Investigating the earthquake cycle of normal faults

Geodetic observations of crustal deformation through the earthquake cycle provide unique opportunities to gain essential knowledge of faulting mechanisms, lithospheric rheology, and fault interaction. Normal faults, an integral geologic unit responsible for crustal deformation, are specifically investigated in this thesis, via three case studies in two significantly different tectonic environments: the 2008 Mw 6.3 Damxung and Mw 7.1 Yutian earthquakes on the Tibetan Plateau, and the 2005 Mw 7.8 Tarapaca earthquake in the northern Chile subduction zone. To move toward realistic slip models, I consider crustal layering for the Damxung earthquake, and non-planar rupture for the Yutian earthquake. The Damxung study shows that assuming a homogeneous crust underestimates the depth of slip and overestimates the magnitude, in comparison to a crustal model with a weak sedimentary lid. A curved fault model composed of triangular dislocation elements (TDEs) for the Yutian earthquake recovers the geodetic observation better than rectangular fault segments. Normal faulting earthquakes are characterized by shallow slip deficit, which is partially compensated by patchy afterslip around, but no deeper than, the coseismic rupture. The complementary and partially-overlapping relationship between coseismic slip and afterslip implies complexity of frictional properties in both down-dip and along-strike directions. Postseismic deformation induced by viscoelastic relaxation (VER) following normal faulting earthquakes is fundamentally different in pattern from that produced by afterslip. This difference enables identification of afterslip as the major postseismic mechanism for the Damxung and Yutian earthquakes, and VER for the Tarapaca earthquake. In addition to understanding the faulting mechanism, I also place constraints on local rheological structure. In central Tibet, where the Damxung earthquake occurred, lack of noticeable VER-related signal allows a lower bound of 1 × 1018 Pa s for the viscosity of the lower crust/upper mantle. In northern Chile, geodetic observations following the Tarapaca earthquake indicate a weak layer with viscosity of 4 – 8 × 10^18 Pa s beneath a higher-viscosity lower crust and mantle lithosphere, and a strong continental forearc. Based on the co- and post-seismic models, I investigate fault interaction from a perspective of static stress change. Stress computation suggests that the 2014 Mw 6.9 strike-slip event close to the Altyn Tagh fault occurred on a fault that was positively stressed by the Yutian earthquake, and the Altyn Tagh fault to the east of the 2014 rupture is a potential locus for future failure. Although the Coulomb stress change on the 2014 Iquique earthquake rupture is negative due to the Tarapaca earthquake and its postseismic VER process, positive loading on the shallow-dipping nodal plane of its M 6.7 preshock suggests that the Tarapaca earthquake may have acted as an indirect trigger of the 2014 Iquique earthquake. Both studies reveal the role played by normal faults in interacting with other types of faults and have implications for seismic hazard assessment

Deep postseismic viscoelastic relaxation excited by an intraslab normal fault earthquake in the Chile subduction zone

The 2005 Mw 7.8 Tarapaca earthquake was the result of normal faulting on a west-dipping plane at a depth of ~ 90 km within the subducting slab down-dip of the North Chilean gap that partially ruptured in the 2014 M 8.2 Iquique earthquake. We use Envisat observations of nearly four years of postseismic deformation following the earthquake, together with some survey GPS measurements, to investigate the viscoelastic relaxation response of the surrounding upper mantle to the coseismic stress. We constrain the rheological structure by testing various 3D models, taking into account the vertical and lateral heterogeneities in viscosity that one would expect in a subduction zone environment. A viscosity of 4–8 × 1018 Pa s for the continental mantle asthenosphere fits both InSAR line-of-sight (LOS) and GPS horizontal displacements reasonably well. In order to test whether the Tarapaca earthquake and associated postseismic relaxation could have triggered the 2014 Iquique sequence, we computed the Coulomb stress change induced by the co- and postseismic deformation following the Tarapaca earthquake on the megathrust interface and nodal planes of its M 6.7 foreshock. These static stress calculations show that the Tarapaca earthquake may have an indirect influence on the Iquique earthquake, via loading of the M 6.7 preshock positively. We demonstrate the feasibility of using deep intraslab earthquakes to constrain subduction zone rheology. Continuing geodetic observation following the 2014 Iquique earthquake may further validate the rheological parameters obtained here

Probing the rheology of continental faults:Decade of post-seismic InSAR time-series following the 1997 Manyi (Tibet) earthquake

The physical processes driving post-seismic deformation after large earthquakes are still debated. As in most cases relatively short observation time periods are being used, it is still challenging to distinguish between the different proposed mechanisms and therefore a longer observation time is needed. The 1997 MW 7.6 Manyi, Tibet, earthquake has an excellent InSAR data archive available to study the post-seismic deformation up to ~13 yr after the earthquake. The coseismic and early post-seismic phases of theManyi earthquakewere already investigated in detail by numerous studies with viscoelastic and afterslip models being used to explain the post-seismic deformation. We use SAR (Synthetic Aperture Radar) data obtained from the ERS and Envisat satellites covering the central part of the Manyi fault from 1997 to 2010 to significantly extend the observation period. We test different viscoelastic (uniform Maxwell, Standard linear solid and Burgers body rheology below an uppermost elastic layer) and afterslip models to assess the most suitable mechanism for post-seismic deformation. While a Maxwell rheology (misfit = 2.23 cm) is not able to explain the observed long timeseries, the standard linear solid (misfit=2.07 cm) and Burgers body models (misfit=2.16 cm) with two relaxation times, cannot reproduce sufficiently the localized deformation patterns. The afterslip model (misfit = 1.77 cm) has the lowest misfit and explains well the temporal and spatial pattern of observed deformation. A combined mechanism model that considers the effects of both afterslip and viscoelastic relaxation is also a feasible process, where the viscoelastic relaxation can slightly improve the fit to the data especially at larger distances from the fault. Themaximum average line-of-sight velocity is~4mmyr-1 during 2008-2010, suggesting that the post-seismic deformation of the Manyi earthquake might be vanishing and gradually stepping into an interseismic phase

Slip distribution of the 2015 Lefkada earthquake and its implications for fault segmentation

It is widely accepted that fault segmentation limits earthquake rupture propagations and therefore earthquake size. While along-strike segmentation of continental strike-slip faults is well observed, direct evidence for segmentation of off-shore strike-slip faults is rare. A comparison of rupture behaviours in multiple earthquakes might help reveal the characteristics of fault segmentation. In this work, we study the 2015 Lefkada earthquake, which ruptured a major active strike slip fault offshore Lefkada Island, Greece. We report ground deformation mainly on the Lefkada Island measured by interferometric synthetic radar (InSAR), and infer a coseismic distributed slip model. To investigate how the fault location affects the inferred displacement based on our InSAR observations, we conduct a suite of inversions by taking various fault location from different studies as a prior. The result of these test inversions suggests that the Lefkada fault trace is located just offshore Lefkada Island. Our preferred model shows that the 2015 earthquake main slip patches are confined to shallow depth (<10 km), with amaximum slip of~1.6 m. In comparison to the 2003 earthquake,which mainly ruptured the northern part of the Lefkada fault, we suggest that the 2015 earthquake closed the seismic gap, at least partially, left by the 2003 earthquake by rupturing the shallow part of the Lefkada fault. The spatial variation in slip distributions for the two earthquakes reveals segmentation along strike, and possibly downdip of the Lefkada fault. A comparison of aftershock locations and coseismic slip distribution shows that most aftershocks appear near the edge of main coseismic slip patches

Reconstructing Ocean-Plate Stratigraphy (OPS) to Understand Accretionary Style and Mélange Fabric:Insights From the Bangong-Nujiang Suture (Tibet, China)

Ocean-plate stratigraphy (OPS) refers to the lithostratigraphic column atop an ocean plate, which becomes scraped off during subduction and preserved in accretionary complex (AC). Herein, based on structural, stratigraphic, and geochronological studies of ACs from the Bangong-Nujiang suture, we demonstrate that OPS can facilitate interpreting structural and compositional heterogeneities in ACs. Carefully correlated OPSs reveal that, on the overall sediment-rich lower plate, different types of basement topography correspond to the accretion of distinct litho-structural assemblages. In particular, subduction of the major, high-relief Zhonggang seamount eroded the earlier margin and was subsequently accreted as coherent seamount slices. In contrast, subduction of the lower-relief, Gaize seamount halted frontal accretion of trailing sediments, which were dragged downward to the seismogenic depth and underplated as pervasive, shear-related broken formations. Such broken formations may fingerprint past lower-relief-seamount subduction in other fossil ACs

Imaging slab-transported fluids and their deep dehydration from seismic velocity tomography in the Lesser Antilles subduction zone

Volatiles play a pivotal role in subduction zone evolution, yet their pathways remain poorly constrained. Studying the Lesser Antilles subduction zone can yield new constraints, where old oceanic lithosphere formed by slow-spreading subducts slowly. Here we use local earthquakes recorded by the temporary VoiLA (Volatile recycling in the Lesser Antilles) deployment of ocean-bottom seismometers in the fore- and back-arc to characterize the 3-D seismic structure of the north-central Lesser Antilles subduction zone. Along the slab top, mapped based on seismicity, we find low Vp extending to 130–150 km depth, deeper than expected for magmatic oceanic crust. The slab's most prominent, elevated Vp/Vs anomalies are beneath the fore- and back-arc offshore Guadeloupe and Dominica, where two subducted fracture zones lie with the obliquely subducting boundary between Proto-Caribbean and Equatorial Atlantic lithosphere. These structures, therefore, enhance hydration of the oceanic lithosphere as it forms and evolves and the subsequent dehydration of mantle serpentinite when subducted. Above the slab, we image the asthenosphere wedge as a high Vp/Vs and moderate Vp feature, indicating slab-dehydrated fluids rising through the overlying cold boundary layer that might induce melting further to the west. Our results provide new evidence for the impact of spatially-variable oceanic plate formation processes on slab dehydration and mantle wedge volatile transfer that ultimately impact volcanic processes at the surface, such as the relatively high magmatic output observed on the north-central islands in the Lesser Antilles

‘Two go together’:Near-simultaneous moment release of two asperities during the 2016 Mw 6.6 Muji, China earthquake

On 25 November 2016, a Mw 6.6 earthquake ruptured the Muji fault in western Xinjiang, China. We investigate the earthquake rupture independently using geodetic observations from Interferometric Synthetic Aperture Radar (InSAR) and regional seismic recordings. To constrain the fault geometry and slip distribution, we test different combinations of fault dip and slip direction to reproduce InSAR observations. Both InSAR observations and optimal distributed slip model suggest buried rupture of two asperities separated by a gap of greater than 5 km. Additional seismic gaps exist at the end of both asperities that failed in the 2016 earthquake. To reveal the dynamic history of asperity failure, we inverted regional seismic waveforms for multiple centroid moment tensors and construct a moment rate function. The results show a small centroid time gap of 2.6 s between the two sub-events. Considering the >5 km gap between the two asperities and short time interval, we propose that the two asperities failed near-simultaneously, rather than in a cascading rupture propagation style. The second sub-event locates ∼39 km to the east of the epicenter and the centroid time is at 10.7 s. It leads to an estimate of average velocity of 3.7 km/s as an upper bound, consistent with upper crust shear wave velocity in this region. We interpret that the rupture front is propagating at sub-shear wave velocities, but that the second sub-event has a reduced or asymmetric rupture time, leading to the apparent near-simultaneous moment release of the two asperities

3D Local Earthquake Tomography of the Ecuadorian Margin in the Source Area of the 2016 Mw 7.8 Pedernales Earthquake

Based on manually analyzed waveforms recorded by the permanent Ecuadorian network and our large aftershock deployment installed after the Pedernales earthquake, we derive three-dimensional Vp and Vp/Vs structures and earthquake locations for central coastal Ecuador using local earthquake tomography. Images highlight the features in the subducting and overriding plates down to 35 km depth. Vp anomalies (∼4.5–7.5 km/s) show the roughness of the incoming oceanic crust (OC). Vp/Vs varies from ∼1.75 to ∼1.94, averaging a value of 1.82 consistent with terranes of oceanic nature. We identify a low Vp (∼5.5 km/s) region extending along strike, in the marine forearc. To the North, we relate this low Vp and Vp/Vs (1.85) which we interpret as deeply fractured, probably hydrated OC caused by the CR being subducted. These features play an important role in controlling the seismic behavior of the margin. While subducted seamounts might contribute to the nucleation of intermediate megathrust earthquakes in the northern segment, the CR seems to be the main feature controlling the seismicity in the region by promoting creeping and slow slip events offshore that can be linked to the updip limit of large megathrust earthquakes in the northern segment and the absence of them in the southern region over the instrumental period