Testing the Classic 1988 Forecast

David D. Jackson

Published June 13, 2018, SCEC Contribution #8052

The Working Group on California Earthquake Probabilities published its first 30-year forecast in 1988. They estimated magnitudes and conditional rupture probabilities for 16 “segments” of the San Andreas, Hayward, San Jacinto, and Imperial faults based on their last rupture date and a quasi-periodic recurrence model.

I applied the Numbers- and the Space-Tests, developed by the Collaboratory for Study of Earthquake Predictability (CSEP), on the forecast (WGCEP 1988). While only later earthquakes were used, the tests are not prospective because test metrics were not agreed in advance.. Forecasts and test protocols are best formalized concurrently. “Association,” that is attaching a prescribed probability to any relevant future earthquake, challenges formal testing. Only the magnitude 6 Parkfield in 2004 can be associated unambiguously with an anticipated outcome. Association of the 1989 magnitude 6.9 Loma Prieta earthquake is ambiguous. Only if it is included does the forecast agree acceptably with the number of events. A “hindcast” in 1988 would have shown that the 1988 – 2018 forecast would likely over-predict the earthquake rate. A simple Alternate “forecast” constructed and tested this year passes both numbers and space tests without the time-dependent recurrence assumptions of the Working Group forecast The comparison shows the importance of calibrating a forecast on past earthquake rates, and that a 30 year forecast test in California alone is unlikely to test the inherent assumptions. The results here have not been, nor could be, through the CSEP process, which requires prescribed protocols and authoritative data sources.

Key Words
forecast, test, California, WGCEP

Jackson, D. D. (2018). Testing the Classic 1988 Forecast. Seismological Research Letters, 89(4), 1288-1297. doi: 10.1785/0220180039.

Related Projects & Working Groups
UCERF, CSEP, WGCEP, Earthquake forecasting and testing