John McCloskey

Earthquake risk on the Sunda trench

On 28 March 2005 the Sunda megathrust in Indonesia ruptured again, producing another great earthquake three months after the previous one. The rupture was contiguous with that of the December 2004 Sumatra–Andaman earthquake, and is likely to have been sparked by local stress, although the triggering stresses at its hypocentre were very small — of the order of just 0.1 bar. Calculations show that stresses imposed by the second rupture have brought closer to failure the megathrust immediately to the south, under the Batu and Mentawai islands, and have expanded the area of increased stress on the Sumatra fault. Palaeoseismologic studies show that the Mentawai segment of the Sunda megathrust is well advanced in its seismic cycle and is therefore a good candidate for triggered failure.

Seismology: Earthquake risk on the Sunda trench

On 28 March 2005 the Sunda megathrust in Indonesia ruptured again, producing another great earthquake three months after the previous one. The rupture was contiguous with that of the December 2004 Sumatra–Andaman earthquake, and is likely to have been sparked by local stress, although the triggering stresses at its hypocentre were very small — of the order of just 0.1 bar. Calculations show that stresses imposed by the second rupture have brought closer to failure the megathrust immediately to the south, under the Batu and Mentawai islands, and have expanded the area of increased stress on the Sumatra fault. Palaeoseismologic studies show that the Mentawai segment of the Sunda megathrust is well advanced in its seismic cycle and is therefore a good candidate for triggered failure.

Structural constraints on the spatial distribution of aftershocks

[1] Calculations of static stress changes due to large earthquakes have shown that the spatial distribution of aftershocks is predictable to first order, with aftershocks primarily occurring in areas experiencing positive stress changes. Delineation of these areas relies on resolving the stress perturbation onto planes with known orientations; common practice is to use poorly constrained regional stress information to compute optimally oriented failure planes, assuming that they exist everywhere. Here we show that this assumption is not supported by observation but rather that aftershock failure planes are controlled by geological structure. We argue that useful aftershock hazard estimates are better made by replacing information on regional stress with statistical measures of structural orientations.

Stress accumulation and increased seismic risk in eastern Turkey

Abstract Unlike the North Anatolian fault zone, which has produced 11 large earthquakes since 1939, the East Anatolian fault zone (EAFZ) has been relatively quiescent in the last century when compared to historical records and has therefore accumulated significant stresses along its length. Determination of the location and likely magnitude of a future probable earthquake along the EAFZ is of interest both because of this history of large earthquakes, (M≈8), and the density of population in the area. Here we calculate stress evolution along the fault zone due to both seismic and tectonic loading since 1822. A sequence of 10 well constrained historical earthquakes is selected and the resulting stresses are calculated, summed with tectonic loading stresses and resolved onto the mapped active faults. We identify two areas of particular seismic risk, one of which might be expected to yield a large event. Our results are sensitive to the previous history of large earthquakes in the region and indicate a need for detailed investigations to constrain the exact rupture geometries of previous earthquakes on these segments.

Observation of diffusion processes in earthquake populations and implications for the predictability of seismicity systems

Scale invariance, either in space or in time, has been shown in many papers to characterize earthquake distributions. Unfortunately, little work has been dedicated to looking at the general space-time scaling invariance of seismicity systems, even though a better understanding of how the two domains (spatial and temporal) link together could help the development of the stochastic dynamical modeling of earthquake populations. In this paper we report the observation of diffusion processes of temporally correlated seismic activity for three different data sets: a mine (Creighton Mine, Canada), the Long Valley Caldera in eastern California, and a 7-year period of recorded seismic activity in southern California. The observed subdiffusion processes are indicative of the general space-time scaling of the system, taking the form of a slow power law growth R(t) similar to t(H) of the mean distance R(t) between the main event arid the temporally correlated afterevents occuring after a delay t. H is found on average to be small (0.1 for Creighton Mine, 0.22 for the Long Valley Caldera, and 0.22 for the southern California main events with magnitude greater than or equal to 1.5) but fluctuates significantly from one main event to the other: the diffusion is found to be intermittent (non-Gaussian) and multiscaling, and except for the Long Valley Caldera, a systematic correlation between the sizes of the main event and subsequent afterevents and the growth exponent H is observed. While classical viscous relaxation models (e.g., elastic listhosphere-plastic asthenosphere coupling, or fluid flow triggered by sudden changes in pore pressure) have been proposed to characterize this relaxation by homogeneous (i.e., nonintermittent) normal (H = 0.5) diffusion processes, the direct implication of the reported results is that seismicity systems, at spatial scales from meters to hundreds of kilometers and small (microearthquakes in a mine) to intermediate magnitudes, relax spatiotemporally in a nonelastic way, revealing the stochastic space-time scale-invariant nature of such systems. Since these diffusion processes correspond to a loss of information with time on the location of the main event, they can be used to investigate the limits of predictability, at all spatial scales, of seismicity systems in terms of the spatiotemporal clustering of temporally correlated earthquakes.

Permeability porosity relationships from numerical simulations of fluid flow

Numerical calculations of permeability are obtained from a lattice Boltzmann simulation of flow in simplified 2D porous media over a range of solid fractions. The evolution equation governing the flow behaviour incorporates the effect of porous medium geometry through the definition of solid density ns, a real number, at each node of the simulation grid. The results obtained for homogeneous media are compared to commonly used theoretical and empirical relationships relating rock properties to permeability. Behaviour consistent with a Kozeny-Carman type relationship between porosity ϕ and permeability k is obtained for low to intermediate solid fractions. At high solid fractions the rapid decrease in k is consistent with a percolation process giving a power-law relationship for ϕ and k. Both the critical porosity and power-law exponent are in agreement with quoted values for the lattice geometry used. A comparison of the results for homogeneous media with k values, obtained by embedding a spanning planar fracture into the matrix, illustrates the importance of matrix-fracture flow interactions. The results for this case are consistent with experimental observations and illustrate the difficulties involved in using simplified assumptions to predict permeability from porosity in fractured porous rock.

Statistical evaluation of characteristic earthquakes in the frequency-magnitude distributions of Sumatra and other subduction zone regions

[1] If subduction zone earthquakes conform to a characteristic model, in which persistent segments fail at predictable stress levels due to the steady accumulation of tectonic loading, historical seismicity may constrain the occurrence of future events. We test this model for earthquakes on the Sumatra-Andaman megathrust and other subduction zones using frequency-magnitude distributions. Using simulations, we show that Poisson confidence intervals correctly account for the counting errors of histogram data. These confidence intervals demonstrate that we cannot reject the Gutenberg-Richter distribution in favor of a characteristic model in any of the real catalogues tested. A visual bias in power-law count data at high magnitudes, combined with a sample bias for large earthquakes, is sufficient to explain candidate characteristic events. This result implies that historical earthquakes are likely poor models for future events and that Monte Carlo simulations will provide a better assessment of earthquake and associated hazards.

Preliminary results from a field investigation of aeolian sand transport using high resolution wind and transport measurements

Preliminary results are presented from two field experiments which employed a new high-sampling-frequency aeolian sand trap together with a single anemometer and wind vane. This system allows simultaneous assessment of wind velocity and sand transport rate every second. Results indicate an almost instantaneous response of transport to wind accelerations which is modified by a short (<6s) memory. The wind velocity/transport relationship is found to be well constrained by a second order polynomial in velocity with a threshold. Transport predictions based on this equation give a high correlation with observation; discrepancies possibly being due to differences in unquantified environmental variables between the two experiments. It is suggested that this approach could be improved to include the effect of other environmental variables, and that useful predictions of transport rate might then be made from existing meteorological databases.

Heterogeneity in a self-organized critical earthquake model

Earthquake dynamics are believed to exhibit self-organized criticality. This belief results from the power-law magnitude frequency distributions of earthquake catalogues, distributions which are accurately reproduced by cellular automata, and from the occurrence of triggered earthquakes. This paper examines the effects of heterogeneity on self-organized criticality in a two-dimensional cellular automaton. The strength heterogeneity is distributed fractally; stress is incremented uniformly. The model produces power-law magnitude frequency distributions. For fractal dimensions above 1.9, the slope of the power-law decreases with increasing fractal dimension. The slope increases weakly with the range of heterogeneity.

Heterogeneity and the earthquake magnitude‐frequency distribution

Regional earthquake populations are widely observed to have power-law magnitude-frequency statistics. Whether earthquake distributions on individual faults are similarly power-law is presently the subject of considerable research effort. Recent careful observations suggest that event statistics on rough faults are power-law whereas earthquake distributions on smooth faults are better described by a characteristic model in which there are more large earthquakes than would be expected by extrapolation from the distribution of small events. Here the relation between heterogeneity and the magnitude-frequency distribution is investigated in a cellular automaton that includes new nearest-neighbor stress transfer rules which produce realistic stress concentrations. The results agree with the suggestion that smooth faults have more characteristic earthquake distributions and support the view that an earthquakes size is controlled by rupture arrest.