According to the USGS, “Preliminary calculations indicate that, as of 10:00 am (PDT) Sept. 27, 2016, there is a 0.03% – 1% chance (1 in 3000 to 1 in 100) of a magnitude 7 or greater earthquake being triggered on the Southern San Andreas fault within the next seven days through October 4, with the likelihood decreasing over time.
This probability range is estimated using several models developed in California to assess foreshock/aftershock probabilities.”
The calculation was made by seismologists at the USGS at its Menlo Park, CA, Pasadena, CA, and Golden, CO, centers, for the California Earthquake Prediction Evaluation Council, of which USGS seismologist Morgan Page is a member.
The calculation is based on the most recent time-dependent Uniform California Earthquake Rupture Forecast model (UCERF3), combined with the most current concepts of the likelihood that an aftershock of the M=4.3 could be larger than its mainshock. This is called an Epidemic Type Aftershock Sequence, or ETAS, developed by the great Japanese seismologist-statistican, Yosihiko Ogata).