-Oki earthquake aptly illustrates this issue, and details of its rupture process remain under debate. Here, we comprehensively compare 32 finite-fault models of the Tohoku-Oki earthquake and analyze the sensitivity of three commonly-used observational data types (geodetic, seismic, and tsunami) to the slip features identified. We first project all models to a realistic megathrust geometry and a 1-km subfault size. At this scale, we observe poor correlation among the models, irrespective of the data type. However, model agreement improves significantly when subfault sizes are increased, implying that their differences primarily stem from small-scale features. We then forward-compute geodetic and teleseismic synthetics and compare them with observations. We find that seismic observations are sensitive to rupture propagation, such as the peak-slip-rise time. However, neither teleseismic nor geodetic observations are sensitive to spatial slip features smaller than 64 km. In distinction, the synthesized seafloor deformation of all models exhibits poor correlation, indicating sensitivity to small-scale slip features. Our findings suggest that fine-scale slip features cannot be unambiguously resolved by remote or sparse observations, such as the three data types tested in this study. However, better resolution may become achievable from uniformly gridded dense offshore instrumentation.

Large earthquakes rupture faults over hundreds of kilometers within minutes. Finite-fault models image these processes and provide observational constraints for understanding earthquake physics. However, finite-fault inversions are subject to non-uniqueness and uncertainties. The diverse range of published models for the well-recorded 2011 $M_w$~9.0 Tohoku-Oki earthquake illustrates this issue, and details of its rupture process remain under debate. Here, we comprehensively compare 32 finite-fault models of the Tohoku-Oki earthquake and analyze the sensitivity of four commonly-used observational data types (geodetic, teleseismic, regional seismic-geodetic, and tsunami) to their slip features. We first project all models to a realistic megathrust geometry and a 1-km subfault size. At this scale, we observe low correlation among the models, irrespective of the data type. However, model agreement improves significantly with increasing subfault sizes, implying that their differences primarily stem from small-scale features. We then forward-compute geodetic and seismic synthetics and compare them with observations available during the earthquake. We find that seismic observations are sensitive to rupture propagation, such as the peak-slip rise time. However, neither teleseismic, regional seismic, nor geodetic observations are sensitive to spatial slip features smaller than 64~km. In distinction, the seafloor deformation predicted by all models exhibits poor correlation, indicating sensitivity to small-scale slip features. Our findings suggest that fine-scale slip features cannot be unambiguously resolved by remote or sparse observations, such as the four data types tested in this study. However, better resolution may become achievable from dense offshore instrumentation.

Earthquakes can be dynamically triggered by the passing waves of events from disconnected faults. The frequent occurrence of dynamic triggering offers tangible hope in revealing earthquake nucleation processes. However, the physical mechanisms behind earthquake dynamic triggering have remained unclear, and contributions of competing hypotheses are challenging to isolate with individual case studies. Therefore, developing a systematic understanding of the spatiotemporal patterns of dynamic triggering can provide insights into the physical mechanisms, which may aid mitigation of earthquake hazards. Here we investigate earthquake dynamic triggering in Southern California from 2008 to 2017 using the Quake Template Matching catalog and an approach free from assuming an earthquake occurrence distribution. We develop a new set of statistics to examine the significance of seismicity-rate changes as well as moment-release changes. We show that up to 70% of global M≥6 events may have triggered earthquakes in southern California and that the triggered seismicity often occurred several hours after the passing seismic waves. On average, earthquakes are triggered about every 4 days in the region, albeit at different locations. Although adjacent fault segments can be triggered by the same earthquakes, the majority of triggered earthquakes seem to be uncorrelated, suggesting that the process is primarily governed by local conditions. Further, the occurrence of dynamic triggering does not seem to correlate with ground motion (e.g., peak ground velocity) at the triggered sites. These observations indicate that nonlinear processes may have primarily regulated the dynamic triggering cases.

Understanding the seismic precursors is essential for deciphering earthquake rupture physics and can aid earthquake probabilistic forecasting. With regional dense seismic arrays, we identify seismic precursors of 527 0.9 ≤ M ≤ 5.4 events of the 2019 Ridgecrest earthquake sequence, including 48 earthquakes with series of precursors. These precursors are likely immediate-foreshocks that are adjacent to the earthquakes. Their corresponding precursory signals share high resemblances with the earthquake P-waves and occur within 100 s of the P-waves. However, attributes of the immediate-foreshocks, including the amplitudes and preceding times, do not clearly scale with the eventual earthquake magnitudes. Our observations suggest that earthquake rupture may initiate in a universal fashion but evolves stochastically. This indicates that earthquake rupture development is likely controlled by fine-scale fault heterogeneities in the Ridgecrest fault system, and the final magnitude is the only difference between small and large earthquakes.

Ocean transform faults often generate characteristic earthquakes that repeatedly rupture the same fault patches. The westernmost Gofar transform fault quasi-periodically hosts ~M6 earthquakes every ~5 years, and microseismicity suggests that the fault is segmented into five distinct zones, including a rupture barrier zone that may have modulated the rupture of adjacent M6 earthquakes. However, the relationship between the systematic slip behavior of the Gofar fault and the fault material properties is still poorly known. Specifically, the role of pore fluids in regulating the slip of the Gofar fault is unclear. Here, we develop a new method using differential arrival times between nearby earthquakes to estimate the in-situ Vp/Vs ratio of the fault-zone materials. We apply this technique to the dataset collected by an ocean-bottom-seismometer network deployed around the Gofar fault in 2008, which recorded abundant microearthquakes, and find a moderate Vp/Vs ratio of 1.75–1.80 in the rupture barrier zone and a low Vp/Vs ratio of 1.61–1.69 in the down-dip edge of the 2008 M6 rupture zone. This lateral variation in Vp/Vs ratio may be caused by both pore fluids and chemical alteration. We also find a 5–10% increase in Vp/Vs ratio in the barrier zone during the nine months before the mainshock. This increase may have been caused by fluid migrations or slip transients in the barrier zone.

Oceanic transform faults accommodate plate motions through both seismic and aseismic slips. However, deformation partition and slip mode interaction at these faults remain elusive mainly limited by rare observations. We use one-year ocean bottom seismometer data collected in 2008 to detect and locate earthquakes at the westernmost Gofar transform fault. The ultra-fast slipping rate of Gofar results in ~30,000 earthquakes during the observational period, providing an excellent opportunity to investigate interrelations between the slip mode, seismicity, and fault architecture at an unprecedented resolution. Earthquake distribution indicates that the ∼100 km long Gofar transform fault is distinctly segmentated into five zones, including one zone contouring a M6 earthquake that was captured by the experiment. Further, a barrier zone east of the M6 earthquake hosted abundant foreshocks preceding the M6 event and halted its active seismicity afterwards. The barrier zone has two layers of earthquakes at depth, and they responded to the M6 earthquake differently. Additionally, a zone connecting to the East Pacific Rise had quasi-periodic earthquake swarms. The seismicity segmentation suggests that the Gofar fault has multiple slip modes occurring in adjacent fault patches. Spatiotemporal characteristics of the earthquakes suggest that complex fault architecture and fluid-rock interaction play primary roles in modulating the slip modes at Gofar, possibly involving multiple concurrent physical processes.