JOHANNESBURG, July 13 (Ridgecrest Online) — A USGS reading showed a preliminary earthquake near Johannesburg in eastern Kern County on Monday morning, around 9:40 a.m. PDT. Early reports put the quake at magnitude 4.1 or 4.3, and at a depth of about 7 kilometers, or 4.3 miles. The USGS placed the epicenter about 18 kilometers, or 11 miles, west-southwest of Johannesburg, near Edwards Air Force Base.
No immediate injuries or damage were reported in the early coverage.
What does “preliminary” mean?
An earthquake is often measured first by a computer system that looks at signals from nearby seismic stations. That first number can change as more data comes in. That is why one report can say 4.1 while another says 4.3. LAist reported that the USGS showed 4.3 on its website and 4.1 in a social-media post.
Preliminary does not mean wrong. It means the agency is still refining the result.
Why can the number change?
Magnitude is a measure of how much energy an earthquake released. The first estimate comes quickly, but it may be revised after more stations send data and scientists review the event. Smaller differences, such as 4.1 versus 4.3, are common early on.
Depth matters too. A shallow quake can shake the ground more strongly near the surface than a deeper one of the same size.
Where was it felt?
USGS shake reports suggested light to moderate shaking in parts of Kern and San Bernardino counties. CBS Los Angeles and ABC7 reported that people also felt it farther away, including in parts of Los Angeles, Orange and Riverside counties. LAist said the USGS ShakeAlert early warning system was activated in some areas.
Did Southern California just have another notable quake?
Yes. VVNG reported a preliminary magnitude 4.2 quake near Frazier Park on Sunday, which would make Monday’s event the second magnitude-4-or-greater earthquake in Southern California in about 48 hours.
The region has also seen recent revisions in other quakes. The Sacramento Bee reported that a June 1 earthquake near Johannesburg was first listed by the USGS as magnitude 3.8 and later upgraded to 4.1. The USGS also received 46 online felt reports for that event. That kind of update is one reason scientists treat the first reading as a starting point.
What is still unknown?
The material available Monday did not include a final USGS event page for the quake, so the official final magnitude, depth and any later damage assessment were not confirmed there. No on-the-record explanation for the 4.1-versus-4.3 difference was included in the available reports.
