The El Salvador earthquakes of January and February 2001: context, characteristics and implications for seismic risk

The small Central American republic of El Salvador has experienced, on average, one destructive earthquake per decade during the last hundred years. The latest events occurred on 13 January and 13 February 2001, with magnitudes M_w 7.7 and 6.6, respectively. The two events, which were of different tectonic origin, follow the patterns of the seismicity of the region although neither event has a known precedent in the earthquake catalogue in terms of size and location. The earthquakes caused damage to thousands of traditionally built houses and triggered hundreds of landslides, which were the main causes of fatalities. The earthquakes have clearly demonstrated trends of increasing seismic risk in El Salvador due to rapid population expansion in areas of high shaking and landslide hazard, exacerbated by deforestation and uncontrolled urbanisation. The institutional mechanisms required for the control of land use and building practice are very weak and present a major obstacle to risk mitigation.     

Mathematical modeling of PCB bioaccumulation in Perna viridis

In the present work, we built a mathematical model of polychlorinated biphenyl (PCB) bioaccumulation in Perna viridis, namely, a one-compartment model with a time dependent incorporation rate R (μg g⁻¹ lipid per ppb water per day), with positive substrate cooperativity as the underlying physical mechanism. The temporal change of the PCB concentration Q (μg g⁻¹ lipid) in the soft tissues of the mussel depends on the competition of the input rate RW and the output rate kQ, where W is the concentration of PCB in water (ppb water) and k is the elimination rate (per day). From our experimental data, k=0.181±0.017 d⁻¹. The critical concentration in water W_c for positive substrate cooperativity was found to be 2.4 ppb. Below W_c, R is a constant. For a water concentration of 0.5 ppb Aroclor 1254, R=24.0±2.4 μg g⁻¹ lipid ppb⁻¹ d⁻¹. Above W_c, positive substrate cooperativity comes into effect and R becomes a function of time and dependent on the concentration Q in a form R=γQ/(Q+δ). This is the case for a water concentration of 5 ppb Aroclor 1254, where γ=15.1 μg g⁻¹ lipid ppb⁻¹ d¹ and δ≈200 μg g⁻¹ lipid. From this model, the uptake is exponentially increasing when the PCB concentration in the mussel is small compared to 200 μg g⁻¹ lipid, and hyperbolically increasing when the concentration is large compared to 200 μg g⁻¹ lipid, which are consistent with the experimental data. The model is useful for understanding the true processes taking place during the bioaccumulation and for risk assessment with higher confidence. Future experimental data which challenge the present model are anticipated and in fact desirable for improvement and perfection of the model.     

Reconciling pyroclastic flow and surge: the multiphase physics of pyroclastic density currents

Two end-member types of pyroclastic density current are commonly recognized: pyroclastic surges are dilute currents in which particles are carried in turbulent suspension and pyroclastic flows are highly concentrated flows. We provide scaling relations that unify these end-members and derive a segregation mechanism into basal concentrated flow and overriding dilute cloud based on the Stokes number (S_T), the stability factor (Σ_T) and the dense-dilute condition (D_D). We recognize five types of particle behaviors within a fluid eddy as a function of S_T and Σ_T: (1) particles sediment from the eddy, (2) particles are preferentially settled out during the downward motion of the eddy, but can be carried during its upward motion, (3) particles concentrate on the periphery of the eddy, (4) particles settling can be delayed or ‘fast-tracked’ as a function of the eddy spatial distribution, and (5) particles remain homogeneously distributed within the eddy. We extend these concepts to a fully turbulent flow by using a prototype of kinetic energy distribution within a full eddy spectrum and demonstrate that the presence of different particle sizes leads to the density stratification of the current. This stratification may favor particle interactions in the basal part of the flow and D_D determines whether the flow is dense or dilute. Using only intrinsic characteristics of the current, our model explains the discontinuous features between pyroclastic flows and surges while conserving the concept of a continuous spectrum of density currents.     

Effective stress analyses of liquefaction-induced deformation in river dikes

A series of effective stress analyses is carried out on the seismic performance of river dikes based on the case histories during the 1993 Hokkaido-Nansei-oki and 1995 Hyogoken-Nambu earthquakes in Japan. Seven case histories selected for the analyses involve a crest settlement ranging from none to 2.7 m in the dikes 3–6 m high with evidence of liquefaction at foundation soil. The effective stress model used is based on a multiple shear mechanism and was developed by one of the authors. The soil parameters are evaluated based on the site investigation and laboratory test results. The results of the analyses are basically consistent with the observed performance of the river dikes. In particular, the effective stress model shows a reasonable capability to reproduce the varying degree of settlements depending on the geotechnical conditions of foundation soils beneath the dikes. The analyses also indicate that the effect of a cohesive soil layer mixed with the liquefiable sand layers beneath the dikes can be a primary factor for reducing the liquefaction-induced deformation of dikes.     

A rifted inside corner massif on the Mid-Atlantic Ridge at 5°S

The structure of the Mid-Atlantic Ridge at 5°S was investigated during a recent cruise with the FS Meteor. A major dextral transform fault (hereafter the 5°S FZ) offsets the ridge left-laterally by 80 km. Just south of the transform and to the west of the median valley, the inside corner (IC – the region bounded by the ridge and the active transform) is marked by a major massif, characterized by a corrugated upper surface. Fossil IC massifs can also be identified further to the west. Unusually, a massif almost as high as the IC massif also characterizes the outside corner (OC) south of the inactive fracture zone and to the east of the median valley. This OC massif has axis-parallel dimensions identical to the IC massif and both are bounded on their sides closest to the spreading axis by abrupt, steep slopes. An axial volcanic ridge is well developed in the median valley both south of the IC/OC massifs and in an abandoned rift valley to the east of the OC massif, but is absent along the new ridge-axis segment between the IC and OC massifs. Wide-angle seismic data show that between the massifs, the crust of the median valley thins markedly towards the FZ. These observations are consistent with the formation of the OC massif by the rifting of an IC core complex and the development of a new spreading centre between the IC and OC massifs. The split IC massif presents an opportunity to study the internal structure of the footwall of a detachment fault, from the corrugated fault surface to deeper beneath the fault, without recourse to drilling. Preliminary dredging recovered gabbros from the scarp slope of the rifted IC massif, and serpentinites and gabbros from the intersection of this scarp with the corrugated surface. This is compatible with a concentration of serpentinites along the detachment surface, even where the massif internally is largely plutonic in nature.     

Efficient methods for solving water flow in variably saturated soils under prescribed flux infiltration

The non-linear solvers in numerical solutions of water flow in variably saturated soils are prone to convergence difficulties. Many aspects can give rise to such difficulties and in this paper we address the gravity term and the prescribed-flux boundary in the Picard iteration. The problem of the gravity term in the Picard iteration is iteration-to-iteration oscillation as the gravity term is treated, by analogy with the time-step advance technique, ‘explicitly’ in the iteration. The proposed method for the gravity term is an improvement of the ‘implicit’ approach of Zhang and Ewen [Water Resour. Res. 36 (2000) 2777] by extending it to heterogeneous soil and approximating the inter-nodal hydraulic conductivity in the diffusive term and the gravity term with the same scheme. The prescribed-flux boundary in traditional methods also gives rise to iteration-to-iteration oscillation because there is no feedback to the flux in the solution at the new iteration. To reduce such oscillation, a new method is proposed to provide such a feedback to the flux. Comparison with traditional Picard and Newton iteration methods for a wide range of problems show that a combination of these two proposed methods greatly improves the stability and consequently the computational efficiency, making the use of small time step and/or under-relaxation solely for convergence unnecessary.     

Combined GPS and InSAR Models of Postseismic Deformation from the Northridge Earthquake

--Models of combined Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (InSAR) data collected in the region of the Northridge earthquake indicate that significant afterslip on the main fault occurred following the earthquake. Additional shallow deformation occurred to the west of the main rupture plane. Both data sets are consistent with logarithmic time-dependent behavior following the earthquake indicative of afterslip rather than postseismic relaxation. Aftershocks account for only about 10% of the postseismic motion. The two data sets are complimentary in determining the postseismic processes. Fault afterslip and shallow deformation dominate the deformation field in the two years following the earthquake. Lower crustal deformation may play an important role later in the earthquake cycle.     

Identification of Mining Blasts at Mid- to Far-regional Distances Using Low Frequency Seismic Signals

—?This paper reports results from two recent monitoring experiments in Wyoming. Broadband seismic recordings of kiloton class delay-fired cast blasts and instantaneous calibration shots in the Black Thunder coal mine were made at four azimuths at ranges from 1° to 2°. The primary focus of this experiment was to observe and to explain low-frequency signals that can be seen at all azimuths and should routinely propagate above noise to mid-regional distances where most events will be recorded by International Monitoring System (IMS) stations.¶The recordings clearly demonstrate that large millisecond delay-fired cast blasts routinely produce seismic signals that have significant spectral modulations below 10?Hz. These modulations are independent of time, the azimuth from the source and the orientation of the sensor. Low-frequency modulations below 5?Hz are seen beyond 9°. The modulations are not due to resonance as they are not produced by the calibration shots. Linear elastic modeling of the blasts that is guided by mine-blast reports fails to reproduce the fine detail of these modulations but clearly indicates that the enhanced “spectral roughness” is due to long interrow delays and source finiteness. The mismatch between the data and the synthetics is likely due to source processes, such as nonlinear interactions between shots, that are poorly understood and to other effects, such as variations of shot time and yield from planned values, that are known to be omnipresent but cannot be described accurately. A variant of the Automated Time-Frequency Discriminant (Hedlin, 1998b), which uses low-frequency spectral modulations, effectively separates these events from the calibration shots.¶The experiment also provided evidence that kiloton class cast blasts consistently yield energetic 2–10 second surface waves. The surface waves are strongly dependent on azimuth but are seen beyond 9°. Physical modeling of these events indicates that the surface waves are due mainly to the extended source duration and to a lesser extent to the slap-down of spalled material. The directionality is largely a path effect. A discriminant that is based on the partitioning of energy between surface and body waves routinely separates these events from the calibration shots.¶The Powder River Basin has essentially no natural seismic activity. How these mining events compare to earthquake observations remains to be determined.     

Particle recycling in volcanic plumes

We have developed a new theoretical model of an eruption column that accounts for the re-entrainment of particles as they fall out of the laterally spreading umbrella cloud. The model illustrates how the mass flux of particles in the plume may increase with height in the plume, by a factor as large as 2.5 because of this recycling. Three important consequences are that (1) the critical velocity required to generate a buoyant eruption column for a given mass flux increases, (2) the total height of rise of the column may decrease, and (3) we infer that in relatively wind-free environments, for eruption columns near the conditions for collapse, the recycling of particles may lead to an unsteady oscillating motion of the plume, which, in time, may lead to the formation of interleaved fall and flow deposits.