.

Historical Seismicity in the 

South San Francisco Bay 

Region

By William H. Bakun1

Open-File Report 2008-1151, v. 1.1 

2008, revised 2009 

U.S. Department of the Interior 

U.S. Geological Survey 

1

 Menlo Park, Calif. 
U.S. Department of the Interior 

DIRK KEMPTHORNE, Secretary 

U.S. Geological Survey 

Mark D. Myers, Director 

U.S. Geological Survey, Reston, Virginia 2008 

Revised 2009 

For product and ordering information: 

World Wide Web: http://www.usgs.gov/pubprod

Telephone: 1-888-ASK-USGS

For more information on the USGS—the Federal source for science about the Earth, 

its natural and living resources, natural hazards, and the environment: 

World Wide Web: http://www.usgs.gov

Telephone: 1-888-ASK-USGS

Suggested citation: 

Bakun, William H., 2008, revised 2009, Historical seismicity in the south San Francisco 

Bay region: U.S. Geological Survey, Open-File Report 2008-1151, v. 1.1 

[http://pubs.usgs.gov/of/2008/1151/].

Any use of trade, product, or firm names is for descriptive purposes only and does 

not imply endorsement by the U.S. Government. 

Although this report is in the public domain, permission must be secured from the 

individual copyright owners to reproduce any copyrighted material contained within 

this report. 
Summary 

Locations (intensity centers) and moment magnitude M for 

24 earthquakes (1858-1911) in the southern San Francisco 

Bay area are estimated from Modified Mercalli intensity 

(MMI) assignments. The uncertainties in location and M

are generally large because there are few MMI assignments 

available. Preferred locations are selected to be consistent 

with a simple model (Oppenheimer et al., 2008; Bakun et 

al., 2008) for seismic activity on the central and southern 

Calaveras fault. Significant seismic activity can be 

explained by repetitive failure of the same fault areas in 

nearly identical earthquakes. 

Significant earthquake activity occurred on both east- and 

west-Bay faults in the ten or so years before the 1868 eastBay earthquake and before the1906 and 1989 west-Bay 

earthquakes. 
2

Introduction 

The 31 October 2007 M5.4 Alum Rock earthquake provided strong support for 

Oppenheimer et al.’s (1990) model for seismicity on the central and southern Calaveras 

fault. Nearly identical waveforms for the 2007 event and the 5 September 1955 ML 5.5 

earthquake (Oppenheimer et al., 2008) showed that both events occurred on 

Oppenheimer et al.’s (1990) Zone V on the central Calaveras fault. Oppenheimer et al. 

(1990) had shown that waveforms for the 26 October 1943 ML 4.9 and the 13 June 1988 

M 5.0 earthquakes were also nearly identical and that both of these events were located 

on a patch of the central Calaveras fault which they labeled Zone IV. These recurring M 

5-6 earthquakes suggest a simple model for seismic activity on the central and southern 

Calaveras fault. Significant seismic activity can be explained by repetitive failure of the 

same fault areas in nearly identical earthquakes. This report presents analyses of seismic 

intensity data necessary to address the question “Is the pre-instrumental seismicity in the 

region consistent with this simple model of seismicity?” This question is addressed 

directly by Oppenheimer et al. (2008b) 

I use Bakun’s (1999) list of significant San Francisco Bay region earthquakes, his Table 2 

(updated to include events since 1999), as the starting point of this study. The locations 

and M given here are consistent with Bakun’s (1999) analysis. The uncertainties in 

location are often large and Bakun (1999) sometimes chose not to list a preferred location 

or an intensity center for poorly located events. This report provides details of location 

and M and their uncertainties that were not given by Bakun (1999). In particular, the 

feasibility of location of historical earthquakes on Oppenheimer et al.’s (1990) Calaveras 

fault Zones I-VI is addressed. 

My preferred locations in Table 1 are the epicenters for post-1911 earthquakes and my 

best guess for the epicenters of the pre-instrumental events. The intensity centers 

provided by the analysis are used for events with sufficient MMI assignments to 

reasonably constrain the intensity center locations. The intensity centers, however, are 

not good estimates of location for many of the events studied here. As an example, 

consider the earthquake that occurred on 30 March 1883 shown in Figure 13. The 

maximum intensities were assigned at a cluster of communities straddling the San 

Andreas Fault east of Monterey Bay. The earthquake generally was not felt at 

communities in San Luis Obispo County to the south, closer to the intensity center 
3

(36.1°N, 122.8°W). MI at the intensity center is 7.1. A magnitude 7 earthquake in 1883 

near the intensity center would have been felt all along the central California coast. For 

this earthquake, the intensity center is a computational artifact, and it should be ignored. 

MMI assignments at distant sites are necessary to constrain the location. A few MMI III 

assignments, if available, would constrain the intensity center to the region of the 

maximum MMI. Lacking those MMI assignments, the algorithm provides an implausible 

intensity center. My preferred location, on the San Andreas Fault, near the cluster on 

maximum MMI, honors the available data and ignores the implausible intensity center. I 

do not list implausible intensity centers in this report lest the casual reader mistakenly 

adopt these artifacts as useful information. 

My preferred locations for some earthquakes are selected to be consistent with 

Oppenheimer et al.’s (2008b) model of repetitive failure of the same Calaveras fault areas 

in nearly identical earthquakes. Specifically, my preferred locations for earthquakes on 

26 November 1858 (#1) and 2 January 1891 (#17) are in Oppenheimer’s (1990) Zone V. 

Similarly, my preferred location for the earthquake on 20 June 1897 (#19) is in 

Oppenheimer’s (1990) Zone II. Other locations are possible for these three events. 

Oppenheimer et al.’s (2008b) model can be extended through the pre-instruimental 

period with these preferred locations. My preferred locations for the other preinstrumental events are elsewhere, i.e., not on the Calaveras fault. 

Instrumental Earthquakes (1911 – 2008)

Earthquakes in the southern San Francisco Bay region that were recorded on 

seismographs (Table 1, Part b) are taken from Oppenheimer et al. (1990) and 

Oppenheimer et al. (2008). The 9 March 1949 ML 5.2 event and the 6 August 1979 M 5.9 

Coyote Lake earthquake occurred on Zones I and II, respectively. The 1 July 1911 M 6.2 

and the 24 April 1984 M 6.2 Morgan Hill earthquakes occurred on Zone III. The 26 

October 1943 ML4.9 and the 13 June 1988 M 5.0 earthquakes occurred on Zone IV. The 

5 September 1955 ML5.5 and the 31 October 2007 M 5.4 earthquakes occurred on Zone 

V. No earthquakes are known to have occurred in Zone VI. Historical (pre-1911) 

earthquakes in the region are listed in Table 1, Part a. 
4

Intensity Data 

Almost all of the MMI assignments used here are listed in Bakun (1998). J. Boatwright 

and H. Bundock (personal communication, 2008) have compiled more-extensive sets of 

MMI assignments for the 4 July 1861 northern Calaveras fault earthquake and for the 21 

October 1868 Hayward fault earthquake. Their MMI assignments are used in the 

analysis of the 1861 event. Toppozada et al.’s (1981) MMI assignments and Boatwright 

and Bundock’s (personal communication, 2008) MMI assignments are used for 

independent analyses of the 1868 earthquake. 

Analysis of Intensity Data 

Following Bakun (1999), I estimate M from individual intensity observations for a trial epicenter 

using 

 MI = mean (Mi), (1) 

where 

Mi = {(MMIi-δMMIi) +3.29 + 0.0206∆h,i}/1.68, (2) 

MMIi, δMMIi and ∆h,i are the MMI value, the site correction and the hypocentral distance, 

respectively, at site i. A depth of 10 km is assumed. The δMMI are listed in Bakun (1998). 

δMMIi is zero if a site correction is not available. 

I find the misfit for each trial epicenter from 

rms [MI

] =[rms (MI – Mi) – rms0(MI – Mi)], (3) 

where rms (MI

 – Mi) = {i[Wi(MI - Mi)]2 / iWi

2

}1/2, rms0(MI – Mi) is the minimum rms (MI – Mi) 

over the grid of trial epicenters, and Wi

 is Bakun and Wentworth’s (1997) distance weighting 

function: 

0.1 + cos [(∆i/150)(/2)] for ∆i <150 km 

Wi = { (4) 

 0.1 for ∆i ≥150 km. 

The intensity center is the trial source location for which rms [MI

] is minimum (Bakun, 1999). 
5

The rms [MI] contours provide a basis for identifying regions of likely epicenter location. The 

rms [MI] contours bound the epicentral region. Bakun and Wentworth (1997) associated rms 

contour values with confidence levels that the epicenter was within the contour, as tabulated in 

the corrected Table 5b of Bakun and Wentworth (1999). MI at trial locations within the 

appropriate confidence-level contours is the best estimate of M for these source locations. 

Analysis of MMI for 24 Historical Earthquakes (1858-1911)

26 November 1858. MMI can be assigned at seven of the 16 localities for which felt 

reports (Toppozada et al., 1981) are available. The maximum MMI was assigned at San 

Jose where brick walls were cracked and chimneys thrown down. Zones III-VI on the 

Calaveras fault are within the 67% confidence contour for location (Figure 1). MI is 6.2 

for zones IV and V. The preferred location is in zone V at the epicenter of the 31 Oct 

2007 event. 

4 July 1861. MMI have been assigned at 13 sites (J. Boatwright and H. Bundock, 

personal communication, 2008). The maximum intensity is VIII-IX in Amador Valley. 

MMI VII-VIII intensities were assigned at residences in San Ramon Valley near the 

location of reported fissures along the Calaveras fault near Dublin (Toppozada et al., 

1981) and the intensity center (Figure 2). The intensity center, the preferred location, is 

located near the trace of the northern Calaveras fault. MI

 is 5.9. 

26 February 1864. MMI can be assigned at five of the 11 localities for which felt 

reports (Toppozada et al., 1981) are available. The maximum assigned MMI VI was 

assigned at Monterey, San Francisco, San Jose, and Watsonville. Zones I-VI on the 

Calaveras fault are within the 67% confidence contour for location (Figure 3). Although 

a location on the Calaveras fault is possible, the maximum MMI at Monterey, San Jose 

and Watsonville are more consistent with a west bay fault. A location near the San 

Andreas Fault is preferred. MI

 is 5.6. 

5 March 1864. MMI can be assigned at eight of the 19 localities for which felt reports 

(Toppozada et al., 1981) are available. The maximum MMI VI was assigned at San 

Francisco, San Jose, and Stockton. Zones I-VI on the Calaveras fault are within the 67% 

confidence contour for location (Figure 4). MI

 is 5.3 for zones IV-VI. A location near the 

Mission siesmic trend is preferred. 
6

21 May 1864. MMI can be assigned at four of the 10 localities for which felt reports 

(Toppozada et al., 1981) are available. The maximum MMI VI was assigned at San 

Francisco and San Jose. Zones I-VI on the Calaveras fault are within the 67% confidence 

contour for location but a location near the San Andreas Fault, at the intensity center, is 

preferred (Figure 5). MI is 5.6. 

24 May 1865. MMI can be assigned at four of the 7 localities for which felt reports 

(Toppozada et al., 1981) are available. The maximum MMI VI was assigned at San Juan 

Bautista. Zones I-III on the Calaveras fault are within the 67% confidence contour for 

location, the maximum MMI at San Juan Bautista is more consistent with a west bay fault 

(Figure 6). MI is 5.9. 

8 October 1865. MMI can be assigned at 27 localities for which felt reports (Toppozada 

et al., 1981) are available. Bakun’s (1999) analysis is adopted. The intensity center, the 

preferred location, is near the Berrocal fault (Figure 7). MI is 6.5. 

26 March 1866. MMI can be assigned at six of the 12 localities for which felt reports 

(Toppozada et al., 1981) are available. The maximum MMI VII was assigned at Gilroy. 

Zones I-II on the Calaveras fault are within the 67% confidence contour for location, but 

a location closer to the San Andreas Fault is preferred (Figure 8). MI is 5.6. 

21 October 1868. MMI were assigned at 70 localities by Toppozada et al. (1981). 

Bakun’s (1999) analysis of that data yields an intensity center on the Hayward fault and 

MI is 6.8 (Figure 9a). J. Boatwright and H. Bundock (personal communication, 2008) 

have reexamined the accounts for the 1868 earthquake and have assigned MMI at 133 

sites. The intensity center based on these MMI assignments is also on the trace of the 

Hayward fault, but farther south near the intersection of the Hayward fault and Mission 

seismic trend (Figure 9b). MI

 is 6.5. The preferred location is at the intensity center in 

Figure 9a. 

17 February 1870. MMI can be assigned at ten of the 21 localities for which felt reports 

(Toppozada et al., 1981) are available. The maximum MMI VII was assigned at Los 

Gatos and Santa Cruz. Zones III-VI on the Calaveras fault are within the 67% confidence 

contour for location, but a location near the 8 October 1865 epicenter is perhaps more 

likely (Figure 10). The preferred location is at the intensity center. MI

 is 5.7. 
7

2 April 1870. MMI can be assigned at nine of the 14 localities for which felt reports 

(Toppozada et al., 1981) are available. The maximum MMI VI was assigned at Oakland, 

San Leandro, San Francisco and Petaluma. The intensity center is on the 1868 Hayward 

fault rupture (Figure 11). The preferred location is at the intensity center. MI is 5.6. 

6 March 1882. MMI can be assigned at nine of the 18 localities for which felt reports 

(Toppozada et al., 1981) are available. The maximum MMI VI was assigned at Salinas, 

Hollister, and Santa Cruz. The preferred location is on the San Andreas Fault (Figure 

12). MI is 5.0.

30 March 1883. MMI can be assigned at nine of the 25 localities for which felt reports 

(Toppozada et al., 1981) are available. The maximum MMI VII was assigned at Old 

Gilroy and Sargents where chimneys were thrown down. The preferred location is on the 

San Andreas Fault (Figure 13). MI is 5.9.

17 September 1888. MMI can be assigned at four sites with MM V at Gilroy, San Jose, 

and Santa Cruz, suggesting a west bay location. The preferred location is on the San 

Andreas Fault (Figure 14). MI is 5.9. 

31 July 1889. MMI can be assigned at seventeen of the 41 localities for which felt 

reports (Toppozada et al., 1981) are available. The maximum MMI VII was assigned at 

San Leandro and West Oakland where chimneys were thrown down. The intensity center 

is near the 1868 Hayward fault rupture (Figure 15). The preferred location is at the 

intensity center. MI is 5.6.

24 April 1890. There are 29 MMI available for this event. Cracks were observed on the 

San Andreas Fault trace near the Pajaro River. The maximum intensity was VIII at 

Corralitos, Green Valley, Pajaro, San Juan Bautista, and Sargents (Toppozada et al., 

1981). The preferred location is on the San Andreas Fault near the intensity center 

(Figure 16). MI

 is 6.3. 

2 January 1891. MMI can be assigned at sixteen of the 31 localities for which felt 

reports (Toppozada et al., 1981) are available. The maximum MMI VI was assigned at 

Alameda, Mt. Hamilton, San Jose, Boulder Creek, and Santa Cruz. Zones II-VI on the 

Calaveras fault are within the 67% confidence contour for location (Figure 17). MI

 is 5.7. 

The preferred location is in zone V at the epicenter of the 31 Oct 2007 event. 
8

13 November 1892. MMI can be assigned at eleven of the 28 localities for which felt 

reports (Toppozada et al., 1981) are available. The maximum MMI VII was assigned at 

Hollister and Sargents where chimneys were thrown down. The preferred location is on 

the San Andreas Fault (Figure 18). MI is 5.8.

20 June 1897. MMI can be assigned at 39 localities for which felt reports (Toppozada et 

al., 1981) are available. The maximum MMI VIII was assigned at Mission San Carlos, 

Gilroy, and San Felipe. Fissures were reported on the Pacheco Pass road near San Felipe 

for the M6.3 1897 event (Toppozada et al., 1981). The trace of the Calaveras fault 

crosses the Pacheco Pass Road near San Felipe, which is located near the north end of 

zone I. Surface slip during the 1979 Coyote Lake earthquake was observed near San 

Felipe (Reasenberg and Ellsworth, 1982). McClellan and Hay (1990) reported triggered 

right-lateral slip on the Calaveras fault at Highway 152 near San Felipe Lake at the time 

of the 1989 Loma Prieta earthquake. Fresh northwest-striking cracks, observed at 

Highway 152 after the 1984 Morgan Hill earthquake, were interpreted as triggered slip 

(Galehouse and Brown, 1987; Harms et al., 1987). The fissures noted in 1897 in the 

same area may also have been triggered slip. Bakun’s (1999) analysis is assumed. Zones 

I-III on the Calaveras fault are within the 67% confidence contour for location (Figure 

19). MI is 6.3. The preferred location is in zone II. 

6 July 1899. MMI can be assigned at twelve localities for which felt reports (Toppozada 

et al., 1981) are available. The pattern of reports is anomalous in that high intensities 

were reported at Pleasanton, Alameda County, and near Watsonville but high intensities 

were not reported at sites between Pleasanton and Watsonville (Toppozada et al., 1981). 

Townley and Allen (1939) suggested that two earthquakes occurred –a larger event near 

Watsonville and a smaller earthquake near Pleasanton. I discount here the account at 

Pleasanton. Zones I and II on the Calaveras fault are within the 67% confidence contour 

for location, but a location closer to the San Andreas Fault is preferred (Figure 20). MI is 

5.7.

30 April 1899. MMI can be assigned at 15 localities for which felt reports (Toppozada et 

al., 1981) are available. The maximum MMI VII was assigned at Green Valley and 

Watsonville where chimneys were thrown down. The preferred location is on the San 

Andreas Fault (Figure 21). MI is 5.8.
9

11 June 1903. MMI can be assigned at 26 localities for which felt reports (Toppozada et 

al., 1981) are available. Zones II-VI on the Calaveras fault are within the 67% 

confidence contour for location (Figure 22). MI is 6.1. The preferred location is at the 

intensity center. 

3 August 1903. MMI can be assigned at 22 localities for which felt reports (Toppozada 

et al., 1981) are available. Zones III-VI on the Calaveras fault are within the 67% 

confidence contour for location (Figure 23). MI is 6.1. The preferred location is at the 

intensity center. 

1 July 1911. MMI can be assigned at 31 localities for which felt reports (Toppozada et 

al., 1981) are available. Bakun’s (1999) analysis is assumed. Zones III-VI on the 

Calaveras fault are within the 67% confidence contour for location (Figure 24). 

Oppenheimer et al. (1990) argue that the 1911 event occurred on Zone III. MI is 6.2. The 

preferred location is in zone III at the epicenter of the 1984 Morgan Hill earthquake. 

Conclusions 

 The preferred locations and M for the historical earthquakes are summarized in Table 1 

and Figure 25. Five of the 11 earthquakes in the 1850-1870 period apparently were on 

east Bay faults; the remaining six events were on west Bay faults. Seven of the 12 

earthquakes in the 25 years before the 1906 earthquake and five of the 6 earthquakes in 

the 25 years before the 1989 Loma Prieta earthquake occurred on east Bay faults. That 

is, significant seismicity in the west Bay apparently preceded the large 1868 east Bay 

event and significant seismicity in the east Bay apparently preceded the large 1906 and 

1989 west Bay events. 

References 

Bakun, W.H. (1998). Modified Mercalli intensities for some recent California 

earthquakes and historic San Francisco Bay region earthquakes, U. S. Geol. Surv 

Open-file Report 98-584, 175 p. 

Bakun, W.H. (1999). Seismic activity of the San Francisco Bay region, Bull. Seism. Soc. Am. 89, 

764-784. 

Bakun, W. H., and C. M. Wentworth (1997). Estimating location and magnitude from 

seismic intensity data, Bull. Seism. Soc. Amer. 87, 1502-1521. 
10

Bakun, W. H. and C. M. Wentworth (1999). Erratum to Estimating earthquake location 

and magnitude from seismic intensity data, Bull. Seismol. Soc. Am. 89, 557. 

Bakun, W.H., D. Oppenheimer, J. Boatwright, and R. Simpson (2008). Earthquakes on 

the Central and Southern Calaveras fault and Earthquake Forecasts: Part II, 1850-

1910 (abstract), Seismol. Res. Lett., vol. 79, p. 343. 

Galehouse, J. S., and B. D. Brown (1979). Surface displacement near Hollister, 

California, in Hoose, S, N., (Ed.), The Morgan Hill, California, earthquake of 

April 24, 1984, U. S. Geol. Surv. Bull. 1639, 69-72. 

Harms, K. K., M. M. Clark, and others (1987). The search for surface faulting, in Hoose, 

S, N., (Ed.), The Morgan Hill, California, earthquake of April 24, 1984, U. S. 

Geol. Surv. Bull. 1639, 61-68. 

Jennings, C. W. (1994). Fault activity map of California and adjacent areas, California 

Geological Data Map Series, California Division of Mines and Geology, map no. 

6. 

McClellan, P. H., and E. A. Hay (1989). Triggered slip on the Calaveras fault during the 

magnitude 7.1 Loma Prieta, California, earthquake, Geoph. Res. Lett. 17, 2027-

2030. 

Oppenheimer, D. H., W. H. Bakun, A. G. Lindh (1990). Slip partitioning of the 

Calaveras fault, California, and prospects for future earthquakes, J. Geophys. Res. 

95, 8483-8498. 

Oppenheimer, D., W. Bakun, R. Uhrhammer, J. Boatwright, and R. Simpson (2008a). 

Earthquakes on the Central and Southern Calaveras fault and Earthquake 

Forecasts: Part I, 1910-Present (abstract), Seismol. Res. Lett., vol. 79, p. 342. 

Oppenheimer, D., W. Bakun, R. Uhrhammer, J. Boatwright, and R. Simpson (2008b). 

The 2007 M5.4 Alum Rock Earthquake: Implications for Future Earthquakes on 

the Central and Southern Calaveras Fault, J. Geophys. Res. in preparation 

Reasenberg, P., and W. L. Ellsworth (1982). Aftershocks of the Coyote lake, California, 

earthquake of August 6, 1979; a detailed study, J. Geophys. Res. 87, 10,637-

10,665. 

Toppozada, T. R., C. R. Real and D. L. Parke (1981). Preparation of isoseismal maps and 

summaries of reported effects for Pre-1900 California earthquakes, Calif. Div. 

Mines and Geol. Open-File Rept. 81-11 SAC, 182 pp. 
Table 1. Southern Bay Area Earthquakes

# Date Preferred Location

M @ 

Preferred 

Location

M (95% 

Conf. 

Range)§ # of MMI

a) Historical Earthquakes

1 26 Nov 1858 Zone V* 37.4°N, 121.8°W 6.2 5.7-6.6 7

2 4 July 1861 N. Calaveras fault 37.7°N, 121.9°W 5.9 5.5-6.2 13

3 26 Feb 1864 San Andreas fault 37.0°N, 121.8°W 5.6 5.0 -6.1 5

4 5 March 1864 Mission fault 37.5°N, 121.9°W 5.3 4.8-5.7 8

5 21 May 1864 San Andreas fault 37.2°N, 122.0°W 5.6 5.0-6.1 4

6 24 May 1865 San Andreas fault 37.0°N, 121.7°W 5.9 5.3-6.4 4

7 8 October 1865 Berrocal fault 37.2°N, 121.9°W 6.5 6.2-6.7 27

8 26 March 1866 San Andreas fault 36.9°N, 121.7°W 5.6 5.1-6.0 6

9 21 October 1868 Hayward fault 37.7°N, 122.1°W 6.8 6.5-7.0 70

10 17 Feb 1870 San Andreas fault 37.2°N, 122.0°W 5.7 5.3-6.1 10

11 2 April 1870 Hayward fault 37.8°N, 122.2°W 5.6 5.2-6.0 9

12 6 March 1882‡ San Andreas fault 36.8°N, 121.6°W 5.9 5.5-6.3 9

13 30 March 1883 San Andreas fault 36.9°N, 121.7°W 5.9 5.5-6.3 9

14 17 Sept 1888 San Andreas fault 37.0°N, 121.7°W 5.0 4.4-5,5 4

15 31 July 1889 Hayward fault 37.7°N, 122.2°W 5.6 5.2-5.9 17

16 24 April 1890 San Andreas fault 37.0°N, 121.8°W 6.3 6.0-6.5 29

17 2 Jan 1891 Zone V* 37.4°N, 121.8°W 5.7 5.3-6.0 16

18 13 Nov 1892 San Andreas fault 36.9°N, 121.7°W 5.8 5.4-6.2 11

19 20 June 1897 Zone II* 37.1°N, 121.5°W 6.3 5.7-6.6 39

20 6 July 1899 San Andreas fault 36.9°N, 121.7°W 5.7 5.3-6.1 11

21 30 April 1899 San Andreas fault 37.0°N, 121.7°W 5.8 5.4-6.1 15

22 11 June 1903 East Bay fault 37.3°N, 121.8°W 6.1 5.9-6.6 26

23 3 August 1903 East Bay fault 37.4°N, 121.9°W 6.1 6.0-6.7 22

b) Instrumental Earthquakes

24 1 July 1911 Zone III* 37.3°N, 121.7°W 6.2 5.9-6.5 31

25 26 Oct 1943 Zone IV* 37.39°N, 121.77°W ML4.9 4.8-5.2

26 9 March 1949 Zone I* 37.02°N, 121.48°W ML5.2 5.0-5.4

27 5 Sept 1955 Zone V* 37.43°N, 121.78°W ML5.5 5.3-5.7

28 28 Nov 1974 Busch fault 36.91°N, 121.48°W 5.1 4.9-5.3

29 6 Aug 1979 Zone II* 37.10°N, 121.50°W 5.9 5.7-6.1

30 24 April 1984 Zone III* 37.32°N, 121.70°W 6.2 6.0-6.2

31 31 March 1986 Mt. Lewis 37.49°N, 121.69°W 5.7 5.5-5.9

32 13 June 1988 Zone IV* 37.39°N, 121.77°W 5.0 4.8-5.2

33 27 June 1988 Lake Elsman 37.13°N, 121.88°W 5.0 4.8-5.2

34 18 October 1989 Loma Prieta 37.04°N, 121.88°W 6.8 6.6-7.0

35 31 Oct 2007 Zone V* 37.43°N, 121.78°W 5.4 5.2-5.6

* Calaveras fault 

‡ There is a transcription error in Bakun (1999) where the date 

 of this event is given as 28 June 1882. 

§ Calculated for earthquakes #1-24 from the number of MMI assignments

 using Bakun and Wentworth (1999). 

 The uncertainty for instrumental M is discussed by Bakun (1999) on page 776. 

11
-122  -121 

37 

38 

6.3

6.1

II

III

VI

V

IV

I

50 km

MMI 

VII-VIII 

VI-VII  V-VI 

<IV  IV-V 

Figure 1. 26 November 1858 earthquake. 

(MMI assignment-Site Correction) at 7 sites (some sites may be off the map) are 

shown as circles relative to Jenning’s (1994) active faults (black lines). The 

contours of MI

 (dashed red lines) are the best estimates of M from the MMI data 

for assumed epicenters on that contour. The rms [MI

] contours corresponding to 

the 67% (innermost contour) and 95% confidence contour (outermost contour) 

for location (Bakun and Wentworth, 1999) are shown as solid green lines. The 

intensity center (Bakun, 1999) is a green triangle. Locations of Oppenheimer et 

al.’s (1990) zones I-VI on the central and southern Calaveras fault are shown as 

violet lines. 

12
-122  -121 

37 

38 

6.0

6.2

50 km

MMI 

VII-VIII 

VI-VII  V-VI 

<IV  IV-V 

III

II

I

IV V

VI

Figure 2. 4 July 1861 earthquake.

(MMI assignment-Site Correction) at 13 sites (some sites may be off the map)

are shown as circles relative to Jenning’s (1994) active faults (black lines). The

contours of MI (dashed red lines) are the best estimates of M from the MMI data

for assumed epicenters on that contour. The rms [MI] contours corresponding to

the 67% (innermost contour) and 95% confidence contour (outermost contour)

for location (Bakun and Wentworth, 1999) are shown as solid green lines. The

intensity center (Bakun, 1999) is a green triangle. Locations of Oppenheimer et

al.’s (1990) zones I-VI on the central and southern Calaveras fault are shown as

violet lines.

13
-122  -121 

37 

38 

5.7

5.9

50 km

II

III

VI

V

IV

I

MMI 

VII-VIII 

VI-VII  V-VI 

<IV  IV-V 

Figure 3. 26 February 1864 earthquake. 

(MMI assignment-Site Correction) at 5 sites (some sites may be off the map) are 

shown as circles relative to Jenning’s (1994) active faults (black lines). The 

contours of MI

 (dashed red lines) are the best estimates of M from the MMI data 

for assumed epicenters on that contour. The rms [MI

] contours corresponding to 

the 67% (innermost contour) and 95% confidence contour (outermost contour) 

for location (Bakun and Wentworth, 1999) are shown as solid green lines. The 

intensity center (Bakun, 1999) is a green triangle. The preferred location is the 

green star. Locations of Oppenheimer et al.’s (1990) zones I-VI on the central and 

southern Calaveras fault are shown as violet lines. 

67%

67%

95%

95%

14
-122  -121 

37 

38 

5.3

5.6

II

III

VI

V

IV

I

50 km

MMI 

VII-VIII 

VI-VII  V-VI 

<IV  IV-V 

Figure 4. 5 March 1864 earthquake. 

(MMI assignment-Site Correction) at 8 sites (some sites may be off the map) are 

shown as circles relative to Jenning’s (1994) active faults (black lines). The 

contours of MI

 (dashed red lines) are the best estimates of M from the MMI data 

for assumed epicenters on that contour. The rms [MI

] contours corresponding to 

the 67% (innermost contour) and 95% confidence contour (outermost contour) 

for location (Bakun and Wentworth, 1999) are shown as solid green lines. The 

intensity center (Bakun, 1999) is a green triangle and id located off the map. The 

preferred location is the green star. Locations of Oppenheimer et al.’s (1990) 

zones I-VI on the central and southern Calaveras fault are shown as violet lines. 

67%

95%

15
-122  -121 

37 

38 

5.6

5.9

50 km

II

III

VI

V

IV

I

MMI 

VII-VIII 

VI-VII  V-VI 

<IV  IV-V 

Figure 5. 21 May 1864 earthquake. 

(MMI assignment-Site Correction) at 4 sites (some sites may be off the map) are 

shown as circles relative to Jenning’s (1994) active faults (black lines). The 

contours of MI

 (dashed red lines) are the best estimates of M from the MMI data 

for assumed epicenters on that contour. The rms [MI

] contours corresponding to 

the 67% (innermost contour) and 95% confidence contour (outermost contour) 

for location (Bakun and Wentworth, 1999) are shown as solid green lines. The 

intensity center (Bakun, 1999) is a green triangle. Locations of Oppenheimer et 

al.’s (1990) zones I-VI on the central and southern Calaveras fault are shown as 

violet lines. 

67%

95%

16
-122  -121 

37 

38 

6.0

6.2

50 km

II

III

VI

V

IV

I

MMI 

VII-VIII 

VI-VII  V-VI 

<IV  IV-V 

Figure 6. 24 May 1865 earthquake. 

(MMI assignment-Site Correction) at 4 sites (some sites may be off the map) are 

shown as circles relative to Jenning’s (1994) active faults (black lines). The 

contours of MI

 (dashed red lines) are the best estimates of M from the MMI data 

for assumed epicenters on that contour. The rms [MI

] contours corresponding to 

the 67% (innermost contour) and 95% confidence contour (outermost contour) 

for location (Bakun and Wentworth, 1999) are shown as solid green lines. The 

intensity center (Bakun, 1999) is a green triangle and is located off the map. The 

preferred location is the green star. Locations of Oppenheimer et al.’s (1990) 

zones I-VI on the central and southern Calaveras fault are shown as violet lines. 

67%

95%

17
-122  -121 

37 

38 

6.6

6.7

MMI 

VII-VIII 

VI-VII  V-VI 

<IV  IV-V  50 km

III

II

I

IV V

VI

Figure 7. 8 October 1865 earthquake. 

(MMI assignment-Site Correction) at 27 sites (some sites may be off the map) 

are shown as circles relative to Jenning’s (1994) active faults (black lines). The 

contours of MI

 (dashed red lines) are the best estimates of M from the MMI data 

for assumed epicenters on that contour. The rms [MI

] contours corresponding to 

the 67% (innermost contour) and 95% confidence contour (outermost contour) 

for location (Bakun and Wentworth, 1999) are shown as solid green lines. The 

intensity center (Bakun, 1999) is a green triangle. Locations of Oppenheimer et 

al.’s (1990) zones I-VI on the central and southern Calaveras fault are shown as 

violet lines. 

18
-122  -121 

37 

38 

5.4

5.8

5.2

50 km

II

III

VI

V

IV

I

MMI 

VII-VIII 

VI-VII  V-VI 

<IV  IV-V 

Figure 8. 26 March 1866 earthquake. 

(MMI assignment-Site Correction) at 6 sites (some sites may be off the map) are 

shown as circles relative to Jenning’s (1994) active faults (black lines). The 

contours of MI

 (dashed red lines) are the best estimates of M from the MMI data 

for assumed epicenters on that contour. The rms [MI

] contours corresponding to 

the 67% (innermost contour) and 95% confidence contour (outermost contour) 

for location (Bakun and Wentworth, 1999) are shown as solid green lines. The 

intensity center (Bakun, 1999) is a green triangle and is located off the map. The 

preferred location is the green star. Locations of Oppenheimer et al.’s (1990) 

zones I-VI on the central and southern Calaveras fault are shown as violet lines. 

67%

95%

19
-122  -121 

37 

38 

7

7.0

6.9

MMI 

VII-VIII 

VI-VII  V-VI 

<IV  IV-V 

III

II

I

IV V

VI

50 km

Figure 9a. 21 October 1868 earthquake. 

(MMI assignment-Site Correction) at 70 sites (some sites may be off the map) 

are shown as circles relative to Jenning’s (1994) active faults (black lines). The 

contours of MI

 (dashed red lines) are the best estimates of M from the MMI data 

for assumed epicenters on that contour. The rms [MI

] contours corresponding to 

the 67% (innermost contour) and 95% confidence contour (outermost contour) 

for location (Bakun and Wentworth, 1999) are shown as solid green lines. The 

intensity center (Bakun, 1999) is a green triangle. Locations of Oppenheimer et 

al.’s (1990) zones I-VI on the central and southern Calaveras fault are shown as 

violet lines. 

20
-122  -121 

37 

38 

6.9

6.7

MMI 

VII-VIII 

VI-VII  V-VI 

<IV  IV-V  50 km

III

II

I

IV V

VI

Figure 9b. 21 October 1868 earthquake. 

(MMI assignment-Site Correction) at 133 sites (some sites may be off the map) 

assigned by J. Boatwright (personal communication, 2008) are shown as circles

relative to Jenning’s (1994) active faults (black lines). The contours of MI

 (dashed 

red lines) are the best estimates of M from the MMI data for assumed epicenters 

on that contour. The rms [MI

] contours corresponding to the 67% (innermost 

contour) and 95% confidence contour (outermost contour) for location (Bakun 

and Wentworth, 1999) are shown as solid green lines. The intensity center 

(Bakun, 1999), a green triangle, is located on the Hayward fault about 20-25 km 

south of Bakun’s (1999) intensity center (violet star) . Locations of Oppenheimer et 

al.’s (1990) zones I-VI on the southern Calaveras fault are shown as violet lines. 

21
-122  -121 

37 

38 

1

5.9

5.8

MMI 

VII-VIII 

VI-VII  V-VI 

<IV  IV-V 

50 km

III

II

I

IV V

VI

Figure 10. 17 February 1870 earthquake. 

(MMI assignment-Site Correction) at 10 sites (some sites may be off the map) 

are shown as circles relative to Jenning’s (1994) active faults (black lines). The 

contours of MI

 (dashed red lines) are the best estimates of M from the MMI data 

for assumed epicenters on that contour. The rms [MI

] contours corresponding to 

the 67% (innermost contour) and 95% confidence contour (outermost contour) 

for location (Bakun and Wentworth, 1999) are shown as solid green lines. The 

intensity center (Bakun, 1999) is a green triangle. Locations of Oppenheimer et 

al.’s (1990) zones I-VI on the central and southern Calaveras fault are shown as 

violet lines. 

22
-123  -122 

37 

38  5.6

5.9

MMI 

VII-VIII 

VI-VII  V-VI 

<IV  IV-V 

50 km

III

II

IV V

VI

Figure 11. 2 April 1870 earthquake. 

(MMI assignment-Site Correction) at 9 sites (some sites may be off the map) are 

shown as circles relative to Jenning’s (1994) active faults (black lines). The 

contours of MI

 (dashed red lines) are the best estimates of M from the MMI data 

for assumed epicenters on that contour. The rms [MI

] contours corresponding to 

the 67% (innermost contour) and 95% confidence contour (outermost contour) 

for location (Bakun and Wentworth, 1999) are shown as solid green lines. The 

intensity center (Bakun, 1999) is a green triangle. Locations of Oppenheimer et 

al.’s (1990) zones I-VI on the central and southern Calaveras fault are shown as 

violet lines. 

67%

67%

23
-122  -121 

37 

38 

6

5.9

5.8

6.0

III

II

I

IV V

VI

50 km

MMI 

VII-VIII 

VI-VII  V-VI 

<IV  IV-V 

Figure 12. 6 March 1882 earthquake. 

(MMI assignment-Site Correction) at 9 sites (some sites may be off the map) are 

shown as circles relative to Jenning’s (1994) active faults (black lines). The 

contours of MI

 (dashed red lines) are the best estimates of M from the MMI data 

for assumed epicenters on that contour. The rms [MI

] contours corresponding to 

the 67% (innermost contour) and 95% confidence contour (outermost contour) 

for location (Bakun and Wentworth, 1999) are shown as solid green lines. The 

intensity center (Bakun, 1999) is a green triangle. The preferred location is the 

green star. Locations of Oppenheimer et al.’s (1990) zones I-VI on the central and 

southern Calaveras fault are shown as violet lines. 

67%

95%

24
-122  -121 

37 

38 

6.0

6.1

50 km

III

II

I

IV V

VI

MMI 

VII-VIII 

VI-VII  V-VI 

<IV  IV-V 

Figure 13. 30 March 1883 earthquake. 

(MMI assignment-Site Correction) at 9 sites (some sites may be off the map) are 

shown as circles relative to Jenning’s (1994) active faults (black lines). The 

contours of MI

 (dashed red lines) are the best estimates of M from the MMI data 

for assumed epicenters on that contour. The rms [MI

] contours corresponding to 

the 67% (innermost contour) and 95% confidence contour (outermost contour) 

for location (Bakun and Wentworth, 1999) are shown as solid green lines. The 

intensity center (Bakun, 1999) is a green triangle. The preferred location is the 

green star. Locations of Oppenheimer et al.’s (1990) zones I-VI on the central and 

southern Calaveras fault are shown as violet lines. 

67%

95%

25
-122  -121 

37 

38 

5

5.2

5.1

50 km

MMI 

VII-VIII 

VI-VII  V-VI 

<IV  IV-V 

III

II

I

IV V

VI

Figure 14. 17 September 1888 earthquake. 

(MMI assignment-Site Correction) at 4 sites (some sites may be off the map) are 

shown as circles relative to Jenning’s (1994) active faults (black lines). The 

contours of MI

 (dashed red lines) are the best estimates of M from the MMI data 

for assumed epicenters on that contour. The rms [MI

] contours corresponding to 

the 67% (innermost contour) and 95% confidence contour (outermost contour) 

for location (Bakun and Wentworth, 1999) are shown as solid green lines. The 

intensity center (Bakun, 1999) is a green triangle. The preferred location is the 

green star. Locations of Oppenheimer et al.’s (1990) zones I-VI on the central and 

southern Calaveras fault are shown as violet lines. 

67%

95%

26
-122  -121 

37 

38 

5.8

5.7

III

II

I

IV V

VI

50 km

MMI 

VII-VIII 

VI-VII  V-VI 

<IV  IV-V 

Figure 15. 31 July 1889 earthquake. 

(MMI assignment-Site Correction) at 17 sites (some sites may be off the map) 

are shown as circles relative to Jenning’s (1994) active faults (black lines). The 

contours of MI

 (dashed red lines) are the best estimates of M from the MMI data 

for assumed epicenters on that contour. The rms [MI

] contours corresponding to 

the 67% (innermost contour) and 95% confidence contour (outermost contour) 

for location (Bakun and Wentworth, 1999) are shown as solid green lines. The 

intensity center (Bakun, 1999) is a green triangle. The preferred location is the 

intensity center. Locations of Oppenheimer et al.’s (1990) zones I-VI on the 

central and southern Calaveras fault are shown as violet lines. 

95%

67%

27
-122  -121 

37 

38 

6.4

6.3

III

II

I

IV V

VI

50 km

MMI 

VII-VIII 

VI-VII  V-VI 

<IV  IV-V 

Figure 16. 24 April 1890 earthquake. 

(MMI assignment-Site Correction) at 29 sites (some sites may be off the map) 

are shown as circles relative to Jenning’s (1994) active faults (black lines). The 

contours of MI

 (dashed red lines) are the best estimates of M from the MMI data 

for assumed epicenters on that contour. The rms [MI

] contours corresponding to 

the 67% (innermost contour) and 95% confidence contour (outermost contour) 

for location (Bakun and Wentworth, 1999) are shown as solid green lines. The 

intensity center (Bakun, 1999) is a green triangle. The preferred location is the 

green star. Locations of Oppenheimer et al.’s (1990) zones I-VI on the central and 

southern Calaveras fault are shown as violet lines. 

28
-122  -121 

37 

38 

5.9

5.9

5.8

II

III

VI

V

IV

I

50 km

MMI 

VII-VIII 

VI-VII  V-VI 

<IV  IV-V 

Figure 17. 2 January 1891 earthquake. 

(MMI assignment-Site Correction) at 16 sites (some sites may be off the map) 

are shown as circles relative to Jenning’s (1994) active faults (black lines). The 

contours of MI

 (dashed red lines) are the best estimates of M from the MMI data 

for assumed epicenters on that contour. The rms [MI

] contours corresponding to 

the 67% (innermost contour) and 95% confidence contour (outermost contour) 

for location (Bakun and Wentworth, 1999) are shown as solid green lines. The 

intensity center (Bakun, 1999) is a green triangle. Locations of Oppenheimer et 

al.’s (1990) zones I-VI on the central and southern Calaveras fault are shown as 

violet lines. 

29
-122  -121 

37 

38 

6.2

5.9

6.0

III

II

I

IV V

VI

50 km

MMI 

VII-VIII 

VI-VII  V-VI 

<IV  IV-V 

Figure 18. 13 November 1892 earthquake. 

(MMI assignment-Site Correction) at 11 sites (some sites may be off the map) 

are shown as circles relative to Jenning’s (1994) active faults (black lines). The 

contours of MI

 (dashed red lines) are the best estimates of M from the MMI data 

for assumed epicenters on that contour. The rms [MI

] contours corresponding to 

the 67% (innermost contour) and 95% confidence contour (outermost contour) 

for location (Bakun and Wentworth, 1999) are shown as solid green lines. The 

intensity center (Bakun, 1999) is a green triangle. The preferred location is the 

green star. Locations of Oppenheimer et al.’s (1990) zones I-VI on the central and 

southern Calaveras fault are shown as violet lines. 

67%

95%

30
-122  -121 

37 

38 

6.4

6.3

III

VI

V

IV

I

II

50 km

MMI 

VII-VIII 

VI-VII  V-VI 

<IV  IV-V 

Figure 19. 20 June 1897 earthquake. 

(MMI assignment-Site Correction) at 39 sites (some sites may be off the map) 

are shown as circles relative to Jenning’s (1994) active faults (black lines). The 

contours of MI

 (dashed red lines) are the best estimates of M from the MMI data 

for assumed epicenters on that contour. The rms [MI

] contours corresponding to 

the 67% (innermost contour) and 95% confidence contour (outermost contour) 

for location (Bakun and Wentworth, 1999) are shown as solid green lines. The 

intensity center (Bakun, 1999) is a green triangle. Locations of Oppenheimer et 

al.’s (1990) zones I-VI on the central and southern Calaveras fault are shown as 

violet lines. 

31
-122  -121 

37 

38 

5.8

5.9

50 km

III

II

IV

V

VI

I

MMI 

VII-VIII 

VI-VII  V-VI 

<IV  IV-V 

Figure 20. 6 July 1899 earthquake. 

(MMI assignment-Site Correction) at 11 sites (some sites may be off the map) 

are shown as circles relative to Jenning’s (1994) active faults (black lines). The 

contours of MI

 (dashed red lines) are the best estimates of M from the MMI data 

for assumed epicenters on that contour. The rms [MI

] contours corresponding to 

the 67% (innermost contour) and 95% confidence contour (outermost contour) 

for location (Bakun and Wentworth, 1999) are shown as solid green lines. The 

intensity center (Bakun, 1999) is a green triangle and id located off the map. The 

preferred location is the green star. Locations of Oppenheimer et al.’s (1990) 

zones I-VI on the central and southern Calaveras fault are shown as violet lines. 

67%

95%

32
-122  -121 

37 

38 

6

5.9

5.8

III

II

I

IV V

VI

50 km

MMI 

VII-VIII 

VI-VII  V-VI 

<IV  IV-V 

Figure 21. 30 April 1899 earthquake. 

(MMI assignment-Site Correction) at 15 sites (some sites may be off the map) 

are shown as circles relative to Jenning’s (1994) active faults (black lines). The 

contours of MI

 (dashed red lines) are the best estimates of M from the MMI data 

for assumed epicenters on that contour. The rms [MI

] contours corresponding to 

the 67% (innermost contour) and 95% confidence contour (outermost contour) 

for location (Bakun and Wentworth, 1999) are shown as solid green lines. The 

intensity center (Bakun, 1999) is a green triangle. The preferred location is the 

green star. Locations of Oppenheimer et al.’s (1990) zones I-VI on the central and 

southern Calaveras fault are shown as violet lines. 

67%

95%

33
-122  -121 

37 

38 

6.3

6.0

50 km

II

III

VI

V

IV

I

MMI 

VII-VIII 

VI-VII  V-VI 

<IV  IV-V 

Figure 22. 11 June 1903 earthquake. 

(MMI assignment-Site Correction) at 26 sites (some sites may be off the map) 

are shown as circles relative to Jenning’s (1994) active faults (black lines). The 

contours of MI

 (dashed red lines) are the best estimates of M from the MMI data 

for assumed epicenters on that contour. The rms [MI

] contours corresponding to 

the 67% (innermost contour) and 95% confidence contour (outermost contour) 

for location (Bakun and Wentworth, 1999) are shown as solid green lines. The 

intensity center (Bakun, 1999) is a green triangle. Locations of Oppenheimer et 

al.’s (1990) zones I-VI on the central and southern Calaveras fault are shown as 

violet lines. 

34
-122  -121 

37 

38 

6.4

6.1

II

III

VI

V

IV

I

50 km

MMI 

VII-VIII 

VI-VII  V-VI 

<IV  IV-V 

Figure 23. 3 August 1903 earthquake. 

(MMI assignment-Site Correction) at 22 sites (some sites may be off the map) 

are shown as circles relative to Jenning’s (1994) active faults (black lines). The 

contours of MI

 (dashed red lines) are the best estimates of M from the MMI data 

for assumed epicenters on that contour. The rms [MI

] contours corresponding to 

the 67% (innermost contour) and 95% confidence contour (outermost contour) 

for location (Bakun and Wentworth, 1999) are shown as solid green lines. The 

intensity center (Bakun, 1999) is a green triangle. Locations of Oppenheimer et 

al.’s (1990) zones I-VI on the central and southern Calaveras fault are shown as 

violet lines. 

35
-122  -121 

37 

38 

6.2

6.4

50 km

III

II

VI

V

IV

I

MMI 

VII-VIII 

VI-VII  V-VI 

<IV  IV-V 

Figure 24. 1 July 1911 earthquake. 

(MMI assignment-Site Correction) at 31 sites (some sites may be off the map) 

are shown as circles relative to Jenning’s (1994) active faults (black lines). The 

contours of MI

 (dashed red lines) are the best estimates of M from the MMI data 

for assumed epicenters on that contour. The rms [MI

] contours corresponding to 

the 67% (innermost contour) and 95% confidence contour (outermost contour) 

for location (Bakun and Wentworth, 1999) are shown as solid green lines. The 

intensity center (Bakun, 1999) is a green triangle. Locations of Oppenheimer et 

al.’s (1990) zones I-VI on the central and southern Calaveras fault are shown as 

violet lines. 

36
-122  -121 

37 

38 

1865

1868

50 km

1989 I

II

III

IV

V

1850-1870

1882-1903

1911-2007

Figure 25. Preferred earthquake epicenters. 

(Preferred locations in Table 1 are shown as triangles relative to Jenning’s (1994) 

active faults (black lines). The M6.5 1865, M6.8 1868, and M6.9 1989 earthquakes 

are plotted as stars. I-VI are Oppenheimer et al.'s (1990) zones of recurring 

earthquakes on the Calaveras fault.