Statistical Seismology and Communication of the USGS Operational Aftershock Forecasts for the 30 November 2018 Mw 7.1 Anchorage, Alaska, Earthquake

The U.S. Geological Survey (USGS) has developed a national capability for aftershock forecasting after significant earthquakes. Use of this capability began in August 2018, and the 30 November 2018 M-w 7.1 Anchorage, Alaska, earthquake provided the first opportunity to apply this capability to a damaging earthquake in an urban area of the United States of America and observe how the forecast was discussed in the media. During this sequence, the forecasts were issued by a seismologist using interactive software that implements the Reasenberg and Jones (1989) model as updated in Page et al. (2016) The forecasts are communicated with a tiered template that provides basic information first before providing a more detailed numerical forecast and are posted on the mainshock's event page on the USGS earthquake program. Experience from the Anchorage sequence showed that the process worked well, and the first forecast was issued only 54 min after the mainshock occurred. Updates over the coming days, weeks, and months adapted the forecast model from the initial generic parameters for the seismotectonic region to Bayesian and sequence-specific models. Media reports accurately reported the forecast, demonstrating that the forecast template was successful except for a few reports that incorrectly merged the probability of one or more events in a given time-magnitude window with the likely range of the number of events. Changes to the template have been made to prevent that confusion in the future. We also released a special report on the possible duration of the sequence to assist in the federal disaster declaration and assistance process. Both our standard forecasts and this special report would benefit from more rapid determination of a sequence-specific decay rate.