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The Latest: Los Angeles to update quake warning threshold

LOS ANGELES (AP) — The Latest on Southern California's strongest earthquake in 20 years (all times local):

1:20 p.m.

The city of Los Angeles is planning to reduce the threshold for public notifications by its earthquake early warning app, but officials say it was in the works before Southern California's big earthquake Thursday.

The ShakeAlert LA app was designed to notify users of magnitudes of 5.0 or greater and when a separate intensity scale predicts potentially damaging shaking.

Robert de Groot of the U.S. Geological Survey says lowering the magnitude to 4.5 was already being worked on and had been discussed with LA as recently as a day before Thursday's magnitude 6.4 quake centered in the Mojave Desert.

The shaking intensity levels predicted for LA were below damaging levels, so an alert was not triggered.

Mayor's office spokeswoman Andrea Garcia also says the lower magnitude threshold has been in the planning stages and an update to the system is expected this month.

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7:05 a.m.

A vigorous aftershock sequence is following the strongest earthquake to hit Southern California in 20 years.

A magnitude 5.4 quake at 4:07 a.m. Friday is so far the strongest aftershock of Thursday's magnitude 6.4 jolt, and was felt widely.

Seismologists had said there was an 80% probability of an aftershock of that strength.

Thursday's big quake struck in the Mojave Desert, about 150 miles (240 kilometers) northeast of Los Angeles, near the town of Ridgecrest, which suffered damage to buildings and roads.

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9 p.m.

The strongest earthquake in 20 years shook a large swath of Southern California and parts of Nevada on the July 4th holiday, rattling nerves and causing injuries and damage in a town near the epicenter, followed by a swarm of ongoing aftershocks.

The 6.4 magnitude quake struck at 10:33 a.m. Thursday in the Mojave Desert, about 150 miles (240 kilometers) northeast of Los Angeles, near the town of Ridgecrest, California.

Kern County Fire Chief David Witt says multiple injuries and two house fires were reported in the town of 28,000. Emergency crews were also dealing with small vegetation fires, gas leaks and reports of cracked roads.

Witt says 15 patients were evacuated from the Ridgecrest Regional Hospital as a precaution and out of concern for aftershocks.

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