SAN FRANCISCO >> For those near the epicenter of the earthquake that struck Southern California on Thursday, the ground jolted so violently that it bounced them off their feet.

But such is the potential devastation of earthquakes in California that seismologists described it as only a moderate one that would not have any bearing on the inevitable arrival of a giant earthquake, which could unleash at least 30 times more seismic energy.

Californians woke early today to a 5.4 magnitude aftershock, a day after the 6.4 magnitude earthquake in the Mojave Desert shook the small desert city of Ridgecrest, causing minor injuries but heralding the end of a period of seismic quiet in California. Thursday’s earthquake was the strongest in Southern California in two decades.

Paul Caruso, a geophysicist with the U.S. Geological Survey in Golden, Colorado, said Thursday’s earthquake did not change the agency’s calculations of when the big one might strike.

“We are not changing our forecast for the San Andreas,” Caruso said. “We still believe there’s a 70% chance of a magnitude 7 or greater in Southern California before 2030.”

The faults around Ridgecrest are part of the larger San Andreas system that runs from the Gulf of California to Mendocino, north of San Francisco.

The San Andreas fault, which runs near heavily populated areas and defines the boundary between the North American and Pacific tectonic plates, is considered the biggest seismic threat to California. Related cracks in the earth, like the Hayward fault that runs through Oakland and Berkeley in the San Francisco Bay Area, are also considered major threats.

Tom Heaton, an earthquake expert at the California Institute of Technology, said the region where Thursday’s earthquake struck seems to be characterized by a complex set of faults that have helped form geological features called basin and ranges.

“These basin and range earthquakes often come in clusters of events,” Heaton said.

Susan Hough, a seismologist with the USGS, said today’s 4:07 a.m. tremor, 9 miles northeast of Ridgecrest near where Thursday’s earthquake took place, would likely be the strongest aftershock, although more powerful ones could follow. There have been about 200 aftershocks since Thursday’s earthquake.

Seismologists have puzzled over why California’s biggest faults have been relatively quiet in recent decades. At the same time, experts have warned that California remains unprepared for a big quake in many keys areas. Few people have bought earthquake insurance, and the state has many buildings that are likely to suffer major damage because of the way they are constructed.

Even though it occurred on minor faults, Thursday’s earthquake could be felt as far away as San Francisco and Phoenix. Around 50,000 people reported on the USGS website that they felt the shaking.

This morning, Giselle Miranda was putting on her shoes in her Palmdale, California, living room when she said she heard a rumbling noise from below and a cracking sound from the walls. Her apartment quivered, and she checked on her sisters. The quake “definitely had me a little shaken up,” Miranda said, even if it lasted for only a few seconds.

Hough said she did not expect the aftershocks to be felt too far from the epicenter of Thursday’s earthquake, although she cautioned that anything could happen. Seismologists are able to understand earthquake patterns in retrospect, but it is much more difficult to predict patterns until events are over, she said.

Hough said a wry line attributed to Charles Richter, the man who developed a scale to measure earthquake strength, held up: “When you get a lot of earthquakes, you get a lot of earthquakes.”

“This is the time in California where you want to stay on your toes,” she said.