Six days after the Loma Prieta quake, aftershocks continued to rattle the Bay Area. Scientists had to use calibration charts to translate the shaking they were seeing on their seismographs into magnitudes.
At the U.S. Geological Survey in Menlo Park, the seismographs used then are now considered relics.
In 1989, it took about eight minutes to calculate the magnitude of an earthquake; these days, calculations are complete in seconds.
"In under a minute, the earthquake has been declared, its been located and its initial estimate of the magnitude has been determined," USGS senior seismologist Jack Boatwright said. Boatwright is the co-director of the USGS Northern California Earthquake Hazard Program.
Scientists no longer physically collect data from their instruments in the field, an information-gathering process that took a month after /*Loma Prieta*/. Now, data is transmitted digitally and detailed shake maps are available to anyone with an Internet connection in a matter of minutes.
Witness accounts of earthquakes are now easily collected online, giving scientists instant feedback on where the strongest shaking or damage may have occurred.
After Loma Prieta, it took a couple of days before anyone learned about one of the fissures -- stretching nearly 500 feet long and 17 feet deep in some points near Summit Road in the Santa Cruz Mountains.
"At the beginning, everyone thought this was the fault," USGS geologist David Schwartz said. Schwartz, with Boatwright, is co-director of the USGS Northern California Earthquake Hazard Program.
As it turned out, the Loma Prieta quake occurred not on the San Andreas Fault, but on a blink fault deep below the earth's surface.
"No one knew that was there before Loma Prieta," Schwartz said.
Schwartz specializes in earthquake faulting and probabilities.
"The Bay Area has the highest density of active faults per square mile of any urban center in the United States," Schwartz said.
Trenches allow scientists to look back in time and examine the history of a fault line.
In Fairfield, scientists work to piece together different strands of the Green Valley Fault.
"This is about 1,000 years ago, this is the level of four earthquakes back, this grey clay layer right here is approximately the ground surface in 1700 AD, about 300 years ago," USGS geologist Jim Lienkaemper said.
The time of the last earthquake along the Green Valley Fault is marked by a "V," essentially a quake fossil on the wall of the trench.
"The earthquake came and it shook and it pulled apart here," Lienkaemper said.
It is the last opportunity scientists have to gather information from the trench on the Green Valley Fault. A year from now, a development is expected to go up on this area, with the fault line being preserved as open space.
Scientists believe the fault is ready, if not past due, for a quake.
"A 6.8 or 6.9 on this fault will be felt throughout the East Bay and it's very likely that a large earthquake on this fault will produce shaking that will damage the levies in the Delta," Schwartz said.
Paleoseismology, the study of ancient earthquakes by examining sediment and rocks, began in earnest in the Bay Area after Loma Prieta. Scientists now have information on every major fault in the Bay Area.
Much attention is now focused on the Hayward Fault, which has not seen a major quake since 1868. Scientists give it a 31 percent chance of a 6.7 or greater in the next 30 years.
A Google Earth flyover details critical infrastructure near or crossing the Hayward Fault, including BART, East Bay MUD, Interstate 580 and Highway 24. It is also a densely populated area.
"We now have houses built right up against the fault, so you'd see that strong damage recur the entire length of the fault," Boatwright said.
The message from scientists is the same as it was 20 years ago: Prepare now, this is earthquake country.
"We've been very lucky that we've gone 20 years without experiencing something like Loma Prieta, but that's going to end and we have to be ready," Schwartz said.
'89 QUAKE FULL COVERAGE:
Web exclusive content commemorating the 20th anniversary of the 1989 Loma Prieta quake. Includes extended interviews with reporters who covered the quake, as well as city officials and first responders who lived through it all.