Magnitude Limits of Subduction Zone Earthquakes

Maximum earthquake magnitude ( m x ) is a critical parameter in seismic hazard and risk analysis. However, some recent large earthquakes have shown that most of the existing methods for estimating m x are inadequate. Moreover, m x itself is ill‐defined because its meaning largely depends on the context, and it usually cannot be inferred using existing data without associating it with a time interval. In this study, we use probable maximum earthquake magnitude within a time period of interest, m p( T ), to replace m x . The term m p( T ) contains not only the information of magnitude limit but also the occurrence rate of the extreme events. We estimate m p( T ) for circum‐Pacific subduction zones using tapered Gutenberg–Richter (TGR) distributions. The estimation of the two TGR parameters, β ‐value and corner magnitude ( m c), is performed using the maximum‐likelihood method with the constraint from tectonic moment rate. To populate the TGR, the rates of smaller earthquakes are needed. We apply the Whole Earth model, a high‐resolution global estimate of the rate of m ≥5 earthquakes, to estimate these rates. The uncertainties of m p( T ) are calculated using Monte‐Carlo simulation. Our results show that most of the circum‐Pacific subduction zones can generate m ≥8.5 earthquakes over a 250‐year interval, m ≥8.8 earthquakes over a 500‐year interval, and m ≥9.0 earthquakes over a 10,000‐year interval. For the Cascadia subduction zone, we include the 10,000‐year paleoseismic record based on turbidite studies to supplement the limited instrumental earthquake data. Our results show that over a 500‐year period, m ≥8.8 earthquakes are expected in this zone; over a 1000‐year period, m ≥9.0 earthquakes are expected; and over a 10,000‐year period, m ≥9.3 earthquakes are expected.