new landslide hazard map

The California Geological Survey has released a new map of landslide hazard that reinforces some of the incontrovertible facts about the equation that governs landslides in the Bay Area:

slope angle + clay expansion + weak rock + rain = landslide

The last variable, rain, is one that is peculiar in the Bay Area. We're moderately arid--not as many inches as Seattle, far more than Barstow. But the signature of Bay Area rains is that they come all at once, in catastrophic storms that funnel into the Bay Area across the Pacific as if by a pipeline. The effect of this is saturation, and the effect of saturation is overland runoff, water literally unable to soak into the receiving clay. This runoff can have an ferocious erosive effect, transforming stable slopes in a matter of hours to undercut, unstable, moving landscapes.

As the San Jose Mercury News has reported, this is bad news for Marin County. Marin is blessed with steep, interesting hills of schist and serpentinite and chert. But the vertiginous fractured Franciscan landscape are both weak and steep, simply waiting for a slight nudge, in the form of water weight and lubrication. And unlike the East Bay, where the mountainous regions are sparsely developed, Marin has extensive building on very steep slopes.

As bad as this is, it gets worse. As was recently discussed at a conference in Sacramento, California faces a risk from periodic "megastorms," called ArkStorms, which can deluge the state with biblical rain.

Earthquake predictions, and the coming storm

Citizens of Rome are frightened that an earthquake is about to occur:

Thousands of people are reported to be staying out of Rome for the next few days, over fears the city will be hit by a huge earthquake.

The panic was sparked by rumours that seismologist Raffaele Bendandi, who died in 1979, predicted the city would be devastated by a quake on 11 May ...

But many people said they were leaving the city to be on the safe side.

There are reports of an 18% increase in the number of city employees planning to stay away from work

The magnitude of this silliness is shocking.

Earthquake prediction has never been able to pinpoint an exact day and time of a quake. This may be a goal forever out of reach of seismologists; the same nonlinear dynamics that make short-term weather prediction impossible more than about a week ahead are involved in the dynamics of earthquakes, meaning that very slight changes in initial conditions produce very different results.

But, in a very real way, this doesn't matter.

Geologists still know 1) where earthquakes will occur in the future, 2) how large they will be in the future, 3) approximately how many people they will kill in a given urban area, based upon existing buildings, and 4) a range of time, in years, when the earthquake, based on past events, is most likely to occur.

For example, on the world's deadliest urban fault, the Hayward Fault, geologists know exactly where the fault lies. In fact, its traces through the San Francisco East Bay are unmistakeable, in thousands of offset foundations, building cracks, and sidewalks buckled by creep. We know where the fault is, and because the subsurface geometry of Hayward goes (unlike many other faults) pretty much straight down, we have a reasonable idea that epicenters will be focused along the fault trace.

We also know approximately how large the coming quake will be. Right now there is thought to be enough stored energy in the rocks to produce a >6.7 Mw quake. That will be quite devastating in proximity to so many homes. We know that the tens of thousands of unreinforced brick buildings in the Bay Area will likely mean casualties on the scale of Japan's 1992 Kobe quake, which killed 1994, which killed over 6400 people.

We also know when we can expect the Hayward to rupture, based on trench work that has revealed a long-term record of past events. According to the great work of Jim Lienkaemper, of the USGS, the recurrence interval of the Hayward ranges from about 161 years, plus or minus 65 years, to 170 years, plus or minus 82 years. For the five most recent big quakes, however, the recurrence interval is only 138 years, plus or minus 58 years.

The last big Hayward Fault event was the quake of 1868, which was 143 years ago. That means by the last-five measurements, we are squarely in the time zone for the next quake, though we, in fact, entered the lower range of this in 1948. By 2064, we are virtually guaranteed to have the quake, though it most likely will occur well before this. Likewise, with the 161 and 170 numbers, we long ago entered the time zone where the quake is primed and ready to occur. This is not good news for us, but it is a prediction, one verified by much scientific research.

So when people ask when the next big quake will be, we can tell them that geologists know where, when, and how bad it will be. This usually comes as a surprise. People are looking for some sort of 3-day warning, which does not exist. People--and municipalities--need to start preparing for the guaranteed seismic event in the near future, rather than imagining scientists are somehow going to come up with a warning as they would with an approaching storm.

quakes can drop land

We often think of sea level as the height of water relative to the land. But geologists know that the land is also in motion, somewhat complicating measurements of rising sea level. (Alaska, which is tectonically rising, records less apparent sea level rise than elsewhere, for example.) In cases where land is subsiding, sea level rise may actually seem augmented. And no part of the Earth underwent a quicker subsidence than areas of coastal Japan during the great quake of 11 March 2011.

As a recent story confirms,

The March 11 earthquake that hit eastern Japan was so powerful it pulled the entire country out and down into the sea. The mostly devastated coastal communities now face regular flooding, because of their lower elevation and damage to sea walls from the massive tsunamis triggered by the quake.

One of my professors of geology remembers camping on a beach the night of the 1992 Mendocino earthquake, and finding in the morning that the coastline had been visibly uplifted, exposing mussell-covered rocks now well-above the high tide line, dooming their sessile molluscan tenants.

While we human like to imagine the solid land as solidly fixed in one place, the reality is far more complex, though our short life spans rarely allow us to the opportunity to observe changes first hand. This the nature of deep time; we can see the long term effects of geologic changes but usually only indirectly observe them occurring.

Japan is an exception:

Some areas in Ishinomaki moved southeast 17 feet (5.3 meters) and sank 4 feet (1.2 meters) lower.

"We thought this slippage would happen gradually, bit by bit. We didn't expect it to happen all at once," says Testuro Imakiire, a researcher at Japan's Geospatial Information Authority, the government body in charge of mapping and survey.

The now permanent situation in Japan should give those of us who live in California a moment of pause to reflect on how quickly the land around may one day change.

magma ocean on Io

Students always come to intro geology courses with the misconception that beneath our thin, rocky crust, there exists an ocean of magma. Intro geology students imagine that volcanoes are simply breaches in this ocean, where pressure forces this liquid rock to spew out.

The reality, of course, is that the mantle is solid, not liquid. The mantle's ability to flow over time makes it seem liquid-like, but it is in fact quite crystalline. Beginning geology students have no end of trouble over this distinction; maybe it's something wrong with the way I teach it, but I'd say at least a quarter of students just never quite get this fundamental fact about the earth. And when I throw in decompression and flux melting, students really start to have difficulty.

Now, to add to the confusion, it seems that Io, one of the moons of Jupiter, does in fact have a "magma ocean." In fact, estimates are that about 10% of the volume of Io involves liquified material. This may account for Io's strikingly-intense volcanic activity. Compared to our relatively tame planet, Io has frequent volcanic activity.