Earthquakes and tectonics II

In 1911, a poorly characterized major earthquake struck the Pamirs, forming the Usoi landslide dam that continues to present a continuing natural hazard if it were to be subject to overtopping of the resultant lake. On 7^th December 2015, a magnitude 7.2 strike-slip earthquake struck the same region, all-be-it differing in fault location and landslide productivity relative to the prior 1911 event of the same magnitude. We apply a wide suite of remote geodetic techniques to determine the displacement field, fault segmentation and slip, with the space-based imaging techniques revealing left-lateral offset along 60 km of the SSW-striking Karakul-Sarez fault (KSF), and numerous coseismic landslides. Sentinel-1 interferograms reveal many metres of left-lateral surface displacement along 40 km of the KSF, with an additional 10-15 km of buried, blind rupture at both ends. This matches the extent of the dislocation we determine from pixel-tracking of pre- and post-event Landsat-8 scenes. Both of these far-field deformation maps indicate that the rupture ended northward around a 3-km step in the fault trace, and southward beneath Sarez Lake. Direct comparison of pre- and post-event SPOT6/7 images shows discontinuous new scarps and small stream offsets along 30 km of the KSF from the shore of Sarez Lake northward, corroborating this surface rupture extent. We difference pre- and post-event topography derived from the tristereo SPOT images, and thus identify through-going strike-slip rupture as the differential lateral advection of steep ridges. Our detailed height-change maps also reveal numerous landslides that may be attributed to the earthquake. In particular, massive slope failures around the shore of Sarez Lake indicate that overtopping of the Usoi dam by a landslide-induced seiche remains one of the principal secondary seismic hazards in the region.  

Present-day deformation in northwestern Tibet is poorly known but is a key to understand the mode of deformation. Interferometric synthetic Aperture Radar (InSAR) has the potential of providing measurements where geodetic data are missing due to harsh field conditions. However, its application in natural environments is hindered by strong decorrelation of the radar phase due to vegetation, relief, and freeze and thaw cycles, but also due to variable tropospheric phase delays across topographic features and long-wavelength residual orbital ramps. Here, we develop methodologies to circumvent these limitations and separate tectonic from other parasitic signals. We process data from the complete Envisat archive on four, 800 km-long orbit tracks from the Tarim Basin to the central part of Tibet using the New Small Baselines Subset (NSBAS) processing software. A specific focus on the permafrost related deformation signal allows us to correctly unwrap interferograms from north to south, in particular across sedimentary basins, and isolate bedrock pixels that are not affected by the permafrost signal for further tectonic analysis. We also analyze the atmospheric signal across the high plateau margin and estimate proxy for the uncertainty on atmospheric corrections. The propagation of individual errors in the time series analysis allows estimating tectonic velocities with higher reliability. The continuous velocity fields identify a strain accumulation around the western extension of the south trace of the Kunlun Fault, redefining the block boundaries in northwestern Tibet. A novel and surprising result is the observation of a clear line of concentrated deformation within the northern piedmont of the Altyn Shan, of around 3 mm/yr within the Tarim basin, trending parallel to the Altyn Tagh Fault trace, as well as thrust signal uplifting terraces at a rate of 1 mm/yr. These findings suggest that the transpressive deformation along the northern edge of Tibet may be decoupled into transform and compressive deformation on deep-seated structures, which may merge at depth into a single lithospheric boundary. We thus explore the geometry of the fault system at depth and associated slip rates using a Bayesian approach and test the consistency of the present-day geodetic surface displacements with this tectonic model. 

The relationship between individual earthquakes and the longer-term growth of topography and of geological structures is not yet fully understood, but is key to our ability to make use of topographic and geological datasets in the context of seismic hazard and wider-scale tectonics. Given that historical records of earthquakes are much shorter than the recurrence times for many faults, data sources with longer temporal coverage than earthquake catalogues and global spatial coverage, such as digital elevation models, surface geomorphology and geology, can be valuable assets for interpreting seismic hazard and regional tectonics. These interpretations rely on understanding the contributions of interseismic, coseismic and postseismic deformation to the total permanent deformation. Here we investigate these relationships at an active fold and thrust belt on the Tarim-Tibet margin, presenting observations of the coseismic and early postseismic stages of the seismic cycle for the 3 July 2015 Pishan Mw 6.3 earthquake in Xinjiang, China. We also compare our results to the cumulative effects of multiple seismic cycles that are recorded in the local geology and geomorphology.
We use Sentinel-1A interferometric synthetic aperture radar (InSAR) and teleseismic body waveform modelling to determine the fault parameters and slip distribution of the mainshock - a gently dipping reverse faulting earthquake in the southwest corner of the Tarim Basin. Our mechanism and location correspond closely to the fault geometry mapped independently from seismic reflection profiles and show that the earthquake was blind and on a pre-existing ramp fault over a depth range of ~9-13km. We further identify a postseismic ground motion signal in Sentinel-1 interferograms, the first seven months of which has a line-of-sight change around one fifth that of the coseismic signal. By mapping the geomorphology of the overlying area using high-resolution optical imagery and digital elevation models, we find long wavelength folding and numerous small scarps distributed over several kilometres across-strike. The geomorphic folding is consistent with blind reverse faulting but the geometry of the folding cannot be fully explained by repeated coseismic slip in events such as the 2015 earthquake. By comparing the coseismic and postseismic slip on faults at depth with the geomorphic and geological constraints on folding, we are able to discuss the mechanism of fold growth and the consequences for models of structure at depth inferred purely from surface data.

Combining space-based geodetic and array seismology observations can provide detailed informations about earthquake ruptures in remote regions. Here we use Landsat-8 imagery and ALOS-2 and Sentinel-1 radar interferometry data combined with data from the European Seismology Network (EU) to describe the source of the December 7, 2015, Mw7.2 Mughrab (Tajikistan) earthquake. The earthquake reactivated a 70 km-long section of the Serez-Karakul fault, a NE oriented sinistral, trans-tensional fault in northern Pamir. Pixel offset data delineate the geometry of the surface break and line of sight ground shifts from two descending and three ascending interferograms constrained the fault dip and slip solution. Two right-stepping, NE-striking segments connected by a more easterly oriented segment, sub-vertical or steeply dipping to the west were involved. The solution shows two main patches of slip with up to 3.5 m of left lateral slip on the southern and central fault segments. The northern segment has a left-lateral and normal oblique slip of up to a meter. Back protection of the high frequency (0.5-2.0 s) band of seismic data recorded by the EU network processed using the Multitaper-MUSIC approach focus sharply along the modeled fault. The time progression of the high frequency radiators shows that, after a 10 seconds initiation phase at slow speed, the rupture progressed in 2 phases at super-shear velocity (~6 km/s) separated by a ~10 seconds interval of slower propagation corresponding to the passage through the restraining bend. The intensity of the high frequency radiation reaches maxima during the early and middle phases of slow propagation and is reduced by ~50% during the super-shear phases of the propagation. These findings are consistent with other studies on other strike-slip faults in continental domain, showing the importance of the fault geometric complexities in controlling the speed of fault propagation and related high frequency radiation pattern.

The coseismic slip of the 1978 M_w 7.3 Tabas-e-Golshan earthquake (eastern Iran) was mostly concentrated at depth. The slip gradient at shallow depth must therefore be relaxed aseismically by postseismic creep, and this postseismic motion plays an important role in the geological evolution of the Tabas fold system. Our previous study (Zhou et al., 2016) based on historical optical and modern satellite imagery reveals ~7 m shallow slip on a high-angle (50°) thrust ramp beneath the Tabas fold. The majority of slip appears to be afterslip with a minimum of ~4.4 m prior to 1991 and ~0.5 m in 1991-2013. Using ESA’s ERS and Envisat data, Copley (2014) mapped postseismic afterslip around the Tabas region, and found that the average slip rate on the high-angle thrust ramp beneath the Tabas fold decreases from ~5 mm/yr between 1996 and 1999 to ~3.3 mm/yr between 2003 and 2010. We related the observed cumulative afterslip s and time t in the form s=αtⁿ (an empirical relationship observed in laboratory experiments of aseismic creep) and found a best-fitting power law exponent of 0.02 (i.e. s=4.4t^0.02) based on the optical correlations and the ERS-derived slip rate (Zhou et al., 2016). To further investigate the time-dependent afterslip and better understand its physical mechanisms, we employ Sentinel-1 data to measure the postseismic deformation between 2014 and 2016. The radar data were processed using the COMET InSAR processing software (LiCSAR, built around GAMMA InSAR software package) with a multilook factor of 4 in azimuth and 20 in range (corresponding to 100 m spatial resolution). The interferograms were stacked to derive a rate map. The Sentinel-1 preliminary results reveal continued afterslip at a present rate of ~1.1 mm/yr in the satellite line-of-sight direction, implying a fault slip rate of ~2.2 mm/yr. This estimated rate of afterslip in 2014-2016 is consistent with the predicted rate (2.6 mm/yr) from our previous study. Combining all the InSAR-derived slip rates (~5 mm/yr between 1996-1999 from ERS, ~3.3 mm/yr between 2003-2010 from Envisat, and ~2.2 mm/yr between 2014-2016 from Sentinel-1), we re-estimate the power law parameters and obtain s=2.6t^0.03 (or ds/dt=0.078t^-0.97), consistent with the previous estimate, both suggesting ~1/t decay in afterslip rate. The observed decay in postseismic motion (~1/t dependence) allows us to investigate the physical mechanisms of afterslip.

References

Copley, A. (2014). Postseismic afterslip 30 years after the 1978 Tabas-e-Golshan (Iran) earthquake: observations and implications for the geological evolution of thrust belts. Geophysical Journal International, ggu023.

Zhou, Y., Walker, R. T., Hollingsworth, J., Talebian, M., Song, X., & Parsons, B. (2016). Coseismic and postseismic displacements from the 1978 M_w 7.3 Tabas-e-Golshan earthquake in eastern Iran. Earth and Planetary Science Letters, 452, 185-196.