The earthquake apparently gave no warning whatsoever, researchers told a meeting of the American Association for the Advancement of Science on Monday.
Quake experts do not even agree how to measure a quake's magnitude to see if an experiment for predicting quake activity has worked, said Thomas Jordan of the Southern California Earthquake Centre.
"Most seismologists, including myself, are pessimistic that in the next five years or 10 years we will ever be able to come up with a silver bullet earthquake solution," Jordan told a news conference, but added: "Never say never."
There are some suggestions that very low-frequency seismic activity might predict some quakes.
US Geological Survey (USGS) researchers have measured low-pitched rumbles from deep under California's San Andreas fault, about 25km southeast of Parkfield.
More study needed
They resemble measurements made at subduction zones in Japan and the Pacific Northwest.
A subduction zone is where one of the Earth's tectonic plates is slipping under another. The Sumatra temblor occurred at one such subduction zone.
"We just don't understand what those signals mean," Jordan said. "As best we can tell, there are no reliable short-term precursors to earthquakes."
David Applegate of the USGS said Parkfield, which has been covered with instruments for 20 years in the hope of catching a quake in the act, was providing useful information, but nothing that could be used to begin predicting a quake in a useful way.
More important will be finding out the highest risk areas so building codes can be brought up to date and emergency services told where the ground will shake the most, so they can respond quickly when a quake does occur.