7.3 !

Movement: Instantaneous 7 meters (22 feet) Horizontal.

Historical Movement: 240 km (160 miles) in the last 5.0 million years.

GEOLOGY OF THE SAN ANDREAS FAULT AT TEJON PASS, CALIFORNIA

Vincent Rex Ramirez

The major conclusions of this seven year study are:

1. All rocks older than late Miocene are now offset 300 km in a right-lateral direction by the San Andreas fault. In particular, the Neenach Volcanic Formation and sedimentary rocks of the Western Antelope Valley Basin were apparently displaced by two episodes of faulting, the first of which was 55+ km between 14 and 5 m.y. ago, and the second which was about 240 km and occurred within the last 5 m.y.

2. Rocks of the Hungry Valley Formation which are 4.5-5.0 m.y. old are offset 220 km in a right-lateral sense from their source terrane located on the south side of the San Bernardino Mountains, near Morongo Valley.

3. Rocks younger than 4.5 m.y. are offset proportionately lesser amounts, according to their ages.

Initiation of strike-slip displacement histories are similar in northern California along the San Andreas fault and in southern California along the San Andreas and San Gabriel faults. Also, a long term rate of displacement of 4.9 (+0.0, -0.5) cm per year is apparent from the offset of the Hungry Valley Formation.

Identification of the rupture zone associated with the 1857 earthquake has revealed several locations which suggest that individual earthquakes are accompanied by displacements of 6 to 8 m per event.

The magnitude (ML) of a future earthquake centered at Tejon Pass is likely to be less than 7.5, and more likely 6.7 to 7.0. This conclusion is based on comparisons with similar events (including the San Francisco earthquake in 1906), calculations of seismic moment, empirical and theoretical relationships which relate seismic moment and magnitude, and analysis of the 1857 event.

Rex Reports