Elastic models of seismic failure in the Valley of Mexico

Restricted parts of the Valley of Mexico have been plagued by swarms of small earthquakes during recent years. The cause of these events is puzzling, but they may relate to massive artificial changes in the hydrology of the lake-bed which underlies most of Mexico City. If the valley responds elastically, plane strain finite element calculations can be used to relate geologic and seismic structure to observed seismicity and overall stability of the valley. These calculations show that unstable zones and areas of local seismicity coincide if the triggering mechanism of the seismicity is related to variations of pore pressure and water load.In order to make such a connection it is necessary to ascribe the generating mechanism of the seismicity to the density and elastic modulus anomalies associated with the valley structure, and to suggest that the phenomenon that triggers this seismicity is related to changes in water content of the near surface formations.     

Forecasting Wave Amplitudes after the Arrival of a Tsunami

The destructive Pacific Ocean tsunami generated off the east coast of Honshu, Japan, on 11 March 2011 prompted the West Coast and Alaska Tsunami Warning Center (WCATWC) to issue a tsunami warning and advisory for the coastal regions of Alaska, British Columbia, Washington, Oregon, and California. Estimating the length of time the warning or advisory would remain in effect proved difficult. To address this problem, the WCATWC developed a technique to estimate the amplitude decay of a tsunami recorded at tide stations within the Warning Center’s Area of Responsibly (AOR). At many sites along the West Coast of North America, the tsunami wave amplitudes will decay exponentially following the arrival of the maximum wave (Mofjeld et al., Nat Hazards 22:71–89, 2000). To estimate the time it will take before wave amplitudes drop to safe levels, the real-time tide gauge data are filtered to remove the effects of tidal variations. The analytic envelope is computed and a 2 h sequence of amplitude values following the tsunami peak is used to obtain a least squares fit to an exponential function. This yields a decay curve which is then combined with an average West Coast decay function to provide an initial tsunami amplitude-duration forecast. This information may then be provided to emergency managers to assist with response planning.     

Stochastic finite element calculation of seismic risk

The probable locations and the times of occurrence of earthquakes can be suggested if a reasonably accurate finite element model of the area is available. This can be done by determining the unstable regions of the model and studying their evolution in time. Instability of a particular area depends on how close such a region is to a limiting strength and this closeness can be interpreted statistically.     

Theory for the rapid solution of ground subsidence near reservoirs on layered and porous media

The use of two-dimensional fourier transforms of reservoir topography allows substantial increases in the speed of computation of loading effects on layered elastic models of the earth and on porous fluid-filled models of the earth. It is possible to solve the compaction equations for arbitrary reservoirs filled at an arbitrary rate in time. The delay involved in the associated diffusion of pressure can be computed and the theory allows for the computation of the effective stress needed to assess seismic risk associated with the filling of large artificial lakes. A numeric example is given for the vertical deflection and the pore pressure as functions of time and depth below a two-dimensional cross-section of a typical lake profile.     

Bursts of seismicity near the Cocos-North American-Caribbean triple junction

Three previously proposed long-term premonitory patterns, pattern Sigma, pattern Swarms, and ‘bursts of aftershocks’, appear to be moderately successfull for large (M 7.3) earthquakes which have occurred in the vicinity of the North American-Caribbean-Cocos triple junction between 1930 and July 1980.