from California Geology, May 1973, Vol. 26, No. 5.

EARTHQUAKE ACTIVITY BETWEEN MONTEREY AND HALF MOON BAY, CALIFORNIA

G. B. Griggs

Associate Professor of Earth Sciences

Division of Natural Sciences

University of California, Santa Cruz

INTRODUCTION

Since the mission days of the early 1800s, the occurrence of earthquakes of varying severity in the Half Moon Bay-Monterey Bay area has been a part of California's written history. Before that time the local Indians experienced earthquakes. Thus, the area has been one of seismic activity as long as man has been here and geological information indicates that the Earth shook here long before he arrived.

The most active region is the San Andreas fault zone which passes through the Santa Cruz Mountains 12 miles northeast of the city of Santa Cruz. Recently this fault has been studied in terms of plate tectonics. It may represent a transform fault along which the Pacific and North American plates are slipping past one another. Seismic activity is not confined to the fault zone, for the entire California coastal area west of the San Andreas is active. This report is a compilation of data on the earthquake history and seismic activity along this coastal area west of the San Andreas fault between Monterey and just north of Half Moon Bay (see maps, pages 106 and 107).

Sources of data include E. S. Holden [14,15], United States Geological Survey bulletins, publications of the U.S. Coast and Geodetic Survey, and bulletins of the Seismological Station of the University of California, Berkeley. Until 1934 only approximate geographic locations, or areas where the earthquakes were felt, were listed. Therefore, it is difficult to determine whether the San Andreas fault itself was the source of these events, or whether it was one of the faults to the west. By analyzing which areas were most strongly affected by individual earthquakes, one can get a reasonably accurate epicenter for some earthquakes. This is, however, a function of population density. From 1934 to 1941 epicenters were plotted and various intensity scales were used to indicate the effects of each earthquake. Holden, in addition, has estimated the intensity of many of the earlier earthquakes recorded in the 1800s. Beginning in 1941, latitude and longitude were listed by the Berkeley Seismological Station and in 1942, Richter magnitudes began to be listed. The overall reliability of epicenters in this region was greatly improved by the establishment of a network of telemetered seismographic stations in central California in 1960-61 [2]. Nonetheless, Brabb [3] reported apparent errors of as much as 30 kilometers and as little as 5 kilometers in locating the 1963 Chittenden, California earthquake by computer methods. Some additional data were taken from the California Department of Water Resources Bulletins 116-2 and 116-3 [7,8] which deal with California earthquakes of Richter magnitude 4 and greater, 1934 through 1961, including damage to hydraulic structures.

Early earthquakes are listed and described in chronological order in the table on page 108. Many earthquakes which were felt in the Monterey Bay area, but which probably were related to movement on the San Andreas fault, have not been included. For some of the earthquakes, intensities according to the Rossi-Forel Scale also are listed. Those earthquakes for which longitude and latitude were listed (subsequent to 1941) are plotted on the map.

SEISMIC ACTIVITY AND FAULTING

At least partial records are available for a large number of earthquakes that occurred between Monterey and Half Moon Bay over the last 135 years. From the more complete records of the past 30 years it appears that an average of five to ten earthquakes are felt each year in this area.

The earthquake activity in this region (exclusive of that along the trace of the San Andreas fault) appears to be concentrated in four main areas (see maps): 1) the coastal and offshore area between Half Moon Bay and the San Andreas fault; 2) the Santa Cruz Mountains between the Ben Lomond and Butano faults; 3) the offshore area between Point Ano Nuevo and Monterey; and 4) the area west of the San Andreas fault in the vicinity of Watsonville and Moss Landing.

Most of the shocks in the first two areas are of Richter magnitude 2.0 to 4.0. The activity in the Santa Cruz Mountains seems to be concentrated in an area that has extensive outcrops of the Butano Sandstone rather than directly along either the Ben Lomond fault or the Butano fault; these epicenters may, however, be inaccurately located due to positions of the seismograph stations.

The seismic activity offshore between Point Ano Nuevo and Monterey has been less than that in the Ben Lomond-Butano faults area. However, the offshore region has had a number of earthquakes of considerable magnitude. The largest recorded earthquake in recent time was one of estimated Richter magnitude 6.1 which occurred in 1926; it was felt from Lompoc, Santa Barbara County to Sacramento [19]. Although the epicenter has not been accurately located, the maximum intensities on shore were recorded in the Capitola-Santa Cruz-Davenport and Point Ano Nuevo areas.

A number of earthquakes of Richter magnitude 4.0 to 5.0 have been recorded in the Half Moon Bay-Monterey Bay area since 1934 [21]. Several occurred in the Watsonville-Moss Landing area, and some earthquakes of this magnitude were in the central portion of Monterey Bay itself. One of these earthquakes, which occurred in August 1970, had a Richter magnitude of 4.9, followed by a 3.5 shock. During March and April 1971, four earthquakes with Richter magnitudes of 4.0,4.4, 2.8, and 4.5, and epicenters in the central portion of the bay were recorded (written communication, R. D. Adams, Associate Research Seismologist, U.C. Berkeley Seismograph Station, 1971).

The Watsonville-Moss Landing area has been one of extensive seismic activity. A large number of earthquakes of Richter magnitude 3.0 to 5.0 have occurred in the coastal area between the San Andreas fault and Monterey Bay. The general orientation of this group of epicenters and also those in Monterey Bay appears to be parallel to the trend of the San Andreas fault.

The locations of these earthquakes support the contention that there is active faulting west of the San Andreas in this area [2]. As previously mentioned, a large number of small magnitude earthquakes have occurred in the vicinity of the Butano and Ben Lomond Faults.

The Zayante fault, extending southeastward from the Ben Lomond fault (see map), has been traced southeast to the area near Corralitos beyond which the surface covering of alluvium has evidently made it difficult to trace the fault [17]. A concentration of earthquake epicenters, however, begins in this area and extends in a broad zone to the southeast. The earthquake data then seem to indicate a zone of active faulting along what can be considered the extension of the Zayante fault to the southeast. However, because of possible errors in locating the epicenters, these events could have occurred on the San Andreas fault rather than on an extension of the Zayante fault.

Another fault in this area, the San Gregorio fault, extends on land for about 27 kilometers from Point Ano Nuevo on the south to San Gregorio on the north. It trends out to sea at both Point Ano Nuevo and San Gregorio, and its position on the continental shelf has been studied by the U.S. Geological Survey using seismic profiling equipment.

Preliminary analysis of the U.S. Geological Survey data indicates a fault zone, rather than a single fault, that extends along the coast in the offshore area from Monterey to Half Moon Bay. Green, [13] in a U.S. Geological Survey open-file report on southern Monterey Bay, designated a zone of faulting approximately 5 miles wide trending N 50W in the southern portion of the bay. More recently this zone has been traced from Point Ano Nuevo, where the San Gregorio fault zone trends out to sea, southeast-ward along the coast and across Monterey Bay where strands of it probably come ashore again on the Monterey peninsula [12]

The San Gregorio fault, which trends about N 20W, may be related to the Seal Cove fault which is well exposed at Moss Beach,just north of Half Moon Bay. According to Glen, [11] the Seal Cove fault cuts the cliff face at Moss Beach where it brings horizontal overlying Pleistocene terrace sands into clearly exposed fault contact with the mid-Pliocene Merced Formation. This site is one of extensive recent landsliding and sea cliff erosion. A ban on any new construction in the immediate area was recently imposed (1972) by the San Mateo County Board of Supervisors.

Geologic evidence for relatively recent fault movement in this area is apparent on the Geologic map of the southwestern Santa Cruz Mountains between Point Ano Nuevo and Davenport, California [9]. At Point Ano Nuevo the Monterey Formation apparently has been thrust over the terrace deposits with a dip-slip component of about 10 feet. Further to the south at Greyhound Rock the Santa Cruz Mudstone and terrace deposits appear to be vertically offset about 20 feet by three closely spaced faults. The terrace deposits on the sea cliffs where the San Gregorio fault goes out to sea are offset 5 to 10 feet [9].

The mollusks in the 100-foot terrace (lowest terrace) in the Santa Cruz area have been dated at 80,000 to 100,000 years [4]. However, problems recently encountered in dating with uranium disequilibrium series in mollusks have cast doubt on this age. Therefore, the dates of terrace deposit displacement by faulting are unknown.

Some of the recent earthquake epicenters appear to be directly related to the San Gregorio fault zone. In regard to the seismicity west of the San Andreas fault in the vicinity of San Francisco Bay, Bolt, Lomnitz, and McEvilly (2. p. 1737) state:

Small earthquakes located in the Santa Lucia Range northwest of PRS [seismograph station], under Monterey Bay, and northward in the Santa Cruz Mountains, show a more-or-less linear trend. On 22 May 1963, a 4.6 [Richter] magnitude earthquake occurred near the San Gregorio fault in the Santa Cruz Mountains. This linear trend suggests an active system of crustal fractures which is west of the San Andreas fault and which strikes approximately N 15W and joins the San Andreas fault near where it becomes submarine off San Francisco.

Earthquakes of Richter magnitude 2.9 and 3.4 that have occurred in the last 20 years have been plotted almost directly on the seaward projection of the San Gregorio fault in the area of Point Ano Nuevo and Greyhound Rock. The largest of these was in 1963. Farther southeast in the offshore area along this same zone, a number of other epicenters of varying magnitude have been located that extend nearly across Monterey Bay. The 6.1 Richter magnitude (estimated) earthquake of 1926 and the earthquakes of August 1970 and the spring of 1971 follow this same trend. The numerous earthquakes felt in the 1800s and early 1900s near Santa Cruz, Point Ano Nuevo, Pigeon Point, Monterey, and other areas adjacent to the bay may well have occurred in this same zone. This trend, first mentioned by Bolt, Lomnitz, and McEvilly [20] on the basis of epicenters, and then noted in sub-bottom profiles by the U.S. Geological Survey, [13] is clearly an active fault zone [5].

Burford [6] has noted a remarkable sequence of fault creep and seismic events along the San Andreas fault system which started early in 1970 near the Pinnacles National Monument and apparently terminated with the 4.9 Richter magnitude Monterey Bay earthquake on 3 August. These events suggest some interrelationship between the activity along the fault zone in Monterey Bay and that on the San Andreas fault zone. The true significance of the Seal Cove-San Gregorio fault zone was established by Cooper [10] when he showed that it crossed the ocean floor outside the Golden Gate and connected with the San Andreas fault near Bolinas Lagoon in Marin County.

1906 San Francisco Earthquake

The San Francisco earthquake of 18 April 1906 of Richter magnitude 8.3 (estimated), was the most destructive to the coastal area from Monterey to Half Moon Bay of any in recent time. Information on the effects of this and subsequent earthquakes discussed below is drawn from the early issues of the Santa Cruz Sentinel, Department of Water Resources Bulletin 116-3, Earthquake damage to hydraulic structures in California; [8] and Lawson, The California earthquake of April 18, 1906[18].

In the city of Santa Cruz itself there were no recorded deaths from the 1906 earthquake although the damage was extensive. The courthouse was almost destroyed as the cupola fell through the ceiling and landed in the basement. Many roofs and walls and at least one brick building collapsed. It was estimated that one third of the chimneys in the city were either destroyed or damaged. Landsliding resulted in many gaping cracks in the earth in various parts of the city, especially along the waterfront. All bridges were reported as being badly wrenched and unsafe. The water supply was shut off by broken mains and the 8-inch city water pipes at Wilders (several miles northwest of the city) were broken and twisted [20]

In Monterey the Hotel Del Monte was nearly destroyed, and four or five people were killed. Brick and stone buildings in Watsonville were cracked, crumbled, and twisted out of shape. Most of the brick or stone chimneys collapsed. The entire Watsonville-Big Creek Power Company was out of commission. The wharf was destroyed and several warehouses thrown down at Moss Landing. At Soquel there were deep and wide cracks on Main Street from which water flowed up through broken pipes. A 6-inch water main extending across the San Lorenzo River on a covered bridge was broken, and each end of the bridge moved 5.5 inches eastward. At Boulder Creek not a single chimney was left standing. On Deer Creek a large landslide came 500 feet down the mountain, covering 25 acres of ground. The slide material, composed of soil, clay, and shale was 300 feet thick.

North of Santa Cruz, an engineering party on their way south noticed fissures in the soil at San Gregorio from a few inches to 15 feet in width from which a little sand and water was being ejected (Santa Cruz Sentinel, 18 April 1906) [20] In the town of Half Moon Bay buildings were badly damaged, some old frame houses and the brick bank building were flattened. Several miles east of San Gregorio a creek was dammed up by a slide. In the town of Pescadero the shock was heavy; nearly all of the brick chimneys fell, and cracks were visible in the streets. At Pigeon Point, the brick lighthouse showed cracking around the inside of its base. At Bolsa Point, just north of Pigeon Point, a concrete pipe, 24 inches in diameter and 6 inches thick embedded in clay was cracked by the shock. In addition, water tanks at Half Moon Bay and Montara Point, San Mateo County collapsed.

Other Earthquakes

Damage to the coastal area subsequent to the large 1906 San Francisco earthquake has been less extensive. Two large earthquakes in October 1926 (estimated Richter magnitude 6.1) and their three aftershocks cracked plaster in Santa Cruz and also brought down a number of chimneys. Numerous plate glass windows were broken along Pacific Avenue. To the north, the city water main was broken at Laguna Creek, and at Davenport groceries were thrown from the shelves of the store (Santa Cruz Sentinel, 23 October 1926) [20,19]. An earthquake in February 1927, believed to be related to those in the fall of 1926, also cracked plaster, and articles fell from shelves in Santa Cruz. The epicenters, located from maximum recorded intensities for all these earthquakes, probably were in or just outside of southern Monterey Bay [19].

TABLE

1836 to 1941EARTHQUAKES MONTEREY AND HALF MOON BAY AREA

From Holden, U.S. Geological Survey; Bulletin university of California, Berkeley, Seismograph Station; and U.S. Coast and Geodetic Survey bulletins (see bibliography); all intensities are reported on the Rossi-Forel scale until 1937; after 1937 intensities are listed in the Modified Mercalli scale. Time gaps in record indicate periods for which data are apparently unavailable.

Earthquakes have been recorded by seismographs only since the 1920s. Prior to that time, the occurrence of earthquakes was recorded by newspaper writers, letter writers, and others.

Year Earthquake Description

1836 9-10 June - Severe shocks from Monterey northward (Hayward fault)

1838 Shocks in June and July (San Andreas fault) 12 May - A very short, slight shock at Monterey, intensity Ill

1841 Summer - At Monterey. The shocks of 120 earthquakes were felt during two successive summer months. The average. however, of two earthquakes a day, is not so frightful as it looks. the shocks being seldom severe and often so slight as to escape the notice of the uninitiated stranger. 3 July - Felt at Monterey and at sea, intensity Ill

1851 26 November - Coast of California from 37 to 40 latitude - 11 shocks.

1854 21 October - Light shock at Monterey, intensity IV.

1856 15 February - Heavy shock at Monterey. 3 September - Strong shock at Santa Cruz, intensity V. 6 September - A smart shock at Santa Cruz, intensity V.

1859 24 September - Slight shock at Half Moon Bay; waters of the bay receded 15 feet and returned suddenly; intensity IV. 18 October - Shock at San Francisco; at Half Moon Bay the water left for several seconds.

1864 26 February - Severe shocks in Santa Cruz, intensity VI. 18 December - Strong shock in Watsonville and Santa Cruz, intensity V.

1865 8 October - A great earthquake in San Francisco. At Mountain Charlie's on Santa Cruz Road, the earth opened in several places and steam and water were thrown up through the cracks. On the Santa Cruz Gap Road, chimneys were thrown down and the roads were more or less obstructed by stones that rolled down from the mountains. Later, observers remarked that "it is a singular fact that the shock was most severe at Santa Cruz and along the lower part of the Pajaro River." 14, 15, 19 October - Monterey, many shocks. 15 October - Shock in Santa Cruz.

1866 26 March - Severe shock at Monterey, intensity V. March - More shocks in Monterey.

1868 24 August - Shock at Santa Cruz. 31 August - Severe shock at Santa Cruz, lasting 10 or 15 seconds, intensity VI. 21 October - Santa Cruz Mountains near Pescadero; great damage done to redwood trees; limbs fell to the ground and large pieces of rock rolled down the mountains. 17 November - Quick and violent shock at Santa Cruz, intensity VI.

1869 29 January - Heavy shock in Watsonville, intensity V. 14 December - Shock in Santa Cruz.

1870 2 September - Shock in Monterey, intensity V+.

1871 6 February - Shock in Santa Cruz. September - Shock in Castroville.

1882 6 March - Two severe shocks in Santa Cruz, intensity III and V.

1883 30 March - Nine violent shocks at Watsonville, intensityVI. Three shocks at Santa Cruz, intensity VI. 8 or 9 (?) October - Severe shock at Santa Cruz, intensity V.

1884 25 March - Severe shocks from Santa Cruz to Petaluma, intensity V.

1885 22 December - Two heavy shocks in Santa Cruz, intensity VIII. 30 December - Slight shock in Santa Cruz, intensity III.

1886 26 May - Four shocks in Santa Cruz.

1887 11 January - Pigeon Point lighthouse; two shocks. 13 August - Santa Cruz light station, intensity III.

1889 14 April - A severe shock in Santa Cruz. 19 May - Santa Cruz, a sharp shock. 26 May - Quite a shock. 31 July - Santa Cruz earthquake lasting several seconds.

1890 24 April - A heavy shock in Santa Cruz, but little damage; many chimneys thrown down in Watsonville. 30 June - Earthquakes in Santa Cruz, shook all the houses in town.

1891 2 January - Heavy earthquake lasting 10 seconds; heaviest in years. 13 June - Monterey. a sharp shock; clocks were stopped; Santa Cruz lighthouse: duration about 2 seconds. 9 August - Monterey; A heavy shock causing buildings to rock; Santa Cruz lighthouse: duration 2 seconds.

1892 13 November - Monterey Bay region; an extremely lengthy and heavy earthquake rattled large buildings, shaking glassware and crockery off shelves, cracking chimneys. Oldest citizens say they have never experienced such a heavy shock.

1893 1 September - Santa Cruz lighthouse; waked sleepers, etc. in the town of Santa Cruz, 2 miles distant, shock described as unusually heavy; intensity VI. 15 October - Santa Cruz; a severe earthquake -undulations from west to east.

1898 2 May - Two distinct shocks, Santa Cruz, Salinas.

1899 30 April - Watsonville; intensity VII to VIII; large plate glass window broken; much crockery broken in surrounding country; chimneys were twisted and some toppled over.

1900 30 April - Monterey County possible central disturbance; heavy in Monterey. 29 July - Shock felt at Santa Cruz and Half Moon Bay. 31 August - Shock felt at Santa Cruz, Half Moon Bay and other places on the coast.

1905 19 April - Santa Cruz

1906 2 May - Santa Cruz 15 May - Point Pinos 27,28 May - Santa Cruz 7 September - Santa Cruz

1907 4 January - Santa Cruz; regular rocking motion. 7 January - Santa Cruz; quick, vicious shaking and twisting; duration 4 seconds; seemed to come from northwest. 8 January - Santa Cruz; sharp jolt.

1908 28 April - Santa Cruz; intensity V. Five sharp shocks of earthquakes were felt here this morning. 23 December - Santa Cruz.

1910 10 March - Monterey Bay Region; shock recorded seismographically over half the earth; felt in an area in excess of 50,000 square miles; possible location under Monterey Bay; intensity VI at Hollister, Salinas, Monterey and Santa Cruz. At Aptos and Watsonville - not less than intensity VII. At Watsonville the excitement reached same proportions; duration 10-30 seconds. 25 July - Aptos and Santa Cruz; fair shock.

1912 24 October - Santa Cruz; aftershock intensity II; recorded only from Santa Cruz.

1914 13 March - Santa Cruz; intensity IV. 28 December - Santa Cruz mountains; felt over 10,000 square miles; most severe at Felton and Boulder Creek; intensity VI; Santa Cruz Intensity V.

1916 8 August - Monterey Bay region; Santa Cruz; abrupt rocking; intensity V; duration 5 seconds, felt by all. 3 November - Santa Cruz; intensity V; abrupt bumping northeast to southwest, lasting 1 second; felt by many.

1919 11 December - Santa Cruz; intensity V.

1920 5 October - Monterey Peninsula; intensity V.

1921 11 June - Felt strongly at Santa Cruz; slight tremor followed by a heavy shock; origin probably at sea off Monterey Bay; duration 20 seconds. 12 June - Felt strongly at Santa Cruz; may be two shocks superimposed.

1924 12 November - Epicenter probably a few miles north of Santa Cruz.

1926 22 October - Rossi-Forel intensity VIII; centered in Monterey Bay; damage to lighthouse on Ano Neuvo Island. In Santa Cruz old brick buildings suffered. Second shock of same strength felt 1 hour later; third shock from same point, less severe; other aftershocks felt over 100,000 square miles. 24 October - Aftershock, Rossi-Forel intensity IV hit Santa Cruz.

1927 12 April - Slight tremor in Santa Cruz.

1929 24 September - Monterey; a series of slight shocks. 5 November - Slight shock at Monterey and Santa Cruz.

1930 &9 January - Series of fairly strong shocks felt in Santa Cruz. 11 February - Shock felt in Santa Cruz and Bonnie Doon. 2 September - Shock at Santa Cruz.

1931 6 June - Earthquake, Rossi-Forel intensity IV at Capitola, III at Santa Cruz. 9 June - Shock felt at Santa Cruz, Point Ano Nuevo (intensity IV), and Pigeon Point lighthouses. 10 June - Shock felt in Santa Cruz, Davenport, Half Moon Bay. 18 October - Shock felt in Carmel, Moss Landing, Aptos and Santa Cruz Light Station, intensity IV. 21 October - Shock felt in Monterey. 21 November - Small earthquake in Monterey Bay. 3 December - Shock at Pigeon Point and Santa Cruz Light Stations, intensity II.

1932 10 January - Watsonville shock, intensity IV. 26 February - Shock in Monterey Bay region, intensity V at Santa Cruz; intensity IV at Aptos and Monterey. 14 June - Earthquake felt in Santa Cruz, San Gregorio, Half Moon Bay; intensity IV. 16 October - Earthquake felt in Aptos and Santa Cruz, intensity IV. 19 October - Shock felt in Monterey and Santa Cruz, intensity IV. 20 October - Watsonville shock, intensity IV. 24 October - Santa Cruz, Soquel and Half Moon Bay shock, intensity IV. 3 November - Santa Cruz, Half Moon Bay, Pigeon Point Light Station shock, intensity IV.

1933 12 January - Mt. Hermon, Soquel, Santa Cruz, intensity IV. 19 May - Santa Cruz, small objects fell, intensity V; Point Ano Nuevo, intensity IV. 16 July - North shore of Monterey Bay, intensity V; felt in communities on the south shore. 11 December - Watsonville, Santa Cruz, intensity IV. 13 December - Santa Cruz, Monterey, intensity IV.

1934 23 April - Santa Cruz and Watsonville, intensity IV. 7 June - Point Ano Nuevo Island, intensity IV. 16 June - Intensity IV at Monterey. 31 July - Point Ano Nuevo Light Station, intensity IV. 30 December - Fairly strong shock probably centering offshore near Santa Cruz. Cracked plaster; slight damage in weakly built houses.

1935 3 January - Santa Cruz, intensity V. 25 October - Santa Cruz, Soquel, Rossi-Forel intensity IV.

1936 7 January - Epicenter probably in Monterey Bay region about 9 miles northwest of Watsonville.

1937 8 January - Watsonville, Monterey Bay region about 9 miles northwest of Watsonville; intensity V. 5 March - Monterey Bay region, Modified Mercalli intensity IV in Santa Cruz. 27 October - Monterey Bay region, Modified Mercalli intensity III in Santa Cruz.

1938 12 February - At sea about 10 miles SW of Santa Cruz. 3000 square miles affected; maximum Modified Mercalli intensity VI. Cracked windows and toppled a chimney in Santa Cruz; rumbling sound accompanied shock.

1941 to present - recorded on map, page 107.

Several earthquakes in late December 1934, again broke the Santa Cruz water supply line on the coast (Santa Cruz Sentinel, 1 January 1935 [20]. The first and largest of the two earthquakes of August 1970 was later blamed for the failure of a city water main in the city of Santa Cruz itself.

It is apparent from this review of the records that earthquakes are common phenomena in the Santa Cruz-Monterey Bay area. Over the years the loss of life has been small but property damage has been considerable. This pattern of tremors has been present through the recorded history of the area and all evidence indicates that it will continue into the future.

CONCLUSIONS

The coastal areas of California are those where the concentrations of people, construction, and development are already high and are continually increasing. Seismic activity in the coastal area is also high. With the aid of marine geological techniques such as seismic reflection profiling, and the increased interest in environmental geology, a more complete study of the coastal areas --both onshore and offshore --should be made to correlate seismic history and the evidence of recent tectonic activity. This kind of information should be made available to planners to aid in reducing the potential of disaster resulting from future seismic events.

ACKNOWLEDGEMENTS

The writer is grateful for the assistance of R. L. Farrington, R. E. Garrison, K. Biddle and J. Childs in the preparation of the manuscript.

REFERENCES

1. Atwater, Tanya. 1970. Implications of plate tectonics for the Cenozoic tectonic evolution of western North America: Geological Society of America Bulletin, vol. 81, pp. 513-3536.

2. Bolt, B.A., C. Lomnitz, and T.V. McEvilly. 1968. Seismological evidence on the tectonics of central and northern California and the Mendocino escarpment: Seismological Society of America Bulletin, vol. 58, pp. 1725-1767.

3. Brabb, E.E. 1967. Chittenden. California. earthquake of September 14. 1963: California Division of Mines and Geology Special Report 91, pp. 45-53.

4. Bradley, W.H., and W.C. Addicott. 1968. Age of the first arine terrace near Santa Cruz., California: Geological Society of America Bulletin. vol. 79, pp. 1203-1210.

5. Brown, R.D.. and W.N.K. Lee. 1971. Active faults and preliminary earthquake epicenters (1969-1970) in the southern part of the San Francisco Bay Region: U.S. Geological Survey Miscellaneous Field Studies Map MF307.

6. Burford, R.O. 1971. Fault creep and related seismicity along the San Andreas fault system between Pinnacles National Monument and San Juan Bautista (Abstract): Geological Society of America Cordilleran Section. pp. 90-91.

7. California Department of Water Resources. 1964. Crustal strain and fault movement investigation: Department of Water Resources Bulletin 116-2, Resources Agency of California, 96 p.

8. California Department of Water Resources. 1967. Earthquake damage to hydraulic structures in California: Department of Water Resources Bulletin 116-3, Resources Agency of California, 200 p.

9. Clark. J.C. 1970. Geologic map of the southwestern Santa Cruz Mountains between Point Ano Nuevo and Davenport, California: U.S. Geological Survey open-file map.

10. Cooper, Allan. 1971. Structure of the continental shelf west of San Francisco. California: California State University at San Jose, unpublished M.S. Thesis.

11. Glen, W. 1959. Pliocene and lower Pleistocene of the western part of the San Francisco Peninsula: University of California Publications in Geological Sciences Bulletin, vol. 36, pp. 147-198.

12. Green,H.G. 1972. Report on USGS studies of the Quaternary in the San Francisco Bay Area: Guidebook for Friends of the Pleistocene.

13. Green, H.G. 1970. Geology of southern Monterey Bay and its relationship to the ground water basin and salt water intrusion: U.S. Geological Survey open-file report.

14. Holden, E.S. 1887. List of recorded earthquakes in California, Lower California, Oregon and Washington terrItory: California Printing Office, Sacramento. 78 p.

15. Holden, E.S. 1887. Earthquakes in California: American Journal of Science, vol. 37, pp. 392-402.

16. Jennings, C.W., and J.L. Burnett. 1961. San Francisco sheet: California Division of Mines Geologic Map of California, Olaf P. Jenkins edition.

17. Jennings, C.W. and R.G. Strand. 1958. Santa Cruz sheet: California Division of Mines Geologic Map of California. Olaf P. Jenkins edition.

18. Lawson. A.C. 1908. The California earthquake of April 18,1906: Report of the State Earthquake Investigation Commission. vol. 1. Carnegie Institution.

19. Mitchell,G.D. 1928. The Santa Cruz earthquakes of October.1926: Seismological Society of America Bulletin, vol. 18, pp. 153-213.

20. Santa Cruz Sentinel, City of Santa Cruz, California, many issues.

21. Seismological Station, University of California, Berkeley.1920-1971. Bulletins, University of California Press, Berkeley.

22. Townley, S.D., and Allen. M.W. 1939. Descriptive Catalog of earthquakes of the Pacific Coast of the United States 1769-1 92 8: Seismological Society of America Bulletin, vol. 29, p. 1-297.

23. United States Geological Survey Bulletins 1889-1895.

24. United States Coast and Geodetic Survey. U.S. Earthquakes 1928-35 and 1936-41.