Abstract

Current seismic hazard models include two major earthquakes (M ∼ 7) in the San Francisco Bay area that are close in space and time: an 1836 event on the northern Hayward fault and an 1838 event on the peninsula section of the San Andreas fault. Analysis and interpretation of the available historical accounts indicate that the 1836 event occurred east of Monterey Bay, far from the Hayward fault, and was of M ∼ 6¼. Also, the 1838 event was not confined to the 60-km peninsula San Andreas as current models indicate. Instead, faulting probably extended from San Francisco to San Juan Bautista (∼ 140 km), indicating a significantly larger earthquake (M ∼ 7½) than previously thought.

Damaging effects of the 1836 earthquake were reported only from Santa Clara to Carmel, and no contemporary effects were reported to the north of Santa Clara or near the Hayward fault. The illusion of an “1836 Hayward earthquake” evolved from a newspaper reminiscence published following the 1868 Hayward earthquake, stating that the 1868 effects in the East Bay were similar to those of an 1836 event. The article describes various strong effects in the East Bay that differ completely from the effects recorded for the 1836 earthquake but are very similar to those documented for the major 1838 San Andreas earthquake that caused extensive damage on both sides of San Francisco Bay. Based on this and other evidence, we conclude that the reminiscence describes the destructive June 1838 effects, but it erroneously indicates the date as June 1836. There is no evidence for any major historical earthquakes in the San Francisco Bay area before the 1838 earthquake, back to the founding of Mission San Francisco Dolores in 1776.

During the 1838 San Andreas fault earthquake, the shaking intensity in Monterey was as strong as or stronger than during the great 1906 San Andreas fault earthquake. This suggests that the 1838 San Andreas fault rupture may have extended to San Juan Bautista as it did in 1906. Numerous probable aftershocks were felt in the area south of San Juan Bautista. These damaged Carmel and Santa Cruz in 1840, and Alisal, 16 km west of the San Andreas fault, in 1841. The northern end of the 1838 faulting was previously assumed to be 25 km south of San Francisco. However, Mission San Francisco Dolores was damaged in 1838 but not in 1906, suggesting that the 1838 faulting extended to San Francisco. Also, the 1838 aftershocks were felt in Oakland as frequently and violently as those following the major 1868 Hayward earthquake, suggesting that the 1838 faulting on the San Andreas extended to the latitude of Oakland.

The 1838 fault segment ruptured again 68 years later as part of the overlapping 1906 San Andreas fault rupture. This, and similar evidence from southern California, indicates that M ∼ 7½ San Andreas fault earthquakes can recur at intervals of 68 years or less when they are followed by M ∼ 8 earthquakes on overlapping segments of the fault.

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