Western Regional Earthquake Risk

 The entire west coast is at high risk for earthquakes. The Insurance Information Institute lists the ten costliest earthquakes in the United States from 1906 through 2018. Of those, six were in California  (Facts + Statistics: Earthquakes and tsunamis.). 

Earthquake Risks in the West

Two other quakes on that list were also on the western end of North America. One quake was a magnitude 6.8 rupture that struck Nisqually, Washington, on February 28, 2001. A magnitude 7.1 shook Anchorage, Alaska, on November 30, 2018.

Of the remaining two costliest earthquakes in the U.S., one was a 6.7 rupture on October 15, 2006, in Kiholo Bay, Hawaii. The other was a much smaller, magnitude 5.8 tremor that rocked the Piedmont region of Virginia on August 23, 2011.

Returning to the west coast, the USGS puts the probability at 60 percent that a magnitude 6.7 or greater earthquake will strike the Los Angeles area within the next 30 years. Probabilities for magnitude 7 and 7.5 ruptures are at 46 and 31 percent, respectively. 

Magnitude    Probability

6.7                60%

7.0                46%

7.5                31%

For the San Francisco area, the USGS projects a 72 percent probability of a magnitude 6.7 or higher hitting the region. There’s a 51 percent chance of a magnitude 7 tremor, and only a 20 percent probability of a 7.5 or larger rupture

Magnitude    Probability

6.7                72%

7.0                51%

7.5                20%

Increasing Western Earthquake Risk

The largest earthquake to strike the U.S. was a 9.2 tremblor that hit Alaska in 1964. Damage from the quake and tsunami was estimated to cost more than $500 million. But, while it was the largest, it didn’t come with the biggest price tag.

The January 17, 1994, Northridge California, earthquake was the costliest, with insured losses estimated at $26.373 billion in 2018 dollars (Facts + Statistics: Earthquakes and tsunamis). Incidentally, “the Northridge earthquake occurred on a previously unrecognized fault” (UCERF3: A new earthquake forecast for California's complex fault system).

Incidentally, if the 1906 magnitude 7.9 earthquake in San Francisco were to happen today, insured loses would probably exceed $105 billion (Background on: Earthquake insurance and risk). 

Regarding the major faults in California, geophysicist Glenn Biasi and paleoseismologist Katherine Scharer analyzed seismic records going back 1,000 years on the five major faults in California: the northern San Andreas Fault, the Hayward Fault, the southern San Andreas Fault, the San Jacinto Fault, and the southernmost San Andreas Fault. 

All together the faults span from north of San Francisco to the Mexico border. What they found is while a single fault might have a 100-year quiet period, the fact that all five have been quiet for over 100 years is unprecedented. Their research showed an average of three or four ground-rupturing quakes each century. Neither the 1989 Loma Prieta quake in the Bay Area, nor the 1994 Northridge earthquake were ground rupturing. Why the faults are quieter than average isn’t known. One possibility is there is something unknown “synchronizing the activity across these five major faults” (California's Eerie 'Earthquake Pause' Is Unprecedented).

Another possibility is the higher than average period from 1800 to 1900, where there were six ground-rupturing earthquakes. Those plus two more in the early 1900s, the 7.9-magnitude 1906 Great San Francisco Earthquake and a 6.7 on the San Jacinto Fault in 1918, may have released more seismic tension where the faults need more time to build up stress.

Moving north, the Cascadia fault, known also as the Cascadia subduction zone, had its last major earthquake in 1700, with a magnitude range of 8.7 to 9.2. Research presented at the May 2018 annual meeting of the Seismological Society of America suggests that the time frame between the megathrust events in the Cascadia subduction zone may be 200 years shorter that previously reported.

The geologic record suggests megathrust earthquakes, along with tsunamis, occurring about every 500 years. However, new research indicates that the southern part of the fault zone may only have a 300-year frequency (The Next Cascadian Megaquake May Be Sooner Than You Think). With the revised estimate, and the last megaquake in 1700, the next one may be due soon.