A tsunami is a giant wave set in motion by a large earthquake occurring under or near the ocean that causes the ocean floor to shift vertically. This vertical shift pushes the water ahead of it, starting a tsunami. These are very long waves (100 to 200 miles [161 to 322 kilometers]) with high speeds (500 mph [805 kph]) that, when approaching shallow water, can grow into a 100-foot (30.5-meter) high wave as its wavelength is reduced abruptly. Ocean earthquakes below a magnitude of 6.5 on the Richter scale, and those that shift the sea floor only horizontally, do not produce these destructive waves. The highest recorded tsunami was 1,719 feet (524 meters) high along Lituya Bay, Alaska, on July 9, 1958. Caused by a giant landslip, it moved at 100 miles (161 kilometers) per hour. This wave would have swamped the Petronas Towers in Kuala Lumpur, Malaysia, which are 1,483 feet (452 meters) high.