Large-magnitude oceanic intraplate seismicity: Implications for lithosphere evolution

ABSTRACT We analyzed 37 large oceanic intraplate earthquakes (M >6). The largest (M >7) are mainly concentrated under the Indian Ocean. Moderate events (6 < M < 7) are sparsely distributed under the Indian Ocean and other oceans where lithospheric ages are between 90 Ma and 20 Ma. Oceanic intraplate events related to mantle plumes or hotspots are rare, though low-velocity anomalies beneath hotspots are a common feature. Tomographic cross sections for Indian Ocean areas with large intraplate earthquakes indicate strong heterogeneity in the mantle. These earthquakes are explained by shallow stress variations caused by a combination of tectonic forces including slab-pull, ridge-push, drag by mantle flow, plume-push, and buoyancy forces as a consequence of low-velocity anomalies in the mantle. Oceanic intraplate seismicity in the Indian Ocean is related to the large-scale, low-velocity anomaly structure around the Ninety East Ridge.

Preliminary study of stress changes evolution on central part of Sumatran Fault influenced by large interplate earthquakes and tectonic stress rates

The inland seismic activity in Great Sumatran Fault (GSF) has significantly increased over the past several decades after the occurrence of historical large interplate earthquakes along the plate boundary. This condition led to some occurrences of historical intraplate earthquakes along Sumatran fault. To quantitatively examine the physical mechanisms between intraplate earthquakes and interplate earthquakes, we estimated the static coseismic stress changes of Coulomb failure function (ΔCFF) using receiver fault approach from large historical-recorded interplate earthquakes and the increase in tectonic stress rates. We examined this research in the central part of GSF since this zone is assumed to have the most heterogeneous stress field and thus became our focus study area. The cumulative ΔCFF models showed almost all segments in the central part of GSF suffered negative changes (<-0.1 MPa) which assumed to be unlikely to rupture in short time. However, the preliminary analysis of the increase in tectonic stress rate indicated that large intraplate earthquakes occurred on Angkola and Siulak segments were dominantly influenced by the increase in interseismic stress rate just after the series of large subduction earthquake occurrences, apart from the decreased stress changes from those major interplate earthquakes.

Influence of crustal lithology and the thermal state on microseismicity in the Wakayama region, southern Honshu, Japan

Here we investigate the influence of the lithology and thermal state of the upper crust on earthquake distributions beneath the Wakayama region, southern Honshu, Japan, to better understand the influence of crustal conditions on regional seismogenesis. The earthquakes are concentrated in the deeper sections of mafic belts and shallower sections of pelitic belts, based on a comparison of relocated hypocenters and estimated subsurface geological structures. We compare the frictional properties of pelitic rocks and basalt, as obtained from petrological experiments, with the hypocenter depth distributions in pelitic and mafic belts to assess the control of crustal lithology on the depth extent of regional seismicity. The earthquake distributions are consistent with the temperature ranges over which the respective rock types are expected to exhibit a velocity-weakening behavior, based on the petrological experiments. The results suggest that the occurrence of shallow intraplate earthquakes is controlled by the temperature- and lithology-dependent friction of the upper crust.

Basement Structure and Styles of Active Tectonic Deformation in Central Interior Alaska

Central Interior Alaska is one of the most seismically active regions in North America, exhibiting a high concentration of intraplate earthquakes approximately 700 km away from the southern Alaska subduction zone. Seismological evidence suggests that intraplate seismicity in the region is not uniformly distributed, but concentrated in several discrete seismic zones, including the Nenana basin and the adjacent Tanana basin. Although the location and magnitude of the seismic activity in both basins are well defined by a network of seismic stations in the region, the tectonic controls on these intraplate earthquakes and the heterogeneous nature of Alaska’s continental interior remain poorly understood. We investigated the crustal structure of the Nenana and Tanana basins using available seismic reflection, aeromagnetic and gravity anomaly data, supplemented by geophysical well logs and outcrop data. We developed nine new two-dimensional forward models to delineate internal geometries and the crustal structure of Alaska’s interior. The results of our study demonstrates a strong crustal heterogeneity beneath both basins. The Tanana basin is a relatively shallow (up to 2 km) asymmetrical foreland basin with its southern, deeper side controlled by the northern foothills of the Central Alaska Range. Northeast-trending left lateral strike-slip faults within the Tanana basin are interpreted as a zone of clockwise crustal block rotation. The Nenana basin has a fundamentally different geometry. It is a deep (up to 8 km), narrow transtensional pull-apart basin that is deforming along the left-lateral Minto Fault. This study identifies two distinct modes of current tectonic deformation in Central Interior Alaska and provides a basis for modeling the interplay between intraplate stress fields and major structural features that potentially influence the generation of intraplate earthquakes in the region.

Scaling relations between seismic moment and rupture area of earthquakes in stable continental regions

This article describes the development of scaling relations between seismic moment and rupture area of earthquakes in stable continental regions (SCRs). The article reviews the relations developed by Somerville and compares them with relations developed by other investigators. It also compares the scaling relations of SCR earthquakes with those in tectonically active continental regions (TCRs). Three different methods of estimating rupture area, based on aftershocks, slip models, and duration methods were used by Somerville to analyze the relation between seismic moment and rupture area, using earthquake source parameters compiled from published literature. For each category of data, the relations obtained were not significantly different from those obtained by constraining them to be self-similar (scale-invariant), so self-similar relations were adopted. The stress drops corresponding to these scaling relations range from 51 to 86 bars, with an average of 65 bars. This value is comparable with the value of 58 bars obtained by Leonard, and it is recommended that the Leonard scaling relations for SCR earthquakes be used for the NGA East Project. To a first approximation, the results of Somerville and those of Somerville et al. indicate that the rupture areas of SCR earthquakes are about half those of TCR earthquakes, and their stress drops are about 2.8 times higher. Allmann and Shearer find less of a difference, presumably because their intraplate category includes some earthquakes that we would assign to TCR instead of SCR. Their study indicates that the rupture areas of intraplate earthquakes are about two-thirds those of TCR earthquakes, and their stress drops are about 2 times higher.

Foreshocks of the 2018 ML 4.0 Shimian Earthquake in the Anninghe Fault and Its Implications for Earthquake Nucleation

Foreshock activity sometimes precedes large earthquakes, but how foreshocks relate to mainshock nucleation is still unclear with limited case studies existing. One way to further the understanding of the foreshock occurrence mechanism is to maximize the resolution of the foreshock characteristics by waveform-based earthquake detection and location. Here, we apply the match and locate method to scan continuous waveforms 30 days before and 44 days after the 2018 ML 4.0 Shimian earthquake in Sichuan, China, and obtain approximately three times more events than reported in a local catalog. The augmented seismicity suggests the existence of a blind small strike-slip fault deep in the east of the Anninghe fault. Forty-one foreshocks of magnitude ranging from ML−0.7 to 3.4 occurred within 4 hr before the mainshock and did not show an accelerating pattern leading up to the mainshock. Focal mechanisms are consistent between the mainshock and foreshocks, implying that the mainshock and foreshock hypocenters are located on the same fault plane. The high-precision relative locations reveal that most of the foreshocks rupture adjacent source patches along the fault plane, with little or partial overlap, which is consistent with cascade stress triggering from foreshocks to foreshocks to the mainshock. Our research is one of the few to focus on the foreshock sequence of moderate mainshocks and provides a new case for studying the mechanism of foreshocks of intraplate earthquakes with a low incidence of foreshocks.

Surface slip distributions and geometric complexity of intraplate reverse-faulting earthquakes

Earthquake ground surface ruptures provide insights into faulting mechanics and inform seismic hazard analyses. We analyze surface ruptures for 11 historical (1968−2018) moment magnitude (Mw) 4.7−6.6 reverse earthquakes in Australia using statistical techniques and compare their characteristics with magnetic, gravity, and stress trajectory data sets. Of the total combined (summative) length of all surface ruptures (∼148 km), 133 km (90%) to 145 km (98%) align with the geophysical structure in the host basement rocks. Surface rupture length (SRL), maximum displacement (MD), and probability of surface rupture at a specified Mw are high compared with equivalent Mw earthquakes globally. This is attributed to (1) a steep cratonic crustal strength gradient at shallow depths, promoting shallow hypocenters (∼1−6 km) and limiting downdip rupture widths (∼1−8.5 km), and (2) favorably aligned crustal anisotropies (e.g., bedrock foliations, faults, fault intersections) that enhanced lateral rupture propagation and/or surface displacements. Combined (modeled and observed) MDs are in the middle third of the SRL with 68% probability and either the ≤33rd or ≥66th percentiles of SRL with 16% probability. MD occurrs proximate to or directly within zones of enhanced fault geometric complexity (as evidenced from surface ruptures) in 8 of 11 earthquakes (73%). MD is approximated by 3.3 ± 1.6 (1σ) × AD (average displacement). S-transform analyses indicates that high-frequency slip maxima also coincide with fault geometric complexities, consistent with stress amplifications and enhanced slip variability due to geometric and kinematic interactions with neighboring faults. Rupture slip taper angles exhibite large variations (−90% to +380% with respect to the mean value) toward rupture termini and are steepest where ruptures terminate at obliquely oriented magnetic lineaments and/or lithology changes. Incremental slip approximates AD between the 10th and 90th percentiles of the SRL. The average static stress drop of the studied earthquakes is 4.8 ± 2.8 MPa. A surface rupture classification scheme for cratonic stable regions is presented to describe the prevailing characteristics of intraplate earthquakes across diverse crustal structural-geophysical settings. New scaling relationships and suggestions for logic tree weights are provided to enhance probabilistic fault displacement hazard analyses for bedrock-dominated intraplate continental regions.