My brother lived in LA and he told me that big earthquakes always happen without warning. You get a big one and then after-shocks but no “pre-shocks”
That seems to be the case. Stress builds up and bam, the big one happens. Here in the SF Bay Area, the Hayward Fault parallels the San Andreas Fault. Big ones on the San Andreas Fault are often followed by a big one on the Hayward Fault a few decades later. Scientists can't predict it, can only make estimates on percentages of probability. It's been 28 years since the 1989 quake on the San Andreas here, and the probability of a big one on the Hayward Fault (which runs directly through Berkeley) is very high. Who knows if the 4.5 in Berkeley relieved the stress, or caused a buildup of stress nearby. As your brother mentioned, no warning of the big one, yikes.