38 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
PAJARO RIVER TO THE NORTH END OF THE COLORADO DESERT. 
By H. W. FAIRBANKS. 
The earthquake of April 18, 1906, opened and displaced the walls of the old fault along 
the Rift as far south as the town of San Juan in San Benito County. The fault-trace 
passes directly under the western span of the Southern Pacific Railroad bridge across 
San Juan River, as shown by the displacement of the piers at the end of the bridge, a 
distance of 3.5 feet. For a distance of nearly half a mile on either side of the bridge, 
the river has established itself in the Rift. To the northwest the steep slopes of Mount 
Pajaro facing the canyon do not show any regular fissure. This does not, however, 
indicate any discontinuity in the fault, for the surface of the whole mountain is more or 
less broken by auxiliary cracks, secondary fissures, and slides. 
Southeast over the hills from the point where the Rift leaves the river, the character- 
istic features of the Rift make their appearance. It is marked by a small pond (plate 17s), 
springs, and a more or less continuous ridge with its steeper face toward the southwest. 
The fissure of the recent earthquake follows this series of features (plate 184), and, at 
a point halfway between the bridge and San Juan, there is shown in a broken fence a 
horizontal displacement of 4 feet. A mile before reaching San Juan, granitic rocks are 
exposed upon the southwest side of the Rift. Shortly beyond this point the Rift leaves 
the hills and traverses the western edge of the valley of the San Benito River. The ridge 
which we have been following is now lost in the level floor of the valley, but as far as 
traceable its course is directly toward the low bluff upon the eastern edge of the town of 
San Juan. The fissure of the recent earthquake is to be seen where it crosses the road 
0.5 mile northwest of San Juan, but has not been noted farther along the old Rift line. 
It appears to bend more easterly, and this probably connects it with the disturbances of 
the earth between Hollister and San Juan. Mr. Abbe, of San Juan, states that the 
earthquake of 1890 opened the old Rift and that the displacement of the walls, tho small, 
was in the same direction as in the recent earthquake. 
The town of San Juan stands upon a bench of gravel which dips gently in a south- 
westerly direction, but upon its northeastern side presents a steep face which, near the 
old mission, has a height of about 50 feet. This bluff is marked thruout its length of 
0.5 mile by several springs; and there can be little doubt that it owes its existence to a 
fault movement uplifting and tilting toward the southwest a portion of the floor of the 
valley, and that it thus originated in the same way as other similar features which we 
shall find to be characteristic of the Rift. The Rift leaves the valley southeast of San 
Juan and gradually rises along the eastern slope of the Gavilan Range. It intersects 
the head of San Juan Canyon, and has here given rise to an interesting modification of 
the drainage. San Juan Canyon is long and narrow and is formed by the union of several 
small streams which, rising upon the higher slopes of the range, pursue a normal course 
toward the San Benito Valley, until reaching the Rift, they turn northwest and slightly 
away from the fracture line, giving rise to San Juan Canyon. At the point where the 
Rift intersects the canyon, the narrow ridge between the canyon and the valley has been 
broken thru, and the whole drainage passes directly down the mountain, abandoning 
the canyon, which is now filling with débris fan material. 
For about 10 miles southeast from the head of San Juan Canyon, the Rift follows the 
eastern slope of the Gavilan Range. It is marked by small valleys and gulches, by 
hollows and ridges upon whose sides oak trees are growing; and it is followed almost 
continuously by a wagon road. One of the most interesting features along this portion 
of the Rift is Green Valley, a broad cienega due to the filling up with gravels and silt 
of a valley lying close under the steeper portion of the Gavilan Range. There are two 
fault-lines below the valley and about 0.25 mile apart, The cienega is due to vertical 
