Comments on 1906 Ground Motion Movies
Color Scheme in Movie
The movies show the propagation of seismic waves away from the epicenter, which lies 2 miles offshore from San Francisco. The residual colors indicate the peak shaking intensity experienced at locations up to the time, in seconds, indicated near the top center of the movie. The current intensity, at the time shown on the view, is indicated by shading of the colors.
Spatial Variations in Shaking Intensity
Note that the rupture front simultaneously propagates to the north and south along the San Andreas fault away from the epicenter. The character of the shaking at any location is influenced by the amount of slip on nearby sections of the fault, properties of the geologic materials that the waves passed through to reach the location, and especially the local ground conditions at the location.
Sites on bedrock are often less susceptible to strong shaking than sites located on unconsolidated sediments. In particular, thick accumulations of deposits in sedimentary basins tend to amplify shaking, not unlike a bowl of jello. For example, Santa Rosa experienced heavy damage in 1906 and it can be seen in the simulation that high intensities of shaking are predicted by the computer model. These high intensities in the simulations result from sedimentary deposits under Santa Rosa, which were included in the the 3-D Geologic Map used as input to the model. Much of the character in the final shaking intensities offshore can also be attributed to the presence of offshore sedimentary basins.
1906 Earthquake Parameters
- Magnitude: Mw 7.8-7.9
- Hypocenter: 122.55 W, 37.75 N, 6 miles (10 km) depth
- Rupture length (total): 300 miles (480 km)
- Rupture length (north): 205 miles (330 km)
- Rupture length (south): 93 miles (150 km)
- Rupture duration (north): 90 s
- Rupture duration (south): 54 s
- Average offset along the fault: 10 ft (3 m)
1906 Wave Propagation
- Average S wave speed: 8000 mph (3.6 km/s)
- Average P wave speed: 14000 mph (6.2 km/s)
- It takes less than 30 seconds for strong shaking to envelope the entire San Francisco Bay area.
|City||Shaking begins||Strong shaking begins||Distance from epicenter||Max ground displacement|
|San Francisco (Ocean Beach)||1.7 s||2.4 s||2 miles (3 km)||~2 ft|
|San Francisco (City Hall)||2.6 s||3.4 s||7 miles (12 km)||~2 ft|
|Oakland||4.9 s||6.2 s||16 miles (25 km)||~2 ft|
|San Jose||13.0 s||20.6 s||46 miles (74 km)||~2 ft|
|Santa Rosa||13.7 s||16.9 s||48 miles (78 km)||~2 ft|
Movies Tracking Fault Rupture
These movies follow the fault rupture by flying along the fault. One movie follows the rupture south while the other follows the first 28 seconds of rupture to the north. The seismic waves seen in the map view movies above are generated by slip on the fault. Note that the fault is deformed horizontally as the rupture travels up the fault, leaving in its wake an offset of 20-30 feet in some places.
- Movie following rupture south: low resolution (640x360, 2.7 MB), high resolution (960x540, 6.9 MB)
- Movie following rupture north: low resolution (640x360, 5.5 MB), high resolution (960x540, 14 MB)
A small, 4 inch high, tsunami was generated by the 1906 earthquake and was measured at the Presidio tidal gauge. The tsunami rapidly decreased in size as it entered San Francisco Bay. Modeling of this tsunami provided useful information regarding the location of the epicenter offshore and the nature of the initial rupture process.