San Andreas San Andreas FaultFault

Introduction

We live next to this fault It’s active It has a history of major earthquakes We had better know something about it ..so that we can better prepare

Features-1

Not recognized (in total) until ~1940 Not recognized as a plate boundary until

~1970 – plate tectonic theory had to happen first!

Named after Lake San Andreas (SW of San Fransico) following the 1906 earthquake

Features-2

Plate boundary between N. American and Pacific plates

Motion is transform with the Pacific plate moving northward

Plate features

Features-3

Active since 23.5 my BP; total = 315 km Long-term rate = 1.3 cm/year NOTE – there have been other strands

and similar faults have occurred in the region since ~~60 my BP; ~~480 km offset of Baja California peninsula

Major California Earthquakes

Complexity in detail

Note that foundations matter

Response of different Earth foundations

Pallett Creek

Work of T. Sieh, ~1987

Ph.D., California Tech.

Pallett Creek

Detailed map of sag-pond sediments across fault

Key to understanding

C-14 dating of organic material Carefully relating ages to offset of pond

sediments

Summation of findings

Covers 1700 years Median frequency = one per 132 years Range 50 – 500 years

Ft. Tejon earthquake

Features

1857 (before reliable, common seismometers and reproducible results)

Magnitude ~8.0 Offset near epicenter ~ 9 m

Ft. Tejon

Rupture and felt area

Note that this break extended to Pallett Creek