Same magnitude, different losses: the missing risk in earthquake risk

Catastrophe models may be underestimating the destructive potential of certain earthquakes, as emerging research into supershear fault ruptures reveals how dramatically ground shaking can differ between events of the same magnitude.

Speaking at ILS Bermuda Convergence, held October 12-14, Volkan Sevilgen, co-founder and CTO of Temblor, warned that supershear earthquakes, in which fault ruptures travel faster than seismic waves, could produce significantly greater damage than conventional earthquake scenarios suggest.

The phenomenon creates an effect comparable to a sonic boom, generating an intense pulse of ground motion capable of causing severe structural damage.

Sevilgen pointed to research involving a simulated 20-storey building in Los Angeles to illustrate the potential consequences.

“If you subject that building to a subshear rupture, same magnitude, the building sustains minimum damage. If you subject that building with the supershear rupture with the same magnitude, the building collapses. They are that different, all because of that killer pulse,” he said.

The implications extend well beyond an isolated scenario. Sevilgen said researchers have identified approximately 22,000 kilometres of major faults potentially associated with supershear ruptures, spanning regions including California, Alaska, Turkey, Japan, Indonesia and New Zealand.

He also cited research suggesting that four of the last six magnitude seven-plus earthquakes on strike-slip faults in California had been recognised as supershear events.

“This is not some rare occurrence. This is occurring more frequent than otherwise,” he said.

According to Sevilgen, existing probabilistic seismic hazard analyses do not adequately account for these ruptures, raising questions about whether catastrophe models fully capture the potential severity of earthquake losses.

Temblor is now working to incorporate supershear ruptures into its own loss models.

The presentation also highlighted another source of earthquake risk: doublets, where two major earthquakes occur close together and interact through the transfer of stress between faults.

Sevilgen cited the devastating 2023 earthquakes in Turkey, when magnitude 7.8 and 7.7 events struck separate but neighbouring faults just nine hours apart.

“The buildings being shaken by the first one had no chance of surviving after the second one,” he said.

For re/insurers and ILS investors assessing earthquake exposure, the concern is that conventional assumptions may not fully reflect how faults interact or how rupture speed influences damage.

Sevilgen cautioned against treating the Turkish earthquakes as a geographically isolated phenomenon.

“There is absolutely no difference between faults in Turkey and faults in California,” he said. “There is no reason in our mind why this interaction wouldn’t occur in San Francisco, or for the same token, why it wouldn’t occur in Los Angeles.”

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