Barometric Pressure Oscillations in Houston Caused by the 1964 Alaska Earthquake

Abstract

The ground motion due to Rayleigh waves of the 1964 Alaska earthquake was recorded by a microbarograph in Houston. This appears to be the only known instance when such waves have been thus recorded; consequently some information about it is hereby given to add to the ample documentation about this earthquake (e.g., Anonymous-I, 1968Anonymous-I, -1973 Wood, 1969). Brief mentions of this observation are made in Donn (1964), McGarr (1965, 1968, and an article in Time magazine (Anonymous-2, 1964). I regret the delay in the present publication.At the time of the earthquake (27 March 1964) the seismographic station at Rice University in Houston, Texas (29~ 95~ was equipped with a three-component modified Press-Ewing seismographic system (De Bremaecker et al., 1963). A microbarograph had been added to record the thermonuclear tests then being conducted by the Soviet Union. The latter purpose was not accomplished, however, since these tests had ended by the time this last instrument started operating.The microbarograph consisted essentially of two chambers separated by a thin metal diaphragm. One of the chambers was open to the outside air; it was connected to the other chamber through a metal tube equipped with a vane, the opening of which could be adjusted. This arrangement reduced the instrumental response to periods greater than a few minutes, but the exact frequency response was unknown. The displacement of the diaphragm was sensed by a capacitor that determined the frequency of an oscillator; this frequency, in turn, was converted to a voltage, the variations of which were recorded on a paper drum by a pen-and-ink rectilinear recorder. Although primitive by today's standards, the instrument operated satisfactorily. It was not calibrated at the time of the earthquake.In the Houston area the ground motion due to the Alaska earthquake caused seiches in pools and other water bodies (Spaeth and Berkman, 1969) and much surprise in the general population. Of course the seismographs immediately went off-scale, but it was a few days before I chanced to look at the record of the microbarograph and saw the train of waves shown in Figure 1. Since the epicentral distance is 5,146 km, and the epicentral time 03:36:13 UT, the group velocity of these waves ranges from approximately 3.06 km/s for T -20 s, to about 2.7 km/s for T = 15 s. They are thus dispersive Rayleigh waves.The amplitude of the ground motion could only be guessed at from the reported seiches and other water disturbances in the area. It was obviously greater than 1 cm, but a more precise determination required calibration of the microbarograph. It should be noted that the velocity of the ground motion was negligible compared to that of sound, so that the cause of the barometric pressure fluctuations was simply the change in elevation due to the passage of the Rayleigh waves. Calibration of the instrument was simple: I took it to a two-story house, turned off the ventilation, and moved the instrument quickly from the ground floor to the second floor, together wi...

Characterization of the Infrasound Field in the Central Pacific

“…Although Hawaiian earthquakes are not inhsonic sources per se, in that the ground displacements associated with seismic arrivals do not appear to couple significant energy into acoustic waves, they are recorded by the sensors due to the physical movement of the ground (Figure 58;De Bremaecker;. They can be easily distinguished fiom inhsonic arrivals by the extremely high trace velocity, appearing to arrive almost simultaneously across the axray.…”