Excerpt from Geologic Trips San Francisco and the Bay Area by Ted Konigsmark ISBN 0-9661316-4-9 GeoPress All rights reserved. No part ofthis book may be reproduced without written permission in writing, except for critical articles or reviews. For other geologic trips see: www.geologictrips.com Trip 6. BAY AREA FAULTS Mussel Rock Hayward Devils Slide South of Market Crystal Springs Reservoir H a yw S a a r n d A fa nd u l r t ea s fau l t San Pacific Francisco Ocean Bay P i la rc i to s fa u l t 5 Miles The bay area is cut by several large faults that are part of the San Andreas fault system. Two of these faults - the San Andreas and Hayward - are very active and have had a number of major earthquakes over the last 150 years. On the trip to the bay area faults you will investigate these faults and see how they affect roads, houses and buildings in the Bay Area. 112 Trip 6. BAY AREA FAULTS Living with Some Big Faults During this trip you will see the San Andreas and Hayward faults, two of the largest and most active faults in the San Andreas fault system. The trip involves a 70-mile drive and several short walks, and should take about a day if you go at a leisurely pace. You will visit these localities: Mussel Rock: The San Andreas fault enters the Pacific at Mussel Rock after a 500-mile journey across southern and central California. On the face of the bluff near Mussel Rock there is a massive landslide formed from crushed rocks of the Merced Formation caught up in the fault zone. Devils Slide: At Devils Slide, Highway 1 travels high along the face of a steep sea cliff that is cut by a large landslide that periodically closes the highway. The slide occurs in weak rocks along the contact zone between Paleocene sedimentary rocks to the north and a large block of granite to the south that makes up most of Montara Mountain. Crystal Springs Reservoir: The Crystal Springs and San Andreas Reservoirs lie in a linear valley carved out along the San Andreas fault. Here you will see where the fault that was formed during the 1906 San Francisco earthquake cut through these reservoirs. Hayward: The Hayward fault cuts through downtown Hayward. In Hayward, you will see a fault scarp near Prospect Street, and where a curb on D Street is being slowly offset by creep along the Hayward fault. You will also drive along Mission Blvd. from Hayward to Fremont and see where recent fault scarps are covered by houses and roads. South of Market: The area south of Market Street was heavily damaged in the 1906 San Francisco Earthquake and again in the 1989 Loma Prieta Earthquake, largely because the area is underlain by thick unconsolidated water-saturated sediments that shake violently during earthquakes. On the trip to this area you will see places where streets and buildings are slowly sinking because of compaction of these soft sediments. 113 Mussel Rock After a 500-mile trip from the Gulf of California and through south and central California, the San Andreas fault leaves the California coast at Mussel Rock and enters the Pacific Ocean. As it leaves the coast, it leaves behind a geological disaster area and a good lesson on how not to use land that is geologically unstable. Several geologic phenomena occur in this area, and all are bad: the area is in a major active fault zone; the rocks in the fault zone are loose and unstable; and the bluff is steep and high. The steep, unstable rocks have caused a major landslide in the bluff. Because of the landslide, the edge of the bluff is rapidly receding, at a rate of up to three feet per year. Land slippage is accelerated when the ground is wet from heavy winter rains. Many houses built at the top of the bluff have been threatened by the landslide and several have been removed as the bluff has receded. Daly City See detailed map of Muscle Rock M us landslide la se nd l R sli oc de k S ky li Mussel ne F Rock r ee Westline Dr w S a an y fa A ul nd t z re on as e Pacific P Ocean al ay 1 m Highw et to A v e MUSSEL ROCK San Andreas Fault Pacific Manor Geologic Site M ano r Dr ive 1/2 Mile The San Andreas fault enters the Pacific Ocean immediately north of Mussel rock. The landslide in face of the bluff is caused by the weak rocks in the fault zone. 114 MUSSEL ROCK LANDSLIDE Geologic Site 500 Feet h n c a a e e Bluff top B e S c l k n y e O o li s z n s e c t i u l f u D i M a r c f a s a P re d T n r A a n ce Landslide a o S f 1 Landfill 9 0 6 Skyline Dr. S a n A n d Mussel re a Rock Toe of s fa landslide u lt Longview Dr.. Longview Westline Dr. Playground Westline Dr. Mussel Rock consists of Franciscan rocks and lies at the south side of the San Andreas fault zone. The fault zone extends from Mussel Rock northward for about one-half mile and includes most of the reentrant in the bluff north of Mussel Rock. The rocks in the fault zone are mainly crushed soft sandstones and shales of the Merced Formation, and it is these rocks that have slumped to form the large landslide that occupies the reentrant. The steep face of the bluff immediately north of the landslide represents the north side of the fault zone. This area has had several land uses over the last century. From 1905 to 1920, the Ocean Shores Railroad had a rail line along the coastline near the toe of the landslide. The rail line was partly destroyed during the 1906 earthquake and had numerous problems with landslides at many other times. The Coastal Highway was opened in 1936 and followed the old rail line along the coast. This highway also had serious problems with landslides and was finally abandoned in 1957 after an M5.3 earthquake that had its epicenter near Mussel Rock. In the early 1960’s, 115 the area along the top of the bluff came under development and many houses were built precariously at the edge of the unstable bluff. From 1963 to 1979, a sanitary landfill was operated in the depression at the base of the landslide. After the sanitary landfill was abandoned, the area of the landfill continued to be used as a dumpsite for construction materials. Three geologic sites will be visited at Mussel Rock. The first site, on Westline Drive, is at the edge of the bluff overlooking the landslide. Here you’ll see cracks along Westline Drive caused by landslides in the unstable rocks at the top of the bluff. The second site, the Longview Playground, is near the fault that was formed during the 1906 San Francisco Earthquake. The third site, on Skyline Drive, is in the San Andreas fault zone and overlooks the landslide scarp formed by the fault zone. Westline Drive To get to this site, follow Highway 1 south from San Francisco to Daly City. Continue on Highway 1 through Daly City. Take the Manor Drive exit 1.5 miles south of the Highway 1 - Highway 35 interchange. Go north on Palmetto Ave. to Westline Drive then follow Westline Drive to the north. Park where Westline Drive turns sharply east at the edge of the bluff. At the bend in Westline Drive you are at the top of the bluff and near the south edge of the San Andreas fault zone. Several homes at the edge of the bluff in this area have been removed and others are seriously threatened as the bluff has receded because of the landslide. You can see evidence of the land slippage in the arc-shaped cracks in the street and sidewalk along the bend in Westline Drive. These cracks indicate active slumping of the edge of the bluff. From here you also get a good view of the landslide along the face of the bluff to the north. Longview Playground Continue east on Westline Drive one block to Longview Drive, then follow Longview Drive one-half block to the Longview Playground. The Longview Playground is at the south side of the San Andreas fault zone and at the edge of the bluff overlooking the landslide in the bluff. The fault zone consists of hundreds of offsets along the San Andreas fault. The most recent of these offsets was caused by the 1906 San Francisco Earthquake. The trace of the 1906 fault crosses Longview Drive 100 feet south of the playground and then goes through the southwest corner of the playground and continues northwest down the landslide scarp just to the north of the steep gully below the playground. 116 From the bend in Westline Drive, this photo looks northeast across the San Andreas fault zone. The road is at the south side of the fault zone and the opposite cliff follows the northeast boundary of the fault zone. The road at this location is close to the edge of the bluff and the arcuate cracks in the road show that the bluff top is slipping downslope into the soft rocks of the fault zone.