Observers pay their respects Friday at the scene where, a day before, driver Haeyoon Miller, 29, of Tustin, was crushed by a 50- to 70-foot eucalyptus tree. (KEVIN CHANG, Daily Pilot / September 16, 2011)

COSTA MESA — What caused a 10-ton tree to fall on a woman's car while she waited at a stop light remained unclear to public safety officials and seismic and horticultural experts Friday.

But theories — from moist soil to trimmed roots to this week's earthquake — emerged from the tragedy that claimed Haeyoon Miller, 29, who died following the impact on 17th Street and Irvine Avenue on Thursday afternoon.

A 3.5 earthquake that took place off Newport Beach about 2:56 a.m. Thursday may have played a role, though there easily could have been other contributing factors, according to Lisa Grant Ludwig, a UC Irvine seismologist.

About 380 people in Costa Mesa reported feeling the earthquake on the U.S. Geological Survey site, according to Ludwig.

"[That's a] good indication right there that that location was shaken relatively strongly," she said. "It's possible."

But soil conditions, whether the ground was wet or slanted, and the health of the tree, are all potential factors that could have felled the 50-foot blue gum eucalyptus, she said.

The city of Newport Beach, which has a contract with the city of Costa Mesa to maintain the trees near the cities' shared borders, said in a statement that the city is investigating what felled the tree.

The tree showed no signs of illness, according to the city.

The trees on the meridian are inspected every six months and were last pruned in April, according to the satement.

Eucalyptus are known for shedding bark and branches, but are not known to topple, said Laura Lyons, nursery manager at the UC Irvine arboretum.

Lyons said the UCI arboretum took out their eucalyptus "in part because of the liability."

Miller, who was in her blue, 2002 Hyundai at a red light at the time of the tree's collapse died of accidental blunt-force trauma, according to Larry Esslinger, supervisor and deputy coroner at the Orange County coroner's office.

She was on her lunch break from her job as an executive assistant at a mortgage firm, DLJ Financial in Newport Beach, according to friends. The victim also ran a children's gym with her boyfriend.

At the time of the accident Miller may have been on her way to pick up a brochure featuring photos she took of a friend's children, said George Osorio, 58, her boyfriend of five years. The couple had been sending text messages earlier in the day.

Miller was from South Korea, but as a violin prodigy moved to the United States with her parents when she was about 10 to attend the Juilliard School in New York, Osorio said. She went onto to perform atCarnegie Hall.

She spent most of her high school years at San Clemente High School. She attended USC but did not graduate, Osorio said.

Osorio and Miller lived in their Tustin home together for about a year, and would take trips up the California coast, to Seattle and Oregon, to "just to see places that were interesting for photographs," he said.

The two met after Miller began adult gymnastic classes at the gym where Osorio taught in Aliso Viejo. Miller had always wanted to do gymnastics but wasn't allowed as a child.

After a divorce, she began taking classes.

In January her love for photography and gymnastics combined at Pix&Flips, a personalized fitness training program for children.