SUMMARY
We propose a theoretical modelling framework for earthquake occurrence and clustering based on a family of invariant Galton–Watson (IGW) stochastic branching processes. The IGW process is a ...rigorously defined approximation to imprecisely observed or incorrectly estimated earthquake clusters modelled by Galton–Watson branching processes, including the Epidemic Type Aftershock Sequence (ETAS) model. The theory of IGW processes yields explicit distributions for multiple cluster attributes, including magnitude-dependent and magnitude-independent offspring number, cluster size and cluster combinatorial depth. Analysis of the observed seismicity in southern California demonstrates that the IGW model provides a close fit to the observed earthquake clusters. The estimated IGW parameters and derived statistics are robust with respect to the catalogue lower cut-off magnitude. The proposed model facilitates analyses of multiple quantities of seismicity based on self-similar tree attributes, and may be used to assess the proximity of seismicity to criticality.
We document and quantify effects of two types of catalogue uncertainties—earthquake location errors and short-term incompleteness—on results of statistical cluster analyses of seismicity in southern ...California. In the main part of the study we analyse 117 076 events with m ≥ 2 in southern California during 1981–2013 from the waveform-relocated catalogue of Hauksson et al. We present statistical evidence for three artefacts caused by the absolute and relative location errors: (1) Increased distance between offspring and parents. (2) Underestimated clustering, quantified by the number of offspring per event, the total number of clustered events, and some other statistics. (3) Overestimated background rates. We also find that short-term incompleteness leads to (4) Apparent magnitude dependence and temporal fluctuations of b-values. The reported artefacts are robustly observed in three additional catalogues of southern California: the relocated catalogue of Richards–Dinger & Shearer during 1975–1998, and the two subcatalogues—1961–1981 and 1981–2013—of the Advances National Seismic System catalogue. This implies that the reported artefacts are not specific to a particular (re)location method. The comparative quality of the four examined catalogues is reflected in the magnitude of the artefacts. The location errors in the examined catalogues mostly affect events with m < 3.5, while for larger magnitudes the location error effects are negligible. This is explained by comparing the location error and rupture lengths of events and their parents. Finally, our analysis suggests that selected aggregated cluster statistics (e.g. proportion of singles) are less prone to location artefacts than individual statistics (e.g. the distance to parent or parent–offspring assignment). The results can inform a range of studies focused on small-magnitude seismicity patterns in the presence of catalogue uncertainties.
Cytoplasmic microtubules (MTs) continuously grow and shorten at their free plus ends, a behavior that allows them to capture membrane organelles destined for MT minus end-directed transport. In ...Xenopus melanophores, the capture of pigment granules (melanosomes) involves the +TIP CLIP-170, which is enriched at growing MT plus ends. Here we used Xenopus melanophores to test whether signals that stimulate minus end MT transport also enhance CLIP-170-dependent binding of melanosomes to MT tips. We found that these signals significantly (>twofold) increased the number of growing MT plus ends and their density at the cell periphery, thereby enhancing the likelihood of interaction with dispersed melanosomes. Computational simulations showed that local and global increases in the density of CLIP-170-decorated MT plus ends could reduce the half-time of melanosome aggregation by ~50%. We conclude that pigment granule aggregation signals in melanophores stimulate MT minus end-directed transport by the increasing number of growing MT plus ends decorated with CLIP-170 and redistributing these ends to more efficiently capture melanosomes throughout the cytoplasm.
River deltas are intricate landscapes with complex channel networks that self‐organize to deliver water, sediment, and nutrients from the apex to the delta top and eventually to the coastal zone. The ...natural balance of material and energy fluxes, which maintains a stable hydrologic, geomorphologic, and ecological state of a river delta, is often disrupted by external perturbations causing topological and dynamical changes in the delta structure and function. A formal quantitative framework for studying delta channel network connectivity and transport dynamics and their response to change is lacking. Here we present such a framework based on spectral graph theory and demonstrate its value in computing delta's steady state fluxes and identifying upstream (contributing) and downstream (nourishment) areas and fluxes from any point in the network. We use this framework to construct vulnerability maps that quantify the relative change of sediment and water delivery to the shoreline outlets in response to possible perturbations in hundreds of upstream links. The framework is applied to the Wax Lake delta in the Louisiana coast of the U.S. and the Niger delta in West Africa. In a companion paper, we present a comprehensive suite of metrics that quantify topologic and dynamic complexity of delta channel networks and, via application to seven deltas in diverse environments, demonstrate their potential to reveal delta morphodynamics and relate to notions of vulnerability and robustness.
Key Points:
Establish a quantitative framework for delta channel connectivity and dynamics
Compute delta's steady fluxes, contributing and nourishment areas algebraically
Construct vulnerability maps to identify hot spots of change
Microtubule-based transport is critical for trafficking of organelles, organization of endomembranes, and mitosis. The driving force for microtubule-based transport is provided by microtubule motors, ...which move organelles specifically to the plus or minus ends of the microtubules. Motor proteins of opposite polarities are bound to the surface of the same cargo organelle. Transport of organelles along microtubules is discontinuous and involves transitions between movements to plus or minus ends or pauses. Parameters of the movement, such as velocity and length of runs, provide important information about the activity of microtubule motors, but measurement of these parameters is difficult and requires a sophisticated decomposition of the organelle movement trajectories into directional runs and pauses. The existing algorithms are based on establishing threshold values for the length and duration of runs and thus do not allow to distinguish between slow runs and pauses, making the analysis of the organelle transport incomplete. Here we describe a novel algorithm based on multiscale trend analysis for the decomposition of organelle trajectories into plus- or minus-end runs, and pauses. This algorithm is self-adapted to the characteristic durations and velocities of runs, and allows reliable separation of pauses from runs. We apply the proposed algorithm to compare regulation of microtubule transport in fish and
Xenopus melanophores and show that the general mechanisms of regulation are similar in the two pigment cell types.
Deltas are landforms that deliver water, sediment and nutrient fluxes from upstream rivers to the deltaic surface and eventually to oceans or inland water bodies via multiple pathways. Despite their ...importance, quantitative frameworks for their analysis lack behind those available for tributary networks. In a companion paper, delta channel networks were conceptualized as directed graphs and spectral graph theory was used to design a quantitative framework for exploring delta connectivity and flux dynamics. Here we use this framework to introduce a suite of graph‐theoretic and entropy‐based metrics, to quantify two components of a delta's complexity: (1) Topologic, imposed by the network connectivity and (2) Dynamic, dictated by the flux partitioning and distribution. The metrics are aimed to facilitate comparing, contrasting, and establishing connections between deltaic structure, process, and form. We illustrate the proposed analysis using seven deltas in diverse morphodynamic environments and of various degrees of channel complexity. By projecting deltas into a topo‐dynamic space whose coordinates are given by topologic and dynamic delta complexity metrics, we show that this space provides a basis for delta comparison and physical insight into their dynamic behavior. The examined metrics are demonstrated to relate to the intuitive notion of vulnerability, measured by the impact of upstream flux changes to the shoreline flux, and reveal that complexity and vulnerability are inversely related. Finally, a spatially explicit metric, akin to a delta width function, is introduced to classify shapes of different delta types.
Key Points:
Define metrics that depict the topologic and dynamic complexity of deltas
Study how complexity relates to an intuitive notion of vulnerability
Project deltas for comparison and physical insight in a topo‐dynamic space
We revisit the significance of the increased number of great earthquakes since December 2004. An analysis of the global seismic moment release during 1918–2014 inferred from the International ...Seismological Centre catalog rejects a null hypothesis of independence between earthquake seismic moments and occurrence times. Our results suggest the existence of temporal variation in the seismic moment distribution on a global scale with decreased moment release during 1975–2004 and a transition to a regime with increased moment release during the 1960s and after December 2004. We use complementary likelihood and regression analyses based on nonparametric resampling and parametric Monte Carlo simulations to construct tests powerful enough to reject the null hypothesis of independence.
Key Points
Global seismic moment release increased since December 2004
Global seismic moment release during 1918–2014 cannot be fit under assumption of independence between earthquake times and seismic moments
Rise of great earthquakes during the 1960s and after December 2004 might be caused by a higher corner moment
SUMMARY
The spatial geometry of seismicity encodes information about loading and failure processes, as well as properties of the underlying fault structure. Traditional approaches to characterizing ...geometrical attributes of seismicity rely on assumed locations and geometry of fault surfaces, particularly at depth, where resolution is overall quite poor. In this study, we develop an alternative approach to quantifying geometrical properties of seismicity using techniques from anisotropic point process theory. Our approach does not require prior knowledge about the underlying fault properties. We characterize the geometrical attributes of 32 distinct seismicity regions in California and introduce a simple four class classification scheme that covers the range of geometrical properties observed. Most of the regions classified as having localized seismicity are within northern California, while nearly all of the regions classified as having distributed seismicity are within southern California. In addition, we find that roughly 1 out of 4 regions exhibit orthogonal seismicity structures. The results of this study provide a foundation for future analyses of geometrical properties of seismicity and new observables to compare with numerical modelling studies.
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