WESTVALLEYFAULT.COM

SPECIAL SERIES: EL NIÑO AND THE BIG ONE — PART 1 OF 5

How El Niño could change the conditions in which the Philippines faces its next major earthquake.

As the Philippines prepares for an intensifying El Niño, an understandable question may arise in an earthquake-prone country:

Can El Niño cause earthquakes—or make a major earthquake such as the feared “Big One” more likely?

The short answer is there is no scientific evidence that the ongoing El Niño increases the likelihood that the West Valley Fault will generate a major earthquake.

But the longer answer is more interesting.

Scientists have found that changes in rainfall, groundwater and other forms of water loading can produce very small changes in stress within the Earth’s crust. Under particular conditions, those changes may slightly influence the timing or frequency of some small earthquakes.

That is very different from saying that El Niño causes major tectonic earthquakes.

Understanding that distinction is important—especially as the Philippines enters what could become an exceptionally strong El Niño.

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A Very Strong El Niño Is Developing

On September 23, 2026, DOST-PAGASA warned that a strong El Niño was already active in the tropical Pacific and was expected to intensify into a very strong El Niño between September and December 2026. PAGASA expects the event to persist into the first half of 2027. Source: DOST-PAGASA

NOAA’s Climate Prediction Center is similarly projecting an unusually intense event. Its September 10 assessment gave El Niño a greater than 90% chance of reaching very strong strength during the Northern Hemisphere fall and winter of 2026–27. Source: NOAA Climate Prediction Center

NOAA has also estimated a 75% chance that the October–December 2026 period reaches a level that would exceed previous El Niño events in its record dating to 1950 under its current Relative Oceanic Niño Index measure. Source: NOAA

For the Philippines, PAGASA says El Niño increases the likelihood of below-normal rainfall, dry spells and drought in some areas, although rainfall can still be above normal in western parts of the country during periods influenced by the southwest monsoon.

Those conditions can affect water supplies, agriculture, electricity demand, public health and emergency-response resources.

But do they affect earthquakes?

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El Niño and Earthquakes Begin in Very Different Systems

El Niño is fundamentally an ocean-atmosphere phenomenon.

It develops when unusually warm sea-surface conditions emerge across parts of the central and eastern equatorial Pacific and interact with atmospheric circulation. The resulting changes can alter rainfall, temperature and weather patterns across large parts of the world.

A tectonic earthquake has a very different origin.

PHIVOLCS defines tectonic earthquakes as earthquakes produced by sudden movement along faults and plate boundaries. Source: DOST-PHIVOLCS

Faults are fractures in the Earth’s crust where blocks of rock move relative to one another. PHIVOLCS classifies faults that have moved during approximately the past 10,000 years and may move again as active faults. Source: DOST-PHIVOLCS

The West Valley Fault belongs to this geological world.

Its earthquake potential comes from tectonic forces acting within the Earth’s crust—not from changes in Pacific Ocean temperature or atmospheric circulation.

There is therefore no established mechanism by which an intensifying El Niño would suddenly load the West Valley Fault with the tectonic energy required to produce a major earthquake.

But that does not mean the Earth’s crust is completely insensitive to what happens at the surface.

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Water Has Weight—and the Crust Responds

Rainfall, reservoirs, groundwater, snow and soil moisture all have mass.

When large quantities of water accumulate on or beneath the Earth’s surface, they add weight. When water disappears during prolonged dry periods or drought, that weight decreases.

The Earth’s crust can respond to those changes.

The movements are usually extremely small—often measured in millimeters—but modern GPS networks can detect them.

The U.S. Geological Survey explains that heavy precipitation, drought, reservoir levels and aquifer changes can alter near-surface loading and produce small changes in stresses acting on faults. Some studies have consequently detected small changes in the rate of microearthquakes associated with seasonal hydrological conditions. Source: U.S. Geological Survey

This is where an important scientific distinction becomes necessary.

A small change in stress acting on a fault is not the same thing as creating the tectonic stress that causes a major earthquake.

Think of a fault that has already accumulated enormous tectonic stress over decades, centuries or longer.

A tiny environmental change might—in certain circumstances—slightly alter the timing of movement on a small fault that was already extremely close to slipping.

But that environmental change did not create the underlying tectonic problem.

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Scientists Have Detected Seasonal Effects on Small Earthquakes

Research from California illustrates this distinction.

Using high-precision GPS measurements, scientists have documented seasonal deformation of the crust caused by changes in water storage. Researchers have also investigated whether those seasonal stress changes correspond with variations in earthquake activity.

A 2017 study published in Science found that seasonal hydrological loading produces measurable crustal deformation and that small-earthquake activity can be modestly correlated with stress conditions favorable to rupture. Source: Science / PubMed

That is scientifically significant.

But it does not mean that rainfall causes large earthquakes.

The USGS reaches essentially the same conclusion in practical terms: there is no observed pattern of large, damaging earthquakes occurring more frequently during either periods of heavy precipitation or periods of drought. Consequently, precipitation is unlikely to materially change seismic hazard. Source: U.S. Geological Survey

That distinction becomes particularly important when discussing El Niño.

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What About Drought?

El Niño can increase the probability of drought and prolonged dry conditions in parts of the Philippines. PAGASA is already warning that the developing event may produce below-normal rainfall, dry spells and drought in some areas.

During prolonged drought:

• soil moisture can decline;
• reservoirs can lose water;
• groundwater levels can fall;
• groundwater extraction may increase; and
• the weight of water stored at or beneath the surface can change.

Those processes can produce measurable crustal deformation.

But measurable deformation does not automatically translate into a meaningful increase in the probability of a destructive tectonic earthquake.

There is no established scientific basis for a statement such as:

“A severe El Niño makes the West Valley Fault more likely to rupture.”

Nor is there evidence that:

“A stronger El Niño means a stronger earthquake.”

The magnitude of a tectonic earthquake depends primarily on the fault rupture itself—including how much of the fault moves and how much accumulated tectonic strain is released—not on whether the Philippines happens to be experiencing El Niño at the time.

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Could El Niño Trigger a Fault That Was Already About to Move?

This is where the answer requires more care than a simple yes or no.

In principle, very small stress changes can sometimes influence a fault that is already extremely close to failure.

Earthquakes can themselves trigger other earthquakes through stress transfer. Human activities such as reservoir filling, mining, geothermal operations and fluid injection have also produced documented cases of induced or triggered seismicity.

Natural hydrological loading can create much smaller stress changes.

So scientists cannot reasonably say that conditions at the Earth’s surface exert zero influence on every earthquake everywhere.

But that is very different from demonstrating that El Niño can trigger a major earthquake along the West Valley Fault.

At present:

There is no validated evidence that the 2026–27 El Niño increases the probability of a West Valley Fault rupture.

Scientists also cannot use El Niño, drought, rainfall or seasonal weather patterns to predict when the West Valley Fault will move.

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The Philippines’ Earthquake Hazard Exists With or Without El Niño

The Philippines experiences earthquakes because of its tectonic setting.

PHIVOLCS notes that the country contains numerous active faults and lies within an environment where major tectonic plates and fault systems interact. Large Philippine earthquakes have repeatedly been traced to movement along active faults or plate boundaries. Source: DOST-PHIVOLCS

Those geological processes continue whether the Philippines is experiencing El Niño, La Niña or neutral climate conditions.

For the West Valley Fault, that means three things should be clearly separated:

El Niño does not create the earthquake hazard.

El Niño does not remove the earthquake hazard.

And El Niño does not tell us when the fault will move.

The fault remains an active geological hazard regardless of climate conditions.

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But That May Not Be the Most Important Question

If El Niño does not meaningfully increase the known probability of the Big One, why should an earthquake-preparedness website be paying attention to it?

Because the probability of an earthquake is only one part of disaster risk.

The other part is what condition society is in when the earthquake arrives.

Imagine two identical earthquakes occurring beneath the same city.

In the first scenario, reservoirs are healthy, water distribution is functioning normally, temperatures are moderate, firefighting resources are fully available and households have intact emergency reserves.

In the second, the region has experienced months of severe heat and drought. Reservoirs are under pressure. Water conservation or rationing is already occurring. Electricity demand is elevated. Emergency agencies are responding to heat, agricultural or water-related problems. Household water reserves are already being stretched.

The earthquake itself has not changed.

But the conditions surrounding it have.

That can change the consequences.

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Hazard Is Not the Same as Vulnerability

This distinction is fundamental to understanding disaster risk.

The hazard is the potentially damaging event—in this case, a major earthquake.

The vulnerability is the condition of the people, infrastructure and systems exposed to it. As WVF has examined separately, building vulnerability can determine how severely earthquake shaking affects communities.

El Niño does not need to make an earthquake more likely in order to matter enormously to earthquake preparedness.

A very strong El Niño could potentially affect:

• water availability;
• reservoir conditions;
• household emergency supplies;
• firefighting capacity;
• electricity demand;
• heat exposure following evacuation;
• hospital operations;
• agriculture and food security;
• government emergency resources; and
• the resilience of critical infrastructure.

Those are not causes of earthquakes.

They are factors that can influence how difficult an earthquake becomes to survive and recover from.

And that is the question WestValleyFault.com will examine in this series. For practical household preparation now, see the WVF Emergency Preparedness Essentials guide; if backup electricity is a concern, our earthquake power-outage guide explains how portable backup power can fit into a household emergency plan.

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The Question We Should Be Asking

Instead of asking only:

Can El Niño cause the Big One?

A more useful preparedness question may be:

What happens if the Big One occurs while the Philippines is already dealing with a very strong El Niño?

The science currently gives us no reason to conclude that El Niño makes the West Valley Fault more likely to rupture.

But the same El Niño could change the environment into which that earthquake arrives.

And those conditions are something we can prepare for.

Next in the series

What If the Big One Strikes During a Major Water Shortage?

Water is essential after a major earthquake—not only for drinking and sanitation but for hospitals, evacuation centers and firefighting.

If El Niño places pressure on Metro Manila’s water supply before a major earthquake occurs, how much more difficult could emergency response become?

Part 2 of El Niño and the Big One examines the water question.