The Western Geologist

I recently went to a very interesting talk about simulating ground motion caused by earthquakes. The talk, which was archived and is available here, with JPG copies of the presentation slides here. The speaker was Brad Aagaard, a geophysicist who works with the U.S. Geological Survey in Menlo Park, California.

Aagaard, along with some other people at the USGS, created simulations of the ground motion caused by the 1906 San Francisco earthquake, the 1989 Loma Prieta earthquake, and then a few different 1906-like earthquakes. These 1906-like earthquakes were the same magnitude as the 1906 earthquake (M 7.8), but nucleated at different points along the San Andreas. The 1906 quake nucleated approximately under the Golden Gate bridge, and the rupture propagated both north and south, extending from Cape Mendocino to San Juan Bautista. Aagaard showed a 1906-like event that nucleated along the northern part of the San Andreas (Bodega Bay), and then one that nucleated along the southern part (San Juan Bautista). If you take a look at slide 82 of his talk (linked above) you can see the ground shaking intensity (using Modified Mercalli Intensities (MMI) ) in the greater Bay Area caused by the these events. A future 1906-like earthquake that nucleated near Bodega Bay would be more destructive in the Bay Area than the original 1906 quake.

The ground motion simulations require three data sets – a rupture model (how the fault moved during an earthquake), a geologic model, and then finally either recorded ground motions or MMI maps that can be used to constrain the model. For the 1906 quake, two groups of geophysicists have created rupture models (I'm not sure how these are created, so unfortunately I can't provide more detail). The geologic models are based on mapping and subsurface observations (seismic lines, boreholes, etc.). For the 1906 quake the ground motion constraints come from records of damage, reported using MMI. These data have been complied by another group of USGS geophysicists (available here). Add all those pieces of information together, and the result is REALLY impressive.

Here is a link to a video of a recreation of the 1906 event. The view is from Palo Alto looking north toward San Francisco. A screenshot is shown below:



The purple/blue in that picture show the arrival of the P-waves, and the yellow is the arrival of the S-waves. The surface waves follow shortly thereafter. It is amazing for me to watch the rupture begin underneath the Golden Gate, and then to see the seismic waves racing down the Peninsula. I should note that the ground motions in the videos have been exaggerated by a factor of 1000. The creation of these videos represents an amazing amount of work. Looking at all of them would be time well spent.

One of the goals of projects like this is to ultimately provide input to engineers so they can determine how structures will behave during an earthquake. These simulations aren't quite at that point yet because they don't include site effects (local variations in the ground motion). For example, the ground motion model of the Loma Prieta event doesn't include the liquefaction in the Marina District. Still, the models at this point do provide a lot of useful information for engineers. Below is a screenshot from a video showing how the Golden Gate Bridge would behave during a 1906-like event (the motion has been exaggerated by a factor of 100).



These recreations do a great job of making the 1906 quake (and future quakes) a lot more real. The pictures from the 1906 quake are amazing, but watching the seismic waves race along the San Andreas fault really hits me at a gut level. I'm grateful to all the people who contributed to this work, they've done an amazing job.