The largest earthquakes are events of colossal scale that reshape coastlines and release energy comparable to thousands of atomic bombs. These rare, massive seismic movements occur where the Earth’s tectonic plates collide, building up stress over centuries before catastrophically releasing it. The power of these mega-events raises a fundamental question: what is the highest magnitude the planet has ever recorded?
Defining the Moment Magnitude Scale
Measuring the true size of an earthquake requires a scale that accurately captures the total energy released from the rupture. Seismologists rely on the Moment Magnitude Scale (\(M_w\)). This scale is based on the seismic moment (\(M_0\)), a physical property directly proportional to the area of the fault that slipped and the average distance the fault moved.
The \(M_w\) scale was developed because the older Richter scale (\(M_L\)) tends to “saturate,” meaning it underestimates the true magnitude of earthquakes above magnitude 7. Earthquakes of that size exceed the measuring capacity of the original Richter instruments and calculation method. The Moment Magnitude Scale is more directly linked to the physical process of the earthquake source.
The scale is logarithmic, meaning that for every whole number increase, the energy released increases by a factor of approximately 32. For example, a magnitude 8.0 earthquake releases about 32 times more energy than a magnitude 7.0 event.
The 1960 Valdivia Earthquake The Record Holder
The largest earthquake ever recorded occurred on May 22, 1960, off the coast of Chile, an event officially measured at magnitude \(M_w\) 9.5. Known as the Valdivia Earthquake, this event took place along the Nazca and South American plates, where the oceanic Nazca plate is subducting beneath the continental South American plate. The rupture zone extended for an estimated 1,000 kilometers along the Chilean subduction zone.
The resulting ground motion and vertical displacement of the seafloor generated an immense tsunami that became the most destructive aspect of the event. Waves up to 11.5 meters high pounded the Chilean coastline, causing widespread devastation and coastal subsidence. The tsunami’s reach extended far across the Pacific Ocean, demonstrating the global impact of such a powerful event.
The massive waves traveled thousands of kilometers, reaching Hawaii about 15 hours later and causing 61 deaths and millions in damage in Hilo. Approximately 22 hours after the initial earthquake, the tsunami struck the shores of Japan, where waves as high as 5.5 meters destroyed more than 1,600 homes and caused 185 deaths or disappearances. The seismic energy released by the 1960 Valdivia earthquake remains the highest ever captured by modern seismological instruments.
Geological Constraints on Earthquake Size
The question of whether an even larger earthquake, perhaps a magnitude 10, could occur is directly linked to the physical limitations of the Earth’s crust. The size of an earthquake is determined by the dimensions of the fault rupture: the length, the width, and the distance of slip. To achieve a magnitude 10, a fault would need to rupture along a length far greater than any known continuous tectonic plate boundary.
The largest earthquakes, known as megathrust events, occur in subduction zones where one plate slides beneath another, providing the greatest potential surface area for rupture. The available length and depth of these zones, combined with the maximum shear stress that rock can sustain before failing, impose a natural upper limit. Most seismologists conclude that a magnitude \(M_w\) 9.5 is likely very close to the geophysical limit for the Earth.