Most high school graduates, having had at least one science class, learned that a cubic foot of fresh water at standard conditions of temperature and pressure weighs 62.4 pounds. That fact may have little impact on you unless you have carried buckets of water for livestock, or for landscape irrigation, or for camping, or have packed "Indian tanks" on your back for firefighting. I have participated in all of those activities and I have a healthy respect for water weight.
As a geologist I have appreciated the importance of underground water pore-pressure in de-stabilizing hill slopes, and I have participated with engineers and other geologists in preparing plans and procedures for controlling concentrations of water to prevent damage of various kinds to both natural landscapes and artificial constructions.
In relatively recent years it has been recognized that water can be an important factor relative to triggering earthquakes, although each step along the way has been highly controversial at the outset. Now it is generally accepted that pumping fluids in or out of deep wells can stimulate earthquakes. Often construction of large dams and reservoirs is also associated with subsequent earthquakes (R.I.S.), even in areas that have been historically stable.
Similarly there are a number of significant papers in the scientific literature that show clear correlations between earthquakes and local flooding or unusually heavy and prolonged precipitation.
Furthermore, the rhythmic oscillations of tide waters have been studied as another means of triggering earthquakes in Coastal areas. Several hundred articles in the world's scientific literature have explored this mechanism, along with the undulatory effects of "earth tides" that affect the interiors of continents, more than a thousand miles from the seashore.
These tidal effects are mainly the result of the phases of the Moon, although the Sun contributes about 42 per cent to the range of ocean tides. When the Earth is in a tug-of-war between the Sun and Moon, the earth tides can attain three feet, and in sea coast areas of unusual topography, such as in the Bay of Fundy, the ocean tides can range more than 50 feet in a six hour period. (Contrast this with the island shores of the mid-Pacific, where tides may range less than two feet.)
All this talk of water loading and unloading needs a quantitative basis for clear understanding. There is no question that the crust of the Earth behaves elastically and massive ice sheets depresses it many hundreds of feet. In fact, in Scandinavia and Canada where continental glaciers exceeded two miles in thickness as recently as 12,000 to 18,000 years ago, the solid crust is still slowly rising in response to relief from the great load of ice.
At the Golden Gate, where a normal range in daily tide is four to five feet, the daily difference between high and low tide often exceeds eight feet at the time of a new or full Moon (syzygy.) The range may reach 8.5 to 9.2 feet on the rare occasions when a syzygy occurs on the same day as the monthly closest approach of the Moon to the Earth (perigee.) This rare event takes place only from two to five times per year and the maximum gravitational force between 1600 AD and 2200 AD took place at such a time on January 4, 1912, the day of the strongest West Coast earthquake in two years. Was this just coincidence? Let us analyze some forces that may cause old Mother Earth to "slip a disk" (or a plate?):
One acre = 43,560 sq. ft. ; One acre-foot of water = 43,560 cu. ft.; x 62.4 lbs./cubic foot = 2,714,439 lbs. = 1,357.22 tons
One square mile =640 acres; 640 x 1,357.22 tons = 868,620 tons/ ft. (of pure water)
Sea water (specific gravity 1.035) weighs more. 1.03.5 x 868,620 = 899,022 tons/sq. mi. (per foot of sea-water). In addition, for each foot of cold turbid sea-water (maximum density at 39.4F), the load for each square mile would be about 920,000 tons. To make it memorable, I refer to the loading as "nearly 1 million tons per square mile."
Now look at the 500 square miles of the San Francisco Bay and Delta, where eight to nine feet of saline water surges back and forth within a six-hour cycle:
9 ft. x 500 sq. mi. x 1 million tons equals a rapid load change totaling about 4.5 BILLION tons No wonder Mother Earth occasionally slips a disk in the San Francisco Bay Area!