Star Tribune
October 22, 1989

By Priscilla Grew

For a while Tuesday night, the East Bay didn't just consist of Oakland, Berkeley, Hayward and the Nimitz Freeway. Minnesota was part of the East Bay, too. Minnesotans made a million phone calls to California.

We worried about family and friends. We thought with shudders about that freeway we'd been on, that bridge we used to drive, tried to remember exactly what it is that people do at 5:04 p.m., like take elevators and drive home. Even if we'd never been to California, the television brought it to us over and over again -- the reality of an everyday afternoon, a baseball game, gone unreal.

At the Minnesota Geological Survey the next morning, I felt like I was right back in California. Press calls all day, interviews, callers with questions I could answer (no, it wasn't caused by the ozone hole), and those I couldn't (I'm sorry, but I don't know what the San Andreas fault is going to do next).

Some people were hoping it had been the Big One. Unfortunately, it would take 10 of those Tuesday quakes going off at once to equal the magnitude 8 earthquake in California's future.

Others hoped that this Santa Cruz earthquake had reduced the risks now for the Bay Area. But the Hayward Fault runs right under Oakland and Berkeley, and is capable of a magnitude of 6 or 7 that would be even more damaging because of its closer proximity to the densely populated metropolitan area.

By Wednesday evening guilt was settling in on the Twin Cities, as we enjoyed our fresh running water, electric lights, undamaged buildings, uninjured children. I was getting questions like, "Aren't we on a fault line too in Minnesota?" What would be the effect of a magnitude 6.9 quake under Minneapolis? Are you sure we can't have a magnitude 8 earthquake on a Minnesota fault?" Minnesotans somehow felt we ought to have our own share of earthquake risks too.

A magnitude 8 earthquake, or even one only one-tenth that big, requires a great active fault system like the San Andreas. Just as we don't face the prospect of a volcanic eruption here, with no Mount St. Helens or Fuji in sight, we are not at risk from a great active fault system in Minnesota. Lake Wobegon's children may be above average, but our geologic active faults are definitely in the substandard category.

Each continent has a central stable part, called a "shield." Minnesota's deep bedrock is part of the southern portion of the Canadian Shield. The Minnesota Valley has some of the oldest rocks in North America, with the ages of 3.5 billions years. To the north, Minnesota has rocks formed from volcanoes and earthquake faults that were active in ancient Precambrian time.

About 1.1 billion years ago, the area that is Minnesota nearly became the East Coast of North America when the continent almost split apart along the Mid-continent Rift. But then Minnesota stabilized, and has been pretty quiet ever since. For example, we see no major fault offsets in limestones deposited in quiet seas that covered parts of Minnesota over 100 million years ago.

Our largest historic earthquake was a 4.8 releasing only a thousandth of the energy of the Santa Cruz event. In contrast, much of California consists of younger geologic crustal blocks that have "moved in" and attached themselves to North America much more recently. These blocks have been produced by the interaction of the North American continental plate and the Pacific oceanic plate, a process still active today along the complex San Andreas Fault plate boundary.

It is this kind of basic geologic background information that is helpful in understanding earthquake risk. Unfortunately, we do not yet know how to predict earthquakes by time, location and size. But earth scientists in recent years have made much progress on the location and size parts of the prediction.

For example, a U.S. Geological Survey 1989 report correctly identified the segment of the San Andreas that broke Oct. 17 as the site of the next earthquake over magnitude 6.5 on the northern half of the San Andreas Fault. The time for the earthquake, however, was a fuzzy forecast; a 30 percent chance before the year 2020.

But knowing the location and size of potential earthquakes is vital for constructing earthquake-resistant structures that protect human life, and for land use decisions to avoid bad ground that amplifies earthquake shaking. So earthquake research is justified; we need not throw up our arms in fatalistic despair and say nothing can be done because scientists can't yet tell us there's a 20 percent chance of an earthquake tomorrow morning, clearing with minor aftershocks in the afternoon.

Minnesotans are already contributing to the relief efforts in California through the Red Cross and the Salvation Army, and they are providing essential emotional support for family and friends who endured the severe stress of the earthquake shock and its aftermath. And though Minnesota will never itself be at risk from an active fault, it nevertheless can be part of the geological action. Here are some suggestions:

# Identify with Californians by recalling that our geology used to be like California's, full of earthquakes and active faults. We just sowed the wild oats of continental youth a little earlier. We had our own San Andreas-type faults quaking over 1.7 billion years ago.

#Invite scientists to come study our fossil fault systems. We have long-dead "strike-slip" faults like those active in California today, and some of them might even contain gold. They are eroded down in the bedrock of northern Minnesota so geologists can see things at the surface that they would have to drill a long way down to see inside the California fault zones.

#Enhance earth-sciences education in Minnesota schools. Even if we feel we are far from California, all those telephone calls show we really care about the people who live there. Educated voters are the ones who will support research finding for people to study rocks, groundwater and earthquakes.

#Tell our Minnesota congressional delegation that we favor federal support for earthquake research the next time it comes up in the budget wars. These funds have been cut in recent years, and fewer students are choosing geology and geophysics as careers. Earthquake research saves lives and money. Property damage in the billions in California costs all of us in tax and insurance premium dollars.

#Promote public education about earthquake hazards outside California. For example, St. Louis and Memphis, with many older structures susceptible to earthquake damage, are at risk from the active New Madrid fault system; more research is needed on the specifics of this risk in the mid-continent. Californians are not alone, and a national earthquake effort would benefit all.

These suggestions should help us deal with our slightly guilty feelings about living in one of the nation's less hazardous earthquake risk zones. We just happen to be residents of one of the older, quieter neighborhoods of the East Bay.

Priscilla Grew moved from Oakland to Minneapolis in 1986 to become director of the Minnesota Geological Survey at the University of Minnesota. She is a former member of the National Subcommittee on Earthquake Research.