Late resonant response at Texcoco, Valley of Mexico, during distant earthquakes

Recordings of the ground motion induced by two shallow (15–25 km deep), distant (300 and 605 km) earthquakes made on deep, soft lacustrine sediments at Texcoco, Valley of Mexico, show a late monochromatic response at 0.48 Hz. Data from a strong-motion recorder array show that this late response is consistent with slow (60 m/s group velocity) Rayleigh waves generated near the 6 km distant soft/stiff soil interface of the ex-lake surface margin. It is concluded that the excitation of local Rayleigh waves in soft soil deposits by arriving earthquake ground motion provides one mechanism to explain the prolonged duration of resonant motion on soft soils, and hence the extreme damage often associated with soft soils responding to distant earthquakes.     

The limits of self-similarity

Numerical experiments on simulating ruptures of a one-dimensional plate boundary suggest that certain basic features of seismicity (including multiple ruptures and aftershock sequences) can be adequately reproduced if one assumes the existence of an elementary particle (or geon) of fault dynamics. Some general consequences of corpuscular assumptions in the earth sciences are discussed, with particular reference to the estimation of hazard.