The Newport-Inglewood Fault runs about 75 kilometres through the heart of the Los Angeles Basin, from the Santa Monica area southeast through Inglewood and Long Beach and out to sea near Newport Beach. It is a right-lateral strike-slip fault whose motion has also pushed up a line of low hills and created the oil-rich structural traps that made the basin one of the most productive fields in the world.
What makes this fault especially dangerous is not its length but its location: it lies directly beneath one of the most densely populated and heavily built regions in the United States. Offshore it may connect with the Rose Canyon Fault near San Diego, raising the possibility of longer, more powerful ruptures than once assumed.
The fault produced the deadly 1933 Long Beach earthquake (magnitude 6.4), which killed around 120 people and caused severe damage to schools, prompting California's landmark Field Act requiring earthquake-resistant school construction. It remains a benchmark example of how a moderate quake on an urban fault can inflict outsized human and economic harm.