A tsunami is a powerful series of ocean waves generated by large-scale disturbances like undersea earthquakes, volcanic eruptions, or landslides that displace significant water. Unlike normal ocean waves, tsunamis have extremely long wavelengths and can travel across entire ocean basins. California’s active seismic zones and extensive coastline make it susceptible to tsunami threats.
Factors Influencing Tsunami Reach
The distance a tsunami travels inland, known as inundation, is influenced by several factors. The tsunami’s source, such as a large undersea earthquake, plays a significant role. Wave characteristics like height and energy also directly impact inland penetration. In the deep ocean, a tsunami wave has a small height but a very long wavelength, often hundreds of kilometers.
As a tsunami approaches shallower coastal waters, its speed decreases, but its height increases dramatically due to energy conservation. This can cause the wave to appear as a rapidly rising flood or a wall of water. Local coastal topography and bathymetry (the underwater depth and shape of the seafloor) also determine inland travel. Shallow bays, river mouths, or low-lying coastal plains can channel and amplify a tsunami’s energy, leading to greater inland penetration. Conversely, steep cliffs or higher elevations act as natural barriers, limiting the wave’s reach.
Mapping California’s Tsunami Zones
Official tsunami inundation maps for California’s coastlines provide information about potential inland reach. State agencies like the California Geological Survey (CGS) and the California Office of Emergency Services (CalOES) develop these maps. Their creation involves complex modeling that incorporates potential tsunami sources, wave characteristics, and coastal geography. Historical tsunami data also informs these models, helping predict worst-case scenarios.
Tsunami inland reach varies significantly along California’s diverse coastline. In some areas, inundation might extend only a few blocks from the shoreline. However, in low-lying river deltas or flat coastal plains, a tsunami could travel several miles inland. For example, a distant-source tsunami could result in water levels 32 feet above mean sea level at San Francisco’s Ocean Beach, while in Alameda, it might be 18 feet. These publicly available maps serve as tools for urban planning and emergency preparedness.
Preparing for a Tsunami
Recognizing natural warning signs is a first step in preparing for a tsunami. If you are near the coast and experience a strong earthquake that makes it difficult to stand, a sudden ocean recession, or hear a loud, roaring sound, these indicate a tsunami may be approaching.
In such instances, move immediately to higher ground without waiting for an official warning. Tsunamis can arrive within minutes of a nearby earthquake, leaving little time for alerts.
Official warnings are broadcast through channels, including Wireless Emergency Alerts (WEA) on mobile phones, local radio and television, and outdoor sirens. Upon receiving an official warning or observing natural signs, evacuate to higher ground, ideally at least 100 feet above sea level or one mile inland.
Evacuation should be done on foot, as roads may become impassable due to damage or traffic congestion. Having an emergency kit readily available, containing essentials like food, water, and first-aid supplies for several days, is also recommended. Knowing your local tsunami evacuation zones and having a family emergency plan, including a designated meeting point and an out-of-state contact, can help ensure safety and facilitate reunification.