worsened by soil liquefaction which causes the loss of foundation support and contributes to dramatic settlement of large buildings. Mexico City's downtown area mostly lies on the silt and volcanic clay sediments of the bed of the historic Lake Texcoco, which are between seven and thirty-seven meters deep and have a high water content. Above this is a layer of sand and above this is a layer of sand and rock. The western and northwestern parts of the city are outside the old lakeshores and are located on sands from eroding volcanic cones that surround the Valley of Mexico. The southern part of the city rests on hardened basalt lava flows. The old lakebed, with its high water content, is easily moved or compressed. The old lakeshore area also has a fairly high water content, allowing movement, though not as much as the lakebed. The old lava flows have little water content or movement in comparison and are therefore more stable. Another factor is that the old lakebed resonates with certain seismic waves and low frequency signals. This lakebed has a natural "pitch" of one cycle every 2.5 seconds making everything built on the bed vibrate at the same frequency. This is the same "pitch" as a number of shallow earthquake waves. This resonance amplifies the effects of the shock waves coming from an earthquake far away. However, only certain types of structures are vulnerable to this resonance effect. Taller buildings have their own frequencies of vibration. Those that are six to fifteen stories tall also vibrate at the 2.5-second cycle, making them act like tuning forks in the event of an earthquake. The low-frequency waves of an earthquake are amplified by the mud of the lakebed, which in turn, is amplified by the building itself. This causes these buildings to shake more violently than the earthquake proper as the earthquake progresses. Many of the older colonial buildings have survived hundreds of years on the lakebed simply because they are not tall enough to be affected by the resonance effect. Earthquake The earthquake occurred in the Pacific Ocean, off the coast of the Mexican state of Michoacán, a distance of more than from the city, in the Cocos Plate subduction zone, specifically in a section of the fault line known as the Michoacán seismic gap. The Cocos Plate pushes against and slides under the North American Plate, primarily along the