Newly Discovered Fault Connections Raise San Francisco Major Earthquake Risk

While San Francisco is no stranger to the ever-looming threat of a catastrophic earthquake, a recent discovery of intersecting faults under San Francisco's San Pablo Bay has dramatically increased the risk of a major earthquake in the next thirty years.

Scientists at the U.S. Geological Survey published their discovery this week in Scientific Advances. The Hayward Fault has long been considered a threat because it runs under densely populated neighborhoods east of San Francisco. The study found that beneath San Pablo Bay, it joins with a Rogers Creek fault, a less active underground fracture to the north. This newly discovered direct link between the faults raises the possibility of a simultaneous rupture of the Hayward and Rodgers Creek faults, a scenario that could result in an earthquake up to a magnitude of 7.4 that would cause widespread property damage, extensive loss of life, and far-reaching ripple-effects on the global economy.

The relationship between these two faults had long been unknown.  Detailed subsurface imaging, geophysical interpretation and kinematic modeling by the U.S.G.S. team demonstrate that the Hayward and Rodgers Creek faults are directly connected at the surface.  The Hayward and Rodgers Creek faults combine to represent a continuous 118 mile-long fault.  If they were to break simultaneously, they could produce a magnitude 7.4 quake. 

An earthquake of this magnitude would be more than five times stronger than the 1989 Loma Prieta quake on the San Andreas Fault that killed over 60 people and caused an estimated $6 billion in property damage.  In addition, the epicenter of the 1989 Loma Prieta earthquake was located 60 miles southeast of San Francisco while the Hayward-Rogers Creek faults run directly beneath the city.  David Ponce, a scientist with the USGS research group, was quoted by Popular Mechanics on this risk:

“You have to understand that there are over 2.4 million people living right along this fault, and the population of this whole area is around 7.5 million. It also turns out that major transportation, gas, water and electrical lines cross this fault. So when it goes, it's going to be absolutely disastrous."

Mr. Ponce was also quoted as saying that there is a 32% chance that such a quake could occur in the next 30 years.

Speculation regarding California earthquakes and “the next big one” is hardly anything new, but with the Hayward-Rodgers Creek discovery this week, it appears we have a new front-runner for how that “big one” could originate.

Posted by Matt Gollinger

A Look Back, and Ahead, to the “California Shakeout”

Two weeks ago, the CAT-Law Navigator reviewed an Earthquake Advisory issued by the California Governor’s Office, which directed the public to prepare for an increased probability of earthquakes through October 7. The Advisory was issued following the observation of an “earthquake swarm” near Bombay Beach, California, that started on Sept. 26, 2016, beneath the Salton Sea, near the southern end of the San Andreas Fault. The U.S. Geological Survey had calculated that there was up to a 1 in 100 chance of a magnitude 7 or greater earthquake occurring on the southern San Andreas Fault through October 4 (later revised to up to a 1 in 500 chance of such an earthquake through October 7). Given those odds, it’s no surprise that a major earthquake did not in fact materialize, but the U.S.G.S. calculation and the Governor’s Advisory reminded us of The Great Southern California Shakeout Scenario, first published by the U.S.G.S. in 2008, which modeled a similar event. 

The goal of the 308-page 2008 Shakeout Scenario was “to identify the physical, social and economic consequences of a major earthquake in southern California and in so doing, enable the users of our results to identify what they can change now—before the earthquake—to avoid catastrophic impact after the inevitable earthquake occurs.” (Shakeout Scenario at p. 2) In service of that goal, the authors outlined the magnitude and locus of an imagined Southern California earthquake along the San Andreas Fault, and then estimated the resulting physical damage, the impact on social systems, and the actions that can still be taken to prepare for and minimize the impact of a Southern California earthquake.  

The authors of the Shakeout Scenario noted that although the particular earthquake scenario they modeled “may never happen . . . [b]ig earthquakes on the San Andreas Fault are inevitable, and by geologic standards extremely common, but probably will not be exactly like this one. The next very damaging earthquake could easily be on another fault.  However, lessons learned from this particular event apply to many other events and could provide benefits in many possible future disasters.” In other words, the earthquake scenario the Shakeout authors imagined is not their prediction, but it provides a thorough analysis of one possible scenario, and the author’s findings have broad application for those interested in the subject.

There is a wealth of information in the extensive Shakeout Scenario document, but here are some headline findings of particular interest to the insurance industry: the authors modeled a magnitude 7.8 earthquake along the southernmost 200 miles of the San Andreas Fault, from the Salton Sea to Lake Hughes (South and East of Los Angeles); the earthquake involves extensive and widespread ground shaking, surface level fault offsets of 30 feet, liquefaction of the earth and landslides in isolated areas, but no tsunami activity due to the distance of the event from the Pacific Ocean. All told, the authors anticipated 1,800 deaths and over $200 billion in economic losses (with property damage of $112.7 billion and business interruption losses of $96.2 billion).  For comparison, the total amount of insured losses (in 2015 dollars) stemming from Hurricane Katrina was $49 billion, still the most expensive insurance catastrophe in U.S. history. The 9/11 Terrorist Attack involved insured losses of $24.6 billion; the 1994 Northridge, California earthquake’s insured losses were $18.6 billion. 

We will return to the Shakeout Scenario in subsequent posts, exploring the authors’ breakdown of the nature and scope of the event itself, the predicted damage to residential and commercial buildings, the impact on California’s infrastructure, and the broader business interruption implications. We will also consider the overall social impact and the authors’ advice for steps that can be taken now to reduce the physical and societal damage of a major earthquake along the San Andreas Fault and other California faults.  

Posted by Dan Millea