But this time it was 2 to 4 metres, which is extraordinary. Japan's Meteorological Agency has now upgraded the intensity of the quake in most areas of Kobe from 6 to 7 - the maximum on Japan's shindo scale, which grades intensity of shaking rather than the overall energy of the quake. The maximum figure of 7 on the shindo scale was only added in 1947, after a strong earthquake hit the town of Fukui, and registered what was then a maximum of 6. The lateral shaking may have been amplified by a phenomenon noted during the Mexico City earthquake in 1985, in which seismic waves bounce off firmer rock below, and intensify the shaking at ground level. They are slower, but just as powerful as the initial waves.
Structures can suffer devastating damage when they're hit by both sets of waves.
Kobe lent itself to this phenomenon because it sits on soft ground made up of debris eroded from the mountain chain behind it. The initial seismic waves echoed off the solid mountains and increased the duration of lateral shaking on the soft ground. Mexican seismologists have been comparing notes with their Japanese colleagues. But while seismic echoes did occur in Mexico City, which sits in a crater ringed by mountains, Japanese seismologists say the recent Kobequake provided the clearest example yet of the phenomenon. The unexpected intensity of the quake may prove a problem for Japanese engineers.