What happened Monday morning?
JOHANNESBURG, July 13 (Ridgecrest Online) — A preliminary earthquake struck near Johannesburg in eastern Kern County at about 9:40 a.m. PDT, according to the U.S. Geological Survey. Early readings placed it at magnitude 4.1 or 4.3, which shows the number was still being reviewed.
The USGS said the epicenter was about 11 miles west-southwest of Johannesburg in the Indian Wells Valley region, near Edwards Air Force Base. The agency reported the depth at about 7 kilometers, or 4.3 miles.
Why did the magnitude differ?
Earthquake magnitudes are often given first as automatic estimates. As more seismic data comes in, the number can change. In this case, the USGS website listed the quake at 4.3 while a USGS social post gave it as 4.1, a sign that the reading was still preliminary.
That kind of difference does not mean two separate earthquakes happened. It means the agency was still refining one event.
How strong was it?
A quake in the low-4 range can be felt across a fairly wide area. Shake reports and eyewitness reports indicated light to moderate shaking across parts of Kern and San Bernardino counties, with additional reports from Los Angeles, Orange and Riverside counties.
The early reports available Monday did not show immediate injuries or damage.
Did anyone get a warning?
The USGS ShakeAlert early warning system was activated, and some areas received alerts before the shaking arrived. Those warnings are automatic and may not reach every location.
Has this area seen recent quakes?
Yes. A separate Johannesburg-area quake on June 1 was later upgraded by the USGS from magnitude 3.8 to 4.1, and the agency received 46 online felt reports for that event.
That earlier quake is a reminder of why scientists treat first earthquake numbers as estimates. The final reading can shift after more data is checked.
