JOHANNESBURG, July 13 (Ridgecrest Online) — The U.S. Geological Survey reported a preliminary earthquake Monday morning near Johannesburg in eastern Kern County, California. In the initial reports available Monday, the quake was listed as magnitude 4.1 in one USGS post and 4.3 on the agency’s website, according to LAist. The quake struck at about 9:40 a.m. PDT, or 16:40:55 UTC, about 18 kilometers (11 miles) west-southwest of Johannesburg and north of Edwards Air Force Base. The USGS said the quake was about 7.0 kilometers (4.3 miles) deep. No immediate injuries or damage were reported in the initial coverage.
What does “preliminary” mean?
A preliminary earthquake report is an early estimate from the USGS. It tells people where the quake likely happened, how deep it was, and how strong it seemed at first. Those first numbers can change as more seismic data comes in.
That is why the Monday morning quake showed two different magnitudes in early USGS material. The size of the quake had not yet been finalized in the material reviewed for this report.
Why were two magnitudes reported?
LAist reported that the USGS website listed the quake at magnitude 4.3, while a USGS social-media post referred to it as magnitude 4.1. Both numbers came from the USGS, but they were part of the agency’s early reporting, not a final event record.
For readers, the main point is that the quake was still being measured and checked. Small changes in magnitude are common in the first stage of earthquake reporting.
Who may have felt the shaking?
USGS shake reports suggested that people in parts of Kern and San Bernardino counties may have felt light to moderate shaking. Other coverage said shaking was reported in Los Angeles, Orange, and Riverside counties.
The USGS shake-report system collects quick responses from people who say they felt an earthquake. Those reports help show how far the shaking reached.
What is ShakeAlert?
LAist reported that the USGS earthquake early warning system, ShakeAlert, was activated and issued warnings in some areas. ShakeAlert is designed to use early seismic signals to warn people after an earthquake starts and before the strongest shaking reaches some locations.
Has this area had recent earthquakes?
Yes. USGS felt-report data from a June 1 Johannesburg-area earthquake showed 46 online reports of weak shaking, and the agency later upgraded that quake from magnitude 3.8 to 4.1, according to the data cited in reporting on the event.
That earlier quake helps explain why people across eastern Kern County were watching closely on Monday. But for the Monday morning event itself, the most reliable takeaway is still the same: the USGS had issued only a preliminary report, and the final numbers could still change.