Landfill, Liquefaction, and Loma Prieta

Geologic map of San Francisco’s Marina District, adapted from US Geological Survey Professional Paper 1551-F

Say what you will about San Francisco’s Marina District. It has its fans and its detractors, and plenty of stereotypes to go around no matter which side your opinion falls on. But today, the anniversary of the M6.9 1989 Loma Prieta earthquake, everyone’s heart should go out at least a little bit to that part of the city that sustained the most damage. While Loma Prieta luckily didn’t come close to destroying San Francisco in a way to compare to 1906, the Marina suffered more structural collapses – and more fire – than anywhere else in town that day.

Unfortunately, though, the reason the Marina came closest to getting wiped off the map that October afternoon ties directly into the way the Marina put itself onto the map to begin with: artificial fill. The land of the Marina is not natural land at all. It’s a combination of sand and brick and wreckage put in place to recapture some real estate from the Bay. And when it comes to earthquakes, that’s a very bad kind of land to be on.

1851 Coast Guard survey map of the cove that eventually became the Marina District (USGS).

The shoreline of San Francisco today has been altered substantially from its natural state, both to make a better docking area for ships and to increase square footage for built development as the city became more established. This early filling was mostly concentrated along the Embarcadero, the Financial District, and SoMa, and it used all kinds of things, from sand and rock to sunken ships. There was some going on in the Marina, but that was effectively the outskirts of town at the time. By the end of the 1800s, some fill had been put in place to round out the edges of the natural cove and cover over a slough, several piers extended out into the Bay, and a seawall had been placed, but this still enclosed a large area of marshy water.

The Marina in 1912, shortly before the landfill for the 1915 Exposition began. The entire watery area enclosed by the seawall is full of houses now (USGS).

The real landfill action in the Marina started in the wake of the 1906 earthquake, though not in the most obvious way. Most of the wreckage was dumped into Mission Bay, though some of it did make it into the northern edge of the Marina. Rather, the final filling in of the Marina came with San Francisco’s way of showing everyone that it had overcome the cataclysm of 1906 and was a world-class city once again: the 1915 Panama-Pacific Exposition. Between April and September of 1912, the remaining water in Marina Cove was filled in with sand and sludge, to make way for the temporary but grand expo buildings. Structures that are not meant to last were probably the best sort of thing to put on this not-really-land, but once they were gone, more permanent ones took their place, setting the stage for the major problems that came with Loma Prieta, and will come with other large Bay Area earthquakes.

So, what’s so bad about this fill? First of all, it shakes harder than solid rock does. When a seismic wave travels through a material, it has a certain energy budget that must be balanced between shaking and moving forward. The more it shakes something side to side, the less fast it can go. With solid rock, everything is consolidated, so there’s not much side to side shaking, which means waves can go through this rock pretty quickly. With softer stuff – and especially with completely unconsolidated fill – each and every particle of it can be shaken side to side quite a lot, so more energy goes into this shaking and less goes into propagating, which leads to stronger shaking for longer. Because the water table in the Marina is still so high, liquefaction – the process of seismic shaking stirring water and soil together until they basically become quicksand – compounds the problem. Adding an earthquake to some artificial fill is a recipe for destabilized foundations, separated pipes and gas lines, and any number of other problems that could come from combining those elements.

I find it simultaneously sobering, ironic, and spooky that fill largely placed for a celebration from recovering from one earthquake led to such concentrated destruction in the next earthquake. It should also absolutely be a cautionary tale that, despite successful recovery from Loma Prieta, the underlying issues that produced the damage have not gone away, and will behave that way in the next one. The ground underneath the Marina is definitely working against it. It certainly wouldn’t be a bad thing if a fanatical fixation on earthquake preparedness worked its way into the stereotypical Marina culture.

This entry was posted in Earthquakes, Geoscience, San Francisco and tagged , , , . Bookmark the permalink.

Leave a Reply

Your email address will not be published. Required fields are marked *