Roberto Quaas

Strong Ground Motion from the Michoacan, Mexico, Earthquake

The network of strong motion accelerographs in Mexico includes instruments that were installed, under an international cooperative research program, in sites selected for the high potenial of a large earthquake. The 19 September 1985 earthquake (magnitude 8.1) occurred in a seismic gap where an earthquake was expected. As a result, there is an excellent descripton of the ground motions that caused the disaster.

Is there truly a “hard” rock site in the Valley of Mexico?

To understand the cause of observed amplification of seismic waves even at hill-zone sites in the Valley of Mexico, digital accelerographs have been installed at three especially chosen sites. Two of these sites, MADI and TEXC, located on hard Pleistocene lava (a few meters in thickness) overlying Oligocene andesites, were expected to be free of site effects. Analysis of the data recorded by these and other accelerographs during three moderate, shallow subduction zone events, however, shows significant amplification at MADI and TEXC between 0.2 and 0.6 Hz with respect to established attenuation relations. The cause of the amplification at hill-zone sites in the Valley of Mexico, including MADI and TEXC, may be pervasive low S-wave velocity in, and complex structure of, the upper layers of the volcanic rocks. If so, then there may not be a truly “hard” rock site in the valley.

“Strong” Ground motions in North America from the Bolivia Earthquake of June 9, 1994 (Mw=8.3)

The Bolivia earthquake was remarkable for being felt in North America, as far as 8680 km from the epicenter. This paper examines the characteristics of ground motions in North America from the perspective of strong motion seismology. Data on ground motions is mostly from high-gain stations, but we also have two strong motion accelerograms. The largest accelerations and highest frequencies are associated with the P and PcP phases, which also correlate with the felt reports in several instances. Felt locations are associated with a combination of favorable factors. Most are in a distance range where P-wave amplitudes tend to be relatively high, as observed in the empirical correction factors for body wave magnitude, and in regions with relatively high values of Q in the lithosphere. In addition, site effects, structural amplification, or both, seem to contribute to the selective detection of the earthquake in the regions where some people noticed it. Motions were far below design spectra for earthquake resistant design, and far below levels likely to be associated with damage.