Magnitude-4.0 quake strikes in major Los Angeles fault zone

The Newport-Inglewood Fault Zone is the likely source of yesterday’s early morning quake that rattled many in the Los Angeles metro region.

By Jennifer Schmidt, Ph. D., Temblor Earthquake News Director (@DrJenGEO)

Citation: Schmidt, J., 2021, Magnitude-4.0 quake strikes Los Angeles, Temblor,

Lennox, CA: A magnitude-4.0 earthquake shook parts of the Los Angeles metro region yesterday, just before 5 a.m. local time. The mainshock occurred just to the south of the SoFi stadium. It did not cause damage but was strong enough to rattle the ground throughout the LA Basin. By 24 hours later, approximately 80 aftershocks had struck in the area. Aftershocks are a normal occurrence following a moderate quake and taper off over time.

Map with red lines showing faults and red circles showing earthquake locations.
The magnitude-4.0 quake struck in Lennox, Calif., just before dawn and has since been followed by many aftershocks.


Shaking rattles residents

The main quake occurred nearly 12 miles (20 kilometers) below the surface, meaning “everyone is at least 20 km away,” said Lucy Jones, a seismologist formerly with the U.S. Geological Survey (USGS) and now at the Dr. Lucy Jones Center for Science and Society and Caltech, in a tweet just after the event. Because the quake was relatively deep, the region was spared from the most damaging shaking that typically occurs closer to the nucleation point of the quake. But that doesn’t mean that it was not jarring for those nearby.

“Everything was shaking,” said Madeline Bauer, who was awakened by the quake. Bauer said that a small item fell from a shelf in her home in Koreatown, approximately 8 miles (12 kilometers) north of the epicenter. Within a day, the USGS “Did You Feel It” system received more than 7,400 shaking reports.

Broad aftershock pattern

The magnitude-4.0 mainshock struck near the Newport-Inglewood Fault Zone, a complex series of faults that accommodate right-lateral motion between eastern and western portions of the LA Basin. The land to the west of the fault zone is moving slowly to the north relative to the land to the east. The Newport-Inglewood Fault Zone runs for a fragmented 40-mile (65-kilometer) stretch between Beverly Hills and Newport Beach, before continuing offshore.

USGS data suggest that the earthquake resulted from a combination of side-to-side motion (known as strike-slip action) and up-and-down motion (from thrusting) along its host fault. This is generally consistent with what scientists known about the Newport-Inglewood Fault; however, the shock struck just to the west of the closest mapped fault segment. It is not yet clear whether the quake struck on a main Newport-Inglewood Fault segment or an unmapped step-over fault between two main sections.

Aftershocks have occurred over a fairly broad area — roughly 1.5 by 4.5 miles (2.5 by 7 kilometers). This pattern is reminiscent of seismicity associated with the fault zone (Hauksson, 1987).

A fault with history

Los Angeles is no stranger to strong quakes. Since 2000, nearly 20 magnitude-4.0 and greater quakes have struck within the metro area. Last October, a magnitude-4.5 quake rattled residents to the east, in the Montebello Hills. The Newport-Inglewood Fault Zone itself hosted the 1933 magnitude-6.4 Long Beach earthquake. That quake struck just offshore near Huntington Beach and caused widespread damage in the region (Hough and Graves, 2020), with greatest shaking in the central LA city of Compton. An earlier study identified at least three other large quakes along the fault that occurred in the last 11,000 years (Grant et al., 1997).

Aftershocks are likely to continue after yesterday’s quake. In the event of shaking, remember to drop, cover, and hold on.

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Grant, L. B., Waggoner, J. T., Rockwell, T. K., & von Stein, C. (1997). Paleoseismicity of the north branch of the Newport-Inglewood fault zone in Huntington Beach, California, from cone penetrometer test data. Bulletin of the Seismological Society of America, 87(2), 277-293.

Hauksson, E. (1987). Seismotectonics of the Newport-Inglewood fault zone in the Los Angeles basin, southern California. Bulletin of the Seismological Society of America, 77(2), 539-561.

Hough, S. E., & Graves, R. W. (2020). The 1933 Long Beach Earthquake (California, USA): Ground Motions and Rupture Scenario. Scientific reports, 10(1), 1-10.