JOHANNESBURG, July 13 (Ridgecrest Online) — A preliminary earthquake struck near Johannesburg in eastern Kern County on Monday morning, and the U.S. Geological Survey said the event was still being checked when early reports went out.
USGS reported the quake at about 9:40 a.m. PDT, with the epicenter about 18 kilometers, or 11 miles, west-southwest of Johannesburg in the Indian Wells Valley region. The agency’s early information also placed the quake at a depth of 7.0 kilometers, or 4.3 miles, and north of Edwards Air Force Base.
Why did some reports say 4.1 and others say 4.3? That can happen during the first minutes after an earthquake. USGS often posts an initial magnitude estimate before later reviewing more data from its sensors. In this case, LAist reported that the agency showed magnitude 4.3 on its website and 4.1 on social media. The difference is a sign that the number was still preliminary, not that two separate quakes happened.
For readers, the key point is that the event was in the moderate range and close enough to be felt over a wide area. USGS shake reports and eyewitness accounts relayed by local outlets showed shaking in parts of Kern and San Bernardino counties, with reports also coming in from Los Angeles, Orange and Riverside counties. Viewers in Los Angeles County, including Burbank and Hollywood, also reported feeling it.
The USGS early warning system, ShakeAlert, was activated in some areas, according to LAist. Early warning alerts can reach people a few seconds before shaking arrives, depending on where they are and how far they are from the epicenter.
So far, there were no immediate reports of injuries or damage in the initial coverage.
What does “preliminary” mean?
When an earthquake first happens, seismologists have only a small amount of data. USGS uses readings from its network of sensors to estimate where the quake started, how deep it was and how large it was. Those first numbers can be updated as more stations are analyzed. That is why early earthquake reports often change a little before the final record is posted.
Why do people feel a quake miles away?
Even a quake centered in a relatively remote part of Kern County can be felt far from the epicenter, especially if the shaking moves efficiently through the ground. That is one reason people in several Southern California counties reported noticing Monday’s quake.
The Johannesburg area has also seen other recent seismic activity. USGS listed a preliminary magnitude 4.2 quake near Frazier Park on Sunday. In an earlier June 1 quake near Johannesburg, USGS later upgraded its first reading from 3.8 to 4.1 and received 46 online felt reports, including from Canyon Country, Ridgecrest, Inyokern and Mojave.
For now, Monday’s Johannesburg-area quake remains a preliminary USGS event. The final magnitude and other details can still change as the agency finishes its review.
