After a magnitude 7.4 earthquake off Mexico's coast, what factors determine whether a tsunami warning is issued?

Quick answer

A tsunami warning is the strongest alert. It is issued when a tsunami that may cause widespread flooding is expected or is already happening, and people are told to move to high ground or inland [3][16][84]. A watch is lower: a tsunami is possible, but the danger level is not known yet [4][17][86]. An advisory is lower still: strong currents and dangerous waves are expected, but flooding of land is not expected [18][85].

In the alert system described by NOAA, there are four levels: Information Statement, Watch, Advisory, and Warning [60]. NOAA’s two Tsunami Warning Centers monitor for earthquakes that might cause tsunamis and issue these messages after a possible tsunami-generating event [2][13]. The key question after a major undersea quake is simple: was a tsunami actually generated [87]?

What officials check about the earthquake

• They look at the earthquake’s location, depth, and magnitude as the three key parameters [55][64].
• The earthquake must be a shallow marine event that displaces the seafloor [38][73]. Quakes deeper than about 60 km generally pose no tsunami threat [65].
• It must be big enough and close enough to the ocean floor to cause vertical movement of the seafloor [46]. Side-to-side movement is unlikely to cause a severe tsunami [94].
• The size and type of fault movement matter. Thrust earthquakes are far more likely to generate tsunamis than strike-slip earthquakes [39][74].
• The amount of seafloor uplift, the size of the area moved, the water depth at the source, and the source depth and fault geometry all affect tsunami size [47][66].
• Most destructive tsunamis come from large, shallow earthquakes with an epicenter or fault line near or on the ocean floor [54].
• Not every quake makes a tsunami [83][91]. About 80% of known tsunamis are triggered by earthquakes [48], but underwater landslides and volcanic eruptions can also create them [10][81][91]. An earthquake too small to make a tsunami directly can still trigger an underwater landslide that does [51].

What a 7.4 magnitude means

Magnitude is one factor, but it is not the whole story [37][72]. General NOAA/Pacific Tsunami Warning Center guidelines say:

• Below about 6.5: very unlikely to trigger a tsunami [40].
• 6.5 to 7.5: usually no destructive tsunami, though small sea-level changes can happen near the epicenter. Damaging tsunamis are rare but have occurred through landslides or submarine slumps [41][76].
• 7.6 to 7.8: may produce destructive tsunamis, especially near the epicenter [42][77].
• 7.9 and above: destructive local tsunamis are possible [43][78].

So a 7.4 quake sits in the “usually not destructive” range [41][76]. But there is a reason it gets close attention: it was once believed that an earthquake needed to reach about 7.4 to have a major impact on tsunami wave generation [95]. Larger quakes do not always create the largest waves [96], and a moderate quake in shallow water can sometimes produce a larger-than-expected tsunami [97]. On its own, 7.4 does not guarantee a warning [37][72].

How the warning decision is refined

The first tsunami message is based on seismic information plus the historical tsunami record for that area [61][63]. It uses conservative criteria because lives are at stake, while also trying to avoid needless evacuations [67][71]. Historical records help warning centers judge how likely an earthquake is to create a tsunami [44][49].

After the initial estimate, ocean data take over:

• If the earthquake meets certain criteria, warning centers use sea-level data to decide whether a tsunami exists and to refine or cancel their messages [56].
• Shore-based tide gauges and deep-ocean buoys with pressure sensors monitor the ocean continuously [23][24]. Visual detection is hard because tsunami waves stay low in the open ocean [28].
• Deep-ocean pressure sensors are the gold standard for sizing up waves, but they are expensive and sparse [89].
• Forecast models combine seismic data, sea-level data, and pre-set scenarios to estimate wave height, arrival times, coastal flooding, and event duration [57][59].
• Forecasts, historical tsunami data, and extra seismic analysis help warning centers adjust or cancel messages [58].
• Instruments that sense wave force send data into the warning system [25][26], but the readings must be interpreted by trained experts [92]. One extra good data point can be enough to support an evacuation call [90].

Location, coastline, and timing

The location and size of the earthquake, plus the shape of the local coastline, affect how bad the tsunami will be [7]. If the quake is very close to the coast, a tsunami can arrive within minutes [8][9][20]. A distant tectonic earthquake may allow four to ten hours of warning time [19]. The danger period can also continue for many hours after a major earthquake [11].

At the coast, natural warning signs include a strong or long earthquake, a sudden rise or fall of the ocean, or a loud roar from the ocean [6][14].

Mexico-specific context

Mexico’s tsunami hazard is classified as high: there is more than a 40% chance of a potentially damaging tsunami in the next 50 years [29], and coastal projects there are told to consider tsunami impacts [30]. But the available evidence does not describe a separate Mexico-only warning trigger. Instead, it says detailed local hazard assessments, inundation modeling, and expert input are needed to determine real coastal impacts [31][32][34][35].

So for an M7.4 off Mexico, the same general factors apply: depth, location, fault type, seafloor displacement, historical record, sea-level observations, and forecast models decide whether the message becomes a full warning [44][55][56][61][64].

Bottom line

A tsunami warning is not automatic after an M7.4 off Mexico. It would be issued if the quake is shallow, under the sea, big enough, and positioned to displace the seafloor in a way that creates a tsunami, and if ocean observations or forecast models show a tsunami that may cause widespread flooding [3][38][46][53][56][59]. If the evidence shows only a possible wave or strong currents, the alert could stay at a watch or advisory instead [4][18]. If there is no tsunami threat, the alert may be only an information statement [5].