Presidio tide gauge graph for April 18 1906

Shortly after the Great San Francisco earthquake of April 18, 1906, a sea level disturbance (tsunami) was recorded at the Presidio tide gauge station in San Francisco (the station is now located nearby at Ft. Point). This disturbance puzzled Lawson (1908), author of the comprehensive report of the earthquake, and Henry Reid, proponent of the elastic rebound theory of earthquakes, primarily because it appeared as a small and solitary negative amplitude wave. This graph shows what the record looked like with the tidal component removed (the red arrow indicates the approximate time of the earthquake). This tsunami record is unusual because the initial lowering of sea level is not followed by a subsequent positive amplitude wave. The lowering of sea level over about 16 minutes was followed by a series of oscillations with an approximate period of 40-45 minutes. Lawson (1908) originally ascribed these later oscillations to reverberation of the tsunami within San Francisco Bay. Further studies, however, indicate that these oscillations are caused by complex wave effects outside the Golden Gate and not within the bay. The central question remains: what type of mechanism (earthquake rupture, landslide, other) generated the tsunami recorded at the Presidio tide gauge station?