Lujia Feng

The 2012 Mw 8.6 Wharton Basin sequence: A cascade of great earthquakes generated by near-orthogonal, young, oceanic mantle faults

We improve constraints on the slip distribution and geometry of faults involved in the complex, multisegment, Mw 8.6 April 2012 Wharton Basin earthquake sequence by joint inversion of high-rate GPS data from the Sumatran GPS Array (SuGAr), teleseismic observations, source time functions from broadband surface waves, and far-field static GPS displacements. This sequence occurred under the Indian Ocean, ∼400 km offshore Sumatra. The events are extraordinary for their unprecedented rupture of multiple cross faults, deep slip, large strike-slip magnitude, and potential role in the formation of a discrete plate boundary between the Indian and Australian plates. The SuGAr recorded static displacements of up to ∼22 cm, along with time-varying arrivals from the complex faulting, which indicate that the majority of moment release was on young, WNW trending, right-lateral faults, counter to initial expectations that an old, lithospheric, NNE trending fracture zone played the primary role. The new faults are optimally oriented to accommodate the present-day stress field. Not only was the greatest moment released on the younger faults, but it was these that sustained very deep slip and high stress drop (>20 MPa). The rupture may have extended to depths of up to 60 km, suggesting that the oceanic lithosphere in the northern Wharton Basin may be cold and strong enough to sustain brittle failure at such depths. Alternatively, the rupture may have occurred with an alternative weakening mechanism, such as thermal runaway.

The mechanism of partial rupture of a locked megathrust: The role of fault morphology

Assessment of seismic hazard relies on estimates of how large an area of a tectonic fault could potentially rupture in a single earthquake. Vital information for these forecasts includes which areas of a fault are locked and how the fault is segmented. Much research has focused on exploring downdip limits to fault rupture from chemical and thermal boundaries, and along-strike barriers from subducted structural features, yet we regularly see only partial rupture of fully locked fault patches that could have ruptured as a whole in a larger earthquake. Here we draw insight into this conundrum from the 25 April 2015 M w 7.8 Gorkha (Nepal) earthquake. We invert geodetic data with a structural model of the Main Himalayan thrust in the region of Kathmandu, Nepal, showing that this event was generated by rupture of a decollement bounded on all sides by more steeply dipping ramps. The morphological bounds explain why the event ruptured only a small piece of a large fully locked seismic gap. We then use dynamic earthquake cycle modeling on the same fault geometry to reveal that such events are predicted by the physics. Depending on the earthquake history and the details of rupture dynamics, however, great earthquakes that rupture the entire seismogenic zone are also possible. These insights from Nepal should be applicable to understanding bounds on earthquake size on megathrusts worldwide.

Asthenosphere rheology inferred from observations of the 2012 Indian Ocean earthquake

The concept of a weak asthenospheric layer underlying Earth’s mobile tectonic plates is fundamental to our understanding of mantle convection and plate tectonics. However, little is known about the mechanical properties of the asthenosphere (the part of the upper mantle below the lithosphere) underlying the oceanic crust, which covers about 60 per cent of Earth’s surface. Great earthquakes cause large coseismic crustal deformation in areas hundreds of kilometres away from and below the rupture area. Subsequent relaxation of the earthquake-induced stresses in the viscoelastic upper mantle leads to prolonged postseismic crustal deformation that may last several decades and can be recorded with geodetic methods. The observed postseismic deformation helps us to understand the rheological properties of the upper mantle, but so far such measurements have been limited to continental-plate boundary zones. Here we consider the postseismic deformation of the very large (moment magnitude 8.6) 2012 Indian Ocean earthquake to provide by far the most direct constraint on the structure of oceanic mantle rheology. In the first three years after the Indian Ocean earthquake, 37 continuous Global Navigation Satellite Systems stations in the region underwent horizontal northeastward displacements of up to 17 centimetres in a direction similar to that of the coseismic offsets. However, a few stations close to the rupture area that had experienced subsidence of up to about 4 centimetres during the earthquake rose by nearly 7 centimetres after the earthquake. Our three-dimensional viscoelastic finite-element models of the post-earthquake deformation show that a thin (30–200 kilometres), low-viscosity (having a steady-state Maxwell viscosity of (0.5–10) × 1018 pascal seconds) asthenospheric layer beneath the elastic oceanic lithosphere is required to produce the observed postseismic uplift.

Imaging the distribution of transient viscosity after the 2016 Mw 7.1 Kumamoto earthquake

Crustal rock strength from outer space The response of crustal rock to stresses is challenging to estimate yet vital for determining risks from events such as earthquakes. Moore et al. take advantage of the recent Mw 7.1 Kumamoto earthquake in Japan to determine the rheology of crustal rocks in the region. The observed inversion of the crustal strain rates demonstrates that certain areas have stiff rock and others (e.g., under the Aso volcanic complex) have much weaker rock. The results match up with expectations, which means that the method can successfully measure rock properties over a wide range of strength and large spatial and temporal scales. Science, this issue p. 163 The combination of GPS and InSAR data after the Kumamoto earthquake in Japan allows for an estimate of regional rock rheology. The deformation of mantle and crustal rocks in response to stress plays a crucial role in the distribution of seismic and volcanic hazards, controlling tectonic processes ranging from continental drift to earthquake triggering. However, the spatial variation of these dynamic properties is poorly understood as they are difficult to measure. We exploited the large stress perturbation incurred by the 2016 earthquake sequence in Kumamoto, Japan, to directly image localized and distributed deformation. The earthquakes illuminated distinct regions of low effective viscosity in the lower crust, notably beneath the Mount Aso and Mount Kuju volcanoes, surrounded by larger-scale variations of viscosity across the back-arc. This study demonstrates a new potential for geodesy to directly probe rock rheology in situ across many spatial and temporal scales.

Upper-mantle water stratification inferred from observations of the 2012 Indian Ocean earthquake

Water, the most abundant volatile in Earth’s interior, preserves the young surface of our planet by catalysing mantle convection, lubricating plate tectonics and feeding arc volcanism. Since planetary accretion, water has been exchanged between the hydrosphere and the geosphere, but its depth distribution in the mantle remains elusive. Water drastically reduces the strength of olivine and this effect can be exploited to estimate the water content of olivine from the mechanical response of the asthenosphere to stress perturbations such as the ones following large earthquakes. Here, we exploit the sensitivity to water of the strength of olivine, the weakest and most abundant mineral in the upper mantle, and observations of the exceptionally large (moment magnitude 8.6) 2012 Indian Ocean earthquake to constrain the stratification of water content in the upper mantle. Taking into account a wide range of temperature conditions and the transient creep of olivine, we explain the transient deformation in the aftermath of the earthquake that was recorded by continuous geodetic stations along Sumatra as the result of water- and stress-activated creep of olivine. This implies a minimum water content of about 0.01 per cent by weight—or 1,600 H atoms per million Si atoms—in the asthenosphere (the part of the upper mantle below the lithosphere). The earthquake ruptured conjugate faults down to great depths, compatible with dry olivine in the oceanic lithosphere. We attribute the steep rheological contrast to dehydration across the lithosphere–asthenosphere boundary, presumably by buoyant melt migration to form the oceanic crust.

Implications of the diffuse deformation of the Indian Ocean lithosphere for slip partitioning of oblique plate convergence in Sumatra

Oblique plate convergence between Indian Ocean lithosphere and continental crust of the Sunda plate is distributed between subduction on the Sunda megathrust and upper plate strike-slip faulting on the Sumatran Fault Zone, in a classic example of slip partitioning. Over the last decade, a destructive series of great earthquakes has brought renewed attention to the mechanical properties of these faults and the intervening forearc crustal block. While observations of forearc deformation over the earthquake cycle indicate that the forearc crust is fundamentally elastic, the spatial pattern of slip vector azimuths for earthquakes sourced by rupture of the Sunda megathrust is strongly inconsistent with relative motion of two rigid plates. Permanent and distributed deformation therefore occurs in either the downgoing lithospheric slab or the overriding forearc crust. Previous studies have inferred from geodetic velocities and geological slip rates of the Sumatran Fault that the forearc crust is undergoing rapid trench-parallel stretching. Using new geological slip rates for the Sumatran Fault and an updated decadal GPS velocity field of Sumatra and the forearc islands, we instead show that permanent deformation within the forearc sliver is minor and that the Sumatran Fault is a plate boundary strike-slip fault. The kinematic data are best explained by diffuse deformation within the oceanic lithosphere of the Wharton Basin, which accommodates convergence between the Indian and Australian plates and has recently produced several large earthquakes well offshore of Sumatra. The slip partitioning system in Sumatra is fundamentally linked with the mechanical properties of the subducting oceanic lithosphere.

Interseismic megathrust coupling beneath the Nicoya Peninsula, Costa Rica, from the joint inversion of InSAR and GPS data

The Nicoya Peninsula, Costa Rica, was struck by a long-anticipated and gap-filling Mw 7.6 earthquake in 2012. To study interseismic strain accumulation on the megathrust beneath the Nicoya Peninsula, we present an improved interseismic coupling model by integrating interferometric synthetic aperture radar (InSAR) and GPS data. Our model reveals three strongly coupled patches. The first strongly coupled patch locates beneath the Nicoya Peninsula and ruptured during the 2012 earthquake. The second strongly coupled patch locates offshore the central Nicoya Peninsula and remained largely unbroken. However, this region is close to and possibly intermingled with shallow slow slip and tremor, suggesting that accumulated strain in this region may be released both seismically and aseismically. The third strongly coupled patch offshore of the southeastern end of Nicoya overlaps part of the coseismic rupture of the 1990 Mw 7.0 Nicoya Gulf earthquake, indicating that significant strain has re-accumulated since this event. Incorporating InSAR data provides a more refined interseismic coupling model than using GPS alone and allows for a more reliable comparison with local seismic and aseismic activities. This comparison indicates that strongly locked regions during the interseismic stage are the loci of coseismic slip, and deep slow slip and low-frequency earthquakes occur in regions of low coupling or transition zones from low to high coupling, while shallow slow slip and tremor commingle with strongly coupled regions. Our study demonstrates that InSAR data can be used to recover small long-wavelength deformation signals with refined resolution in challenging subduction zone environments when integrated with GPS observations.

Transient rheology of the Sumatran mantle wedge revealed by a decade of great earthquakes

Understanding the rheological properties of the upper mantle is essential to develop a consistent model of mantle dynamics and plate tectonics. However, the spatial distribution and temporal evolution of these properties remain unclear. Here, we infer the rheological properties of the asthenosphere across multiple great megathrust earthquakes between 2004 and 2014 along the Sumatran subduction zone, taking advantage of decade-long continuous GPS and tide-gauge measurements. We observe transient mantle wedge flow following these earthquakes, and infer the temporal evolution of the effective viscosity. We show that the evolution of stress and strain rate following these earthquakes is better matched by a bi-viscous than by a power-law rheology model, and we estimate laterally heterogeneous transient and background viscosities on the order of ~1017 and ~1019 Pa s, respectively. Our results constitute a preliminary rheological model to explain stress evolution within earthquake cycles and the development of seismic hazard in the region.The rheology of the upper mantle is key to understanding how plate tectonics may evolve. Here, using GPS and tide-gauge measurements along the Sumatran subduction zone, the authors’ show that a bi-viscous rheology model is needed to explain the stress and strain evolution of the upper mantle following earthquakes.

Fault Slip and GPS Velocities Across the Shan Plateau Define a Curved Southwestward Crustal Motion Around the Eastern Himalayan Syntaxis

Characterizing the 700-km-wide system of active faults on the Shan Plateau, southeast of the eastern Himalayan syntaxis (EHS), is critical to understanding the geodynamics and seismic hazard of the large region that straddles neighboring China, Myanmar, Thailand, Laos and Vietnam. Here we evaluate the fault styles and slip rates over multi-timescales, reanalyze previously-published short-term GPS velocities and evaluate slip-rate gradients to interpret the regional kinematics and geodynamics that drive the crustal motion. Relative to the Sunda plate, GPS velocities across the Shan Plateau define a broad arcuate tongue-like crustal motion with a progressively northwestward increase in sinistral shear over a distance of ~700 km followed by a decrease over the final ~100 km to the syntaxis. The cumulative GPS slip rate across the entire sinistral-slip fault system on the Shan Plateau is ~12 mm/yr. Our observations of the fault geometry, slip rates and arcuate southwesterly-directed tongue-like patterns of GPS velocities across the region suggest that the fault kinematics is characterized by a regional southwestward distributed shear across the Shan Plateau, compared to more block-like rotation and indentation north of the Red River fault. The fault geometry, kinematics and regional GPS velocities are difficulty to reconcile with regional bookshelf faulting between the Red River and Sagaing faults or localized lower crustal channel flows beneath this region. The crustal motion and fault kinematics can be driven by a combination of basal traction of a clockwise, southwestward asthenospheric flow around the EHS and gravitation or shear-driven indentation from north of the Shan Plateau.

Sibling earthquakes generated within a persistent rupture barrier on the Sunda megathrust under Simeulue Island

A section of the Sunda megathrust underneath Simeulue is known to persistently halt rupture propagation of great earthquakes, including those in 2004 (Mw 9.2) and 2005 (Mw 8.6). Yet the same section generated large earthquakes in 2002 (Mw 7.3) and 2008 (Mw 7.4). To date, few studies have investigated the 2002 and 2008 events, and none have satisfactorily located or explained them. Using near-field InSAR, GPS, and coral geodetic data, we find that the slip distributions of the two events are not identical but do show a close resemblance and largely overlap. We thus consider these earthquakes “siblings” that were generated by an anomalous “parent” feature of the megathrust. We suggest that this parent feature is a locked asperity surrounded by the otherwise partially creeping Simeulue section, perhaps structurally controlled by a broad morphological high on the megathrust.