Seismic damage accumulation in multiple mainshock–aftershock sequences

Earthquakes are generally clustered, both in time and space. Conventionally, each cluster is made of foreshocks, the mainshock, and aftershocks. Seismic damage can possibly accumulate because of the effects of multiple earthquakes in one cluster and/or because the structure is unrepaired between different clusters. Typically, the performance-based earthquake engineering (PBEE) framework neglects seismic damage accumulation. This is because (i) probabilistic seismic hazard analysis (PSHA) only refers to mainshocks and (ii) classical fragility curves represent the failure probability in one event, of given intensity, only. However, for life cycle assessment, it can be necessary to account for the build-up of seismic losses because of damage in multiple events. It has been already demonstrated that a Markovian model (i.e., a Markov chain), accounting for damage accumulation in multiple mainshocks, can be calibrated by maintaining PSHA from the classical PBEE framework and replacing structural fragility with a set of state-dependent fragility curves. In fact, the Markov chain also works when damage accumulates in multiple aftershocks from a single mainshock of known magnitude and location, if aftershock PSHA replaces classical PSHA. Herein, this model is extended further, developing a Markovian model that accounts, at the same time, for damage accumulation: (i) within any mainshock–aftershock seismic sequence and (ii) among multiple sequences. The model is illustrated through applications to a series of six-story reinforced concrete moment-resisting frame buildings designed for three sites with different seismic hazard levels in Italy. The time-variant reliability assessment results are compared with the classical PBEE approach and the accumulation model that only considers mainshocks, so as to address the relevance of aftershocks for life cycle assessment.     

Residual structural capacity evaluation of steel moment‐resisting frames with dynamic‐strain‐based model updating method

This paper presents a method for evaluating the residual structural capacity of earthquake‐affected steel structures. The method first quantifies the damage severity of a beam by computing the dynamic‐strain‐based damage index. Next, the model used to analyze the structure is updated based on the damage index, to reflect the observed damage conditions. The residual structural capacity is then estimated in terms of changes in stiffness and strength, which can be applied by structural engineers, via a nonlinear static analysis of the updated model. The main contributions of this paper are in performance evaluation of the dynamic‐strain‐based damage index for seismically induced damage using a newly developed substructure testing environment, consideration of various damage patterns in composite beams, and extension of a local damage evaluation technique to a residual capacity estimation procedure by incorporating the model‐updating technique. In laboratory testing, the specimens were damaged quasi‐statically, and vibration tests were conducted as the damage proceeded. First, a bare steel beam–column connection was tested, and then a similar one with a floor slab was used for a more realistic case. The estimated residual structural capacities for these specimens were compared with the static test results. The results verified that the proposed method can provide fine estimates of the stiffness and strength deteriorations within 10% for the specimen without the floor slab and within 30% for that with the floor slab. Copyright © 2017 John Wiley & Sons, Ltd.     

Numerical simulations of impacts involving porous bodies: II. Comparison with laboratory experiments

In this paper, we compare the outcome of high-velocity impact experiments on porous targets, composed of pumice, with the results of simulations by a 3D SPH hydrocode in which a porosity model has been implemented. The different populations of small bodies of our Solar System are believed to be composed, at least partially, of objects with a high degree of porosity. To describe the fragmentation of such porous objects, a different model is needed than that used for non-porous bodies. In the case of porous bodies, the impact process is not only driven by the presence of cracks which propagate when a stress threshold is reached, it is also influenced by the crushing of pores and compaction. Such processes can greatly affect the whole body's response to an impact. Therefore, another physical model is necessary to improve our understanding of the collisional process involving porous bodies. Such a model has been developed recently and introduced successfully in a 3D SPH hydrocode [Jutzi, M., Benz, W., Michel, P., 2008. Icarus 198, 242-255]. Basic tests have been performed which already showed that it is implemented in a consistent way and that theoretical solutions are well reproduced. However, its full validation requires that it is also capable of reproducing the results of real laboratory impact experiments. Here we present simulations of laboratory experiments on pumice targets for which several of the main material properties have been measured. We show that using the measured material properties and keeping the remaining free parameters fixed, our numerical model is able to reproduce the outcome of these experiments carried out under different impact conditions. This first complete validation of our model, which will be tested for other porous materials in the future, allows us to start addressing problems at larger scale related to small bodies of our Solar System, such as collisions in the Kuiper Belt or the formation of a family by the disruption of a porous parent body in the main asteroid belt.     

Exploring typhoon variability over the mid-to-late Holocene: evidence of extreme coastal flooding from Kamikoshiki,Japan

Sediment cores from two coastal lakes located on the island of Kamikoshiki in southwestern Japan (Lake Namakoike and Lake Kaiike) provide evidence for the response of a backbarrier beach system to episodic coastal inundation over the last 6400 years. Sub-bottom seismic surveys exhibit acoustically laminated, parallel to subparallel seismic reflectors, intermittently truncated by erosional unconformities. Sediment cores collected from targeted depocenters in both lakes contain finely laminated organic mud interbedded with coarse-grained units, with depths of coarse deposits concurrent with prominent seismic reflectors. The timing of the youngest deposit at Kamikoshiki correlates to the most recently documented breach in the barrier during a typhoon in 1951 AD. Assuming that this modern deposit provides an analog for identifying past events, paleo-typhoons may be reconstructed from layers exhibiting an increase in grain-size, a break in fine-scale stratigraphy, and elevated Sr concentrations.Periods of barrier breaching are concurrent with an increase in El Niño frequency, indicating that the El Niño/Southern Oscillation has potentially played a key role in governing typhoon variability during the mid-to-late Holocene. An inverse correlation is observed between tropical cyclone reconstructions from the western North Atlantic and the Kamikoshiki site, which may indicate an oscillating pattern in tropical cyclone activity between the western Northern Atlantic and the western North Pacific, or at least between the western Northern Atlantic and regions encompassing southern Japan. The two kamikaze typhoons which contributed to the failed Mongol invasions of Japan in 1274 AD and 1281 AD occur during a period with more frequent marine-sourced deposition at the site, suggesting that the events took place during a period of greater regional typhoon activity.     

Degree of impactor fragmentation under collision with a regolith surface—Laboratory impact experiments of rock projectiles

Some meteorites consist of a mix of components of various parent bodies that were presumably brought together by past collisions. Impact experiments have been performed to investigate the degree of target fragmentation during such collisions. However, much less attention has been paid to the fate of the impactors. Here, we report the results of our study of the empirical relationship between the degree of projectile fragmentation and the impact conditions. Millimeter‐sized pyrophyllite and basalt projectiles were impacted onto regolith‐like sand targets and an aluminum target at velocities of up to 960 m s^?1. Experiments using millimeter‐sized pyrophyllite blocks as targets were also conducted to fill the gap between this study and the previous studies of centimeter‐sized rock targets. The catastrophic disruption threshold for a projectile is defined as the energy density at which the mass of the largest fragment is the half of the original mass. The thresholds with the sand target were 4.5 ± 1.1 × 10⁴ and 9.0 ± 1.9 × 10⁴ J kg^?1, for pyrophyllite and basalt projectiles, respectively. These values are two orders of magnitude larger than the threshold for impacts between pyrophyllite projectiles onto aluminum targets, but are qualitatively consistent with the fact that the compressive and tensile strengths of basalt are larger than those of pyrophyllite. The threshold for pyrophyllite projectiles and the aluminum target agrees with the threshold for aluminum projectiles and pyrophyllite targets within the margin of error. Consistent with a previous result, the threshold depended on the size of the rocks with a power of approximately ?0.4 (Housen and Holsapple 1999). Destruction of rock projectiles occurred when the peak pressure was about ten times the tensile strength of the rocks.     

Electrical conduction and polarization of calcite single crystals

The electrical conductivity and polarization properties of calcite single crystals with three orientations, namely, a (00.1) plane perpendicular to the crystallographic c axis (10.0) plane parallel to the crystallographic c axis, and a (10.4) cleavage plane, were studied by both complex impedance and thermally stimulated depolarization current (TSDC) measurements. Conductivities for (00.1)-, (10.0)-, and (10.4)-oriented single calcite crystals at 400–600?°C were 1.16?×?10^?7?–?1.05?×?10^?5, 7.40?×?10^?8?–?4.27?×?10^?6, and 4.27?×?10^?7?–?2.86?×?10^?5 Ω^?1 m^?1, respectively, and the activation energies for conduction were 112, 103, and 101?kJ?mol^?1, respectively. The TSDC spectra verified the electrical polarizability of calcite crystals. The activation energy for depolarization, estimated from TSDC spectra, of the (00.1)-, (10.0)-, and (10.4)-oriented calcite substrates were 112, 119, and 114?kJ?mol^?1, respectively. Considering the correlation between the processes of conduction and electric polarization, we proposed the mechanisms of conduction and polarization in calcite on the assumption of oxide ion transport.     

Isotopic evidence for trapped fissiogenic REE and nucleogenic Pu in apatite and Pb evolution at the Oklo natural reactor

A part of the boundary layer of reactor zone 10 at the Oklo natural reactor shows a unique petrologic texture, which contains high-grade uraninite and massive apatite concretions. In order to study distribution behavior of fission products around the boundary between the reactor zone and the wall rock and to clarify the relation of migration mechanisms of fission products with geochemical factors, in-situ isotopic analyses of Nd, Sm, Gd, Pb and U in uraninite and apatite from the sample were performed by Sensitive High Resolution Ion Microprobe (SHRIMP). Sm and Gd isotopic ratios of uraninite and apatite show evidence of neutron irradiation with fluence between 4.4-6.8×10¹⁹ n/cm². Judging from the isotopic anomalies of Nd and U, the apatite coexisting with the uraninite plays an important role in trapping fissiogenic LREE and nucleogenic ²³⁹Pu into the structure. Systematic Pb isotopic data from apatite, uraninite, galena and minium suggest the following chronological interpretations.

1.

The apatite formed 1.92±0.01 Ga ago and trapped fissiogenic light REE and nucleogenic ²³⁹Pu that migrated from the reactor during the criticality.

2.

The uraninite around the boundary between reactor and sandstone dissolved once 1.1∼1.2 Ga ago.

3.

Galena grains were formed by U-Pb mobilization in association with the intrusion of dolerite dyke 0.45∼0.83 Ga ago.

4.

Minium was derived from recent dissolution of galena under locally oxidizing conditions.

     

Topography and volume effects on travel distance of surface failure

The authors investigated factors affecting the travel distance of a surface failure and developed a simple model for estimating the distance based on soil properties and topographic factor. The authors conducted field surveys and various soil tests for a number of surface failures that occurred during torrential rain at the end of August 1998 in southern Fukushima Prefecture, Japan. The authors studied the effects of various factors such as landslide volume, pore-water pressure, slope inclination, and the internal friction near the slip surfaces on the travel distances. The analyses showed no clear relationship between landslide volume and equivalent coefficient of friction, which was likely attributable to the very small range of volume compared to other studies on catastrophic landslides. The effect of excess pore-water pressure was likely negligible because undrained conditions were not to be maintained at the shallow flow depths. A positive correlation was shown between slope inclination and equivalent coefficient of friction. This correlation was attributable to two factors, one was positive correlation between the internal friction on the slip surface and slope inclination, and the other was the kinetic energy dissipation of moving mass that occurred at the inclination changing point between a slope and a sedimentary flat surface. The authors then developed a predictive model for critical flow inclination of landslide by installing the factors of soil properties and slope inclination factors. The model predicted equivalent coefficient of frictions, which were very similar to the observed values, thus verifying the effectiveness of the model.     

Building Damage and Casualties after an Earthquake

The relationship between building damage patterns and human casualties in Nishinomiya City – one of the most heavily damaged cities in the 1995 Hanshin-Awaji Earthquake Disaster – was investigated using photographs of damaged buildings. First, the photographs of buildings in which casualties occurred were identified, and the building damage patterns were judged based on the photographs considering the existence of survival space. Then the relationship between the building damage pattern and casualty occurrence, and the characteristics of casualty distribution, were investigated. The main findings were as follows: Most casualties occurred in relatively old two-story wooden buildings in which the ground floor completely collapsed without survival space; casualties occurred at all building damage levels including ``no damage', and it can be seen that building damage is the major, but not the sole cause, of casualties in an earthquake; in Nishinomiya City, the regional distributions of casualties due to the collapse of buildings that left no survival space is similar to that of casualties due to other types of building damage.