Statistical Seismology and Communication of the U.S. Geological Survey Aftershock Forecasts for the Southwest Puerto Rico Earthquake Sequence of 2019–2020

Session: Early Results from the 2020 M6.4 Indios, Puerto Rico Earthquake Sequence
Type: Oral
Date: 4/30/2020
Time: 09:30 AM
Room: 115
Description: 

A sequence of earthquakes offshore near Guayanilla and Guánica, Puerto Rico began in mid-December, 2019 and is continuing through mid-January, 2020. The USGS began issuing Operational Aftershock Forecasts for this sequence after the first M5.0 event occurred on 29 December. These initial forecasts used the Reasenberg and Jones model (Science, 1989) as updated in Page et al. (BSSA, 2016). That model ties all aftershocks to the mainshock and so the forecasts for the 29 December event were superseded when a M5.8 earthquake occurred on 6 January. And that forecast was superseded when a M6.4 event occurred the next day.

After a M5.9 aftershock on 11 January, the Reasenberg and Jones model did not sufficiently capture the complexity of this sequence. Hence, we began using an Epidemic Type Aftershock Sequence (ETAS) model (Ogata, JAmStat, 1988) to better capture the temporal evolution of the sequence. Our ETAS aftershock forecasting model does not include a background rate and so the forecasts are based solely on interevent triggering statistics. Preliminary experiments suggested that about 10% of the earthquake rate was due to an increased background rate before the M6.4. After that the background rate was negligible. We are developing swarm forecast models with a background rate, but they are not ready for use. While this sequence has some characteristics of a swarm it has not been critical to include a background rate in the forecasts, through mid-January.

Our standard aftershock forecasts are presented on the USGS earthquake event pages and contain text commentary and a table of expected numbers of events and probabilities. To help people interpret the probabilities, we developed a set of three scenarios based on experience from past sequences in New Zealand. Those scenarios were released via the USGS web site in English and Spanish. To support FEMA and local government decisions, a report on the expected duration of the sequence will be released.

Presenting Author: Andrew J. Michael

Authors