A Spatially Varying Scaling Method for InSAR Tropospheric Corrections Using a High‐Resolution Weather Model

Variation in tropospheric delay is a major limiting factor on the accuracy of interferometric synthetic aperture radar (InSAR) measurements. This is particularly the case when deformation and topography are correlated. To address limitations of previous InSAR tropospheric correction methods, here we present a new approach that combines the use of both external weather model data and the interferometric phase. We assume that vertical refractivity profiles calculated from a high‐resolution weather model data can generally describe the form of the relationship between tropospheric delay and height but that the magnitude can be incorrect. We estimate a magnitude correction by scaling the original delays to best match the interferometric phase. We validated our new method using simulated data and demonstrate that both coseismic and interseismic signals can be separated from strong tropospheric delays. We also applied our algorithm to the central portion of the Altyn Tagh Fault in northern Tibet, where deformation correlates strongly with topographic relief of 6,000 m, and show that the derived velocity field is more internally consistent and agrees better with independent Global Positioning System measurements

Current slip rates on conjugate strike-slip faults in central Tibet using synthetic aperture radar interferometry

This is the published version. Copyright 2006 American Geophysical Union. All Rights Reserved.We estimate the current slip rates on active conjugate strike-slip faults in central Tibet using repeat-pass synthetic aperture radar interferometry (InSAR). The conjugate fault systems are centered along the east trending Late Jurassic–Early Cretaceous Bangong-Nujiang suture zone and are composed of NE striking left-slip faults to the north and NW striking right-slip faults to the south. The surface displacement field obtained from InSAR data show 30- to 60-km-wide zones of concentrated shear that coincide with active fault traces observed in the field. The radar data indicate that, within a 200- to 300-km-wide belt, the deformation regime defined by the conjugate strike-slip faults is accommodating ∼5 mm yr−1 of pure shear contraction oriented in the N10°E direction and ∼6 mm yr−1 of right-lateral simple shear in the N110°E direction. The observation of localized strain along faults indicates that faulting is the dominant mode of deformation in central Tibet with seismogenic depths extending down to ∼25 km. Furthermore, extrapolating the current slip rates estimated for the central Tibet faults, the total magnitude of fault slip based on geological observations would only require the faults to have initiated sometime in the past 2–3 Myr. This appears to contradict the few geochronologic constraints on fault initiation at 8 Ma or even earlier. This discrepancy suggests that the fault slip rates may not have remained constant through time but have accelerated in the recent period. With the exception of the slip rate on the Gyaring Co fault, the slip rates that we determined on the conjugate strike-slip faults in central Tibet are significantly slower than the rates on faults that bound the Tibetan Plateau, such as the left-slip Altyn Tagh fault to the north. This observation suggests that although deformation is active within central Tibet, plateau-bounding structures are dominant in absorbing Indo-Asian convergence

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

Modelling co- and post-seismic displacements revealed by InSAR, and their implications for fault behaviour

The ultimate goal of seismology is to estimate the timing, magnitude and potential spatial extent of future seismic events along pre-existing faults. Based on the rate-state friction law, several theoretical physical earthquake models have been proposed towards this goal. Tectonic loading rate and frictional properties of faults are required in these models. Modern geodetic observations, e.g. GPS and InSAR, have provided unprecedented near-field observations following large earthquakes. In theory, according to the frictional rate and state asperity earthquake model, velocity-weakening regions holding seismic motions on faults should be separated with velocity-strengthening regions within which faults slip only aseismically. However, early afterslip following the 2011 MW 9.1 Tohoku-Oki earthquake revealed from GPS measurements was largely overlaid on the historical rupture zones, which challenged the velocity weakening asperity model. Therefore, the performance of the laboratory based friction law in the natural events needs further investigation, and the factors that may affect the estimates of slip models through geodetic modelling should also be discussed systematically. In this thesis, several moderate-strong events were investigated in order to address this important issue. The best-fit co- and post-seismic slip models following the 2009 MW 6.3 Haixi, Qinghai thrust-slip earthquake determined by InSAR deformation time-series suggest that the maximum afterslip is concentrated in the same area as the coseismic slip model, which is similar to the patterns observed in the 2011 Japan earthquake. In this case, complex geometric asperity may play a vital role in the coseismic nucleation and postseismic faulting. The major early afterslip after the 2011 MW 7.1 Van mainshock, which was revealed by one COSMO-SkyMed postseismic interferogram, is found just above the coseismic slip pattern. In this event, a postseismic modelling that did not allow slip across the coseismic asperity was also tested, suggesting that the slip model without slip in the asperities can explain the postseismic observations as well as the afterslip model without constraints on slip in the asperities. In the 2011 MW 9.1 Tohoku-Oki earthquake, a joint inversion with the GRACE coseismic gravity changes and inland coseismic GPS observations was conducted to re-investigate the coseismic slip model of the mainshock. A comparison of slip models from these different datasets suggests that significant variations of slip models can be observed, particularly the locations of the maximum slips. The joint slip model shows that the maximum slip of ~42 m appears near the seafloor surface close to the Japan Trench. Meanwhile, the accumulative afterslip patterns (slip >2 m) determined in previous studies appear in spatial correlation with the Coulomb stress changes generated using the joint slip model. As a strike-slip faulting event, the 2011 MW 6.8 Yushu earthquake was also investigated through co- and post-seismic modelling with more SAR data than was used in previous study. Best slip models suggest that the major afterslip is concentrated in shallow parts of the faults and between the two major coseismic slip patterns, suggesting that the performance of the rate and state frictional asperity model is appropriate in this event. Other postseismic physical mechanisms, pore-elastic rebound and viscoelastic relaxation have also been examined, which cannot significantly affect the estimate of the shallow afterslip model in this study. It is believed that the shallow afterslip predominantly controlled the postseismic behaviour after the mainshock in this case. In comparison to another 21 earthquakes investigated using geodetic data from other studies, complementary spatial extents between co- and post-seismic slip models can be identified. The 2009 MW 6.3 Qinghai earthquake is an exceptional case, in which the faulting behaviours might be dominated by the fault structure (e.g. fault bending). In conclusion, the major contributions from this thesis include: 1) the friction law gives a first order fit in most of natural events examined in this thesis; 2) geometric asperities may play an important role in faulting during earthquake cycles; 3) significant uncertainties in co- and post-seismic slip models can appreciably bias the estimation of fault frictional properties; 4) new insights derived from each earthquake regarding their fault structures and complex faulting behaviours have been observed in this thesis; and (5) a novel package for geodetic earthquake modelling has been developed, which can handle multiple datasets including InSAR, GPS and land/space based gravity changes

Holocene slip rate along the Gyaring Co Fault, central Tibet

Although geodetic measurements of interseismic deformation in interior Tibet suggest slow strain accumulation, active slip along the right-lateral Gyaring Co Fault is suggested to be between 8 and 21 mm/yr. Reliable geologic constraints on the slip rate along this fault are sparse. Here we document 12 ± 2m of right-lateral displacement of lacustrine shorelines across the Gyaring Co Fault. Optically stimulated luminescence ages of the shorelines are tightly clustered between 4.1 and 4.4 ka. These data require an average slip rate of 2.2-3.2mm/yr along the central Gyaring Co Fault during the latter half of the Holocene. Consideration of seismic cycle effects allows the possibility of slightly higher average slip rates, up to 2.2-4.5mm/yr. Overall, our results suggest that the slip rate along the Gyaring Co Fault is similar to other strike-slip faults in interior Tibet, supporting the notion that active deformation in this region is distributed among numerous, slowly moving faults

Investigating the postseismic deformation of strike-slip earthquakes on the Tibetan Plateau

InSAR is a useful technique to detect large-scale surface deformation from space. To place constraints on the rheological structure of the lithosphere in the Tibetan Plateau, two strike-slip earthquakes have been investigated. One is the Mw 7.6 Manyi earthquake, which occurred in the north-central Tibetan Plateau. The other is the Mw 6.5 Jiuzhaigou earthquake, which happened that on the eastern part of the Tibetan Plateau. My InSAR data cover 12 years following the Manyi earthquake, much longer than previous researchers’ dataset. I test three viscoelastic models (Maxwell, Standard linear solids, and Burgers body) and one afterslip model. The viscoelastic models cannot match the observed temporal–spatial deformation patterns. The distributions of deformation in the viscoelastic models extend into the far field and the residuals tend to increase, which are inconsistent with the data. The afterslip model has the lowest misfit and explains the temporal and spatial pattern of the observed deformation with decent result. A combined model that considers the effects of both afterslip and viscoelastic relaxation has also been tested. In this combined model, the viscoelastic relaxation that occurs with an elastic layer of thickness of 30 km over a half-space place, produce an estimate for viscosity of 5 × 1019 Pa s for this area. Therefore, either the afterslip model or the combined model can be used to explain the 12 years postseismic deformation of Manyi earthquake. The long time series of the Manyi earthquake enable us to distinguish between afterslip and viscoelastic relaxation. The seismogenic fault of the Jiuzhaigou earthquake was previously unidentified and no surface rupture is found after the earthquake. I first determined the fault geometry and calculated coseismic slip model. The slip model indicates a left-lateral strike-slip pattern, which is consistent with focal mechanisms were determined by different agencies. There is no visible postseismic deformation signal of the fault, which means the surface deformation generated by fault creeping is smaller than the noise of our observation method over that period. Therefore, I try to find the lower bound of the viscosity for this area. My preferred minimum possible viscosity of the underlying half-space is ∼6 × 1017 Pa s. Together with previous geodetic studies, the viscosities obtained from central Tibet show at least one order of magnitude difference with the viscosities obtained from the eastern Tibet. The heterogeneity indicates the rheology has a relatively large spatial change through the whole Plateau. The viscoelastic model always been proposed to explain long-term postseismic deformation and afterslip is used to explain the short-term deformation or localised deformation. Sometimes, the viscoelastic deformation signal is invisible in the moderate earthquakes as the stress is not large enough to generate observable deformation

Change of apparent segmentation of the San Andreas fault around Parkfield from space geodetic observations across multiple periods

Sequences of earthquakes are commonly represented as a succession of periods of interseismic stress accumulation followed by coseismic and postseismic phases of stress release. Because the recurrence time of large earthquakes is often greater than the available span of space geodetic data, it has been challenging to monitor the evolution of interseismic loading in its entire duration. Here we analyze large data sets of surface deformation at different key episodes around the Cholame, Parkfield and creeping segments of the San Andreas Fault that show evidence of significant deceleration of fault slip during the interseismic period. We compare the average fault slip rates before and after the 2004 Mw6 Parkfield earthquake, in the 1986–2004 and 2006–2012 periods, respectively, avoiding 2 years of postseismic deformation after 2004. Using a combination of GPS data from the Plate Boundary Observatory, the Southern California Earthquake Center Crustal Motion Map and the Bay Area Velocity Unification networks and interferometric synthetic aperture radar from the Advanced Land Observing Satellite (ALOS) and Envisat satellites, we show that the area of coupling at the transition between the Parkfield and Cholame segments appears larger later in the interseismic period than it does earlier on. While strong plate coupling is uniform across the Parkfield and Cholame segments in the 1986–2004 period, creep occurs south of the 2004 epicenter after 2006, making segmentation of the San Andreas Fault south of Parkfield more clearly apparent. These observations indicate that analyses of surface deformation late in the earthquake cycle may overestimate the area of plate coupling. A fault surface creeping much below plate rate may in some case be a region that does not promote earthquake nucleation but rather just be at a slower stage of its evolution. Our analysis also shows signs of large variation of slip velocity above and below plate rate in the creeping segment indicating that cycles of weakening and hardening can also be at play in dominantly aseismic areas

Spatiotemporal evolution of surface creep in the Parkfield region of the San Andreas Fault (1993-2004) from synthetic aperture radar.

International audienceThe Parkfield section of the San Andreas Fault (SAF) is defined as a transitional portion of the fault between slip-release behavior types in the creeping section of the SAF to the northwest and the apparently locked section to the southeast. The Parkfield section is characterized by complex frictional fault behavior because it represents a transition zone from aseismic creep to stick-slip regime. At least six historic earthquakes of Mw ~6 have occurred in this area in 1881, 1901, 1922, 1934, 1966, and 2004. It was observed in the 2004 Mw 6.0 Parkfield earthquake that ~70% of the total (coseismic and postseismic) moment release occurred aseismically. To understand the SAF behavior in this area, it is of particular interest to measure and analyze, not only the spatial evolution of the surface displacement in this area, but also its evolution over time. Using radar data acquired by the European Space Agency's European Remote Sensing (ERS1-2) satellites, we constructed descending interferograms and retrieved time series of surface displacements along the central SAF for the decade preceding the 2004 Parkfield earthquake. We focus on characterizing the space and time evolution of surface creep in the Parkfield and Cholame sections. The spatial pattern of the interseismic displacement rate indicates that tectonic strain was not uniformly distributed along the strike of the fault between 1993 and 2004. Our data indicate not only a decrease in the creep rate from the Parkfield section to south of Highway-46 from 1.4 ±0.3 cm/y to 0.6 ±0.3 cm/y, but also a small but significant creep-rate increase in the Cholame section to 0.2 ±0.1 cm/y. The evidence for episodic creep in the Cholame section of the SAF south-east of Parkfield is in contrast with previously published interpretations of GPS and trilateration data. The Cholame section of the SAF merits close monitoring because it was likely the nucleation site of the 1857 Fort Tejón earthquake and because it has shown recent evidence of deep slow slip as revealed by deep tremors

Improving InSAR geodesy using global atmospheric models

Spatial and temporal variations of pressure, temperature and water vapor content in the atmosphere introduce significant confounding delays in Interferometric Synthetic Aperture Radar (InSAR) observations of ground deformation and bias estimatesof regional strain rates. Producing robust estimates of tropospheric delays remains one of the key challenges in increasing the accuracy of ground deformation measurements using InSAR. Recent studies revealed the efficiency of global atmospheric reanalysis to mitigate the impact of tropospheric delays, motivating further exploration of their potential. Here, we explore the effectiveness of these models in several geographic and tectonic settings on both single interferograms and time series analysis products. Both hydrostatic and wet contributions to the phase delay are important to account for. We validate these path delay corrections by comparing with estimates of vertically integrated atmospheric water vapor content derived from the passive multi-spectral imager MERIS, onboard the ENVISAT satellite. Generally, the performance of the prediction depends on the vigor of atmospheric turbulence. We discuss (1) how separating atmospheric and orbital contributions allows one to better measure long wavelength deformation, (2) how atmospheric delays affect measurements of surface deformation following earthquakes and (3) we show that such a method allows us to reduce biases in multi-year strain rate estimates by reducing the influence of unevenly sampled seasonal oscillations of the tropospheric delay

Blind Thrusting, Surface Folding and the Development of Geological Structure in the Mw 6.3 2015 Pishan (China) Earthquake

The relationship between individual earthquakes and the longer-term growth of topography and of geological structures is not fully understood, but is key to our ability to make use of topographic and geological datasets in the contexts of seismic hazard and wider-scale tectonics. Here we investigate those relationships at an active fold-and-thrust belt in the southwest Tarim Basin, Central Asia. We use seismic waveforms and interferometric synthetic aperture radar (InSAR) to determine the fault parameters and slip distribution of the 2015 Mw 6.3 Pishan earthquake - a blind, reverse-faulting event dipping towards the Tibetan Plateau. Our earthquake mechanism and location correspond closely to a fault mapped independently by seismic reflection, indicating that the earthquake was on a pre-existing ramp fault over a depth range of ˜9–13 km. However, the geometry of folding in the overlying fluvial terraces cannot be fully explained by repeated coseismic slip in events such as the 2015 earthquake nor by the early postseismic motion shown in our interferograms; a key role in growth of the topography must be played by other mechanisms. The earthquake occurred at the Tarim-Tibet boundary, with the unusually low dip of 21° . We use our source models from Pishan and a 2012 event to argue that the Tarim Basin crust deforms only by brittle failure on faults whose effective coefficient of friction is ≤0.05±0.025. In contrast, most of the Tibetan crust undergoes ductile deformation, with a viscosity of order 10²⁰–10²² Pa s. This contrast in rheologies provides an explanation for the low dip of the earthquake fault plane