24,495 research outputs found
Electro-Magnetic Earthquake Bursts and Critical Rupture of Peroxy Bond Networks in Rocks
We propose a mechanism for the low frequency electromagnetic emissions and
other electromagnetic phenomena which have been associated with earthquakes.
The mechanism combines the critical earthquake concept and the concept of crust
acting as a charging electric battery under increasing stress. The electric
charges are released by activation of dormant charge carriers in the oxygen
anion sublattice, called peroxy bonds or positive hole pairs (PHP), where a PHP
represents an with ,
i.e. an in a matrix of of silicates. We propose that PHP are
activated by plastic deformations during the slow cooperative build-up of
stress and the increasingly correlated damage culminating in a large
``critical'' earthquake. Recent laboratory experiments indeed show that
stressed rocks form electric batteries which can release their charge when a
conducting path closes the equivalent electric circuit. We conjecture that the
intermittent and erratic occurrences of EM signals are a consequence of the
progressive build-up of the battery charges in the Earth crust and their
erratic release when crack networks are percolating throughout the stressed
rock volumes, providing a conductive pathway for the battery currents to
discharge. EM signals are thus expected close to the rupture, either slightly
before or after, that is, when percolation is most favored.Comment: 17 pages with 3 figures, extended discussion with 1 added figure and
162 references. The new version provides both a synthesis of two theories and
a review of the fiel
Spectral-element simulations of long-term fault slip: Effect of low-rigidity layers on earthquake-cycle dynamics
We develop a spectral element method for the simulation of long-term histories of spontaneous seismic and aseismic slip on faults subjected to tectonic loading. Our approach reproduces all stages of earthquake cycles: nucleation and propagation of earthquake rupture, postseismic slip and interseismic creep. We apply the developed methodology to study the effects of low-rigidity layers on the dynamics of the earthquake cycle in 2-D. We consider two cases: small (M ~ 1) earthquakes on a fault surrounded by a damaged fault zone and large (M ~ 7) earthquakes on a vertical strike-slip fault that cuts through shallow low-rigidity layers. Our results indicate how the source properties of repeating earthquakes are affected by the presence of a damaged fault zone with low rigidity. Compared to faults in homogeneous media, we find (1) reduction in the earthquake nucleation size, (2) amplification of slip rates during dynamic rupture propagation, (3) larger recurrence interval, and (4) smaller amount of aseismic slip. Based on linear stability analysis, we derive a theoretical estimate of the nucleation size as a function of the width and rigidity reduction of the fault zone layer, which is in good agreement with simulated nucleation sizes. We further examine the effects of vertically-stratified layers (e.g., sedimentary basins) on the nature of shallow coseismic slip deficit. Our results suggest that low-rigidity shallow layers alone do not lead to coseismic slip deficit. While the low-rigidity layers result in lower interseismic stress accumulation, they also cause dynamic amplification of slip rates, with the net effect on slip being nearly zero
Enhancing Students' Understanding of Risk and Geologic Hazards Using a Dartboard Model
This article describes the use of a model to express the magnitude-frequency relationships of natural hazards. The model consists of a dartboard whose rings can be drawn to represent magnitude, exceedence probability, average recurrence interval, or other statistical information. Students are engaged by "playing" the dart game through conducting a thought experiment, actually throwing at a physical dartboard, or simulating events based on a computer program. This type of model is applicable to any sequence of events that can be described by random sampling. It helps emphasize the random nature of such events, and provides a means for presenting hazard recurrence information in an easily visible form. In addition, it helps mitigate students' misconceptions about risk and average recurrence intervals, and provides a way to teach probability concepts without the use of sophisticated mathematics. Educational levels: Graduate or professional
Current challenges for preseismic electromagnetic emissions: shedding light from micro-scale plastic flow, granular packings, phase transitions and self-affinity notion of fracture process
Are there credible electromagnetic (EM) EQ precursors? This a question
debated in the scientific community and there may be legitimate reasons for the
critical views. The negative view concerning the existence of EM precursors is
enhanced by features that accompany their observation which are considered as
paradox ones, namely, these signals: (i) are not observed at the time of EQs
occurrence and during the aftershock period, (ii) are not accompanied by large
precursory strain changes, (iii) are not accompanied by simultaneous geodetic
or seismological precursors and (v) their traceability is considered
problematic. In this work, the detected candidate EM precursors are studied
through a shift in thinking towards the basic science findings relative to
granular packings, micron-scale plastic flow, interface depinning, fracture
size effects, concepts drawn from phase transitions, self-affine notion of
fracture and faulting process, universal features of fracture surfaces, recent
high quality laboratory studies, theoretical models and numerical simulations.
Strict criteria are established for the definition of an emerged EM anomaly as
a preseismic one, while, precursory EM features, which have been considered as
paradoxes, are explained. A three-stage model for EQ generation by means of
preseismic fracture-induced EM emissions is proposed. The claim that the
observed EM precursors may permit a real-time and step-by-step monitoring of
the EQ generation is tested
Simplified Estimation of Economic Seismic Risk for Buildings
A seismic risk assessment is often performed on behalf of a buyer of
commercial buildings in seismically active regions. One outcome of the assessment is that a probable maximum loss (PML) is computed. PML is of
limited use to real-estate investors as it has no place in a standard financial
analysis and reflects too long a planning period. We introduce an alternative
to PML called probable frequent loss (PFL), defined as the mean loss resulting from shaking with 10% exceedance probability in 5 years. PFL is approximately related to expected annualized loss (EAL) through a site economic hazard coefficient (H) introduced here. PFL and EAL offer three
advantages over PML: (1) meaningful planning period; (2) applicability in financial analysis (making seismic risk a potential market force); and (3) can
be estimated using a single linear structural analysis, via a simplified method
called linear assembly-based vulnerability (LABV) that is presented in this
work. We also present a simple decision-analysis framework for real-estate
investments in seismic regions, accounting for risk aversion. We show that
market risk overwhelms uncertainty in seismic risk, allowing one to consider
only expected consequences in seismic risk. We illustrate using 15 buildings,
including a 7-story nonductile reinforced-concrete moment-frame building in
Van Nuys, California, and 14 buildings from the CUREE-Caltech Woodframe Project
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