After a relatively quiet period of seismic activity in the Los Angeles area, the last five months have been marked by five earthquakes larger than magnitude 4.0.
That hasn't occurred since 1994, the year of the destructive Northridge earthquake that produced 53 such temblors.
The Los Angeles Times' Rong-Gong Lin II looked into the issue. Here are some basic questions and answers.
Q: What does this increase in quakes mean?
A: Experts say it's hard to know whether the recent string of quakes suggests that a larger one is on the way. "Probably this will be it, and there won't be any more 4s. But the chance we will have a bigger earthquake this year is more than if we hadn't had this cluster,"
Q: Where have these quakes occurred?
A: The largest this year was a magnitude 5.1 in La Habra, which caused several million dollars in damage. Others hit Fontana and Rowland Heights. But scientists are particularly intrigued by the other two quakes, which were centered along the 405 Freeway under the Santa Monica Mountains. In addition to a 4.4 quake March 17 in Encino and Sunday's 4.2 temblor a few miles away in Brentwood, 15 earthquakes between magnitude 1.0 and 2.5 hit between January and March in the Santa Monica Mountains near Wilacre Park. The quakes show that seismic activity underneath the Santa Monica Mountains is increasing, Caltech seismologist Egill Hauksson said. The mountains were formed by earthquake activity over millions of years.
Q: Any sense of the causes?