The fault line runs under thousands of older buildings, many made with concrete and never properly retrofitted. What’s worse, the softer soil in the Los Angeles Basin would enhance a quake’s ripple effect like a stone in water. As a result, a magnitude-7.5 earthquake on this fault could kill 3,000 to 18,000 people and cause as much as $250 billion in damage, says the US Geological Survey (USGS).
By comparison, estimates are that an even larger magnitude 8 quake along the San Andreas, further north and west in more rural areas, would cause 1,800 deaths.
In 1987 a 5.9 quake on the Puente Hills fault named the Whittier Narrows killed 8 people and caused more than $350 million in damage to homes, cars, roads, and utility lines.
USGS seismologist Lucy Jones was quoted in the Los Angeles Times as saying that Friday night’s initial 5.1 quake was caused by the fault slipping for half a second underground, resulting in about 10 seconds of shaking at the surface. But because of its horizontal orientation – unlike the vertical cracks that characterize other faults – a 7.5 quake on the Puente Hills fault could cause it to slip for 20 seconds, producing shaking at the surface that would last far longer and be spread over a larger area, possibly 375 square miles.