Bilinearity in the Gutenberg‐Richter Relation Based on M L for Magnitudes Above and Below 2, From Systematic Magnitude Assessments in Parkfield (California)

Several studies have shown that local magnitude, M L, and moment magnitude, M , scale differently for small earthquakes ( M  < ~2) than for moderate to large earthquakes. Consequently, frequency‐magnitude relations based on one or the other magnitude type cannot obey a power law with a single exponent over the entire magnitude range. Since this has serious consequences for seismic hazard assessments, it is important to establish for which magnitude type the assumption of a constant exponent is valid and for which it is not. Based on independently determined M , M L and duration magnitude, M d , estimates for 5,304 events near Parkfield, we confirm the theoretically expected difference in scaling between the magnitude types, and we show that the frequency‐magnitude distribution based on M and M d follows a Gutenberg‐Richter relation with a constant slope, whereas for M L it is bilinear. Thus, seismic hazard estimates based on M L of small earthquakes are likely to overestimate the occurrence probability of large earthquakes.