A slow seismic event recorded in Pasadena

A prominent long‐period wave with a duration of 2000 sec or longer was recorded with a very‐broadband system in Pasadena on June 18, 1988. This wave was not observed elsewhere, and is considered of local origin. The acceleration amplitude is 2.5×10 −5 cm/sec² in the northwest direction, with the vertical component less than 10% of the horizontal. The horizontal acceleration can be interpreted as due to a tilt of the ground of 2.5×10 −8 radians to the northwest. A slowly propagating pressure wave with an amplitude of about 15 mbars could be the cause of the tilt; however, there were no reports suggesting such pressure changes. A more likely cause is a slow tectonic event near Pasadena. The required magnitude of such a slow event is M w =0, 2, and 4, for a distance of 0.1, 1, and 10 km respectively. This event could be part of a tectonic episode associated with the larger earthquakes which occurred in southern California around this time, especially the December 3, 1988, Pasadena earthquake ( M L =4.9) which occurred six months later within 4 km of the Pasadena station.