Tectonic Geomorphic of Sulawesi Island and Its Implication for Future Large Earthquake

This paper analyzes how resource of past and prospective great earthquake on the Central Sulawesi Arm, adhere on topography analysis from several space-based source. To answer the question, we analysis the tectonic geomorphic, stream pattern, exhumed fault, geological mapping and seismicity data. Detailed tectonic geomorphic studies in Sulawesi still lacking due to tectonic and fault obscures. For instance, Palu Koro Fault (PKF) was unpredictable, because the historical seismic records inevitably remain poorly documented and unrecognized fault strand, which was buried beneath abundant Quaternary alluvium subsequently obscured the fault trace. In other hand, the faults have been active during Quaternary must take into account because potentially dangerous, also the inactive faults during instrumental period must be re-evaluated in order to have awareness for large future large earthquake. Surprisingly, recent seismic activity of PKF generate super shear rupture a Mw 7.5 earthquake on 28th September 2018 with average slip 41 mm/year, which over the past two decade quiet from any seismic activity. The seismic potential for large fault is essential, since it has been silent during the instrumental period. Therefore, our motivation in this study to produce detail tectonic geomorphic map of the region in local scale, which is currently not available to prepare better knowledge and awareness for the large future earthquake. We have use Shuttle Radar Topography Mission (SRTM) with resolution ~30m, which run by ArcGIS software to observed tectonic geomorphic evidence of fault system and supplement with structural, geological and bathymetric data’s as ware available to us. We relate this analysis with seismicity data from Centroid Moment Tensor Solution (CMT) to recognize the seismic source. Our results show the tectonic geomorphic of Central Sulawesi Arm due to nature extension of NNW-SSE left-lateral slip curving to WNW-ESE of Palu-Koro Fault (PKF), then transcript to N-S circular normal fault of Poso Fault (PF). The PF indicate replica of PKF curving, where has not been mapped previously. We have mapped 60 major onshore fault systems, 10 faults showed evidence maximal to rapid rate tectonic activity along instrumental periods. Based on our CMT analysis, Sulawesi Island is greatly dominated by oblique fault.

Ground motions variability in Israel from 3-D simulations of M 6 and M 7 earthquakes

In Israel, due to low seismicity rates and sparse seismic network, the temporal and spatial coverage of ground motion data is insufficient to estimate the variability of moderate-strong (M > 6) ground motions required to construct a local ground motion model (GMM). To fill this data gap and to study the ground motions variability of M > 6 events, we performed a series of 3-D numerical simulations of M 6 and M 7 earthquakes. Based on the results of the simulations, we developed a statistical attenuation model (AM) and studied the residuals between simulated and AM PGVs and the single station variability. We also compared the simulated ground motions with a global GMM in terms of peak ground velocity (PGV) and significant duration (Ds 595). Our results suggest that the AM was unable to fully capture the simulated ground motions variability, mainly due to the incorporation of super-shear rupture and effects of local sedimentary structures. We also show that an imported GMM considerably deviates from simulated ground motions. This work sets the basis for future development of a comprehensive GMM for Israel, accounting for local sources, path, and site effects.

Experimental Study on the Influence Mechanism of the Structural Plane to Rockbursts in Deeply Buried Hard Rock Tunnels

During the excavation of a large number of deeply buried tunnels and mining projects, rockburst disasters occur frequently due to the complex geologic environment in deep underground, including high initial geostress, adverse tectonic actions, and excavation disturbance. Many rockbursts have been found to be induced by some small-scale structural planes in the area around the tunnels during the construction of Jinping II hydropower station. In order to study the influence mechanisms of the structural plane to rockbursts, the physical simulation tests of rockbursts under biaxial stress conditions are carried out using marble samples by considering different relative positions of the structural plane with tunnels, namely, in tunnel spandrel, in tunnel sidewall, and at the intersection with the tunnel. The digital image correlation (DIC) technique is used to trace the evolution of the deformation on the surface and the rockburst process of the marble sample. The results reveal that three types of rockbursts are identified, namely, fault-slip rockburst, split bulking rockburst, and shear rupture rockburst, and their evolution processes are reproduced. The presence of small-scale structural planes in the vicinity of deep tunnels could be one of the major influence factors in triggering rockbursts. The findings could provide helpful references for predicting the development process and the design of burst-resistant measures for this type of rockbursts.

Numerical Investigation on the Triaxial Compression Behavior of Large-Scale Jointed Coal

Accurate estimation of the triaxial compression behavior of coal mass is essential for coal mining. In this study, a numerical synthetic rock mass method was used to study the triaxial compression behavior of coal mass. The jointed-coal specimens were constructed based on in-situ joint measurements and microparameter calibration against laboratory tested data. A series of triaxial compression tests on jointed-coal specimens with different loading orientations and confining pressures were performed to obtain joint and confining-pressure effects and to reveal the related failure mechanism. The results suggest that jointed coal has a strong joint effect and confining-pressure effect. Joints weaken the strength and elastic modulus, reduce the lateral deformation, and affect the geometries of the shear-rupture surface. With increase in the confining pressure, the peak strength and residual strength increase but the elastic modulus remains stable; the lateral strain decreases, especially at low confining pressure; the mechanical behavior transitions from brittleness to ductility; the failure mode transitions from shear-rupture surface to plastic flow; and the joint effect diminishes and even disappears. The shear-rupture surface is formed by the combined effect of shear stress and joints at low confining pressure, and the contribution of joints decreases with increase in confining pressure.

The criterion of block media strength and geomechanical back-calculation

Relevance and problematics. The mechanism of block medium and jointed rock mass deformation and breaking differs notably from continuum mechanics ideas, making up the crucial fundamental problem of geomechanics and distinguishing it from other engineering sciences connected with the mechanics ISSN 0536-1028 «Известия вузов. Горный журнал», № 6, 2020 47 of deformable solids. However, due to poor understanding of block media breaking mechanism, rock masses strength is overwhelmingly estimated by means of empirical dependences obtained by simulation in laboratory conditions. It is apparent that this approach does not solve the crucial fundamental problem and the accuracy of predicting mechanical characteristics of jointed rock masses may reach 100%. Research methodology. Based on the earlier theoretical research by the methods of variational, integral, and differential calculus, a new mechanism of block mass breaking at a point was substantiated. It implies translation and rotational character of blocks displacement in a rock mass in the process of its deformation making it possible to substantiate the criterion of jointed rock masses breaking. Research results, analysis and recommendations for use. The research was the first to obtain the block medium strength criterion. It has been shown that block mass strength is made up of the strength along the slip joint and block rotation resistance. Besides, within some range of loads, the ultimate strength of the rock mass is proportional to the tensile strength of a structure block. The work also proposes a range of analytical dependences obtained based on the solid mechanics making which make it possible to back calculate the rock mass strength based on the curvature of the shear rupture surface and assess the full tensor of the plain field of strain by the base axes orientation.

Back-propagating super-shear rupture in the 2016 M7.1 Romanche transform fault earthquake

<p>Rupture propagation of an earthquake strongly influences potentially destructive ground shaking. Variable rupture behaviour is often caused by complex fault geometries, masking information on fundamental frictional properties. Geometrically smoother ocean transform fault (OTF) plate boundaries offer a favourable environment to study fault zone dynamics because strain is accommodated along a single, wide zone (up to 20 km width) offsetting homogeneous geology comprising altered mafic or ultramafic rocks. However, fault friction during OTF ruptures is unknown: no large (M<sub>w</sub>>7.0) ruptures had been captured and imaged in detail. In 2016, we recorded an M<sub>w</sub> 7.1 earthquake on the Romanche OTF in the equatorial Atlantic on nearby seafloor seismometers. We show that this rupture had two phases: (1) up and eastwards propagation towards the weaker ridge-transform intersection (RTI), then (2) unusually, back-propagation westwards at super-shear speed toward the fault’s centre. Deep slip into weak fault segments facilitated larger moment release on shallow locked zones, highlighting that even ruptures along a single distinct fault zone can be highly dynamic. The possibility of reversing ruptures is absent in rupture simulations and unaccounted for in hazard assessments.</p>

Direct observations of a dynamic earthquake rupture in the lower crust

<p>A significant number of studies in recent years have demonstrated that earthquakes in the lower crust are more abundant than previously thought. Specifically in continental collision zones, earthquakes are suggested to play a crucial role in permitting fluid infiltration and driving metamorphic transformation processes in crustal portions that are typically considered dry and metastable. However, the mechanisms that trigger brittle failure in the lower crust remain debated and the sequence of events that ultimately lead to seismic slip is unclear. To further understand this process we performed field and microstructural observations on an amphibolite facies fault (0.9-1 GPa) in granulite facies anorthosite from the Bergen Arcs, Western Norway. The fault preserves an exceptional record of brittle deformation and frictional melting that allows us to constrain the temporal sequence of deformation events. Most notably, the fault is flanked on one side by a damage zone where wall rock minerals are fragmented with little to no shear strain (pulverization). The fault core consists of a zoned pseudotachylyte bound on both sides by fine-grained cataclasites. Spatial relationships between these structures reveal that asymmetric pulverization of the wall rock and comminution preceded the seismic slip required to produce melting. These observations are consistent with the propagation of a dynamic shear rupture. Our study implies that high differential stress levels may exist within the dry lower crust of orogens, causing brittle faulting and earthquakes in a portion of the crust that has long been assumed to be characterized by ductile deformation.</p>

Factors controlling shear rupture roughness: An insight from field and laboratory experiments

<p>Faults and fracture surfaces record the history of slip events through a range of structural features in tectonically active zones. Slickensides, among them, prove to be the most prominent evidences of such slip movements. These linear features give us crucial information about the mechanical processes associated with shear surface roughness formation. We conducted extensive field survey in the Singhbhum Shear Zone, Eastern India, and report shear fractures of varying surface roughness from deformed quartzites. Shear surfaces encountered in the field study varied from very smooth, devoid of any lineation to strongly rough with prominent slickenlines.</p><p>For better understanding of the varied surface roughness, we performed analogue laboratory experiments. The experimental results suggest that the fracture orientation and the mode of shear failure are potential factors that control the fracture roughness. We used cohesive sand-talc models for the analogue experiments with varying sand:talc volume ratio, ranging from pure sand to pure talc variant. Experimental models with pure sand composition underwent Coulomb failure in the brittle regime. With subsequent increase in talc content, the behavior of failure switched to plastic yielding in the ductile regime. This transition from coulomb failure to plastic yielding produced a remarkable variation in the shear surface roughness characteristics. Shear surfaces formed by Coulomb failure are smooth and devoid any slickenlines, whereas, those formed by plastic yielding show prominent presence strongly linear roughness, defined by cylindrical ridge-grooves along the slip direction.</p><p>Shear surface roughness defined by linear irregularities become more prominent with increasing fracture orientation (θ) to the compression direction (θ = 30° to 60°). Increase in θ promotes the formation of smooth slickenlines at the cost of rough zones. For critical analysis and understanding of these features we develop a new computational technique. The technique is based on controlled optical images to map the shear surface geometry from field casts and laboratory samples. Binarization of the irregular surface images (cantor set) provides 1D fractal dimension (D), which is used to quantify the roughness variability, and the degree of their anisotropy in terms of ΔD (difference in D across and along the slip direction). From numerical models, we finally show onset of wave instability in the mechanically distinct rupture zone as an alternative mechanism for slickenlines formation.  </p>