[Rockhounds] The weird way the Los Angeles basin alters earthquakes

[Kreigh Tomaszewski]

Southern California has been shaken by two recent earthquakes greater than
magnitude 4.0. The way they were experienced in Los Angeles has a lot to do
with the sediment-filled basin the city sits upon.

A little over an hour after sunset on 6 August 2024, a sparsely populated
belt of farmland near Bakersfield, Southern California, was shaken from a
restful evening. A magnitude 5.2 earthquake
<https://earthquake.usgs.gov/earthquakes/eventpage/ci40865184/executive>,
followed by hundreds of smaller aftershocks
<https://earthquake.usgs.gov/earthquakes/map/?currentFeatureId=ci40865184&extent=35.00497,-119.23737&extent=35.18587,-118.96271&autoUpdate=false&range=search&sort=largest&baseLayer=terrain&settings=true&search=%7B%22name%22:%22Search%20Results%22,%22params%22:%7B%22latitude%22:%2235.0955%22,%22longitude%22:%22-119.1002%22,%22maxradiuskm%22:%2210%22,%22endtime%22:%222025-08-16T05:00:00.000Z%22,%22limit%22:%2220000%22,%22orderby%22:%22magnitude%22,%22starttime%22:%222024-07-08T04:09:56.760Z%22%7D%7D>,
shuddered through the area as a fault near the southern end of the Central
Valley ruptured.

It wasn't a terribly unusual event, by California's standards. The state is
the second-most seismically active
<https://www.usgs.gov/faqs/which-state-has-most-earthquakes-cause-damage-which-state-has-most-earthquakes-not-human>
 in the United States behind Alaska
<https://journals.sagepub.com/doi/10.1177/87552930231215428>, with Southern
California experiencing an earthquake on average every three minutes
<https://www.science.org/doi/full/10.1126/science.aaw6888>. While most are
too small to be felt, around 15-20 events exceed magnitude 4.0 each year
<https://www.usgs.gov/programs/earthquake-hazards/cool-earthquake-facts>.

This latest magnitude 5.2 earthquake is the largest to hit Southern
California in three years. The epicenter was about 17 miles (27km) south of
Bakersfield, California, and people reported shaking
<https://earthquake.usgs.gov/earthquakes/eventpage/ci40865184/dyfi/intensity>
nearly
90 miles (145km) away in portions of Los Angeles and as far away as San
Diego. Then, a few days later, another jolt rattled the Los Angeles
<https://earthquake.usgs.gov/earthquakes/eventpage/ci40699207/executive> area
due to a rupture on a small section of the dangerous Puente Hills fault
system
<https://pubs.geoscienceworld.org/gsa/geology/article-abstract/45/3/227/195267/Accelerating-slip-rates-on-the-Puente-Hills-blind?redirectedFrom=fulltext>.
The resulting magnitude 4.4 earthquake had its epicentre just four miles
northeast of the city's downtown area.

Although there was minimal damage caused by both quakes, they have
highlighted just how the geology under California's largest city can alter
the effects of fault movements in the area
<https://eartharxiv.org/repository/view/1274/>. The relatively shallow
depth of the 6 August earthquake appeared to create more intense or
prolonged shaking
<https://earthquake.usgs.gov/earthquakes/eventpage/ci40865184/map?shakemap-code=40865184&shakemap-source=ci&shakemap-intensity=true&shakemap-mmi-contours=false&shakemap-stations=true>
in
some parts of the city, while others felt almost nothing at all.

https://www.bbc.com/future/article/20240816-california-earthquakes-why-the-los-angeles-basin-is-like-a-bowl-of-jelly