Displays

The accommodation of motion on faults spans a large spectrum of slip modes, ranging from stable creep to earthquakes. While seismic slip modes certainly have the largest impact on the surface due to the induced ground shaking, it has been recognized that slow aseismic slip modes relax most of the accumulated stresses on a fault. It has also been suggested that aseismic slip controls seismic events, thus making this kind of slip mode key for earthquake prediction.

Despite the importance of aseismic slow slip, its underlying physical mechanisms are still unclear. Commonly, slow slip events are modeled in terms of frictional failure, employing a rate-and-state model of fault friction, often also invoking fluids that alter frictional properties on the fault. However, at larger depths, frictional processes become increasingly difficult to activate due to the increase in ambient pressure and ductile processes are more likely to dominate deformation.

Here we therefore investigate deep aseismic slip processes governed by ductile deformation mechanisms using 2D numerical models, where we employ a composite viscoelastic rheology combined with grain size reduction and shear heating as weakening processes. We show that the collaborative action of these two weakening mechanisms is sufficient to create the entire spectrum of aseismic slip, ranging from stable creep to long-term slow slip events. The results show that ductile deformation does not necessarily result in stable slip and induces slip modes with considerably larger velocities than the far-field plate velocities. Moreover, the propagation of ductile ruptures induces large stresses in front of the rupture tip which may also trigger short-term seismic events.

How to cite: Thielmann, M. and Duretz, T.: Going from stable creep to aseismic slow slip events in the ductile realm, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18554, https://doi.org/10.5194/egusphere-egu2020-18554, 2020.

Recent seismological observations highlighted that both aseismic silent slip and/or foreshock sequences can precede large earthquake ruptures (Tohoku-Oki, 2011, Mw 9.0  (Kato et al., 2012); Iquique, 2014, Mw 8.1 (Ruiz et al, 2014; Socquet et al., 2017); Illapel, 2015, Mw 8.3 (Huang and Meng, 2018); Nicoya, 2012, Mw 7.6 (Voss et al., 2018)). However, the evolution of such precursory markers during earthquake nucleation remains poorly understood. Here, we report for the first time, experimental results regarding the nucleation of laboratory earthquakes (stick slip events) conducted on Westerly Granite saw-cut samples under both dry and fluid pressure conditions. Experiments were conducted under stress conditions representative of the upper continental crust, i.e confining pressures from 50 to 95 MPa; fluid pressures (water) ranging from 0 to 45 MPa.

At a given effective confining pressure, different precursory slip behaviors are observed. In dry conditions, we observe that slip evolves exponentially up to the main instability and is escorted by an exponential increase of acoustic emissions. With pressurized fluids, precursory slip evolves first exponentially then switches to a power law of time. There, precursory slip remains silent, independently of the fluid pressure level. The temporal evolution of precursory fault slip and seismicity are controlled by the fault’s environment, limiting its prognostic value. Nevertheless, we show that, independently of the fault conditions, the total precursory moment release scales with the co-seismic moment of the main instability. The relation follows a semi- empirical scaling relationship between precursory and co-seismic moment release by combining nucleation theory (Ida, 1972; Campillo and Ionescu, 1992) with the scaling between fracture energy and co-seismic slip which has been demonstrated experimentally (Nielsen et al., 2016; Passelègue et al., 2016), theoretically (Viesca and Garagash; 2015) and by natural observations (Abercrombie and Rice; 2005). We then compile data from natural earthquakes, and show that, over a range of Mw6.0 to Mw9.0 the proposed scaling law holds for natural observations. In summary, the amount of moment released prior to an earthquake is directly related to its magnitude, increasing therefore the detectability of large earthquakes. The scaling relationship between precursory and co-seismic moment should motivate detailed studies of precursory deformation of moderate to large earthquakes.

How to cite: Acosta, M., Passelègue, F., Schubnel, A., Madariaga, R., and Violay, M.: On the scaling between precursory moment release and earthquake magnitude: Insights from the laboratory., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13690, https://doi.org/10.5194/egusphere-egu2020-13690, 2020.

The frictional behavior of rocks under shear offers tremendous complexity depending among others on rock type, temperature, fluid content, and sliding velocity. A large body of laboratory experiments documents these effects, but a unifying theoretical framework linking these observations is still missing. Here, I present a constitutive law based on multiple temperature and fluid activated healing processes and a fluid lubrication phase to capture fault behavior in the brittle field in all conditions relevant to the seismic cycle. Distinct healing processes are activated at different temperatures, pore fluid pressures, and depths based on their respective activation enthalpy. A fluid phase is rapidly formed at the high temperatures facilitated by shear heating, allowing strong weakening at high slip velocity. The model explains the intricate change of frictional behavior of carbonate rocks at various temperatures, including simultaneous velocity-strengthening and temperature-weakening at temperatures lower than 70ºC, transitioning to simultaneous velocity-weakening and temperature-hardening at higher temperatures. With different parameters, the model explains the frictional properties of quartz and granitic rocks in hydrothermal conditions with velocity-strengthening behavior in nominally dry conditions, transitioning to velocity-weakening between 100ºC and 350ºC in wet conditions. Inclusion of a lubrication phase formed between the solidus and the liquidus of the host rocks explains the strong weakening at high slip velocity in a variety of rocks. The unified constitutive framework allows modeling of faults in varying temperature and pore pressure conditions, including for example injection of pore fluids in natural faults or shear heating of the host rocks. 

How to cite: Barbot, S.: Temperature and fluid activation of contact healing and fault lubrication in rate-and-state friction, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4192, https://doi.org/10.5194/egusphere-egu2020-4192, 2020.

Slow slip events (SSEs) represent a significant mechanism of strain release along several subduction zones, and understanding their occurrence and relations with major earthquake asperities is essential for a comprehensive understanding of the seismic cycle. Here, we focus on the Mexican subduction zone, characterized by the occurrence of recurrent large slow slip events (SSEs), both in the Guerrero region, where the SSEs are among the largest observed worldwide, and in the Oaxaca region, where smaller, more frequent SSEs occur. Up to now, most slow slip studies in the Mexican subduction zone focused either on the detailed analysis of a single event, were limited to a small area (Guerrero or Oaxaca), or were limited to data before 2012 [e.g.1-4]. In this study, our aim is to build an updated and consistent catalog of major slow slip events in the Guerrero-Oaxaca region.

We use an approach similar to Michel et al. 2018 [5]. We analyze the GPS time series from 2000 to 2019 using Independent Component Analysis (ICA), in order to separate temporally varying sources of different origins (seasonal signals, SSEs and afterslip of major earthquakes). We are able to isolate a component corresponding to seasonal loading, which matches the temporal evolution of displacement modeled from the GRACE data. The sources (independent components) identified as tectonic sources of deep origin are inverted for slip on the subduction interface. We thus obtain a model of the spatio-temporal evolution of aseismic slip on the subduction interface over 19 years, from which we can isolate around 30 individual slow slip events of M_w > 6.2.

 The obtained catalog is coherent with previous studies (in terms of number of events detected, magnitude and duration) which validates the methodology. The observed moment-duration scaling is close to M₀~T³ as recently suggested by Michel [6] for Cascadia SSEs, and our study extends the range of magnitude considered in their analysis. Finally, we also investigate the spatio-temporal relations between the SSEs occurring in the adjacent regions of Guerrero and Oaxaca, and their interaction with local and distant earthquakes.

References:

1. Kostoglodov, V. et al. A large silent earthquake in the Guerrero seismic gap, Mexico. Geophys. Res. Lett 30, 1807 (2003).
2. Graham, S. et al. Slow Slip History for the Mexico Subduction Zone: 2005 Through 2011. Pure and Applied Geophysics 1–21 (2015). doi:10.1007/s00024-015-1211-x
3. Larson, K. M., Kostoglodov, V. & Shin’ichi Miyazaki, J. A. S. The 2006 aseismic slow slip event in Guerrero, Mexico: New results from GPS. Geophys. Res. Lett. 34, L13309 (2007).
4. Radiguet, M. et al. Slow slip events and strain accumulation in the Guerrero gap, Mexico. J. Geophys. Res. 117, B04305 (2012).
5. Michel, S., Gualandi, A. & Avouac, J.-P. Interseismic Coupling and Slow Slip Events on the Cascadia Megathrust. Pure Appl. Geophys. (2018). doi:10.1007/s00024-018-1991-x
6. Michel, S., Gualandi, A. & Avouac, J. Similar scaling laws for earthquakes and Cascadia slow-slip events. Nature 574, 522–526 (2019) doi:10.1038/s41586-019-1673-6

How to cite: Radiguet, M., Kazachkina, E., Maubant, L., Cotte, N., Kostoglodov, V., Gualandi, A., and Chanard, K.: Systematic characterization of slow slip events along the Mexican subduction zone from 2000 to 2019, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17441, https://doi.org/10.5194/egusphere-egu2020-17441, 2020.

After large earthquakes at subduction zones, the plate interface continues moving due to mostly frictional afterslip processes. Below depths of 60 km, little frictional afterslip is to be expected on the plate interface due to low shear strength, lack of apparent geodetic interseismic locking, and low seismic moment release from aftershocks. However, inversion models that consider an elastic crust above a mantle with viscoelastic rheology result in a significant portion of afterslip at depths > 60 km. In this study, we present a forward 3D geomechanical-numerical model with power-law rheology that simulates dislocation creep processes for the crust and upper mantle in combination with an afterslip inversion. The linear rheology case is also considered for comparison. We estimate the cumulative viscoelastic relaxation and the afterslip distribution for the first six years following the 2010 M_w 8.8 Maule earthquake in Chile. The cumulative afterslip distribution is obtained from the inversion of the residual surface displacements between continuous GPS (cGPS) observations and predicted displacements from viscoelastic forward modelling. We investigate three simulations: two with the same dislocation creep parameters in the slab and upper mantle but different ones in the continental crust, and another with elastic properties in the crust and slab and a linear viscoelastic upper mantle. Our preferred simulation is the one with power-law rheology in the crust and upper mantle with a weak continental crust since the corresponding afterslip distribution shows the best overall fit to the cGPS displacements (cumulative and time series) as well as having a good correlation with aftershock activity. In this simulation, most of the viscoelastic relaxation occurs in the continental lower crust beneath the volcanic arc due to dislocation creep processes. The resulting afterslip pattern from the inversion is reduced at depths > 60 km, which correlates well with the spatial distribution of cumulative seismic moment release from aftershocks. We conclude that by allowing for non-linear stress relaxation in the continental lower crust due to dislocation creep processes, the resulting afterslip distribution is in better agreement with the physical constraints from the shear strength of the plate interface at depth, the predicted locking degree, and the aftershock activity.

How to cite: Peña, C., Heidbach, O., Moreno, M., Bedford, J., Ziegler, M., Tassara, A., and Oncken, O.: Effect of Rheology on Afterslip and Viscoelastic Patterns Following the 2010 Mw 8.8 Maule, Chile, Earthquake, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4288, https://doi.org/10.5194/egusphere-egu2020-4288, 2020.

The San Andreas Fault creeping section is generally considered as slipping continuously and aseismically, at a rate of about 35 mm/yr. However, recent studies, using either Global Positioning System (GPS) network or Interferometric Synthetic Aperture Radar (InSAR) data, have highlighted spatial and temporal variations of slip rate. Here, we combine GPS, InSAR, creepmeter and seismicity data over the 2008-2018 period, taking advantage of their complementary spatial and temporal resolutions, to detail a comprehensive picture of episodic acceleration and deceleration slip patterns. For this purpose, we use a variational Bayesian Independent Component Analysis (vbICA) decomposition to separate geodetic deformation due to non-tectonic sources from signals of tectonic origin. The fault slip kinematics is reconstructed by linear inversion of each Independent Component related to transient tectonic activity. We document aseismic slip acceleration transients and discuss their origin.

How to cite: Michel, S., Jolivet, R., Gualandi, A., Guardonio, B., Lengliné, O., Dalaison, M., and Benoit, A.: Reconstructing 10 years of spatio-temporal aseismic slip history along the San Andreas Fault, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17101, https://doi.org/10.5194/egusphere-egu2020-17101, 2020.

It is increasingly apparent that the progression to eventual failure of large subduction earthquakes can be captured by continuous networks that record anomalous seismic and geodetic signals in the late interseismic period. Such precursory signals are generally understood to be related to a gradual decoupling of the mainshock area of the fault and can last from days to years. These natural observations are consistent with various numerical and laboratory models in which similar late-interseismic signals are generated.

Here we analyse the continuous GNSS records of the final 5 years leading to the 2010 Mw 8.8 Maule, Chile and 2011 Mw 9.0 Tohoku-oki, Japan earthquakes. We implement the Greedy Automatic Signal Decomposition - a regression approach that builds upon existing tectonic trajectory models - to model the daily GNSS displacement time series as the sum of background seasonal oscillations, step functions, linear (1^st order polynomial) motion, and a sparse number of multi-transient functions. The multi-transient functions are simply the sum of decay functions (e.g. exponential, logarithmic) that begin at the same time but have different characteristic decay constants. The inclusion of these versatile multi-transients allows the model to capture a variety of transient motion. We see that both subduction margins exhibit variability in their interseismic velocities. The most striking of these motions occur in the 5-7 months directly before both the Maule and Tohoku-oki earthquakes during which the sense of motion reverses in the trench-perpendicular component. These reversals manifest themselves as wobbles in the displacement time series with a peak-to-peak displacement between 4-8 mm and occur on a spatial scale in the order of thousands of kilometres. After investigating fluid loading and possible reference frame artifacts, we conclude that the wobbles are most likely of a tectonic origin.

In the pre-Tohoku-oki case, for which we have a much denser surface coverage, kinematic models indicate an initial extension in the Philippine Sea Plate followed by a viscoelastic rebound. The spatial scale and approximate onset of this apparent extension are in agreement with the anomalous GRACE gravity signals reported in earlier work of Panet et al. (2018, Nature Geoscience). Furthermore, the speed that the trench-wards transient migrates along-strike of the subduction zone before Tohoku-oki indicates that deep slow-slip is also occurring. In the pre-Maule case, we see a similar reversal but lack the number of measurements to track any migration of the velocity front. Nevertheless, from inclusion of vertical displacement in analyses of both networks, we suspect that these late interseismic reversal signals are caused by a sudden enhanced slab pulling. Such an enhanced slab pull might be caused by sudden densification of metastable slab. Therefore, a main message of this work is that large asperities, while they might fail gradually local to the mainshock region, might also brought to failure by changes in the slab pull boundary conditions that can be several hundreds of km deep.

How to cite: Bedford, J., Moreno, M., Deng, Z., Oncken, O., Schurr, B., John, T., Báez, J. C., and Bevis, M.: Reversals in geodetically observed surface motions suggests enhanced slab pull in the months preceding Maule Mw 8.8 and Tohoku-oki Mw 9.0 earthquakes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18166, https://doi.org/10.5194/egusphere-egu2020-18166, 2020.

The precursory activity leading up to the Tohoku-Oki earthquake of 2011 has been suggested to feature both long- and short-term episodes of decoupling and suggests a particularly complex slow slip history. The analysis of the F3 solution of the Japanese GPS network suggested that an accelerated slip occurred in the deeper part of the seismogenic zone during the 10 years preceding the earthquake (Heki & Mitsui, EPSL 2013; Mavrommatis et al., GRL 2014; Yokota & Koketsu, Nat. Com. 2015). During the two months preceding the earthquake, no anomaly in the GPS position time series has been revealed so far, although several anomalous geophysical signals have been reported (an extended foreshock crisis near the future hypocenter (Kato et al., Science 2012), a synchronized increase of intermediate-depth background seismicity (Bouchon et al., Nat Geosc. 2016), a signal in ocean-bottom pressure gauges and on-land strainmeter time series (Ito et al., Tectonoph. 2013), and large scale gravity anomalies that suggest deep-seated slab deformation processes (Panet et al., Nat. Geosc. 2018 ; Wang & Burgmann, GRL 2019)).

We present novel results based on an independent analysis of the Japanese GPS data set. We perform a full reprocessing of the raw data with a double-difference approach, a systematic analysis of the obtained time-series, including noise characterization and network filtering, and make a robust assessment of long- and short-term tectonic aseismic transients preceding the Tohoku-Oki earthquake. An accelerated slip on the lower part of the seismogenic zone over the last decade is confirmed, not only below the epicenter of Tohoku-Oki earthquake but also further south, offshore Boso peninsula, which is a worrying sign of an on-going slow decoupling east of Tokyo. At shorter time-scale, first results seem compatible with a slow slip close to the epicenter initiating ~ 2 months before the mainshock.

How to cite: Socquet, A., Marill, L., Marsan, D., Rousset, B., Radiguet, M., Burgmann, R., Cotte, N., and Bouchon, M.: Revisiting the deformation transients before the 2011 Tohoku-Oki Megathrust Earthquake with GPS , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21477, https://doi.org/10.5194/egusphere-egu2020-21477, 2020.

With the accumulation of seismic and other geophysical data and update of methodologies, the accuracy and reliability of seismic risk assessment can be improved. In particular, the introduction of GPS observation data leads to better understanding of earthquake origins and sequences. For this, we cross-compare the pre- and post-seismic deformation of the 2011 Tohoku Mw9.1 earthquake in Japan, the 2010 off shore Maule Mw8.8 earthquake in Chile, the 2018 Kodiak Mw7.9 earthquake in the Gulf of Alaska, and the 2016 Kaikoura Mw7.8 earthquake in New Zealand derived from GPS observations with integral characteristics of the regional seismic regime, including the accumulated length of seismic sources derived from the catalogs of earthquake hypocenter parameters. We found that (a) the area on top the 2011 Tohoku mega-thrust keeps moving at speed of about 10 cm per year, (b) eventually, the 2016 Kaikoura unidirectional strike-slip resulted in the current position retreat nearby epicenter and steady increase on the opposite edge of its rupture zone, (c) the four cases show up different deformation vs seismicity correlation patterns in advance and after the catastrophic event, and (d) GPS data confirm the existence of intermittent long periods of regionally stable levels of seismic regime controlled by the Unified Scaling Law for Earthquakes that may switch as the result of mid- or even short-term bursts of activity associated with major catastrophic earthquakes.

The study supported from the RFBR Project No. 19-35-50059 “Study of pre- and post-seismic displacements in the areas of the strongest earthquakes in the world".

How to cite: Liu, T. and Kossobokov, V.: Cross-comparing GPS and seismic data in advance and after great earthquakes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1373, https://doi.org/10.5194/egusphere-egu2020-1373, 2020.

Enhanced interplate coupling has been found for segments adjacent along-strike to megathrust faults after the 2003 Tokachi-Oki and the 2011 Tohoku-Oki earthquakes, NE Japan, and was interpreted as acceleration of the subducting Pacific Plate slab. A similar enhanced coupling was also reported for the segments to the north of the rupture area of the 2010 Maule earthquake, central Chile. We utilize available GNSS data to find such enhanced coupling in worldwide subduction zones including NE Japan, central and northern Chile, Sumatra, and Mexico to investigate their common features. Our study revealed that the accelerations of landward movement of 2.1-9.0 mm per year appeared in adjacent segments following the 2014 Iquique (Chile), the 2007 Bengkulu (Sumatra), and the 2012 Oaxaca (Mexico) earthquakes. We also confirmed that the enhanced coupling is associated with the increase of seismicity for all these six cases. We found that the degree of enhancement depends on the length of the slab and the magnitude of the earthquake, which is consistent with the simple 2-dimensional model proposed earlier.

How to cite: Yuzariyadi, M. and Heki, K.: Enhancement of Interplate Coupling after Recent Megathrust Earthquakes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2550, https://doi.org/10.5194/egusphere-egu2020-2550, 2020.

Lauriane Baylé (1), Romain Jolivet (2), Nadaya Cubas (1) and Laetitia Le

Pourhiet (1)

(1) Institut des Sciences de la Terre de Paris, UMR 7193, UPMC UniversitéParis 6, CNRS, Paris,

France

(2) Laboratoire de Géologie, Département de Géosciences, École Normale Supérieure, CNRS UMR 8538,

PSL ResearchUniversity, Paris, France

Recent studies have pointed out to a discrepancy between the short- and long-

term deformation of overriding plates in subduction zones. This led to debates

about when and how permanent deformation is acquired. This contradiction

has notably been observed along the Central Andes Subduction Zone, where

the coast subsides during and shortly after major earthquakes while a coastal

uplift with rates ranging between 0.1 and 0.3 mm/yr has been inferred the

last 4000 ky. For instance, during the 15th September 2015 Mw 8.3 Illapel

earthquake the geodetics (GPS and InSAR) data show a coastal subsidence

along the line-of-sight of 20 cm in InSAR.

To reconcile the seemingly contradictory observations, we here propose to

provide a seismic cycle uplift balance by constrainning inter-, co- and post-

seismic vertical velocities from InSAR time series. The study focuses on La

Serena peninsula (71.3°W, 30°S, Chile) along which the Illapel earthquake

occurred and for which long-term uplift rates have been provided by previous

geomorphological studies.

To build this seismic cycle balance, we use InSAR data (Sentinel-1) acqui-

red between the September 15, 2015 and January 19, 2019. The time series

for the ascendant orbite is calculated and the accumulated vertical displace-

ment extracted providing co- and post-seismic displacement. The co-seismic

displacement are similar to those previously obtain. To constrain the displa-

cement during the inter-seismic period, data on both sides of the peninsula

are used. In that respect, we aim determining when, during the seismic cycle,

and where, along the coast, the uplift occurs.

The deduced time series will then be confronted to numerical modelling

to provide the short- and long-term mechanics reproducing the short- and

long-term observations.

How to cite: Bayle, L., Jolivet, R., Cubas, N., and Le Pourhiet, L.: Build seismic cycle balance deformation with InSAR in Northen Chile, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20862, https://doi.org/10.5194/egusphere-egu2020-20862, 2020.

subduction zone ruptured by the high slip patch of the 1960 Mw9.5 Valdivia earthquake. The density structure of the upper plate was generated by using a 2-D forward modeling schema (GGrad) offshore, and 3D inversion onshore (GRAV3D) with a database composed by a recompilation of previous marine and onshore gravity measurements, complemented by 113 new gravimetric station acquired by our group. The modelling was constrained by independent seismological data, active seismic information and electromagnetic soundings registered during the project. The joint analyze of the obtained density model with magnetic data and seismic models, provide new insight about the structure of the upper plate forearc, where an East-West segmentation of physical parameters (perpendicular to the margin) is associated with first order changes in the surface geology, deep structural style and seismotectonics characteristics of the margin. The systematical comparison of the results observed in this segment with surrounding regions of the Chilean margin, suggests a causal link between complex sequence of large earthquakes ruptures and changes of rheology/lithology along the interplate boundary, determined in turn, by the long term tectonic and geodynamic evolution of the subduction zone.

How to cite: Maksymowicz, A., Montecinos, D., and Díaz, D.: Deep structure of the Central-Chile continental wedge and its implications for large megathrust earthquakes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12165, https://doi.org/10.5194/egusphere-egu2020-12165, 2020.

Megathrust earthquakes at subduction zones are one of the most devastating natural phenomena. Understanding the relationships between their temporal recurrence, spatial segmentation and the frictional structure of the megathrust is of primary relevance. We analyzed the common spatial variability of gravity anomalies, geodetic locking and wedge taper basal friction (three independent proxies for megathrust frictional structure) along the Chilean margin. A marked along-strike segmentation has emerged that is organized into three hierarchical levels. At a subcontinental-scale (10³ km), we observe a first-order difference between Central (18-32°S) and Southern (32°-46°S) Andes. This is marked by a dominance of positive/negative gravity, high/low locking, high/low friction along the Central/Southern segments. We explain this as mainly reflecting the combined effect on effective normal stress (σ_eff) of a high/low density forearc and low/high pore pressure along both megathrust segments, in agreement with the geological structure of the forearc, sediment input at the trench and the long-term architecture of the Andes. Inside this large-scale subdivision, we identify a number of segments (10² km) that are limited by marked small-scale (10¹ km) changes in the first-order tendency of the three proxies coinciding with geological features of both plates. When we compare this against the paleoseismic, historic and instrumental record of past earthquakes in Chile, we note that segments largely coincide with seismic asperities, i.e. those regions of the megathrust concentrating the largest fraction of coseismic slip. Bridging these two scales, the rupture length of giant (Mw 8.5-9.5) earthquakes, which encompassed several asperities, define an intermediate hierarchic level of organization (10²-10³ km). Considering this segmentation into the conceptual framework of the rate-and-state friction (RSF) law, we infer that asperities inside the rate-weakening seismogenic zone of the Central Andean megathrust are dominantly unstable (i.e. σ_eff>σ_c = the critical stress defined by RSF parameters) and therefore prone to initiate and concentrate the coseismic rupture. In contrast, most of the asperities along the Southern mega-segment are likely characterized by a conditionally-stable behavior (σ_eff<σ_c) that allows a rich and complex seismogenic behavior where interseismic creep and locking are both possible and large coseismic slip propagation is dominant. This can explain the apparent difference in the recurrence of giant earthquakes along both mega-segments, since the synchronization of unstable asperities in the Central Andean megathrust (2000-3000 yr recurrence time) is less probable than in the case of conditionally-stable asperities in the Southern segment (300-500 yrs). We will test these hypothesis developing numerical simulations of multiple seismic cycles with setups representing the inferred contrast on the physical properties of the megathrust along the Chilean margin, and we will present preliminary results of this exercise. 

How to cite: Molina, D., Tassara, A., Ampuero, J.-P., and Melnick, D.: Investigating seismic segmentation and recurrence patterns of great earthquakes along the Chilean megathrust. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12150, https://doi.org/10.5194/egusphere-egu2020-12150, 2020.

Two major fault systems host M_w>7 earthquakes in Central and Northern Sulawesi, Indonesia: the Minahassa subduction interface and the Palu-Koro strike-slip fault. The Celebes Sea oceanic lithosphere subducts beneath the north arm of Sulawesi at the Minahassa subduction zone. At the western termination of the Minahassa subduction zone, it connects to the left-lateral Palu-Koro strike-slip fault zone. This fault strikes onshore at Palu Bay and then crosses Sulawesi. Interseismic GNSS velocities indicate that the Palu-Koro fault zone accommodates about 4 cm/yr of relative motion in the Palu Bay area, with a ~10 km locking depth. This shallowly locked segment of the Palu-Koro fault around the Palu Bay area ruptured during the devastating, tsunami-generating, 2018 M_w7.5 Palu earthquake. This complex event highlights the high seismic hazard for the island of Sulawesi.

We have a >20-year record of GNSS velocities on Sulawesi, where the densest cluster of monument sites surrounds the Palu-Koro fault, specifically around Palu Bay, whereas the rest of the island is less densely covered. High quality estimates of interseismic velocities reveal second-order complex patterns of transient deformation in the wake of major earthquakes: the velocities in northern Sulawesi and around the Palu-Koro fault do not follow their interseismic trends after a major subduction earthquake has occurred, for several years after the event. This effect of transient deformation reaches more than 400km away from the epicentre of the major earthquakes. Surprisingly, a deviation from the background slip rate on the Palu-Koro fault is not accompanied by a deviation from the background (micro)seismic activity.

We construct a 3D numerical model based on the structural and seismological data in the Sulawesi region. We investigate the post-seismic relaxation pattern from a subduction earthquake and determine whether the slip rate on the Palu-Koro fault changes due to this earthquake through forward model calculations. With a modelling focus on the 1996 M_w7.9 and 2008 M_w7.4 earthquakes that ruptured the Minahassa subduction interface, this study outlines the triggering of transient deformation and continual interaction between the Minahassa subduction interface and the Palu-Koro strike-slip fault.

How to cite: Nijholt, N., Simons, W., and Riva, R.: Continual interaction between the Minahassa subduction interface and the Palu-Koro strike-slip fault in Sulawesi, Indonesia., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9831, https://doi.org/10.5194/egusphere-egu2020-9831, 2020.

The Hikurangi subduction margin offshore of the east coast of New Zealand displays along-strike variations in subduction-thrust slip behavior. Geodetic observations show that the subduction-thrust of the southern segment of the margin is locked on the 30-100 year scale and the northern segment displays periodic slow-slip on the 1-2 year scale. It is hypothesised that spatial variations in pore-pressure may play a role in this contrasting phenomenon. Higher pore-pressures would result in lower effective stresses, which promote slow-slip of the subduction-thrust. In addition, the presence of a sedimentary wedge with very low shear wave-speeds in the northern Hikurangi margin has been proposed to fit the ultra-long duration of ground motions observed following the 2016 Kaikoura earthquake. Compressional (P-) wave velocities (V_p) of the subsurface provide useful information about the lithological composition. Combined with shear (S-) wave velocities (V_s), the V_p/V_s ratio which is directly related to Poisson’s ratio can be obtained. This is a diagnostic property of a rock’s consolidation and porosity. Typical V_p/V_s ratio of consolidated and crystalline rocks range from 1.6 to 1.9 and that of unconsolidated sediments can range from 2.0 to 4.0.

We use the controlled sources of R/V Marcus G Langseth recorded by a profile of 49 multi-component ocean bottom seismometers (OBS) along the Hikurangi margin forearc for the Seismogenesis at Hikurangi Integrated Research Experiment (SHIRE) to derive the V_s structure and estimate the V_p/V_s ratio. The orientations of the horizontal components of each OBS are found by a hodogram analysis and by an eigenvalue-decomposition of the covariance matrix. Using the orientations, the horizontal components of each OBS are rotated into radial and transverse components. P to S converted phases are identified on the radial and transverse components considering their linear moveout, polarisation angle, and ellipticity. We confirm incoming S-waves to OBSs by comparing them with their hydrophone components. We identify both PPS (up-going P-wave after reflection or refraction converts to an S-wave at an interface) and PSS (down-going P-wave from the controlled source converts to an S-wave at an interface) type conversions. The identified conversion interfaces are the sediment-basement interface and the top of the subducting crust. The travel-time delay of a PPS type conversion relative to its P-wave arrival is indicative of V_s above the converting interface. The linear-moveout of PSS type conversions are indicative of V_s along the raypath after the conversion. Preliminary results from the southern Hikurangi margin suggest V_p/V_s ratios of ~1.70 for the basement rocks above the subducting crust and ~1.90 for the sediments overlying the basement rocks. These values indicate that the basement rocks are consolidated and less porous than the overlying sediments.

We expect to estimate the V_p/V_s ratios in the northern Hikurangi margin to assess the role played by pore-pressure in the along-strike variation in subduction-thrust slip behavior. We also expect to ascertain the presence and estimate the thickness of the low-velocity sediment wedge in the northern Hikurangi margin.

How to cite: Herath, P., Stern, T., Savage, M., Bassett, D., Henrys, S., Barker, D., Van Avendonk, H., Bangs, N., Arnulf, A., Arai, R., Kodaira, S., and Mochizuki, K.: Using P- to S- wave conversions from controlled sources to determine the shear-wave velocity structure along Hikurangi Margin Forearc, New Zealand, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11940, https://doi.org/10.5194/egusphere-egu2020-11940, 2020.

In recent years, Ocean Bottom Seismometers (OBSs) have become widely used to expand the coverage of seismic networks onto the ocean. This study takes advantage of offshore observations at the northern end of the Hikurangi margin and southwestern Okinawa Trough to study the tectonics in both regions.

In the Hikurangi subduction zone, slow slip events (SSEs) have been observed, which are caused by the subduction of the Pacific Plate under New Zealand. The behaviour of SSEs and how they influence the physical properties of Earth materials are open to question. From 2014 to 2015, 15 OBSs were deployed offshore Gisborne, New Zealand on the Hikurangi margin. Ambient noise data from the OBSs are used to study velocity changes related to SSEs. Single station cross-component correlations and auto-correlations are computed, from which coda waves are used to monitor the velocity changes before, during and after the SSEs in 2014 and 2015 to analyse the slow earthquake behaviour and its relation to stress changes. Different rotation on horizontal components is tested by rotating horizontal components to N-E direction and parallel-perpendicular to the coastline. The dv/v computed by different components or rotation show different changes. The averaged dv/v displays a 0.1% velocity decrease during the SSE in October 2014.

The southwestern Okinawa Trough tapers towards Taiwan. How the back-arc crust accommodates the narrowing processing remains to be understood. At various times between 2010 and 2017, 22 OBSs on a small scale (~0.2°×0.3°) were deployed in Southwestern Okinawa Trough offshore northeast Taiwan. Ambient noise recorded on vertical velocity and pressure sensors is used to retrieve Scholte waves for studying shear wave velocity structure. Phase velocities are forward-modeled according to a model proposed by Kuo et al. 2015 and shear strength and density results from ODP1202. Phase velocity dispersion curves are measured from cross-correlations and unwrapped according to the modeled phase velocities. The fundamental mode phase velocities averaged from different station pairs are 0.62 km/s at 3 s period and 1.56 km/s at 6.5 s period. A 3D inversion will be conducted for a shear wave velocity structure from the basin center to the edge.

How to cite: Wang, W., Savage, M., Yates, A., Hung, S.-H., Luo, Y., Stern, T., Lin, P.-Y. P., Yang, H.-Y., Kuo, B.-Y., Fry, B., and Webb, S.: Studies of seismic velocities in subduction zones from continuous OBS data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12063, https://doi.org/10.5194/egusphere-egu2020-12063, 2020.

The strength of faults is subject of an important debate throughout various Earth Scientific disciplines. Different scientific communities have different perspectives with respect to appropriate values for friction coefficients μ. Geodynamicists with a long-term perspective require very low effective strengths (μ<0.05), while at the same time realizing mountains need to be sustained as well. Geologists and seismologists typically start from Byerlee friction coefficients of 0.6<μ<0.85, whereas rock mechanics experiments at high seismic slip rates show short-term low dynamic friction values of 0.03<μ<0.3. Here I show that both long- and short-term approaches can be made more compatible through considering that a regional or global frictional strength should be approached as a strain-averaged quantity. Doing this accounts for large variations of strain in both time and space. What matters for large-scale models is that most deformation occurs over a very small space and time during which friction is exceptionally low, thus making the representative long-term strength low. This is supported by seismo-thermo-mechanical models that self-consistently simulate the dynamics of both long-term subduction and short-term seismogenesis. The latter sustain mountain building, while representative earthquake-like events occur on faults with pore fluid pressure-effective static friction coefficients between 0.125 and 0.005 (or 0.75<Pf/Ps<0.99). These low friction values suggest faults are weak and suggest the dominant role of fluid pressures in weakening faults in subduction zones. This is confirmed in analytical considerations based on mechanical energy dissipation, which provide an equation to calculate the long-term fault strength as a strain-average quantity. Constraining the four parameters in this equation by observations confirms that fluid weakening is more important for long-term weakening than dynamic frictional weakening and low static friction coefficients. From the short-term perspective of modeling earthquake rupture dynamics it is now also becoming evident that fluid overpressured faults are preferable. They namely facilitate the incorporation of laboratory-observed dynamic weakening (70-90%) by limiting the stress drop to reasonable values. In summary, this cross-scale perspective supports long-term effective friction values in the range of about 0.03 to 0.2.

How to cite: van Dinther, Y.: Bridging Long- and Short-term Behavior Shows Fault Strength as a Strain-average, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20374, https://doi.org/10.5194/egusphere-egu2020-20374, 2020.

Earthquake sequences reflect the repetitive dynamic processes of stress accumulation and release on a fault. Understanding earthquake sequences is fundamental for the research of induced and natural earthquakes and may ultimately help to better assess long-term seismic hazard. Numerical models are well-suited to overcome limited spatiotemporal observations and improve our understanding on this topic. However, large models in 3D are still computational time and memory consuming. Moreover, this may not be optimal if the aspects of lateral or depth variations within the results are not needed to answer a particular objective. This motivated us to investigate the advantages and limitations of various dimensional models by simulating earthquake sequences in 0D, 1D, 2D and ultimately 3D. We applied a C++ numerical library GARNET [1] to deal with the various dimensional models in one simulator. This library uses a fully staggered finite difference scheme with a rectilinear adaptive grid. It also incorporates an automatic discretization algorithm and combines different physical ingredients such as visco-elasto-plastic rheology and quasi- and fully dynamic approaches into one algorithm.

Here we present numerical experiments of a strike-slip fault under rate-and-state friction, surrounded by an elastic medium with constant tectonic loading and, test them under different parameters and initial conditions. By adding one dimension at a time, we simulate a more detailed structure of the seismic cycle. The higher dimensional models present both the validity and the limitations of the lower dimensional ones. For example, inertial waves are not possible to present in 0D while a quasi-dynamic radiation damping term can be added here instead. Another example is that due to lack of grid extension along the fault, both 0D and 1D model fail to reveal an earthquake nucleation phase. However, some important observables, such as the seismic cycle period, maximum/minimum stress and slip rates, are calculated accurately in lower dimensional models, which are much faster than higher dimensional models. We also implemented and compared quasi- and fully dynamic models in the same way. Our results indicate that both the size of simulated seismic events and their interval are reduced in quasi-dynamic models. This could provide us with guidance to identify the appropriate model complexity for various problems. We will also present 3D modeling results, which will be compared to their 2D equivalent. Finally, we present our results for the SCEC SEAS benchmarks [2] and compare them to other participating codes.

[1] Pranger, C. C., L. Le Pourhiet, D. May, Y. van Dinther, and T. Gerya (2016). “Self- consistent seismic cycle simulation in a three-dimensional continuum model: method- ology and examples.” AGU Fall Meeting Abstracts.

[2] Erickson, B. A., et al. (2019). The Community Code Verification Exercise for Simulating Sequences of Earthquakes and Aseismic Slip (SEAS). Poster Presentation at 2019 SCEC Annual Meeting.

How to cite: Li, M., Pranger, C., Dal Zilio, L., and van Dinther, Y.: Dynamic and quasi-dynamic modelling of earthquake sequences from zero to three dimensions: choose model complexity as needed , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10210, https://doi.org/10.5194/egusphere-egu2020-10210, 2020.

Modern seismotectonic studies are aimed at obtaining a self-consistent explanation of fault zone heterogeneity, the rupture process, recurrence times and rupture mode of large earthquake sequences. In subduction regions large earthquakes are often characterized by very long source zones and complex long-term postseismic processes following the coseismic release of accumulated elastic stresses. A set of mechanical models was proposed to describe the generation of strongest earthquakes based on the idea of the synchronous failure of several adjacent asperities.

In this study we propose a model which is based on verified numerical schemes, which allows us to quantitatively characterize the process of generation of strong earthquakes. The model takes into account the fault-block structure of the continental margin and combined the ideas of a possible synchronous destruction of several adjacent asperities, mutual sliding along a fault plane with a variable coefficient of friction and subsequent healing of medium defects under high pressure conditions.

The applicability of the proposed model is shown by the example of the recent seismic history of the Kuril subduction zone. Kuril island arc is one of the most tectonically active regions of the world due to very high plate convergence rate. Heterogeneities in the mechanical coupling of the interplate interface in this region lead to the formation of the block structure of the continental margin, which is confirmed by various geological and seismological studies.

GPS observations recorded at different stages of seismic cycle related to the 2006–2007 Simushir earthquakes allow us to model geodynamic processes of slow strain accumulation and its rapid release during the earthquake and the subsequent posteseismic process. We use parameters describing the regional tectonic structure and rheology obtained from the inversion of geodetic data to construct a 2D model of generation of large earthquakes in central Kurils. Analysis of paleoseismic data on dates and rupture characteristics of previous major earthquakes shows a good agreement between the modeled and observed seismic cycle features. The predicted horizontal displacements of the seismogenic block at the coseismic stage are consistent with satellite geodetic data recorded during the 2006 Simushir earthquake.

The proposed model provides new insights into the geodynamic processes controlling the occurrence of strong subduction earthquakes.

How to cite: Vladimirova, I., Gabsatarov, Y., Alekseev, D., and Lobkovsky, L.: The role of the fault-block structure of the continental margin in the generation of the strongest subduction earthquakes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8854, https://doi.org/10.5194/egusphere-egu2020-8854, 2020.