Racing continued Friday at the northern California track where a jockey fell off his horse and later died.

Jorge Herrera, 33, died Thursday at Eden Trauma Center in Castro Valley from a head injury.

He was racing in his second event of the day at the track in Pleasanton when he was thrown from his horse, Morito, which appeared to clip the heels of another horse and stumble. Herrera lurched forward and went over Morito's head as the horse righted itself and continued.

"He was stepped on by either his horse or another horse and suffered severe head trauma," April Mitchell, a spokeswoman for the Alameda County Fair, told CNN affiliate KPIX. "We deeply mourn this loss today."

Mitchell told CNN that racing began at its usual time Friday, with a moment of silence after the national anthem. A memorial service was scheduled for Sunday, the last day of the three-week meet.

The racing death is the first since jockey Mark Villa died at a New Mexico track in September 2010 and is a stark reminder of the physical toll the sport can take.

Jockeying is the most dangerous job in sports based on the number of deaths and injuries, said Terry Meyocks, national manager for the Jockeys' Guild, based in Nicholasville, Kentucky.

Since 2007, there have been five deaths -- including Herrera -- and last year 19% of the active riding members of the guild were out on temporary disability. There are roughly 650 active riders in the guild.

Within the past month the guild has begun work on a jockey injury database, which looks not only at the number of injuries, but at the racing conditions and what gear the rider was wearing, Meyocks said. The goal is to find patterns that might help prevent injuries in the future.

Meyocks said there has been an average of about two jockey deaths per year in the United States -- 152 total -- since the guild started compiling a fatalities database in 1940.

An ambulance was following the horses and medical personnel responded immediately, a statement from the association said.

Fair CEO Rick Pickering said the event has five paramedics within 100 feet of the track. For the rest of the meet, an additional paramedic will join emergency medical technicians in an ambulance that follows the horses, he said.

The fair exceeds standards for responding to emergencies, Pickering said.

Herrera had 1,010 thoroughbred mounts and 55 victories in his career, according to equibase.com. He had been injured three times, Meyocks said. The guild is looking into what those injuries were.

Herrera was a quiet, polite man who generally kept to himself, said Darrell Haire, regional manager for the Jockeys' Guild.

"He was a nice kid; everybody liked him," he said. "Like a lot of riders he was trying to get started and get lucky. He didn't get the opportunities that a lot of guys get."

Haire said he thought Herrera, originally from Mexico, lived with an uncle.

It isn't uncommon for horses to clip heels during a race, Haire said.

"A lot of times when they clip heels they'll stumble, but when they clip just right they'll do a nose dive," he said. "It's like a crash. They just go down."

It also isn't uncommon for jockeys to fall, though usually they'll break collarbones and shoulders, or injure their backs, he said.