Earthquake forewarning in the Cascadia region

Open-File Report 2015-1151
By: , and 



This report, prepared for the National Earthquake Prediction Evaluation Council (NEPEC), is intended as a step toward improving communications about earthquake hazards between information providers and users who coordinate emergency-response activities in the Cascadia region of the Pacific Northwest. NEPEC charged a subcommittee of scientists with writing this report about forewarnings of increased probabilities of a damaging earthquake. We begin by clarifying some terminology; a “prediction” refers to a deterministic statement that a particular future earthquake will or will not occur. In contrast to the 0- or 100-percent likelihood of a deterministic prediction, a “forecast” describes the probability of an earthquake occurring, which may range from >0 to <100 percent. When the time window is short (days to months) and the forecast is formulated for operational utility, this term may be “operational earthquake forecasting.” The subcommittee considered short-term forecasts only, herein referred to as “forewarnings,” but not their formulation into messages or their applications, which will be addressed by NEPEC in subsequent activities.
The subcommittee considered “direct” and “indirect” forewarnings. Direct forewarnings originate with observed changes in geologic processes or conditions, which may include

  • Increased rates of M>4 earthquakes on the plate interface north of the Mendocino region 
  • Changes in shallow seismicity patterns
  • Increased rates of moderate earthquakes within the subducting plate
  • Changes in the pattern of slow slip on the plate interface and other major faults

Indirect forewarnings are based largely on model predictions of increased earthquake-occurrence probabilities. In this context, “models” refers to simulations of the processes believed to affect earthquake occurrence, as implemented in computer software, laboratory experiments, or some analog natural system. These indirect forewarnings likely will be more uncertain and difficult to interpret than direct forewarnings. This report also highlights the challenges of assessing the significance of forewarnings, which mostly will be extraordinary events with little or no historical precedent in the Cascadia region.


Suggested Citation

Gomberg, J., Atwater, B., Beeler, N., Bodin, P., Davis, E., Frankel, A., Hayes, G., McConnell, V., Melbourne, T., Oppenheimer, D., Parrish, J., Roeloffs, E., Rogers, G., Sherrod, B., Vidale, J., Walsh, T., Weaver, C., and Whitmore, P., 2015, Earthquake forewarning in the Cascadia region: U.S. Geological Survey Open-File Report 2015–1151, 8 p.,

ISSN: 2331-1258 (online)

Study Area

Table of Contents

  • Executive Summary
  • Purpose
  • Geography
  • Forewarning Events
  • Examples of Plausible Forewarning Events in the Cascadia Region
  • Precedents for Forewarning Advisories
  • Assessment of Earthquake Probabilities
  • Acknowledgments
  • References
Publication type Report
Publication Subtype USGS Numbered Series
Title Earthquake forewarning in the Cascadia region
Series title Open-File Report
Series number 2015-1151
DOI 10.3133/ofr20151151
Year Published 2015
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Earthquake Science Center
Description iii, 8 p.
Country Canada, United States
Other Geospatial Cascadia region
Online Only (Y/N) Y
Additional Online Files (Y/N) N
Google Analytic Metrics Metrics page
Additional publication details