Electronic Supplement to
Potentially Induced Earthquakes during the Early Twentieth Century in the Los Angeles Basin

by Susan E. Hough and Morgan Page

This electronic supplement contains summary of background information about the Wilmington oil field, information about revisited earthquakes between 1900 and 1935, tables of felt events, and accounts of three earthquakes.

The Wilmington Oil Field

Analyzing data from leveling surveys dating back to 1906, Gilluly and Grant (1949) showed that differential subsidence centered on the original Wilmington oil field was found to be trivial, indeed if it existed at all, prior to 1934, but became notable by summer of 1937, reaching nearly 1.3 m by January 1945 (Gilully and Grant, 1949). By 1932, subsidence centered around Signal Hill (part of the Long Beach oil field; Fig. 1 of the main article) was detectable (13 cm).

Although most of the subsidence within the Wilmington field was not associated with detectable seismic events, by 1949 it was recognized that large slides on an established fracture system sometimes generated abrupt disturbances that were recorded seismically (Richter, 1961; see Data and Resources). Notable so-called slippage events occurred on 14 December 1947 (ML 2.4), 17 November 1949 (ML 3.2), 15 August 1951 (ML 2.4), 25 January 1955 (ML 3.3), and 4 April 1961 (ML 3.3). In personal correspondence, Richter indicated that he did not regard these events as true earthquakes per se, noting their especially shallow depths around 510 m and their distinct mechanism, although he also noted that they were sometimes large enough to generate seismic signals (Richter, 1961; see Data and Resources). The depths of the events were established precisely by slippage on planes that crossed oil wells. Newspaper articles of the time, however, did not comment clearly on the causative relationship between the events and oil wells. For example, following the 1949 event, an article in the San Diego Union (19 November 1949) described the substantial damage caused to oil wells on Terminal Island, noting that the island had been sinking for 10 years because of oil pumping, but not stating explicitly that the recorded seismic event was caused by pumping or subsidence. In 1955, an article in the San Diego Union quoted seismologists’ reports that the disturbance was localized and due to a slippage of rocks underlying Terminal Island at a depth of about 2000 ft, and went on to note damage to oil wells. The article further concluded with Richter’s report that the temblor was no ordinary earthquake that could be measured on instruments as to magnitude and intensity, but not because the earthquake was induced, or had a fundamentally different mechanism than a natural earthquake, rather because it was shallow in nature. A graphic published in a Los Angeles Times article following the April 1961 event showed wells crossing the Cerritos and Wilmington faults, but again did not state explicitly that oil pumping had caused the earthquake.

An association between earth slips and oil pumping was discussed even earlier in both private correspondence by leading experts of the day (Buwalda, 1932; see Data and Resources) and local newspaper articles. A 1932 article in the Los Angeles Times notes that earth slips had hit oil fields at Buena Vista Hills in the San Joaquin valley, and in recent years at other fields including the Coyote Hills, Inglewood, and Ventura fields. The article further noted that an earth tremor three years ago had struck the Santa Fe Springs oil field, and that a well that had been producing oil regularly began spouting hot water (Los Angeles Times, 19 September 1932). Like the later articles, however, this article did not state explicitly that the tremor had been caused by pumping.

Historical Events

For this study, we consider in detail the 14 events in the Los Angeles basin between 1900 and 1935 that reportedly caused damage and/or have estimated magnitudes of 4.0 and above as reported by the Uniform California Earthquake Rupture Forecast (UCERF3; Field et al., 2013) catalog. Results for key events are presented in the main article; results for minor events are summarized here. For each event, we include the event number from Table 1 in the main article.

21 October 1913 Orange County (1)

An earthquake at 01:38 a.m. (local time [LT]) on 21 October 1913 was reported as slight in Los Angeles and on Catalina Island. According to Townley and Allen (1939), it was reportedly heavy enough in Monrovia (northeast of Los Angeles) to awaken a number of sleepers. The UCERF3 catalog lists Mw 4.0, with a location at 33.8° N, 118.0° W, as estimated by Toppozada et al. (1978). The reports summarized in the Townley and Allen (1939) catalog are inconsistent: an earthquake centered near Monrovia would not have been felt on Catalina Island, over 100 km away, unless it was a sizeable event. A significantly larger (than Mw 4.0) event, however, would have been more strongly felt at other locations in the Los Angeles area. It is possible that two separate small events occurred close in time, and the accounts were conflated; it is also possible that there were transcription or reporting problems with early accounts. We conclude that the location and magnitude of this event are grossly uncertain. For lack of an alternative interpretation, we preserve the UCERF2 catalog solution (Table 1 and Fig. 1 of the main article).

8 November 1914 Offshore Santa Monica (2)

An earthquake at 03:40 a.m. (LT) on 8 November 1914 was widely felt, with reported intensity Rossi–Forel (RF) IV–V in four locations including Los Angeles, Whittier, and San Fernando. Shaking (RF III–IV) was reported on Catalina Island. The UCERF3 catalog lists Mw 4.5, with a location at 34.0° N, 118.5° W, near Santa Monica. Shaking was also reported in Ojai, some 85 km west-northwest of Santa Monica, and at Arroyo Grande in San Luis Obispo County, some 225 km northwest. The reported time of shaking at Arroyo Grande was 03:30 a.m. (LT). The event was weakly recorded in Berkeley, but it was not possible to determine an epicentral distance (Townley and Allen, 1939). The fact that the earthquake was recorded in Berkeley suggests a magnitude of 4.5–5, but the location is grossly uncertain. In light of the fact that no location reported intensities higher than RF V, we suggest that this event was farther offshore than the catalog location. We propose a location of 34.0° N, 118.6° W; while highly uncertain, this location is more consistent with available data than the catalog location, which places the event immediately along the coast near Santa Monica.

6 March 1918 Santa Monica (5)

An earthquake at 10:20 a.m. (LT) on 6 March 1918 was felt at a number of locations in the Los Angeles region, with the most severe shaking in Santa Monica and Venice (Townley and Allen, 1939). At Venice (33.99° N, 118.46° W), an abrupt rocking accompanied by explosive rumbling was felt by many, resulting in some broken crockery. In Santa Monica, shaking reportedly caused doors to swing. Shaking was felt in the western and northwestern portions of Los Angeles, as well as in Pasadena and in the San Fernando valley. The UCERF3 catalog lists a magnitude of 4.0 and a location of 34.0° N, 118.5° W, from Toppozada et al. (1978). We consider this solution reasonable; although both the magnitude and location are clearly highly uncertain, unlike the 21 October 1913 event, macroseismic observations do suggest a location close to the coast.

18 June 1920 Offshore (6)

An earthquake at 02:08 a.m. (LT) was widely felt in the Los Angeles region on 18 June 1920. People were reportedly awakened both in the city of Los Angeles and on Catalina Island. According to Townley and Allen (1939), seismograms showed the event to be about the same size as the Inglewood earthquake that occurred about three days later, but concluded that the reports indicated an offshore location. The fact that the earthquake was recorded on early instruments supports the inference of a magnitude of 4.5–5. The UCERF3 catalog lists a magnitude of 4.5 and a location of 33.5° N, 118.25° W, from Toppozada et al. (1978). We consider this solution reasonable, although clearly highly uncertain.

8 October 1927 Glendale (15)

The Townley and Allen catalog includes a reportedly damaging earthquake with an estimated location east of Compton that occurred at 11:14 a.m. (LT) on 8 October 1927; we have found no magnitude estimate for this event, which is not included in the UCERF3 catalog. As summarized by Townley and Allen (1939), the highest reported intensity (broken plate glass windows) was about 20 miles northwest of the origin, which was fixed by instrumental records as east of Compton. The shock was reportedly felt over a wide area, as far east as Riverside and as far south as Santa Ana. In the Los Angeles Times (9 October 1927), it was reported as a slight earthquake. In the Seismological Notes section of the Bulletin of the Seismological Society of America, it is described as having generated RF IV, with a low booming sound heard in Pasadena and mention of broken windows in Glendale. Wood and Richter (1931) mention this event, noting that limited instrumental recordings from three stations suggest a location close to a swarm that occurred near Compton on 30–31 December 1928. We conclude that this inference, based on very limited early data, was incorrect and that it was likely located near Glendale. Given that it was felt to a distance of ~50 km, we estimate Mw 3.7.

13 September 1929 Catalina Island (17)

Heck and Bodle (1930) list an event at 5:23 a.m. (LT) on 13 September 1929, noting that, per an indication from Wood, the event was located off the north point of Catalina Island. Shaking was apparently strongest in Long Beach, where sleepers were awakened. It was reported felt in San Pedro and Los Angeles. The UCERF3 catalog gives Mw 4.0, with an epicenter at 33.63° N, 118.2° W, which places the epicenter closer to the coast than the location indicated by Wood. Depending on the location assumed, the felt radius was ≈40 or ≈60 km. The inferred catalog magnitude is reasonable, stipulating it to be especially uncertain. An inferred offshore location is supported by the fact that the felt radius was relatively large, but shaking was only weakly felt even along the coast.

31 March 1931 Yorba Linda (19)

Neumann (1932) lists an event at 12:33 p.m. (LT) on 31 March 1931, noting that the Pasadena Seismological Laboratory reported a location near Pomona and described the event as feeble. Shaking was apparently strongest in Yorba Linda, where a table was reported displaced; the only other felt report was from Pomona, ~20 km from Yorba Linda. The UCERF3 catalog gives Mw 4.0, with an epicenter at 34.1° N, 117.8° W. Given that the event was not reported from the other locations, we suggest that it was likely located between Yorba Linda and Pomona, with a poorly constrained epicenter at 33.94° N, 117.79° W. The felt radius suggests a modest magnitude Mw ≈3.5.

24 April 1931 Redondo Beach (20)

Neumann (1932) lists an event at 10:28 a.m. (LT) on 4 April 1931, noting that the Pasadena Seismological Laboratory reported an offshore location at 33.77° N, 118.48° W, with slight damage at several beach towns including Redondo Beach. This event was felt to a distance of ≈100 km inland, indicating a more sizeable event than the apparently unrelated 31 March earthquake. The UCERF3 catalog gives Mw 4.4, with the same epicenter as reported by Neumann (1932). We conclude that both the magnitude and location are reasonable estimates, although uncertain.

4 November 1931 Wilmington (21)

The UCERF3 catalog includes an estimated Mw 4.0 event that occurred on 4 November 1931. The catalog location, 33.8° N, 118.3° W, is west/northwest of central Wilmington. The Seismological Notes section of the Bulletin of the Seismological Society of America mentions two felt events (estimated intensity III) three miles east of Torrance, within the Wilmington tank farm, on 13 and 14 August 1931. On 4 November 1931 (3 November LT), an earthquake lasting only a few seconds was reportedly felt, also with intensity III, in Compton and Torrance. This event was also felt in the city of Los Angeles, a distance of ~15 km. Unlike the 1920 Inglewood and 1929 Santa Fe Springs event, no report on the 4 November 1931 earthquake was published. The Los Angeles Times (4 November 1931) described it as a slight quake, apparently centered in the vicinity of Redondo, with an ~15 km radius of the felt extent. One picture reportedly dropped from the wall in a home in Compton. The article quotes Charles Richter’s report that the shock was ~30 miles from Pasadena. A felt radius of ~15 km suggests a magnitude of ~3.2.


Table S1. Felt events in the greater Los Angeles area between January 1900 and December 1931.

Table S2. Effects of 22 June 1920 Inglewood earthquake.

Table S3. Effects of 8 July 1929 Whittier earthquake from field survey by C. F. Richter (unpublished notes, 1929; see Data and Resources).

Table S4. Effects of 8 July 1929 Whittier earthquake from United States Earthquakes (Heck and Bodle, 1931) and other archival sources.

Data and Resources

C. F. Richter’s unpublished field notes on the 1929 earthquake and private correspondence are available from the Caltech Archives (Papers of C. F. Richter), Pasadena, California. Private correspondence of J. P. Buwalda also available from Caltech Archives (Papers of J. P. Buwalda), Pasadena, California.


Field, E. H., G. P. Biasi, P. Bird, T. E. Dawson, K. R. Felzer, D. D. Jackson, K. M. Johnson, T. H. Jordan, C. Madden, A. J. Michael, et al. (2013). Uniform California earthquake rupture forecast, version 3 (UCERF3): The time-independent model, U.S. Geol. Surv. Open-File Rept. 2013-1165.

Gilluly, J., and U. S. Grant (1947). Subsidence in the Long Beach harbor area, California, Geol. Soc. Am. Bull. 60, no. 3, 461–530, doi: 10.1130/0016-7606(1949)60[461:SITLBH]2.0.CO;2.

Heck, N. H., and R. R. Bodle (1931). United States earthquakes, 1929, U.S. Dept. of Commerce Coast and Geodetic Survey Report, Serial No. 511, U.S. Government Printing Office, Washington, D.C., 57 pp.

Neumann, F. (1932). United States earthquakes, 1931, Coast and Geodetic Survey Report, Serial No. 553, U.S. Government Printing Office, Washington, D.C., 26 pp.

Taber, S. (1920). The Inglewood earthquake in southern California, June 21, 1920, Bull. Seismol. Soc. Am. 10, 129–145.

Toppozada, T. R., D. L. Parke, and C. T. Higgins (1978). Seismicity of California 1900–1931, Special Report 135, California Division of Mines and Geology, Sacramento, California, 39 pp.

Townley, S. D., and M. W. Allen (1939). Descriptive catalog of earthquakes of the Pacific Coast of the United States, 1769 to 1928, Bull. Seismol. Soc. Am. 29, 1–296.

Wood, H. O., and C. F. Richter (1931). Recent earthquakes near Whittier, California, Bull. Seismol. Soc. Am. 21, 183–203.

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