Low Seismic Zones Research Articles

Subsurface structures around the subducting seamount illuminated by local earthquakes at the off-Ibaraki region, southern Japan Trench

The off-Ibaraki region is a convergent margin at which a seamount subducts in the southern Japan Trench. At this off-Ibaraki region, an Ocean Bottom Seismometer (OBS) experiment was conducted from before to after the 2011 Tohoku-oki earthquake. In this study area, a source region of the largest aftershock (Mw7.9) of the Tohoku-oki earthquake occurred and many subsequent aftershocks were recorded in the OBSs. An intensive event location was performed around the subducting seamount to reveal the spatiotemporal variation of the seismicity and the regional seismotectonics of this region. By applying a migration-based event location method to an Ocean Bottom Seismic network record of both P- and S-waves, over 20,000 events were determined in the off-Ibaraki region below ~ M4. The seismicity showed clear spatiotemporal patterns enough to identify the seismicity changes and geometry of the seismic interface. At the updip side, the shallow tectonic tremors and earthquakes are shown to be spatially complementary bounded by an updip limit of the seismogenic zone. At the downdip side, a semicircular low-seismicity zone was identified, which is possibly a rupture area of the Mw7.9 event. The event depth profile exhibited a gently sloped planar downdip interface subparallel to the subducting slab. This seismic plane is located about 8 km deeper than the plate interface and appears to be stably active from 2008 to 2011, although this study could not reveal its mechanism. A comparison with the active source seismic survey profiles exhibits that this planar downdip interface is several kilometers deeper than the top of the oceanic crust. After the Mw7.9 event, a high-angle downdip seismic interface was activated above the planar interface. Further, below the planar downdip interface, broadly scattered events occurred with a swarm manner. We successfully illuminated the complicated subsurface structures around the subducting seamount. It is suggested that most of the events occur along or below the plate interface as the top of the oceanic crust which could not be explained by the existing seamount subduction models. Further study in the future is needed to validate the conclusion of this study and then to construct a mechanism for this peculiar seismic interface.Graphical

Geoelectric studies in earthquake hazard assessment: the case of the Kozlodui nuclear power plant, Bulgaria

AbstractThe study presents the results of geoelectric research for seismic risk assessment on the example of the Kozlodui nuclear power plant in Bulgaria. The image of the geoelectric structure in the study area was obtained using one-dimensional inverse electrical resistivity modeling of the full five-component magnetotelluric data and quasi-three-dimensional inverse conductivity modeling of the geomagnetic responses recorded during the summer 2021 field campaign. According to the presented results, the geoelectrically anomalous structure is divided into two levels. The near-surface anomalous structure in the immediate reach of human geotechnical activity corresponds to the electrically conductive sedimentary fill. The mid-crustal layer is coincident with the low seismic velocity zone at the brittle and ductile crust interface, revealed in previous studies. The presented results imply that the geological environment is not affected by large faults capable of transmitting seismic energy from tectonically active areas, however, in further studies, attention should be paid to the strike-slip fault systems adjacent to the study area.

Seismic performance of precast bridge bents with prestressed piles and new pocket connection design: Experimental and numerical study

Pile-slab bridges with prestressed high-strength concrete (PHC) piles or prestressed and reinforced high-strength concrete (PRC) piles are widely used in low-seismic zones. To further understand the seismic performance of pile-slab bridges and to promote their applications in high-seismic regions, specimens of PHC bent (PHCB) or PRC bent (PRCB) from pile-slab bridges with a new bent-cap-pile pocket connection are studied through quasi-static cyclic tests. Seismic performance of the PHCB and PRCB is evaluated and compared in terms of damage development, failure mode, lateral stiffness, residual displacement, peak strength, displacement and energy-dissipation capacity. The experimental results show that both PHCB and PRCB fail due to non-ductile fracture of prestressing rebars in the piles, and the proposed bent-cap-pile pocket connection remains functional throughout the tests. Additional mild rebars of PRCB significantly increase displacement capacity of PRCB against PHCB. Finite element (FE) models are then developed and validated by the experimental results, after which the influence of initial prestressing force level and longitudinal prestressing reinforcement ratio of piles is investigated through parametric study based on the validated models. The proposed FE models can accurately predict the overall hysteretic behavior. Based on the parametric study, the seismic performance of both PHCB and PRCB can be improved by decreasing initial prestressing force level or increasing longitudinal prestressing reinforcement ratio.

A lower crust shear zone facilitates delamination and continental subduction under the Apennines

Physical properties and structure of the lithosphere are the first step to constrain the evolution of mountain belts. Here we show detailed shear wave velocity profiles of the lithosphere in the Apennines that clarify a controversial aspect of continental subduction: the intricate mechanism of crust delamination from the downgoing plate. From the analysis of complete and dense teleseismic Receiver Function data set, we find that the delamination of the continental lithosphere is favored by the development of a low seismic shear wave velocity zone in the middle-lower crust. We observe a double Moho below the external portions of the present mountain range, suggesting the progressive formation of the shallow interface. The delamination edge is located in the forearc, far eastward than expected, implying that the re-equilibration of the thermal unbalance, generated by the mantle substitution, may last 10-7 Myr.

Parametric study on seismic response modification factor of strap-braced cold-formed steel systems

While cold-formed steel structures have been extensively studied for seismic-resistant construction, existing regulations, and implementation guidelines do not comprehensively address the impact of key design parameters on the seismic behavior of CFS strap-braced shear wall systems. This study investigates the lateral resistance and seismic behavior factor of strap-braced shear walls and compares them with code values, and tries to fully understand the effect of different design parameters on the seismic R-factor of these systems. The effect of parameters such as the strap cross-sectional area, aspect ratio, and distance between the studs is studied using ANSYS finite element software to create datasets for various configurations. The pushover analysis results obtained from this analysis are utilized to perform incremental dynamic analysis, under a set of 22 ground motion records, to evaluate the seismic response modification factor for different residential buildings. For this purpose, one to four-story residential buildings are designed based on four seismic hazard zones: low, moderate, high, and very high and then, they are analyzed. Results of pushover analysis show that increasing the strap cross-sectional area and aspect ratio increases capacity, but changing the distance between studs does not have a significant effect, and the R factor presented by codes is conservative. Incremental dynamic analysis results also indicate that the behavior of studied buildings varies depending on the structure and selected record and the suggested R factor by codes for diagonal straps braced cold-formed steel structures is acceptable for buildings up to three stories in low, medium, and high seismic zones, and for buildings up to two stories in very high seismic zones. Overall, this study shows that strap-braced cold-formed steel shear walls have the potential to achieve higher seismic R-values in comparison to design codes.

Lithosphere Removal in the Sierra Nevada de Santa Marta, Colombia

AbstractThe Sierra Nevada de Santa Marta (SNSM) in northwestern Colombia is one of the world's highest coastal mountains, with an elevation above 5 km. Gravity measurements show that the SNSM has a positive Bouguer anomaly (>+130 mGal), indicating that the mountain lacks a crustal root. In this work, we test the hypothesis that these observations can be explained by gravitational removal of the dense lower lithosphere. We use 2D numerical models to examine the dynamics of lithosphere removal and its effect on surface elevation and gravity anomaly. The models consist of continental lithosphere that includes a pre‐thickened crustal region, representing the SNSM. In our preferred model, the dense mantle lithosphere and crustal root are gravitationally unstable and undergo removal as local drips within ∼10 Ma from the onset of foundering. This creates an area of thinned crust (∼38 km) underlain by a buoyant sublithospheric mantle where melting and low seismic velocities are predicted. Subsequent non‐isostatic forces maintain a topography of 3.3 km with a 2D Bouguer gravity anomaly of +103 mGal. Parameter tests show that a strong lower‐crustal rheology provides greater support for the high topography and that a weak mantle lithosphere rheology produces faster removal. The models demonstrate that local lithosphere dynamics can explain the first‐order observations in the SNSM. We propose that lithosphere removal could have occurred recently (∼2 Ma), explaining the localized low seismic velocity zone below the SNSM, or at 56–40 Ma, inducing anomalous short‐lived Eocene magmatism.

Identification of active faults and tectonic features through heat flow distribution in the Nankai Trough, Japan, based on high-resolution velocity-estimated bottom-simulating reflector depths

Estimates of heat flow can contribute to our understanding of geological structures in plate convergent zones that produce great earthquakes. We applied automated velocity analysis to obtain the accurate seismic profiles needed for precise heat flow estimates using six new seismic profiles acquired during R/V Kaimei KM18-10 voyage in 2018. We calculated heat flow values in the accretionary wedge of the Nankai Trough off the Kii Peninsula, Japan, from the positions of widespread bottom-simulating reflectors (BSRs) in seismic reflection profiles. Calculated conductive heat flow values from the depth of the BSR agree with previous studies where a regional trend is observed from ~ 50 mW/m2 to < 40 mW/m2 60 km landward from the deformation front. This trend is caused by thickening of accretionary sediments and the subduction of the Philippines Sea plate. Segments of profiles are marked by anomalous high heat flow values. Such anomalies represent alterations of the shallow crustal thermal structure caused either by a combination of topographic affects, surface erosion of the seafloor, or by fluid flow that transports heat by advection. We interpret heat flow anomalies (~ 100 mW/m2) as indicators of active faulting, which correspond to low seismic velocity zones along faults. Our results also showed relatively high heat flow at the landward end of several survey lines close to the Kii Peninsula, which we interpret to the possible presence of plutonic rocks that underlie the Kii Peninsula and extend offshore and may be the cause of geothermal springs, steep geothermal gradients, and high heat flow.Graphical abstract

Seismic characterisation of aluminium fusible links in single‐storey steel buildings

AbstractThis work is part of the European RFCS “FISHWALL” project entitled “Fire and Seismic performances of hybrid fire walls in case of single‐storey industrial and commercial steel buildings”. The aim of the project is to design a hybrid firewall solution using sandwich panels for single‐storey buildings connected with the unprotected steel structure by means of fusible links, made of aluminium bolts. In this respect, the paper shows the results of preliminary numerical analyses performed on existing case studies of single‐storey steel buildings located in low and moderate seismicity zones and they were used: i) to estimate the seismic forces acting on the fusible links and ii) to subsequently design the specimens to be tested in the laboratory. In this respect, several finite elements models were developed in order to consider the widest range of configurations of the firewall with respect to the direction of the portal frame composing the steel buildings, i.e. firewall parallel and orthogonal to the portal frames. Based on the estimated forces, the design of the six different specimens was performed and it is thoroughly shown in the paper.

Application of TOPSIS model in active tectonic prioritization: Madeira watershed, South America

The Andes orogenic belt is one of the most tectonically active zones located between the convergent margins of the South American plate and the Pacific plate. From the dissected western belt to the expanded alluvial plain of Amazon basin, the Madeira watershed bears recent tectonic imprints. The tectonic signature is well documented by the spatial and linear characteristics of the five sub watersheds along with the Madeira trunk river and channels. The level of tectonic activeness has been assessed on the sub watersheds of the Madeira River, which is one of the main right-hand tributaries of the Amazon River. On the five defined sub-watersheds and the main channel watershed of Madeira, several morphometric indices, viz., asymmetry factor (AF), tilt angle (β), sinuosity index (SI), channel concavity (ɵ), elongation ratio (Re), circularity ratio (Rc), transverse topographic symmetry factor (T) and basin shape index (Bs), have been used. To rank the watersheds based on tectonic activity, we applied the Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) The active tectonics was also tested by a comparative study of linear indicators as Hack profiles, longitudinal profiles and segment-wise stream gradient index (SL) of watersheds 2 (rank 1) and 3 (rank 6). The results reveal that the most tectonically active watershed 2 is located in a high seismic magnitude zone with several deep earthquake epicenters. The least tectonically active watershed 6 is located in a low seismic zone.

Behaviour factor and seismic safety of reinforced concrete structures designed according to Eurocodes

Eurocode 8 allows force-based seismic design and safety assessment of buildings as if they were indefinitely linearly elastic. However, to implicitly account for their inelastic deformation capacity, Eurocode 8 allows reduction of the design values of seismic forces through a structure-dependant behaviour factor. Recent studies showed that this design approach yields structures with different probabilities of failure in different sites. However, for optimal design and investment of economic resources, a structural code should ensure similar risk level (lower than an acceptable risk level) for all structures that are designed according to it. To achieve this goal, the use of risk-targeted behaviour factors may be considered; therefore, the behaviour factors adopted for the design of the same structure in zones with different seismicity levels should be different.In this study, reinforced concrete moment-resisting frame buildings designed according to Eurocodes with different design values of seismic acceleration and number of storeys were considered, and their “available” behaviour factors and probabilities of failure were evaluated through nonlinear static analyses. The potential dependency of the available behaviour factor and probability of failure on different design parameters, as well as their dependency on the model adopted to reproduce the nonlinear response of reinforced concrete members, were investigated, and are discussed in this paper. The calculation of the probabilities of failure of the case-study structures showed that their probability of failure in low-seismicity zones was significantly lower than the acceptable risk level, which was not optimal. Conversely, the structures designed for high-seismicity zones had probability of failure similar to or higher than the acceptable risk level, which was optimal or not admissible, respectively. Thus, based on the assessed values of the available behaviour factor and probability of failure, a modification to the current equations for the calculation of the behaviour factor is presented. This modification implies the use of hazard-dependent values for the behaviour factor to have structures with similar and hazard-independent probability of failure. Notably, because the probabilities of failure for structures designed for low-seismicity zones were significantly lower than the acceptable risk level, the proposed hazard-dependent behaviour factor was higher than that currently adopted according to the Eurocode.

Mapping crustal structure across southern Australia using seismic ambient noise tomography

The rocks of southern Australia record over three billion years of Earth’s evolution, but the basement geology is veiled by sediment. Geophysical data are needed to unveil the geology. The 2018–2022 Lake Eyre Basin and 2020–2022 AusArray SA seismic arrays expand seismic coverage in South Australia. In conjunction with permanent and preceding temporary arrays, we extracted Rayleigh wave dispersion data from ambient noise recordings at a total of 501 seismic stations spanning the transition from Precambrian to Phanerozoic Australia. These data were used to develop Rayleigh wave phase velocity maps of southern Australia at periods 3–20s, and in turn, a shear wave velocity model to 20km depth. At upper-crustal depths, low velocity structure tracks Phanerozoic sedimentary accumulations. The Moyston Fault, regarded as the boundary between the Delamerian and Lachlan Orogens, has an intermittent expression in the shear velocity model: it has no obvious expression at depths shallower than ∼10km, but in the mid-crust is marked by a velocity contrast in southern Victoria and a velocity contrast tracing the southern edge of the Darling Basin in western New South Wales. An arcuate velocity contrast characterising the western edge of deep, sediment-filled troughs of the Darling Basin is a candidate for the transition from Precambrian to Phanerozoic crust. Fluid derived from neotectonic metamorphic devolatilization and/or remnant hydrated mantle is our preferred hypothesis for explaining seismicity and coincident seismic and conductivity anomalies in the mid-to-lower crust beneath the Ikara–Flinders Ranges. Our model suggests that the Olympic Dam and Carrapateena IOCG deposits reside above the margin of a mid-crustal low seismic velocity zone. We surmise that this might reflect past metalliferous fluid movement associated with the Olympic Cu-Au Province, akin to low reflectivity and low resistivity zones evident in 2D reflection seismic and magnetotelluric profiles, respectively.

Seismic Analysis of Soft Storey Building in Earthquake Zones

In this paper (G+8) building is modeled like a bare frame, a bare frame with the shear wall, and a bare frame with X bracing by changing the soft storey to different floors. The static analysis effect is determined for all three models with zone IV and zone V using Staad pro-V8i software. The main objective of the research was to assess the impact of a soft storey in various earthquake zones and by varying places of the soft storey from first to the top floor and for frames with different column shapes by seismic analyses in staad pro. The results of variable building models are obtained from the research regarding various parameters such as displacement, storey drift, and base shear. More significantly, comparing different structural systems revealed a reduction in lateral displacement and story drift. The shear wall reduced the Storey Displacement by 98.838% and storey drift by 99.86%. The Steel bracing reduced the Storey Displacement by 97.846 % and storey drift by 92.6%. Finally, it has been found that the Shear wall reduces lateral displacement and storey drift, thus significantly contributing to greater structural stiffness. The analysis results recommended that the shear wall use reinforced concrete frames for the seismic hazard zones and the Steel bracing recommended for the low seismic zones.

Seismic Risk Assessment of Nagpur City using the Geographic Information System

Earthquakes are the major threat to mankind accounting for almost half of the fatalities brought about by all of the natural hazards and desert their traces in societies for years. It is well known that seismic risk cannot be eliminated but minimized. Risk studies lay down the roadmap to prepare and minimize the aftermath of any disastrous event. Geographic Information System (GIS) has been proved as an effectual tool for seismic risk assessment study. This study presents the seismic risk assessment of Nagpur city using the GIS framework. Albeit per Indian Standard (IS) 1893-2016 [1] seismic zonation of Nagpur city comes under zone II (relatively low seismic hazard zone), the seismic risk being a combination of hazard, elements at risk and, vulnerability cannot be denied. Similar to other Indian cities the city of Nagpur had a heterogeneous development pattern. Even though Nagpur city is municipalized in well distributed wards, many intra wards distributions show housing clusters of different socioeconomic status. Therefore, using the satellite imagery different socioeconomic housing clusters were identified and mapped in the ArcGIS platform. Six socioeconomic clusters were identified using satellite imagery for Nagpur. From the field survey of randomly selected typical clusters, 14 prevailing Model Building Types (MBTs) were identified. Census data were used to find the population density and its distribution throughout the city. Seismic hazard has been estimated from IS 1893-2016 [1] as well as using the probabilistic hazard map of NDMA [2]. Selected MBTs were assessed for their seismic vulnerability, seismic risk in terms of casualties and damage is calculated. Finally, thematic maps showing damages arrived from the various approach are shown and discussed.

Stress conditions and seismicity around the rupture zone of the mainshock of the 2016 Kumamoto earthquake sequence in Kyushu, southwest Japan

The 2016 Kumamoto earthquake sequence in Kyushu included a foreshock (Mw 6.2) on 14 April and the mainshock (Mw 7.0) on 16 April, both of which were caused by fault ruptures at the intersection of the Futagawa and Hinagu fault zones. However, not all sections of the two fault zones were ruptured during the mainshock; in particular, although the northernmost (Takano–Shirahata) section of the Hinagu fault zone ruptured, the rupture did not propagate to southern sections of the fault zone. We examined fault geometry, geological structure, and seismicity around the fault zones, and conducted numerical Coulomb stress change and slip tendency analyses to investigate rupture conditions around the source faults. Fault geometry and slip tendencies indicated that before the foreshock, the source fault (the Futagawa section of the Futagawa fault zone) was favorable for rupture. Seismicity analysis showed that, before the foreshock, a remarkable low-seismicity zone existed where the rupture ultimately terminated. A material or structural boundary in the fault zones probably caused this anomalous seismicity and affected rupture propagation during the mainshock. Coulomb stress change analysis indicated a positive stress change in the Hinagu section just after the mainshock. In addition, the strikes of the optimum slip planes were parallel to that of the Hinagu section just after the mainshock, and slip tendencies in the section increased at that time. These results suggest that the Hinagu section was brought closer to failure just after the mainshock. In contrast, later in the post-mainshock period, the strikes of the optimum slip planes were oblique to that of the Hinagu section and the slip tendencies of the section were low. These results suggest that the favorability for a future rupture of the Hinagu section has declined since 2016 Kumamoto earthquake sequence.

Seismic and thermo-compositional insights into the uppermost mantle beneath the Northern Andes magmatic arc

The mantle wedge beneath the Northern Andes magmatic arc is expected to be latitudinally heterogeneous as suggested by the highly variable composition of recovered xenoliths, a discontinuous pattern of low seismic velocity zones, and unusually high Vp/Vs ratios below volcanic centers. Such heterogeneity might be controlling the development of the modern arc which currently shows a narrow northern volcanic segment between ∼4° and 6°N (<75 km wide), a volcanic gap of hundreds of km in its central portion (∼2°-4°N), and a wider southern volcanic expression between ∼2°S and 2°N (>120 km wide). Through an analysis of Pn and Sn inter-station wave speed estimates, the seismic structure dissimilarity and latitudinal thermo-compositional insights were constrained within the uppermost mantle beneath the arc. Results, integrating wave speed estimates, anisotropy, and thermo-compositional inferences obtained by wave velocity comparison using physical properties of minerals and phase equilibria modeling, suggest a seismically slower structure, higher anisotropy, and warmer conditions beneath the northern region (>4°N), whereas a faster, less anisotropic, and colder mantle towards the south (<2°N). We interpret the warmer and higher degree of anisotropy in the northern uppermost mantle as influenced by the Caldas tear, prompting hot mantle influx. Beneath the gap region, seismic speeds are similar to those in the north, yet a colder thermal state is suggested. We interpret the colder temperatures and the abrupt reduction in surface volcanism as reflecting an absence, or a reduced amount of magma pounding within the Moho vicinity. The southern upper mantle is seismically faster and colder compared to the northern portions of the arc. We interpret the fast velocities, and a wider volcanic expression on the surface, as influenced by the Carnegie ridge interaction prompting a shallower subduction with respect to the north. Our wave speed estimate and thermo-compositional inferences draw an increase in temperature and seismic anisotropy in the uppermost mantle from south to north along the Northern Andes magmatic arc. The main controlling factor of the general anisotropy below the entire arc seems to be the preferred orientation of olivine and pyroxene rather than an aligned melt fabric.

Model updating of a masonry tower based on operational modal analysis: The role of soil-structure interaction

Vibration-based finite element model (FEM) updating of cultural heritage assets is gaining so much attraction these days since destructive tests are usually not allowed to be performed. In this study, a framework for developing three-dimensional (3D) FEMs is proposed using 3D laser scanners and applied on Slottsfjell tower, a stone masonry tower in Tønsberg, Norway. Operational modal analysis (OMA) was done based on the ambient vibration testing (AVT) data to define the frequency values and corresponding mode shapes of the tower. Mechanical properties of the tønsbergite stone were utilized to derive the base values of the material properties of the homogenized masonry for performing sensitivity analysis and FEM updating. To investigate the effect of the soil-structure interaction (SSI) on the FEM updating results, three FEMs are developed. The fixed-base model is the FEM without considering the SSI effects, and two other FEMs are developed using the substructure and direct methods for simulating the SSI effects. Sensitivity analysis was performed to investigate the effective parameters on the dynamic characteristics of the models. FEM updating was conducted on the three FEMs, and results are compared to each other to show the role of the SSI on the FEM updating results. The resonance effect can cause damages to buildings located even in low seismicity zones. For this aim, the risk of resonance effect has been evaluated for the tower. Finally, linear dynamic analysis was performed on the three calibrated models, and the results were compared to each other.

Overview of the seismic probabilistic safety assessment applied to a nuclear installation located in a low seismicity zone

Deterministic and probabilistic nuclear safety analysis methodologies have been developed and updated based on operational experience, investigation of past incidents or accidents, and analysis of postulated initiating events in order to maintain the protection of workers, the public and the environment. The evaluation of accident sequences and the total radiological risk resulting from off-site releases are general objectives addressed by these methodologies. There are hazards that continually challenge the safety of a nuclear facility or its nearby area. In particular, seismic events represent a major contributor to the risk of a nuclear facility. Different levels of ground motion induced by earthquakes may be experienced by the structures, systems and components (SSCs) of the installation. In this context, a seismic hazard analysis, seismic demand analysis and seismic fragility analysis must be carried out in order to characterize the local seismic hazard and what are the seismic demands on SSCs, allowing an adequate seismic classification of SSCs, even in installations located in sites with low seismicity. In this article, a general description of the Seismic Probabilistic Safety Assessment (Seismic PSA) methodology is presented, with emphasis on their support studies, aiming at applying the methodology described in this article to an experimental nuclear installation containing a PWR reactor designed for naval propulsion to be installed in a low seismicity zone in Brazil.

NONLINEAR DYNAMIC ANALYSIS OF TWO STOREY RC BUILDING MODEL

Peninsular Malaysia lies in a low seismic zone, but its building structures had come across the concrete deterioration due to the seismic ground motion originated from far or near field. Notably, most of the building structures in this country are designed based on wind load only. Moreover, current practice to analyze or design a building such as FEMA 368 and EC8 underestimated the effect of repeated excitations. These guidelines only considered single vibrations to evaluate the framed structure. Therefore, the objective of this study was to assess the performance of private educational institute reinforced concrete building with generic 3D two storey frame structure under multiple seismic motions. Structural model was examined under series of earthquake motions which include pre-shock, main shock and aftershock scenario. Total of 7 seismic ground motions were selected to quantify the structural frame model by nonlinear dynamic time history analyses. Pseudo-dynamic ground motions were recorded on shaking table ranging from 0.18 g to 0.82 g were applied onto the building model for assessment. The outcome of this study has identified that the low-rise building model survived at higher PGA values. Moderate damages (0.25 ≤ DI &lt; 0.40) were recorded after passing through multiple ground motions. Moreover, low seismic vibrations with large ground movement had caused ground floor storey act as soft storey. The study concluded that low rise building model had higher tendency to absorb lower to higher ‘g’ values and resist the earthquake loading due to the strength of framed structure.

Imaging the Subsurface Structure of Mount Agung in Bali (Indonesia) Using Volcano-Tectonic (VT) Earthquake Tomography

Local seismic tomography is a well-known and commonly used method for obtaining detailed information about the internal structure of volcanoes. The eruption of Mt. Agung in 2017 was a vital opportunity scientifically because it is the first eruption that had sufficient seismic observation networks to carry out local seismic tomography at this volcano. In this study, we investigate the subsurface structure of Mt. Agung in Bali, which is one of the highest risk volcanoes in Indonesia. We conducted travel-time tomography using P- and S-wave arrival times of volcano-tectonic (VT) events to determine the three-dimensional (3D) Vp, Vs, and Vp/Vs ratio structure beneath Mt. Agung. We used 1,926 VT events, with corresponding 9482-P and 8683-S wave arrival times recorded by eight seismic stations over an observation time spanning from October 18 to December 31, 2017. We obtain the hypocenter solution for VT events using the maximum likelihood estimation algorithm and use an optimum 1D velocity model as input for the Joint 3-D seismic tomographic inversion. Local earthquake tomography revealed five anomalous regions that are useful to describe the overall seismic activity around Mt. Agung. We interpret these anomalous regions qualitatively due to limited data resolution in this study. We have successfully localized a high Vp/Vs ratio (∼1.82), low Vs (−1.9%) and high Vp (+3.8%), within a low seismicity zone at depths between 2 and 5 km below the Mt. Agung summit, which may be related to a shallow magma reservoir. There is also an anomalous region between Mt. Agung and Batur with moderate to high Vp/Vs ratios (1.76–1.79) where most of the earthquakes recorded before the 2017 eruption originated. We interpret this anomaly to be related to the existence of sub-vertical dyke complex at depths between 8 and 14 km. The results of our study provide new insights into the subsurface structure of the magma plumbing system beneath Mt. Agung, which can be used to improve the quality of determining the location of the hypocenter and source modeling for future eruption forecasting.

Seismic Anisotropy and Its Impact on Imaging the Shallow Alpine Fault: An Experimental and Modeling Perspective

AbstractThe transpressional Alpine Fault in New Zealand has created a thick shear zone with associated highly anisotropic rocks. Low seismic velocity zones and high seismic reflectivity are recorded in the Alpine Fault Zone, but no study has explored the underlying physical rock parameters of the shallow crust that control these observations. Protomylonites are the volumetrically dominant lithology of the fault zone. Here we combine experimental measurements of P‐wave speeds with numerical models of elastic wave anisotropy of protomylonite samples to explore how the fault zone can be seismically imaged. Numerical models that account for the porosity‐free real samples' fabric elastic tensors from electron backscatter diffraction (EBSD) are calculated by MTEX and a finite element model (FEM), while microfractures are modeled with differential effective medium (DEM) theory. At effective pressures representative of the Alpine Fault brittle zone, experimental wave speeds are lower than those predicted by MTEX/FEM. A possible DEM model suggests that a combination of random and aligned microfractures with aspect ratios increasing with pressure can explain the experimental wave speeds for pressures &lt;70 MPa. Such microporosity in the form of foliation‐ and mica basal plane‐parallel microfractures and grain boundaries is validated with synchrotron X‐ray microtomography and transmission electron microscopy (TEM) images. Finally, by modeling anisotropy of seismic reflection coefficients with angle of incidence, we demonstrate that the high reflectivity and low‐velocity zone (LVZ) observed at the Alpine Fault can only be explained if this microporosity is accounted for throughout the brittle fault zone, even at depths of 7–10 km.