What is tsunami? A tsunami is a series of ocean waves that can cause dangerous coastal flooding and other damage. Tsunamis are usually caused by a severe earthquake or landslide that displaces a large amount of ocean water. Tsunami waves can last for many hours; the first wave is almost never the largest. Tsunamis can also cause strong, unpredictable currents off beaches and in marinas and harbors, making it dangerous to be in those areas. The National Tsunami Warning Center (NTWC) issues four general types of tsunami bulletins: