'Palmdale bulge': new study turns geological 'mountain' into molehill
They called it the "Palmdale bulge" -- a major and rapid uplift of an extensive area of southern California between 1959 and 1974. Fears that it presaged an earthquake abounded.
Now it appears that the bulge is a mirage, an artifact of systematic errors in the relevant geodetic data.
"The inference of widespread aseismic [not caused directly by earthquake] uplift in southern California is not justified," say David D. Jackson, Wook B. Lee, and Chi-Ching Liu of the University of California at Los Angeles (UCLA). "Corrected data show no significant vertical motion except at the time of the [ 1971] San Fernando earthquake," they add.
In other words, crustal movement associated with a known earthquake in the area stands out. But the aseismic uplift -- that intriguing inexplicable bulge -- is no more. It only seemed to be there because of errors in reading the measuring rods used to detect changes in elevation over the nearly two-decade period.
Although the UCLA geodesists report their findings in the current issue of Science, the Palmdale bulge has been suspect for some time. Professor Jackson and Dr. Lee raised the point nearly a year ago at the 1979 fall meeting of the American Geophysical Union. They made the point again at the AGU spring meeting. At that time, William Strange of the US National Geodetic Survey also said that the leveling data, as they are called, seem to have systematic errors and correcting for them removed much of the bulge.
However, it takes more than suspicion to deflate so widely acknowledged a phenomenon as the anomalous southern California uplift -- a rise of as much as 450 millimeters above a tidal bench mark at San Pedro. At the spring AGU meeting, for example, Ross Stein of the US Geological Survey said there were indeed data errors. But he found them to be small and random, unlikely to deceive geodesists about something as large as the Palmdale bulge.
Since then, the UCLA group has further refined its study and believes its major conclusion to be firmly established. The discussion now, Lee says, is over what the sources of the errors are and which of them is most important, not over whether or not they exist.
Surveying for land elevation is a painstaking process involving precise readings of measuring rods that are used repeatedly over long distances. Typically, two rods, each three meters long, are placed equidistant from the observer. He or she then finds a level line of sight between the rods. A difference in elevation between the rods shows up in reading the graduated scale each rod carries.
If the rods themselves are slightly distorted at one or more places, the elevation readings will be in error. Another source of error is atmospheric refraction. When air density near the ground changes because of local heating or cooling or because of changes in humidity, light rays are bent. Thiscauses mirages including the familiar "water-on-water-road" effect. It can also mislead a surveyor.
The UCLA group thinks that rod distortion is the main source of errors. others, such as William Strange, have argued that refraction is a major factor. To the extent that refraction is to blame, the Palmdale bulge would literally be a mirage.
Once the error sources are pinned down, Lee says, the geodetic data can probably be corrected and will be useful for geophysical studies. Thus, the fact that they have been found to be systematically in error does not mean that the cost and effort of gathering them has to be written off as a total loss. But their most spectacular indication does seem to be spurious.
The bulge intrigued geophysicists not so much for its size as for the speed with which its appeared to develop. A typical high rate of uplift found in the geological record is on the order of 10 millimeters (mm) a year averaged over something like half a million years, including earthquake-caused displacements occurring irregularly during that time. The southern California uplift appeared in places to average 25 mm a year between 1961 to 1974. Between 1961 and 1962, it seemed to rise at over 100 mm a year between San Pedro and Palmdale.
Now geophysicists no longer have to try to explain how such a thing could happen. And residents of southern California need no longer look anxiously in Palmdale's direction. However, Lee notes, this does not mean they can become complacent about earthquake hazards. Southern California, he warms, remains an area of substantial earthquake risk and this must continue to be recognized in regional planning.