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Tsunami 101: What You Should Know

In this article, we will review some of the basic science behind tsunamis and show you how to set up your phone to receive alerts when a warning is issued.

A Rinconvenient Truth spoke with Mr. Aurelio Mercado, a retired professor and physical oceanographer from the University of Puerto Rico, Mayagüez, to enlighten us about how a tsunami forms and what we should do to be ready.

Tsunami warning sign in San Juan; Lawrence Eaton

The word tsunami originates from the Japanese word for “harbor wave” - a shallow water wave that can form from any process that abruptly displaces a large quantity of water, not just earthquakes. Other processes can include an underwater landslide, or a above water landslide (ex. a cliffside falling into the ocean), a large scale underwater explosion (a volcano), an extraterrestrial object impacting the sea surface (i.e. meteor), and lastly a meteotsunami or meteorological tsunami. “These types of tsunami-like waves are generated when rapid changes in barometric pressure cause the displacement of a body of water”, says Mercado.

But if we focus on earthquakes alone, what do we need to know to be prepared? For example, in general a tsunami is not triggered unless the magnitude of the earthquake is 6.7 and above – but that is not always the case. Mercado warns that even “low magnitude earthquakes can destabilize accumulated sediments lying on a steep slope and, thus, produce a landslide tsunami”.

Types of forces that generate tsunamis, by Adam Switzer

Lately, much of the seismic activity has been occurring along the south coast – La Parguera, Guanica, Guayanilla – yet some of these earthquakes are felt on the west coast. Consequently, some residents even evacuated in response to the recent 6.4 earthquake in January. What is causing all this activity? A.R.T. asked geologist Dr. James Joyce for an explanation, The faulting and resulting earthquakes are related to differential movements within and around the island as a consequence of the impact of the westward movement of the North American Plate below northern Puerto Rico pulling the northern and central portions of the island more to the Northwest than the southwestern and southern coastal portions of the island.” (see Fault Map below).

But will earthquakes produced in the south that do trigger tsunamis have any impact on us living along the west and north coasts?

Mercado says it depends on the magnitude of the earthquake and the direction of the fault where it occurred. “The north coast is obviously more immune, but even a tsunami which does not produce any coastal flooding can produce coastally-trapped waves that can propagate to the north coast and generate dangerous currents, and maybe do damage to boats”, states Mercado.

Earthquake damage in Mayaguez after the 1918 San Fermin

We are not immune to seismic activity on the west coast. The proximity to the Puerto Rican trench, where active tectonic plate movement occurs, means that our region should be closely monitored for earthquakes and residents should know how to be prepared. “One of the most seismically active areas of the plate boundary is the western end of the Puerto Rico Trench where the plate boundary transitions through the northern Mona Canyon the eastern end of the Septentrional Fault,” says Joyce while cautioning residents that the northwest coast is the most susceptible to a tsunami threat.

Fault map with plate movements, by Dr. James Joyce

The last tsunami to hit the island occurred in 1918. It resulted from a 7.1 magnitude earthquake known as the San Fermín earthquake, with an epicenter in the Puerto Rican trench. The tsunami was estimated to have struck the island within 4-7 minutes after the earthquake. These natural disasters claimed over 100 lives, primarily in Mayagüez, with around 40 people drowning from the 15-19ft tsunami waves. “That wave was probably 5 meters at the Rincón lighthouse and covered the old train tracks where the road to the lighthouse is now” remarks Joyce.

How to Be Prepared

Mercado suggests that all citizens residing in floodable areas should take the most precaution. Know your evacuation route and have critical medicines and supplies ready to take with you. If have time to leave the area safety and swiftly, then proceed to evacuate to at least 33ft elevation – “If you think you won’t have the time, [then] do vertical evacuation”, he states, such as climbing to your roof or second story. Additionally, do not forget your pets! It’s important that you do not leave them behind if you’re able to quickly get them in the car with you. Do not return to your home until the tsunami warning has been canceled. Check your local radio station or some of the resources that we have listed below in the "How to Stay Alert" section of the article.

How to Stay Alert

In the aftermath of hurricane María, not every coastal town’s tsunami warning system is still active. Although recent infrastructure improvements have been occurring, it’s also best to know of other options to check when the time comes to question whether or not to evacuate.

Flood map of the Rincón area in the event of a tsunami

The Puerto Rico Seismic Network has a useful map to show every municipality where tsunami waves would cause the most flooding, and who needs to evacuate when a warning is issued.

Their Facebook page is a well-known source for the fastest, latest reports that are often issued ahead of the national warning system.

The Pacific Tsunami Warning Center is the authority that issues the final warning or advisory to Puerto Rico once they receive the seismic information transmitted from our gauges. They review the information and then alert Puerto Rico as to whether or not a warning is advisable.

There are many earthquake apps that can be downloaded for Android and Apple devices, yet most free options do not provide tsunami alerts to your phone. We reviewed a simple way to receive notifications via Twitter from the Pacific Tsunami Warning Center, and we confirmed it works.


Below are the directions to add this free notification system to your phone:

  1. To set up alerts on your phone, you will need an account on Twitter. After setting up an account, head over to Settings and Privacy.

  2. Click on Notifications, scroll down to Preferences and select Push Notifications.

  3. Enable Push Notifications. By enabling the notifications, it does not necessarily mean you will be bombarded by notifications from everyone you follow – you must now select those accounts in which to receive those notices.

  4. In the search bar on the Twitter home interface, type in @NWS_PTWC for the Pacific Tsunami Warning Center which is responsible for alerting Puerto Rico of a tsunami threat.

  5. Click on the button to “Follow” the account

  6. Finally, tap the little bell icon next to their name to indicate you want to receive push notifications from this account.

  7. You’re all set! Next time we have an earthquake here or in the area that is large enough to trigger an alert, you will receive a notification about a tsunami threat.


How to Stay Connected

In addition to setting up phone alerts regarding tsunamis, it is also important to consider how information can travel within your neighborhood during a need for evacuation. Consider creating a neighborhood text messaging group to share details for preparation and evacuation. However, for your neighbors who may not use social media or group texting, it may also be relevant to know how to assist your elderly neighbors in the event of an evacuation. This can be as simple as placing a call to their landline or cellphone, and then knocking on their door as you or your family leave the area.

Here are the steps to set up group messaging on Android and iPhone so you can create a message with your neighbors:


1. Create a group Message by first ensuring that your Group Messaging settings are enabled. Find details to do this here.

2. Then, open your Messaging app and tap the New Message icon to bring up the prompt to select contacts.

3. Choose the icon at the top right that indicates the option to select multiple people at once, then select the phone numbers of the people you want to be a part of the group message.

*Note that sense Android treats group messages as SMS messages unless MMS is enabled, so there is a chance that some of your contacts may not see the entire group message if they have an Android device without MMS enabled. If you are concerned about this, then check other group messaging options like WhatsApp.

4. Alternatively, you can set up a Group of Contacts to send group messages. You will still need to ensure that group messaging is enabled.


1. Select the iMessaging app. Start a new message by tapping the message icon.

2. In the “To” field, type the names of your contacts that you wish to add to your group message

3. Alternatively, you can also create a Group of Contacts using your computer and iCloud. You can then select that Group as the recipient of your message.


A Rinconvenient Truth thanks Mr. Aurelio Mercado and Dr. James Joyce for their insightful knowledge, commentary and graphics to assist the development of this story.

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