Bay Area residents are familiar with earthquakes, but nearly 500 in three weeks is odd.

This has been the reality for San Ramon, which has felt 535 earthquakes in 22 days. It began with a micro earthquake at 8:50 a.m. on Oct. 13 that registered a magnitude of 0.8 and peaked with a 3.6 tremor on Oct. 19 at 4:21 p.m., according to the United States Geological Survey.

Ninety-one of the 535 earthquakes were a magnitude of 2.0 or higher, but according to USGS Information Specialist Susan Garcia the seismic activity has died down in the past few days. Since Oct. 30 there have been less than 10 registered earthquakes, despite the slight increase on Halloween.

“The drop in activity has been drastic,” Garcia said. “Swarms like these do happen but aren’t necessarily common. In many cases they are decades apart.”

According to the USGS, the last time San Ramon had a swarm of earthquakes was in 2003, which produced 120 in 31 days that included a 4.2 magnitude quake. The largest recorded swarm in the area was in 1990 when nearby Alamo had 351 earthquakes in 42 days. Neither of the swarms was followed by or included an earthquake over a magnitude of 4.2.

Moira Kelly works at Bay Books in San Ramon and has been on duty during several of the earthquakes.

“Some of us felt a few during the day,” Kelly said. “They weren’t really strong. Nothing fell and we haven’t had any damage.”

The USGS earthquake reporting system, “Did You Feel It” has received over 500 reports since the swarm began three weeks ago. The series of quakes have been reported from throughout the area including Danville, Alamo, Pleasanton, Dublin and Concord. On Oct. 27 and 28, San Ramon had over 30 earthquakes with nearly half of them registering at 2.0 or greater.

According to the USGS website, the earthquakes are strike-slip events that are occurring within the Calaveras Fault Zone near the north end of the Pleasanton Fault. According to the UC Berkeley Seismological Laboratory, strike-slip faulting is when the blocks of land are vertical to each other and the movement is horizontal, which are commonplace when it comes to seismic activity.

Since 1997, UC Berkeley has completed or begun approximately $500 million in seismic improvements to structures on the campus. As of July, 64 of the campuses 200 plus buildings are listed as poor or very poor on their seismic rating list. According to UCBSML, retrofitting includes redesigning the structure of the foundation to slide or roll in order to prevent collapsing during a major earthquake.

San Ramon Assistant City Manager Eric Figueroa said that despite the massive number of recent seismic activity there hasn’t been any major damage reported. According to Garcia, the history of swarms in the area indicates that this one could last for over a month and a large earthquake has not followed previous swarms. Many businesses in the area like Bay Books have not been retrofitted to sustain a major earthquake.

Retrofitting and the term “earthquake safe” is fairly new and has been primarily in effect since the 1989 Prieta-Loma earthquake that rocked the Bay Area. Most buildings built before 1990 use an old bolt style foundation system while many of those built after rely on the new system aimed to prevent collapsing.