In the realm of earthquakes, one should always anticipate the unexpected. This is the message conveyed by seismologists Professor Eric Curry from Ecole Normale Supérieure (ENS) in Paris, and Jean François Ritz, the Director of CNRS Laboratoire Géosciences in Montpellier.

At the core of their counsel lies the fact that earthquakes can occur in unexpected places. These enigmatic occurrences, known as intraplate earthquakes, manifest in geologically tranquil locations, distant from the active boundaries of tectonic plates.

The French scientists are dedicated to comprehending and elucidating these phenomena.

Unpredictable and Destructive

The blocks of rock forming the fragile outer shell of our planet move gradually across the Earth’s surface, at a pace akin to the growth rate of a human fingernail.

While the majority of geological activity of note transpires where plates converge, intraplate earthquakes diverge from this norm, occurring within plates, far from their peripheries.

Curry and Ritz have a compelling motive to shine a light on this topic, given that intraplate earthquakes are infrequent, with a limited number of notable occurrences compared to earthquakes at plate boundaries. Professor Curry noted that only around 20 earthquakes measuring 6 or more in magnitude have been recorded since 1974. This amounts to less than half the percentage of similar-sized earthquakes observed at plate edges during the same timeframe. Their scarcity and protracted duration render them challenging to forecast, yet they have the potential to inflict considerable devastation on unprepared urban centers that have never viewed earthquakes as a pressing concern.

Intraplate earthquakes can transpire wherever geological faults exist within the Earth’s crust. Over the past centuries, they have been documented in locations as diverse as Basel, Switzerland, New York, Boston in the United States, and the St. Lawrence River in Canada.

More recently, they wrought havoc in the Australian city of Newcastle, as well as in Botswana and Puebla, Mexico in 2017, resulting in nearly 400 fatalities in the latter.

The Magnitude of the Problem

Curry and Ritz garnered attention for a magnitude 5 earthquake near the Rhone Valley village of Le Teil in 2019, while a magnitude 5.2 earthquake shook the Lincolnshire town of Market Larsen in England in 2008. Termed the “Larsen Earthquake” by local newspapers, it caused one injury and incurred damages estimated at around £20 million. The seismic events in the UK and France tend to be minor, contrasting with occurrences in other global regions.

The most devastating intraplate earthquake of modern times took place in 2001, with a magnitude of 7.6, striking Bhuj, Gujarat, India. This catastrophic event razed an estimated 300,000 edifices and claimed the lives of up to 20,000 individuals. Looking back to 1886, a around magnitude 7 earthquake hit Charleston on the US east coast, resulting in 60 casualties and widespread devastation. A few years later, the New Madrid, Missouri area endured three potent intraplate earthquakes measuring up to magnitude 7.5, inducing violent tremors across the vicinity.

The rarity of these seismic episodes, combined with their potential for extensive destruction, underscores the urgency for a deeper understanding of intraplate earthquakes.

Increasing Tension

Both intraplate and plate margin earthquakes share a common operational mechanism. Essentially, strain builds up over time on geological faults within the Earth’s crust until it reaches a critical threshold, leading to fault rupture or slippage, thereby generating earthquakes. The release of this built-up energy in the form of seismic waves alleviates the strain. However, the process begins anew as strain accumulates again. Although the process mirrors itself in both types of earthquakes, the triggers that prompt rupture likely differ.

Curry and Ritz propose that while fault rupture at plate margins is predominantly instigated by plate movements, intraplate earthquakes within the plate’s interior are spurred by discrete triggers that occur rapidly on geological time scales. Such triggers could encompass various phenomena such as unloading due to ice sheet melting, surface erosion, rain infiltration, or fluid displacement from the Earth’s mantle.

Intraplate Complexity

It’s worth noting that a fault primed for rupture can be triggered by an equivalent pressure to a handshake. Consequently, even though millions of years may have been necessary for strain to accumulate on ancient intraplate faults, their activation could unfold swiftly over a brief period. Curry and Ritz explored the Le Teil earthquake of 2019 and concluded that it was probably triggered by the shedding of the upper crust following the region’s glacier recession post the Ice Age, possibly triggered by a nearby quarry.

The unloading and deformation of the Earth’s crust post the rapid melting of colossal ice sheets about 20,000 to 10,000 years before the present epoch is presumed to have catalyzed numerous intraplate earthquakes, including those at New Madrid, Charleston, and Basel. At the decline of the Ice Age, Norway and Sweden witnessed a surge in seismic events as the 3 km thick Scandinavian ice sheet melted rapidly, unburdening intraplate faults underneath it, and releasing accumulated strain over thousands of years.

This period witnessed several sizable earthquakes with one heaving about 8,200 years ago, instigating a massive underwater landslide off Norway’s coast, engendering a North Atlantic Ocean tsunami with crest heights reaching 20 meters across the Shetland Islands and 6 meters along Scotland’s eastern coastline.

Prediction Problems

The intricacies of predicting intraplate earthquakes pose a formidable challenge, as Curry highlights, stating, “For these peculiar earthquakes, calculating future risk is highly intricate, particularly given their sporadic nature in specific locales. Objective indicators for evaluating future intraplate seismicity are lacking.”

Despite the convolutions associated with predicting intraplate earthquakes, research concerning the peril posed by these events in historically affected regions is critical. The burgeoning urbanization in areas with past intraplate earthquake history is cause for concern.

Currently, more than half of the global populace resides in urban centers, with cities in regions susceptible to intraplate earthquakes witnessing substantial expansion. Basel, Switzerland, for instance, the nation’s second-largest urban conurbation with a populace of approximately 500,000, serves as a key hub for banking and the chemical sector. In the event of an earthquake akin to the one in 1356, the outcomes would be significantly more severe, portending thousands of casualties and severe property damages.

Similarly, Charleston in the United States, with a population exceeding 550,000, now finds itself at the heart of a bustling city characterized by stone and concrete edifices, rendering it vulnerable to calamitous consequences if struck by an earthquake akin to the 1886 event.

Looking towards the future, the specter of global warming looms large, with the potential to increase intraplate seismic activity as glacial and ice sheet melts diminish the underlying crust’s load, sparking fault ruptures and strain release accumulated over millennia.

The ramifications of such seismic events reverberate across a broad cross-section of society, driving home the importance of preparedness and vigilance in regions prone to intraplate earthquakes.