M. Y. Thomas, and H. S. Bhat, 2018. Dynamic Evolution Of Off-Fault Medium During An Earthquake: A Micromechanics Based Model, Geophysical Journal International, v. 214, p1267-1280.
Geophysical observations show a dramatic drop of seismic wave speeds in the shal- low off-fault medium following earthquake ruptures. Seismic ruptures generate, or re- activate, damage around faults that alter the constitutive response of the surrounding medium, which in turn modifies the earthquake itself, the seismic radiation, and the near-fault ground motion. We present a micromechanics based constitutive model that accounts for dynamic evolution of elastic moduli at high-strain rates. We consider 2D in-plane models, with a 1D right lateral fault featuring slip-weakening friction law. The two scenarios studied here assume uniform initial off-fault damage and an observation- ally motivated exponential decay of initial damage with fault normal distance. Both scenarios produce dynamic damage that is consistent with geological observations. A small difference in initial damage actively impacts the final damage pattern. The sec- ond numerical experiment, in particular, highlights the complex feedback that exists between the evolving medium and the seismic event. We show that there is a unique off-fault damage pattern associated with supershear transition of an earthquake rupture that could be potentially seen as a geological signature of this transition. These scenarios presented here underline the importance of incorporating the complex structure of fault zone systems in dynamic models of earthquakes.
Y. Zhou, M. Y. Thomas, B. Parsons, R. T. Walker, 2017. Time-dependent postseismic slip following the 1978 Mw 7.3 Tabas-e-Golshan, Iran earthquake revealed by over 20 years of ESA InSAR observations, Earth and Planetary Science Letters, v. 483, p. 64-75.
We use over 20 yrs (1996–2017) of the European Space Agency's (ESA) radar interferometry (InSAR) observations to investigate the postseismic deformation of the Tabas fold segment following the 1978 7.3 Tabas-e-Golshan earthquake in eastern Iran. We generated maps of satellite line-of-sight (LOS) velocity using two ERS descending tracks (1996–1999), one Envisat descending track (2003–2010), one Sentinel-1A descending track (2014–2017) and one Sentinel-1A ascending track (2014–2017). The LOS velocity shows afterslip continuing for at least 40 yrs after the earthquake. Elastic dislocation modelling based on the InSAR measurements reveals a decrease in postseismic velocities from 5.0 ± 0.8 mm/yr in 1996–1999 to 3.9 ± 0.6 mm/yr in 2003–2005, 3.0 ± 0.4 mm/yr in 2006–2010, and a present rate of 2.3 ± 0.6 mm/yr in 2014–2017. The rates decay with time, t, as , consistent with the predictions of a simple block-slider model. We then combine the InSAR rates and our previous estimates of the total earthquake slip derived from optical image matching and DEM differencing to explore the frictional behaviour of the Tabas fold. We obtained a rate-and-state parameter a-b ≈ 0.003, indicating rate-strengthening frictional behaviour of the Tabas fault. We also inferred a minimum coseismic slip of 4.7 m, which might have driven bedding-plane shear at shallow depth, resulting in distributed fold growth and secondary faulting observed in the field. The results imply that both coseismic slip and afterslip have occurred in the same location. One possible mechanism to explain such a phenomenon is that the frictional parameter a-b is small enough to allow dynamic ruptures to propagate into rate-strengthening regions.
M. Y. Thomas, J.-P. Avouac, N. Lapusta, 2017. Dynamic modeling of earthquakes sequences on the Longitudinal Valley Fault: implications for friction properties, Journal of Geophysical Research-solid Earth, v. 112, p. 3115-3137.
The Longitudinal Valley Fault (LVF, Taiwan) is a fast-slipping fault (∼5 cm/yr), which exhibits both seismic and aseismic slip. Geodetic and seismological observations (1992–2010) were used to infer the temporal evolution of fault slip. This kinematic model is used here to estimate spatial variations of steady state velocity dependence of fault friction and to develop a simplified fully dynamic rate-and-state model of the LVF. Based on the postseismic slip, we estimate that the rate-and-state parameter decreases from ∼1.2 MPa near the surface to near velocity neutral at 19 km depth. The inferred (a − b) values are consistent with the laboratory measurements on clay-rich fault gouges comparable to the Lichi Mélange, which borders the LVF. The dynamic model that incorporates the obtained estimates as well as a velocity-weakening patch with tuned rate-and-state properties produces a sequence of earthquakes with some realistic diversity and a spatiotemporal pattern of seismic and aseismic slip similar to that inferred from the kinematic modeling. The larger events have moment magnitude (Mw ∼6.7) similar to the 2003 Chenkung earthquake, with a range of smaller events. The model parameterization allows reproducing partial overlap of seismic and aseismic slip before the earthquake but cannot reproduce the significant postseismic slip observed in the previously locked patch. We discuss factors that can improve the dynamic model in that regard, including the possibility of temporal variations in (a − b) due to shear heating. Such calibrated dynamic models can be used to reconcile field observations, kinematic analysis, and laboratory experiments and assess fault behavior.
M. Y. Thomas, H. S. Bhat, and Y. Klinger, 2017. Effect of Brittle off-fault Damage on Earthquake Rupture Dynamics, AGU monograph on “Fault Zone Dynamic Processes: Evolution of Fault Properties During Seismic Rupture”, v. 227, p. 255-280
In the shallow brittle crust, following earthquake ruptures, geophysical observations show a dramatic drop of seismic wave speeds in the shallow off-fault medium. Seismic ruptures gen- erate, or reactivate, damage around faults that alter the constitutive response of the surrounding medium, which in turn modifies the earthquake itself, the seismic radiation and the near-fault ground motion. This numerical study aims to assess the interplay between earthquake rup- tures and dynamically evolving off-fault medium and to underline the damage-related features pertinent to interpret geophysical observations. We present a micro-mechanics based consti- tutive model that account for dynamic evolution of elastic moduli at high-strain rates. We consider 2-D inplane models, with a 1-D right lateral fault featuring slip-weakening friction law. We demonstrate that the response of the damaged elastic solid is different in the com- pressional and tensional quadrant. We observe that dynamic damage induces a reduction in elastic moduli and produces slip rate oscillations which result in high frequency content in the radiated ground motion, consistent with strong motion records. We underline the importance of incorporating off-fault medium history in earthquake rupture processes. We find that dy- namic damage generation is sensitive to material contrast and that it introduces an additional asymmetry beyond that of a bimaterial fault, in agreement with experimental studies.
M. Y. Thomas, J.-P. Avouac, J.-P. Gratier, and J.-C. Lee, 2014. Lithological control on the deformation mechanism and the mode of fault slip on the Longitudinal Valley Fault, Taiwan, Tectonophysics, v632, p. 48-63.
The Longitudinal Valley Fault (LVF) in Taiwan is creeping at shallow depth along its southern half, where it is bounded by the Lichi Mélange. By contrast, the northern segment of the LVF is locked where it is bounded by forearc sedimentary and volcanoclastic formations. Structural and petrographic investigations show that the Lichi Mélange most probably formed as a result of internal deformation of the forearc when the continental shelf of South China collided with the Luzon arc as a result of the subduction of the South China Sea beneath the Philippine Sea Plate. The forearc formations constitute the protolith of the Lichi Mélange. It seems improbable that the mechanical properties of the minerals of the matrix (illite, chorite, kaolinite) in themselves explain the aseismic behavior of the LVF. Microstructural investigations show that deformation within the fault zone must have resulted from a combination of frictional sliding at grain boundaries, cataclasis (responsible for grain size comminution) and pressure solution creep (responsible for the development of the scaly foliation and favored by the mixing of soluble and insoluble minerals). The microstructure of the gouge formed in the Lichi Mélange favors effective pressure solution creep, which inhibits strain-weakening brittle mechanisms and is probably responsible for the dominantly aseismic mode of fault slip. Since the Lichi Mélange is analogous to any unlithified subduction mélanges, this study sheds light on the mechanisms which favor aseismic creep on subduction megathrust.
M. Y. Thomas, J.-P. Avouac, J. Champenois, J.-C. Lee, and L.-C. Kuo, 2014. Spatiotemporal evolution of seismic and aseismic slip on the Longitudinal Valley Fault, Taiwan, Journal of Geophysical Research-solid Earth, v. 119.
The Longitudinal Valley Fault (LVF) in eastern Taiwan is a high slip rate fault (about 5 cm/yr), which exhibits both seismic and aseismic slip. Deformation of anthropogenic features shows that aseismic creep accounts for a significant fraction of fault slip near the surface, whereas a fraction of the slip is also seismic, since this fault has produced large earthquakes with five Mw > 6.8 events in 1951 and 2003. In this study, we analyze a dense set of geodetic and seismological data around the LVF, including campaign mode Global Positioning System(GPS) measurements, time series of daily solutions for continuous GPS stations (cGPS), leveling data, and accelerometric records of the 2003 Chenkung earthquake. To enhance the spatial resolution provided by these data, we complement them with interferometric synthetic aperture radar (InSAR) measurements produced from a series of Advanced Land Observing Satellite images processed using a persistent scatterer technique. The combined data set covers the entire LVF and spans the period from 1992 to 2010. We invert this data to infer the temporal evolution of fault slip at depth using the Principal Component Analysis-based Inversion Method. This technique allows the joint inversion of diverse data, taking the advantage of the spatial resolution given by the InSAR measurements and the temporal resolution afforded by the cGPS data. We find that (1) seismic slip during the 2003 Chengkung earthquake
occurred on a fault patch which had remained partially locked in the interseismic period, (2) the seismic rupture propagated partially into a zone of shallow aseismic interseismic creep but failed to reach the surface, and (3) that aseismic afterslip occurred around the area that ruptured seismically. We find consistency between geodetic and seismological constraints on the partitioning between seismic and aseismic creep. About 80–90% of slip on the southern section of LVF in the 0–26 km, seismogenic depth range, is actually aseismic. We infer that the clay-rich Lichi Mélange is the key factor promoting aseismic
creep at shallow depth.
occurred on a fault patch which had remained partially locked in the interseismic period, (2) the seismic rupture propagated partially into a zone of shallow aseismic interseismic creep but failed to reach the surface, and (3) that aseismic afterslip occurred around the area that ruptured seismically. We find consistency between geodetic and seismological constraints on the partitioning between seismic and aseismic creep. About 80–90% of slip on the southern section of LVF in the 0–26 km, seismogenic depth range, is actually aseismic. We infer that the clay-rich Lichi Mélange is the key factor promoting aseismic
creep at shallow depth.
M. Y Thomas, N. Lapusta, H. Noda, H. and J.-P. Avouac, 2014. Quasi-dynamic versus fully-dynamic simulations of earthquakes and aseismic slip with and without enhanced coseismic weakening, Journal of Geophysical Research-solid Earth, v. 119, p. 1986-2004
Physics-based numerical simulations of earthquakes and slow slip, coupled with field observations and laboratory experiments, can, in principle, be used to determine fault properties and potential fault behaviors. Because of the computational cost of simulating inertial wave-mediated effects, their representation is often simplified. The quasi-dynamic (QD) approach approximately accounts for inertial effects through a radiation damping term. We compare QD and fully dynamic (FD) simulations by exploring the long-term behavior of rate-and-state fault models with and without additional weakening during seismic slip. The models incorporate a velocity-strengthening (VS) patch in a velocity-weakening (VW) zone, to consider rupture interaction with a slip-inhibiting heterogeneity. Without additional weakening, the QD and FD approaches generate qualitatively similar slip patterns with quantitative differences, such as slower slip velocities and rupture speeds during earthquakes and more propensity for rupture arrest at the VS patch in the QD cases. Simulations with additional coseismic weakening produce qualitatively different patterns of earthquakes, with near-periodic pulse-like events in the FD simulations and much larger crack-like events accompanied by smaller events in the QD simulations. This is because the FD simulations with additional weakening allow earthquake rupture to propagate at a much lower level of prestress than the QD simulations. The resulting much larger ruptures in the QD simulations are more likely to propagate through the VS patch, unlike for the cases with no additional weakening. Overall, the QD approach should be used with caution, as the QD simulation results could drastically differ from the true response of the physical model considered.
T. Ader, J.-P. Avouac, J. Liu-Zeng, H. Lyon-Caen, L. Bollinger, J. Galetzka, J. Genrich, M. Thomas, K. Chanard, S. N. Sapkota, P. L. Shrestha, S. Rajaure, D. Lin, and M. Flouzat, 2012, Convergence rate across the Nepal Himalaya and interseismic coupling on the Main Himalayan Thrust, implications for seismic hazard, Journal of Geophysical Research-Solid Earth, v 117, p. B04403
We document geodetic strain across the Nepal Himalaya using GPS times series from 30 stations in Nepal and southern Tibet, in addition to previously published campaign GPS points and leveling data and determine the pattern of interseismic coupling on the Main Himalayan Thrust fault (MHT). The noise on the daily GPS positions is modeled as a combination of white and colored noise, in order to infer secular velocities at the stations with consistent uncertainties. We then locate the pole of rotation of the Indian plate in the ITRF 2005 reference frame at longitude = − 1.34° ± 3.31°, latitude = 51.4° ± 0.3° with an angular velocity of Ω = 0.5029 ± 0.0072°/Myr. The pattern of coupling on the MHT is computed on a fault dipping 10° to the north and whose strike roughly follows the arcuate shape of the Himalaya. The model indicates that the MHT is locked from the surface to a distance of approximately 100 km down dip, corresponding to a depth of 15 to 20 km. In map view, the transition zone between the locked portion of the MHT and the portion which is creeping at the long term slip rate seems to be at the most a few tens of kilometers wide and coincides with the belt of midcrustal microseismicity underneath the Himalaya. According to a previous study based on thermokinematic modeling of thermochronological and thermobarometric data, this transition seems to happen in a zone where the temperature reaches 350°C. The convergence between India and South Tibet proceeds at a rate of 17.8 ± 0.5 mm/yr in central and eastern Nepal and 20.5 ± 1 mm/yr in western Nepal. The moment deficit due to locking of the MHT in the interseismic period accrues at a rate of 6.6 ± 0.4 × 1019 Nm/yr on the MHT underneath Nepal. For comparison, the moment released by the seismicity over the past 500 years, including 14 MW ≥ 7 earthquakes with moment magnitudes up to 8.5, amounts to only 0.9 × 1019 Nm/yr, indicating a large deficit of seismic slip over that period or very infrequent large slow slip events. No large slow slip event has been observed however over the 20 years covered by geodetic measurements in the Nepal Himalaya. We discuss the magnitude and return period of M > 8 earthquakes required to balance the long term slip budget on the MHT.
C. Hamelin, L. Dosso, B. B. Hanan, M. Moreira, A. P. Kositsky, and M. Y. Thomas, 2011. Geochemical portray of the Pacific Ridge: New isotopic data and statistical techniques, Earth and Planetary Science Letters, v. 302, p. 154-162
Samples collected during the PACANTARCTIC 2 cruise fill a sampling gap from 53° to 41° S along the Pacific Antarctic Ridge (PAR). Analysis of Sr, Nd, Pb, Hf, and He isotope compositions of these new samples is shown together with published data from 66°S to 53°S and from the EPR. The recent advance in analytical mass spectrometry techniques generates a spectacular increase in the number of multidimensional isotopic data for oceanic basalts. Working with such multidimensional datasets generates a new approach for the data interpretation, preferably based on statistical analysis techniques.
Principal Component Analysis (PCA) is a powerful mathematical tool to study this type of datasets. The purpose of PCA is to reduce the number of dimensions by keeping only those characteristics that contribute most to its variance. Using this technique, it becomes possible to have a statistical picture of the geochemical variations along the entire Pacific Ridge from 70°S to 10°S. The incomplete sampling of the ridge led previously to the identification of a large-scale division of the south Pacific mantle at the latitude of Easter Island. The PCA method applied here to the completed dataset reveals a different geochemical profile. Along the Pacific Ridge, a large-scale bell-shaped variation with an extremum at about 38°S of latitude is interpreted as a progressive change in the geochemical characteristics of the depleted matrix of the mantle. This Pacific Isotopic Bump (PIB) is also noticeable in the He isotopic ratio along-axis variation. The linear correlation observed between He and heavy radiogenic isotopes, together with the result of the PCA calculation, suggests that the large-scale variation is unrelated to the plume–ridge interactions in the area and should rather be attributed to the partial melting of a marble-cake assemblage.
Principal Component Analysis (PCA) is a powerful mathematical tool to study this type of datasets. The purpose of PCA is to reduce the number of dimensions by keeping only those characteristics that contribute most to its variance. Using this technique, it becomes possible to have a statistical picture of the geochemical variations along the entire Pacific Ridge from 70°S to 10°S. The incomplete sampling of the ridge led previously to the identification of a large-scale division of the south Pacific mantle at the latitude of Easter Island. The PCA method applied here to the completed dataset reveals a different geochemical profile. Along the Pacific Ridge, a large-scale bell-shaped variation with an extremum at about 38°S of latitude is interpreted as a progressive change in the geochemical characteristics of the depleted matrix of the mantle. This Pacific Isotopic Bump (PIB) is also noticeable in the He isotopic ratio along-axis variation. The linear correlation observed between He and heavy radiogenic isotopes, together with the result of the PCA calculation, suggests that the large-scale variation is unrelated to the plume–ridge interactions in the area and should rather be attributed to the partial melting of a marble-cake assemblage.