It has been 20 years since Southern California experienced a major earthquake, a powerful 6.7-magnitude temblor that rolled through Northridge, killing 57 people. But this stretch of seismic calm, though welcome in obvious ways, has undermined efforts to force Los Angeles to deal with what officials describe as potentially lethal deficiencies in earthquake preparation.

That may be changing. Since two back-to-back earthquakes Friday evening — a relatively small one with a magnitude of 3.6, followed by a long and rolling 5.1 quake — Los Angeles has been shaken by nearly 175 smaller aftershocks.
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Mr. Garcetti said he was preparing a plan to deal with what Ms. Jones, a familiar figure in California known as the Earthquake Lady, identified as the three most serious shortcomings in the city’s earthquake preparation. The first challenge is to fix unreinforced buildings that could collapse in a big quake, an issue that promises to stir debates about acceptable risk and who should pay for that very expensive endeavor.

The other two are to plan for catastrophic collapses in water supply and basic communication, as many of the aqueducts and Internet cables that supply Los Angeles cross the San Andreas Fault.