Earthquakes occur when two tectonic plates suddenly slip past one another. This releases energy in the form of seismic waves that radiate out from the point where the rupture occurred. The seismic waves cause shaking and rolling motions as they travel through the earth’s crust. But what happens if an earthquake occurs underneath the ocean? Can the shaking still be felt if you are out at sea?
Can you feel an earthquake on a boat?
Yes, it is possible to feel an earthquake while on a boat at sea. The seismic waves generated by an underwater earthquake will still propagate through the water and can produce noticeable shaking and rolling motions of boats floating on the surface. However, the effect may be diminished compared to on land.
There are a few reasons for this:
- The water helps dampen the shaking to some degree. Water is less rigid than solid ground, so some of the earthquake energy gets absorbed by the water as the waves pass through.
- Being on a boat, you are not directly coupled to the shaking earth. On land, your feet are planted firmly on the ground so you feel the full force. On a boat, there is some isolation from the shaking thanks to the buoyancy of the vessel.
- The wavelength of seismic waves gets shortened in water compared to the ground. Shorter wavelengths mean quicker oscillations and less rolling motion.
So while the shaking may be less intense on a boat, it can still be felt. The boat will sway and rock as the seismic waves pass through the water. This motion can range from subtle to severe depending on factors like the earthquake’s magnitude, distance from the epicenter, and your location in the water.
What factors determine if a seaquake can be felt?
There are a few key factors that determine whether an earthquake occurring at sea will produce noticeable shaking on boats or coastal land:
- Magnitude – More intense earthquakes naturally produce stronger seismic waves and ground shaking. Larger magnitude seaquakes are more likely to be felt across a wider area.
- Distance from the epicenter – The closer you are to the underwater point where fault slippage occurred, the stronger the shaking will be. Effects diminish with distance.
- Depth – Shallower quakes typically produce more intense shaking at the seafloor surface. The seismic waves have less distance to travel through the crust, so lose less energy.
- Seafloor composition – The types of sediment and rock that seismic waves pass through impacts wave propagation. Harder seafloor rock transmits shaking more efficiently.
- Coastal topography – The shape of the coastline and offshore bathymetry can focus or defocus incoming seismic waves, altering shaking levels in coastal zones.
In general, seaquakes with magnitudes above 5.0 occurring within a few hundred kilometers stand the best chance of producing noticeable shaking on ships or land. But lower magnitude events can sometimes be felt as well if all the factors line up right.
What does it feel like to experience a seaquake?
Those who have been through a significant earthquake at sea describe an unnerving experience. Here are some of the most common sensations reported by mariners:
- A foreboding rumbling noise coming from underwater as the seismic waves approach.
- The boat begins gently rocking back and forth, steadily increasing in intensity, as the first P-waves arrive.
- Rolling from side to side as the stronger S-waves and surface waves hit.
- Sudden jolts and jerking motions as the waves pass through.
- Strange swirling of surface currents and frothy water as wave energy is transmitted.
- A general feeling of seasickness from the rocking and rolling.
- Difficulty standing or walking steadily on deck during the heaviest shaking.
- Hearing rigging, masts, and other hardware clatter and rattle.
The actual motions felt can vary depending on the boat type and where you are positioned. Smaller boats will tend to rock more intensely compared to massive ocean liners with their greater inertia and dampening effects. Additionally, being deep below deck provides some insulation from the motions compared to being topside.
Can tsunamis be triggered by seaquakes?
Yes, one of the biggest dangers posed by large underwater earthquakes is the potential to generate devastating tsunamis. Tsunamis are essentially massive waves triggered by sudden seafloor displacement during an earthquake. If the slippage along the fault plane is vertical enough, it pushes up or drops down large regions of water to create wave energy.
The long wavelength of these wave trains means they can propagate over huge distances with little loss of power. As they approach coastal areas, the waves slow down and stack up, growing enormously in height. The resulting tsunami waves can be over 100 feet high as they crash into shorelines, causing catastrophic flooding and destruction.
Some of the most destructive tsunamis in history were generated from massive earthquakes occurring in subduction zones offshore. This includes disasters like:
- The 2004 Indian Ocean tsunami triggered by a M9.1 quake off Sumatra, which killed over 200,000 people.
- The 2011 Tohoku tsunami in Japan, generated by a M9.0 quake, causing over 15,000 deaths.
- The 1964 Alaska tsunami from the M9.2 Good Friday earthquake, which struck the West Coast of North America.
Tsunamis are a ever-present hazard for coastal areas located near offshore fault zones. Advance warning systems have been developed to detect and warn of approaching tsunamis after seaquakes occur.
How are seaquakes detected and measured?
There are several ways that earthquakes originating under the ocean are detected and analyzed:
- Seismic monitoring stations – Networks of land-based seismographs and seafloor sensors can pick up seismic waves from offshore events. Data on the timing and size of the waves is used to pinpoint the epicenter location and estimate the magnitude.
- Tsunami monitoring buoys – These floating detection stations dispersed across the oceans can sense the passage of tsunami waves. The timing between buoys helps track direction and speed.
- Satellite altimetry – Spaceborne radar can measure tiny changes in sea surface height associated with tsunamis, helping track their propagation.
- Modeling software – Advanced computer models account for factors like seafloor geometry and wave dynamics to forecast tsunami arrival times and heights at specific coastal locations.
Integrating data from seismic networks, tsunami buoys, satellites, and predictive models enables timely and accurate hazard assessment following major underwater quakes. Warnings can then be issued to trigger evacuation procedures if needed.
Challenges detecting seaquakes
There are some unique challenges when it comes to detecting and analyzing earthquakes that occur at sea:
- Seismic waves get distorted and dampened traveling through water, making quake locations harder to constrain.
- The lack of seismometers directly at the rupture zone means some signal info is lost.
- Underwater topography and seafloor composition are often less well mapped compared to onshore, complicating modeling.
- Unusual ocean currents can sometimes mimic tsunami waves on detection buoys, causing false alerts.
But continued improvements in seafloor mapping and increased numbers of offshore seismic stations are helping better capture data on seaquakes. Advanced real-time wave gliders also promise to collect valuable in situ measurements.
Famous examples of notable seaquakes
Some of the most significant and damaging earthquakes in recorded history occurred in ocean settings. Here are a few notable examples:
2004 Indian Ocean Earthquake & Tsunami
- M9.1 megaquake off the coast of Sumatra.
- Triggered devastating tsunamis throughout the Indian Ocean basin.
- Over 227,000 fatalities making it one of the deadliest natural disasters ever.
2011 Tohoku Earthquake & Tsunami
- M9.0 off the Pacific coast of Japan.
- Generated tsunami waves over 130 feet high.
- Caused Fukushima nuclear disaster and over 15,000 deaths.
1906 Ecuador-Colombia Earthquake
- Estimated M8.8 quake occurred along the plate boundary offshore South America.
- Generated a destructive trans-Pacific tsunami hitting as far as California.
- Total casualties unknown but likely over 1,000 deaths.
1946 Aleutian Islands Earthquake
- M8.6 quake in the oceanic trench offshore Alaska.
- Produced a local tsunami wave reaching 109 feet in height.
- In total 165 people were killed, mostly in Hawaii.
These examples illustrate the immense hazard posed by large seaquakes and resulting tsunamis. Developing resilience requires robust warning systems, preparedness plans, and public education.
Conclusion
In summary, earthquakes originating underwater can indeed be felt out at sea, but the effects are typically diminished compared to being on land. The shaking sensation on boats ranges from subtle rocking to severe rolling depending on factors like magnitude, distance, and seafloor composition. Larger seaquakes can pose a major tsunami threat as well, capable of causing catastrophic coastal inundation over great distances. A variety of monitoring networks and modeling tools track seaquake activity and tsunami potential. While advances continue to improve detection capabilities, underwater quakes still represent one of the most destructive natural hazards facing coastal regions worldwide.