The Wrights tunnel is an abandoned railroad tunnel that crosses the San Andreas fault, and was damaged and deformed during the 1906, San Francisco earthquake. A plot showing post-earthquake measurements made in the tunnel appears in the Report of the State Earthquake Investigation Commission (1908). The amount of offset shown on this plot (1.5 m) has been used in several studies as being representative of the amount of fault offset in 1906. However, historical research shows that different observers reported differing amounts of fault offset in the tunnel, and that the 1.5 m given on the plot is not a surveyed measurement. In addition, the plot of the tunnel has been interpreted in several previous studies as evidence of a broad (1.5 km) zone of faulting beneath Summit Ridge. However, our analysis shows that this plot does not require a broad zone of deformation. Our historical research and modeling indicate that faulting was confined to a zone less than 400 m wide, and that 60 - 85% of the coseismic slip occurred across a single fault plane. There is no evidence for offset across a second shear zone beneath Summit Ridge in 1906. This implies that surface fractures reported on Summit Ridge in 1906 (similar to those that occurred in 1989) were not associated with deformation of the tunnel. Our modeling also indicates that total coseismic slip across the San Andreas fault zone in the Wrights tunnel in 1906 was at least 1.7 - 1.8 m.
AGU Index Terms: 7215 Earthquake Parameters; 7299 General or miscellaneous; 8199 General or miscellaneous; 1734 Seismology
Keywords/Free Terms: 1906 earthquake, Loma Prieta earthquake, Wrights Tunnel, San Andreas Fault.
JGR-Solid Earth 96JB02934
Vol. 102
, No. B1
, p. 635