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TS4.7

EDI
EDITH: Deciphering the seismic cycle using different methods/approaches

One of the key challenges in earthquake geology is the characterization of the spatial distribution of fault-slip and its partitioning during the coseismic, interseismic, and post-seismic periods. We now have new approaches and techniques for validating the assumption that repeated seismic cycles accommodate the long-term tectonic strain and for disentangling such a complex strain partitioning in both time and space. In fact, the temporal and spatial slip accumulation for an active fault is essential to understand the hazard posed by the fault. As a matter of fact, destructive earthquakes are infrequent along any active fault and this is an inherent limitation to knowledge towards reconstructing the seismic cycle. For example, the occurrence of the 2021 Alaska earthquake Mw 8.2 within the rupture zone of the Mw 8.2 1938 Alaska earthquake, and 2021 Haiti earthquake Mw 7.2 within the same fault zone of the 2010 earthquake Mw 7.0 (which claimed 300,000 lives), reflects how much the characterization of the seismic cycle and earthquakes’ recurrence is critical for cities and regions which are under the constant seismic threat.
Modern techniques such as Remote Sensing, Geodesy, Geomorphology, Paleoseismology, and Geochronology play a vital role in constraining part of or full seismic cycles, with increased accuracy and temporal coverage of the long-term deformation. To fully understand these observations there is a need for a better understanding and integration of such techniques to be applied across different fault systems, globally.
The goal of this session is to bring together innovative approaches and techniques, to take a comprehensive look at the earthquake cycle for plate boundary fault systems to fault systems sitting far away from the plate boundary.

Co-organized by GM2/SM4
Convener: Shreya AroraECSECS | Co-conveners: Zoe MildonECSECS, Franz Livio, Pia Victor, Sambit NaikECSECS, Shalev Siman-Tov
Presentations
| Wed, 25 May, 13:20–14:38 (CEST)
 
Room K2

Wed, 25 May, 13:20–14:50

13:20–13:26
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EGU22-561
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ECS
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Highlight
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Virtual presentation
Gurvinder Singh et al.

Himalayas are seismically very active regions of the world due to ongoing continent-continent collision between India and Eurasia. The Himalayas are known to have hosted deadliest earthquakes in the past century and considering the exponential growth of population in megacities of Gangetic plains, a proper seismic hazard evaluation is very critical in this region. In this regard, the present and past slip rates along the Himalayan Frontal Thrust (HFT) are very important for understanding the convergence pattern and recurrence intervals of major earthquakes. Although geodetically derived short-term convergence rates are consistence with geologically derived long-term slip rates, this correlation is based on selected studies of uplifted Holocene terraces reporting geologically derived slip rates in Central and North-West Himalayas. There is no such reporting of Geological uplift rates from Nahan Salient in NW Himalayas. We have identified uplifted and truncated quaternary terraces along HFT in Nahan Salient Northwest Himalayas through cartosat-I stereo data. We mapped and dated the uplifted terraces in order to understand the long-term convergence rates over Holocene time period. The vertical incision rates are then calculated with the help of OSL ages and height of terraces. Assuming the vertical uplift is due to repeated past earthquakes along HFT dipping at 30°, vertical uplift rates are calculated to be 2.6 mm/yr, which equates to a fault slip rate of 5.16 mm/yr and a horizontal shortening rate of 3 mm/yr. Along with that last tectonic activity along HFT is also bracketed using age of uplifted terraces and unfaulted capping units from an exposed section of HFT fault plane along river section. The OSL ages suggest that the HFT was active between 3.8±0.4Ka and 0.706±0.15Ka. Assuming that no deformation has occurred along HFT after 0.706±0.15Ka a slip deficit of 3.6 m has been accumulated which is sufficient to generate a large earthquake in the Nahan Salient NW Himalayas.

How to cite: Singh, G., Thakur, M., and Malik, J. N.: Incision and Fault slip rates along Himalayan Frontal Thrust in Nahan Salient in Northwestern Himalayas: Implications for seismic hazard assessment., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-561, https://doi.org/10.5194/egusphere-egu22-561, 2022.

13:26–13:32
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EGU22-1557
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ECS
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On-site presentation
Bar Oryan et al.

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.

13:32–13:38
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EGU22-3457
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ECS
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Virtual presentation
Sambuddha Dhar and Jun Muto

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.

13:38–13:44
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EGU22-6070
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ECS
Main features of the formation and development of the fault zone of the 2011 Tohoku earthquake derived from GPS data
(withdrawn)
Irina Vladimirova and Yurii Gabsatarov
13:44–13:50
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EGU22-6095
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ECS
Study of seismic activization of the Chilean subduction zone at the beginning of the XXI century based on GPS data
(withdrawn)
Yurii Gabsatarov and Irina Vladimirova
13:50–13:56
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EGU22-7145
Yohei Kinoshita

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.

13:56–14:02
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EGU22-7507
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Virtual presentation
Andrea Walpersdorf et al.

The 2013 April 16 Mw 7.7 Saravan earthquake, an intra-slab earthquake with a normal faulting mechanism at of 50 km depth, occurred in the western part of the Makran subduction zone, where the Arabian oceanic lithosphere subducts northward under Iran and Pakistan. This event was the first instrumental recorded earthquake with a magnitude larger than Mw 6 since the last century. Studying this earthquake using geodetic and seismological data brings a unique opportunity to measure surface displacement due to the earthquake and assess causative fault parameters. Furthermore, it enables us to address some problems in the Makran subduction zone including slab dip angle, depth of dip angle change.

We used interferograms generated from RADARSAT-2 Synthetic Aperture Radar (SAR) data and coseismic GPS velocity field to combine with teleseismic P-wave data to model source fault parameters. First, we apply uniform slip modeling using a Bayesian bootstrap optimization nonlinear inversion method to find causative fault parameters. We specify search grids based on the LOS displacement map and focal mechanism solutions for each fault parameter to find the best solutions. These parameters include length, width, depth, strike, dip, rake, slip, location of the fault plane, rupture nucleation point, and origin time. Based on some prior tests and seismological information of earthquake, we decreased the search area of each parameter: depth 30- 70 km, dip 40˚- 80˚, strike 200˚-250˚, length 50-120 km, width 30-50 km, rake -150˚ -80˚ slip 1-4 m and let rupture nucleation point and origin time to be wide enough implying that all possible and reasonable fault geometry and kinematics parameters can be explored. Synthetic static displacements and seismic waveforms in a layered medium were computed with the Green's functions calculated using QSSP and PSGRN/PSCMP, respectively (Wang et al., 2006; Wang et al., 2017). A Green's function store contains pre-calculated Green's functions on a grid for combinations of source depth and source-receiver surface distance. For the layered half-space medium, we used the velocity structure of the GOSH seismic station to derive the Green Functions (Sebastian et al., 2016). After 450,000 iterations, the waveform fits, subsampled surface displacements as observed, modeled, and residual maps based on the best model are resolved. The distributions and resulting confidence intervals indicate that the parameters were well constrained. The joint inversion's best result indicates that the Saravan 2013 causative fault is a North-dipping normal fault with a dip of ~ 67°. The earthquake source length and width are approximately 120 and 80 km respectively.  In the second step, we model the derived fault plane in the previous step to retrieve the distributed slip model, allowing the slip to vary across the fault plane. In this step, all the parameters assumed fixed except slip. We extend fault length and width to 150 km and 100 km to prevent unwanted slip in the corners. The slip variation along the causative fault is characterized by one significant patch at the depth between 30-65 km with a maximum magnitude of about 4 m at 42-52 km.

How to cite: Walpersdorf, A., Amiri, M., Pathier, E., Mousavi, Z., Khorrami, F., and Samsonov, S. V.: Slip model of the 2013 April 16 Mw 7.7 Saravan intra-slab earthquake (Makran subduction zone) derived from InSAR, GPS, and Teleseismic P-wave modeling, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7507, https://doi.org/10.5194/egusphere-egu22-7507, 2022.

14:02–14:08
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EGU22-7630
Zahra Mousavi et al.

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.

14:08–14:14
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EGU22-8195
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ECS
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Virtual presentation
Wei Peng et al.

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.

14:14–14:20
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EGU22-8527
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Highlight
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Virtual presentation
Bob Elliott

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 30th 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.

14:20–14:26
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EGU22-10556
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ECS
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Highlight
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On-site presentation
James Biemiller et al.

Despite decades-long debate over the mechanics of low-angle normal faults dipping less than 30°, many questions about their strength, stress, and slip remain unresolved. Recent geologic and geophysical observations have confirmed that gently-dipping detachment faults can slip at such shallow dips and host moderate-to-large earthquakes. Here, we analyze the first 3D dynamic rupture models to assess how different stress and strength conditions affect rupture characteristics of low-angle normal fault earthquakes. We model observationally constrained spontaneous rupture under different loading conditions on the active Mai’iu fault in Papua New Guinea, which dips 16-24° at the surface and accommodates ~8 mm/yr of horizontal extension. We analyze four distinct fault-local stress scenarios: 1) Andersonian extension, as inferred in the hanging wall; 2) back-rotated principal stresses inferred paleopiezometrically from the exhumed footwall; 3) favorably rotated principal stresses well-aligned for low-angle normal-sense slip; and 4) Andersonian extension derived from depth-variable static fault friction decreasing towards the surface. Our modeling suggests that subcritically stressed detachment faults can host moderate earthquakes within purely Andersonian stress fields. Near-surface rupture is impeded by free-surface stress interactions and dynamic effects of the gently-dipping geometry and frictionally stable gouges of the shallowest portion of the fault. Although favorably-inclined principal stresses have been proposed for some detachments, these conditions are not necessary for seismic slip on these faults. Finally, we explore how off-fault damage and slip on steeper splay faults in the hanging wall of a detachment fault influences shallow rupture patterns and coseismic surface displacement during large earthquakes. We present a new suite of models with synthetic or antithetic splay faults dipping 45°, 60°, or 75° that incorporate off-fault plastic failure for different host rock strengths. Coseismic splay fault reactivation limits shallow slip on the detachment and localizes surface displacements outboard of the detachment trace, most strongly when synthetic shallowly-dipping splay faults are present. Our results demonstrate how integrated geophysical and geologic observations can constrain dynamic rupture model parameters to develop realistic rupture scenarios of active faults that may pose significant seismic and tsunami hazards to nearby communities.

How to cite: Biemiller, J., Gabriel, A., and Ulrich, T.: Mechanics of shallow slip in low-angle normal fault earthquakes: insight from 3D dynamic rupture models constrained by multi-timescale observations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10556, https://doi.org/10.5194/egusphere-egu22-10556, 2022.

14:26–14:32
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EGU22-10732
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ECS
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Virtual presentation
Miho Furukawa et al.

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.

14:32–14:38
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EGU22-12020
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ECS
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Highlight
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On-site presentation
Ben Johnson et al.

The 1949 Mw7.4 Khait and 1907 Mw7.6 Karatag earthquakes are the two largest earthquakes of the last ~100 years within Tajikistan, in a zone of convergence between the Pamir and Tian Shan ranges at a rate of ~1cm/yr. The historical nature of these events means seismological and geodetic data are lacking. As such, their locations and source parameters have been very uncertain – preventing our understanding of how they fit into the tectonic model of the north-western Pamir.  

Here we present calibrated earthquake relocations for the 1949 earthquake and focal mechanisms determined from digitised seismograms for the 1949 and 1907 earthquakes. We also present a catalog of precise relocations for moderate magnitude earthquakes from 1949 to the present in vicinity of the Vakhsh Thrust. Finally, we present earthquake surface rupture mapping from the Vakhsh Valley, determined from ultra-high resolution elevation models derived from satellite stereo-imagery.  

We find that the 1949 Khait earthquake did not occur on the Vakhsh Fault, a major right-lateral fault that bounds the northern margin of the Pamir, as previously thought. Instead it occurred on an unmapped fault in the Tian Shan basement. However, 10-20m scarps observed on the south Vakhsh valley show this fault is capable of producing large earthquakes. This tells us the Pamir–Tian Shan convergence is distributed across several basement faults capable of producing large earthquakes. It also tells us that the largest earthquakes may occur on faults which may appear minor in the landscape, which has implications for seismic hazard in the region.  

How to cite: Johnson, B., Kulikova, G., Bergman, E., Krueger, F., Pierce, I., Hollingsworth, J., Copley, A., Kendall, M., and Walker, R.: Source parameters and locations of the 1949 Mw7.4 Khait and 1907 Mw7.6 Karatag earthquakes: implications for how mountain ranges collide, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12020, https://doi.org/10.5194/egusphere-egu22-12020, 2022.