Two-thirds of all the water used  in the nation's second-largest city comes from a far-flung network of streams, reservoirs and aqueducts that stretches for hundreds of miles down the backbone of the Sierra Nevada.

   A recent swarm of earthquakes near Mammoth Lakes. believed to be caused by magma fracturing rock or pressurizing liquid four miles underground. has heightened concern that it is only a matter of time before the region experiences another violent episode in its long and tumultuous volcanic history. And that raises troubling questions about the potential impact of an- other eruption on the city's chief source of water.

   "We know it would be bad." said Winston K. Wu. a geotechnical engineer with the Los Angeles Department of Water and Power. "Our water supply up there will not be usable depending on the nature of the eruption."

   In fact, Assistant General Manager James F. Wickser acknowledged that the DWP's emergeney plan for the Owens Valley anticipates a major earthquake. but not a volcanic event He promised that the plan will be revised.

   "Clearly the volcanic threat was not on the horizon in the early 1970s," said Wickser, who is in charge of the city's water system. "It could have dramatic effects, far more pronounced than an earthquake."

   After a magnitude 4.9 earthquake struck the Mammoth Lakes area Nov. 30-the strongest of thousands of quakes since the summer-U.S. Geological Survey scientists came close to  "yellow volcano watch" for the region.