TS4.7

Simple elastic dislocation models have been widely used to describe the surface displacements associated with subduction zone earthquake cycles. To first order, these assume a portion of the plate interface is locked during the interseismic period, inducing subsidence in the offshore domain and uplift in the onshore region. In contrast, megathrust earthquakes will impart the opposite surface displacement with offshore uplift and onshore subsidence. Such a purely elastic description of the earthquake cycle implies that interseismic deformation should be entirely compensated by large megathrust earthquakes, amounting to effectively zero deformation over numerous cycles. Recent studies however propose that spatial patterns of interseismic (short-term) deformation are reflected in long-term trends of coastal uplift (Jolivet et al., 2020), as well as in the morphology of subduction margins, which is shaped over 100s of kyrs by the interaction of tectonic and surface processes (Malatesta et al., 2021). This suggests that the repetition of seemingly elastic cycles somehow leads to non-recoverable long-term deformation.

We postulate that a small increment of inelastic deformation accumulates during each interseismic phase, leading to a long-term unbalance of co-, post- and interseismic strain. To test this hypothesis, we evaluate the variations in upper plate stress imparted by down-dip gradients in megathrust locking during the interseismic period in the Chile and Cascadia subduction zones. We add these changes to the estimated background stress state of the upper plate, and assess the extent of frictional yielding within the forearc as a function of interseismic slip deficit and upper plate strength. We find that the onset of yielding in the late interseismic phase coincides with observed areas of microseismicity at these subduction margins, typically located above the downdip end of the locked zone.

We then estimate the permanent surface uplift imparted by this upper plate yielding employing a statistical approach. We model frictional yielding of the forearc as incremental slip on a population of small faults whose spatial distribution reflects the fraction of the interseismic phase duration spent at yield. We further assume that the temporal distribution of these slip follows a Gutenberg-Richter distribution of parameters consistent with the observed microseismicity. Upon summing the displacements due to each of these dislocations, we estimate the irreversible surface displacement field associated with multiple seismic cycle.  This ultimately amounts to permanent uplift concentrated above the transition from freely slipping to fully coupled megathrust, and is consistent with the geometry and rates of long-term uplift recorded in Chile. We also demonstrate how our model can explain the recently reported correlation between location of downdip locking limit and shelf break in many active margins.

How to cite: Oryan, B., Olive, J.-A., Jolivet, R., Bruhat, L., and Malatesta, L.: Long-term coastal uplift due to non-recoverable forearc deformation during the interseismic phase of the subduction earthquake cycle, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1557, https://doi.org/10.5194/egusphere-egu22-1557, 2022.

The postseismic deformation in the aftermath of the 2011 Tohoku-oki earthquake showed a stronger surface movement in northeastern Japan (NE) with subsequent decays over time. In response to the coseismic stress perturbation, afterslip on the megathrust interface is held responsible for the short-term deformation while viscoelastic relaxation in the surrounding lithosphere largely contributes to the long-term crustal deformation (e.g., Ozawa et al. 2012, JGR). On the contrary, decade-long studies on the postseismic model implied the prevalence of viscoelastic flow during the early phase of postseismic deformation (e.g., Sun et al. 2014, Nature, Watanabe et al. 2014, GRL, Freed et al. 2017, EPSL; Muto et al. 2016, GRL). Although geodetic displacement at any GNSS station may not indicate the single domination of either viscoelastic relaxation or afterslip over the longer period after the earthquake, the densely deployed nationwide GNSS observations (GEONET) till ~2021 provides a definite opportunity to resolve the contributions of various source mechanisms and their evolution over time.

Time series of geodetic observations are mainly explained using a numerical simulation of the source mechanisms (e.g., Agata et al. 2019 Nat. commun.; Luo & Wang 2021, Nat. Geosci.; Muto et al. 2019, Sci. Adv.; Fukuda & Johnson 2021, JGR) or non-linear regression of a fitting function (Tobita 2016, EPS). Utilizing the lesson learnt from the postseismic model built on laboratory-derived constitutive laws, we proposed an analytical fitting function for the GNSS time-series over the NE Japan. We deploy statistical approaches to ensure its stability and robustness. Our analytical function can be used to fit and predict the postseismic displacements at GNSS stations and understand the relative contributions of source mechanisms in lesser efforts.  We conclude that the afterslip at the downdip of the main rupture zone may continue for several decades following the megathrust earthquake. The decade-long records of repeating earthquakes on the plate boundary reiterate a similar conclusion concerning the longer persistence of afterslip in the Japan subduction zone (Igarashi & Kato 2021, Commun. Earth Env.; Uchida 2019, PEPS).

Our results also show that viscoelastic relaxation dominates immediately following the mega-earthquake at most inland GNSS stations. This conclusion can be supported by comparing the geodetic displacements with aftershock decay patterns (Morikami & Mitsui 2020, EPS), including recently developed stress-dependent postseismic deformation models (Agata et al. 2019, Nat. Commun; Fukuda & Johnson 2021, JGR; Muto et al. 2019, Sci. Adv).  Nevertheless, the previous studies indicate a change in the dominant mechanism of the postseismic deformation after the year ~2013-2015, particularly evident in the vertical motion (Morikami & Mitsui 2020, EPS; Yamaga & Mitsui 2019, GRL). We suggest that the transient deformation of the viscoelastic mantle decayed significantly during the ~3-4 years of the postseismic period, allowing the afterslip rate to supersede. The higher uplift rate along the Pacific coast of NE Japan, even after a decade, may reflect the shift in the dominant mechanism to the afterslip, persisting at the downdip of the main rupture zone.

How to cite: Dhar, S. and Muto, J.: Geodetic inference on decadal afterslip following the 2011 Tohoku-oki earthquake, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3457, https://doi.org/10.5194/egusphere-egu22-3457, 2022.

At the end of 2020, anomalous transient surface deformation was observed by an operational GNSS network at the Noto peninsula, Japan. Although the Noto peninsula locates far from the plate boundary, seismic observations recorded that seismic swarms were accompanied with this transient deformation. Nishimura et al. (2021, presentation at the 2021 Geodetic Society of Japan) estimated that this deformation and swarms may be associated with the intrusion of water from the subducting oceanic plate. Here I performed Sentinel-1 InSAR time series analysis to obtain more detailed view of this transient displacement and to investigate the mechanism of this phenomenon.
In the analysis, at first I created interferograms from Sentinel-1 IW SLCs using ISCE2 software. Then these interferograms were used for the LiCSBAS time series analysis. Orbital and topographic fringes were modeled and removed based on precise orbit information and SRTM 1-arcsecond DEM. No atmospheric corrections were applied. I used both ascending and descending paths so that I could calculate 2.5 dimensional analysis to derive quasi-horizontal and quasi-vertical displacements.
The result of Sentinel-1 time series showed that the transient displacement seems to start since the end of 2020, which is consistent with the result from the GNSS observation. The estimated largest surface velocities became 13 mm/year in ascending and 15 mm/year in descending. The 2.5 dimensional analysis suggested that the uplift was concentrated at the eastern front of the peninsula, which is also consistent with the GNSS observation. The derived displacement fields suggested that there is an inflation source but this need to be further investigation by, for example, using elastic spherical and/or rectangular fault models.
By the presentation, I will perform the InSAR atmospheric correction and source modelling and show these results.

How to cite: Kinoshita, Y.: Transient small displacement since the end of 2020 at Noto peninsula, Japan, revealed by Sentinel-1 InSAR time series analysis, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7145, https://doi.org/10.5194/egusphere-egu22-7145, 2022.

In the last decades, GNSS constraints and geological estimates of the fault slip rates improve the understating of the kinematics of faulting across Iran, particularly in the northeastern part of the country. Here, we complete the sparse GNSS vectors from previous reported studies around the Baghan Quchan fault zone (BQFZ) in northeastern Iran, by processing the Sentinel-1 archives covering this zone. According to tectonic and geodetic studies, the right-lateral BQFZ and the left-lateral Esfarayen fault constitute the northeastern and southern limits, respectively, of the easternmost part of the South Caspian Basin. While the BQFZ is limiting the SCB towards Eurasia, the Esfarayen fault is its border towards the Iranian microplate. We constructed 452 interferograms with 102 images from 2014.10.29 to 2019.10.27 (5 years) in descending geometry using the NSBAS package. We combined all three swaths (iw1, iw2, iw3) to cover the area of interest. The revisit time is 24 days between 2014.10.29 and 2017.02. 15, and 12 days from 2017.02.15 to 2019.10.27. To remove the hydrogeological land displacement effect (charge and discharge of aquifers), we chose the first (2014.10.29) and the last image (2019.10.27) at the same time of the year. Following the SBAS time series analysis approach, we created interferograms with short temporal (one or two months) and spatial baselines. Also, to avoid introducing any artificial signal in the mean velocity map, we created some interferograms with longer temporal baselines (maximum one year). We removed the neutral atmospheric delay using global reanalysis data provided by the European Centre for Medium-Range Weather Forecasts (ECMWF). Then, we filtered and unwrapped the generated interferograms. We applied the SBAS time series analysis on the generated interferograms to obtain displacement variations in time and a mean velocity map in the line of sight (LOS) direction of the satellites. The first noticeable point is the LOS mean velocity change across the BQFZ fault reaching up to ~1.5 mm/yr in the LOS direction, compatible with right-lateral displacement. Moreover, the mean velocity map varies significantly across the Esfarayen fault, in a sense coherent with left-lateral displacement. This velocity map points out that the NW motion of the South Caspian basin is effectively accommodated by the Esfarayen fault, while previous work based on the sparse GNSS network (Mousavi et al., 2013) suggested that the Bojnord fault further north is accommodating this NW motion. In particular, the new InSAR map indicates that the velocity vector of the permanent GNSS station ESFN used by Mousavi et al. (2013) is contaminated by subsidence motion and cannot be representative of a tectonic motion. This study brings new information for assessing seismic hazard in NE Iran with large population centers. Moreover, the retrieved mean velocity map indicates significant subsidence in Nishabour and Jajarm cities and Joveyn, Chahar Borj, Chenaran, Faruj and Ribat Jaz villages in Iran, as well as in the Yashklik city in Turkmenistan. This is the first report of subsidence occurring in Turkmenistan.

How to cite: Mousavi, Z., Walpersdorf, A., Pathier, E., and Walker, R.: InSAR constraints on interseismic slip-rate of the Esfarayen fault, northeastern Iran, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7630, https://doi.org/10.5194/egusphere-egu22-7630, 2022.

The Chihshang fault in Taiwan serves as one of the best examples of faults with a primarily thrust component that rapidly creep at the surface (2-3 cm/yr), while it is also known to have produced magnitude 6 earthquakes. The deeper portion of this thrust fault is typically offshore, where land-based geodetic measurements are insensitive to fault slip at greater depth. The understanding of inter-seismic slip rate at depth therefore, remains elusive. Taking advantages of slip rates inferred from repeating earthquake sequences (RES) at greater depth, here we present a modified method that embeds RES derived slip rate into the neighboring fault patch for geodetic data inversion. Using the geodetic and seismological data from 2007 to 2011, we reach the higher resolution of interseismic slip rate distribution below the depth of 15 km. The inferred low coupling ratio establishes the extensive creeping area that coincides with the location of abundant repeating earthquakes and swarm events. The inferred high coupling ratio on the other hand, delineates the locked area corresponding to the co- seismic slip zone of the 2003 Mw6 Chengkung earthquake. The postseismic area however, is found to mainly overlapped with the low coupling ratio area at shallow depth (freely creeping) but not where the microseismcity, repeating and swarm events are located (partially creeping). We propose that the strongly locked area is concentrated in the middle of the fault extending from near surface to the depth of 25 km, surrounded by the creeping areas where microseismicity, repeating and swarm events are taking place. We estimate that a slip rate deficit equivalent to Mw 6.26 has accumulated annually, which may be able to generate greater than Mw 7 event over an interval of 20 years. It is thus importance, to follow up by time-dependent kinematic model in the future for better estimate of large earthquake potential in this creeping fault.

How to cite: Peng, W., Radiguet, M., Pathier, E., and Chen, K. H.: Spatial distribution of creep on a creeping thrust fault: Joint inversion using geodetic data and repeating earthquakes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8195, https://doi.org/10.5194/egusphere-egu22-8195, 2022.

Bob Elliott, Ken McCaffrey, Richard Walters (all Durham University), Dave Mackenzie (3vGeomatics), Laura Gregory (Leeds University)

Characterising near-fault deformation can improve understanding of how major co-seismic slip at depth is transferred to the surface. Deformation observed close to the fault scarp can identify where there has been shallow slip deficit, and the role of minor faults adjacent to the main faults as controlling influences in co-seismic slip distribution. However, field work and remote sensing techniques such as InSAR and GNSS are often inefficient or unreliable in characterising near-fault deformation due to exposure and data resolution issues. We use high resolution topographical models from optical satellite data from the Pleiades constellations to help identify the co-seismic deformation associated with the 30^th October 2016 Norcia earthquake.  We jointly inverted a total of 11 datasets including Pleiades-derived DEM difference data, InSAR and GNSS (far-field and short baseline)) for slip at depth following the method of Okada (1985). Compared to previous models derived from geodetic datasets, we used a relatively complicated fault geometry set-up in the area covered by the Pleiades datasets. By combining the near-fault input provided by the Pleiades data with far-field data we were able to model near-surface slip as well as slip at depth with a good fit to the Pleiades data, without losing the fit to the far-field data. The results show remarkable detail of slip transfer from the main faults onto minor structures in the hanging wall of the Monte Vettore fault within the top 2 km below the surface. Slip vectors near the surface also display considerable divergence from slip vectors at depth. This research provides valuable insight into the distribution of near-fault co-seismic slip in an area of complex faulting, 

How to cite: Elliott, B.: Characterising near-fault deformation in the Apennines through the use of high-resolution Pleiades optical satellite data, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8527, https://doi.org/10.5194/egusphere-egu22-8527, 2022.

Continental earthquakes often nucleate at the brittle-plastic transition zone in the upper crust. Since the strength of the crust reaches the maximum here, it is inferred that strain is localized, leading to seismic rupture. Fault rock deformation experiments under pressure-temperature conditions simulating the brittle-plastic transition are key to unravel the processes triggering continental earthquakes. We investigated the mechanical behavior and post-mortem microstructure of simulated quartz-feldspar gouges using a Griggs-type solid medium apparatus. The samples consist of mixtures of powdered quartz: albite = 50 : 50 (wt%), which were sheared under pressure-temperature conditions simulating depths of 7 to 30 km, realizing a geothermal gradient of 30 °C/km and a lithostatic pressure corresponding to a granitoid rock density of 2700 kg/m³. Specifically, experiments were carried out at temperatures ranging from 210 °C to 900 °C and confining pressures ranging from 185 MPa to 870 MPa. The bulk shear strain rate was sequentially stepped between ~10^-3 and ~10^-4 /s. After the experiments, each sample was analyzed using optical and scanning electron microscopy.

Experimental results show a clear positive dependence of the shear strength on temperature and pressure up to 720 °C and 750 MPa, suggesting the dominance of brittle deformation. On the contrary, when the condition rises to 900 °C and 870 MPa, the strength dropped by about 550 MPa compared with that of at 720 °C and 750 MPa. This may imply that the plastic deformation gradually has taken over the deformation. Microstructural observation revealed elongated grains with their long axes intersecting with the direction of a Riedel-1(R₁) shear plane (i.e., similar to a S-C fabric). Some grains were reduced in size to the nanometer range. Our observations suggest that shear strain was highly concentrated within fine-grained zones, which, we speculate, may lead to catastrophic rupture. Crack distributions illuminated by image analysis indicate that the formation mechanism of crack changes with temperature and pressure. At the lower temperature (~ 240 °C) and pressure (~ 212 MPa), cracks are short and oriented to various directions. However, as the temperature and pressure increase to 300 °C and 265 MPa, they become longer and the ratios of R₁- and Y- shears increase. This implies that cracks coalesce in the kinematically favored orientations for slip, making it easy to cause a rapid seismic rupture. Since such microstructural changes occur at relatively low temperatures (below 720 °C), it is expected that the structures at higher temperatures (720 °C or higher) show predominance of the plastic deformation. Our results imply that the brittle-plastic transition gradually takes place at the microscopic scale, even within the range where the bulk mechanical behavior indicates brittle deformation.

How to cite: Furukawa, M., Verberne, B. A., Muto, J., Takahashi, M., and Nagahama, H.: Shear of simulated quartz-feldspar aggregates under conditions spanning the brittle-plastic transition, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10732, https://doi.org/10.5194/egusphere-egu22-10732, 2022.