Fault dimensions and maximum magnitude

Many researchers apply rupture dimension statistics to mapped fault dimensions to estimate maximum possible earthquake size. Here we examine the assumptions involved in this process, using seismic and geological data for a selection of southern California and northern Baja earthquakes. The hypothesis that mapped fault dimensions limit future earthquake size depends on two assumptions: that actual fault surfaces lie within the mapped areas, and that future ruptures will not cross the actual boundaries of the fault. For long shallow faults, the two assumptions imply that rupture will stop at both ends of their mapped traces. We show that earthquake ruptures rarely stop at the boundaries of previously mapped faults. Instead, they frequently extend beyond the mapped fault boundaries, and some extend into previously unmapped faults and/or may rupture un-faulted rock. Another frequent assumption is that presently available information, such as fault maps and 3-D geologic mapping, include the full scope of all important faults capable of producing large-magnitude earthquakes. However, it is also known that we may not be able to accurately locate a blind thrust fault. How do we resolve these two opposing views? Often, it is still assumed that a majority of the major large faults are known, and then we make further assumptions on how faults may or may not rupture together. These assumptions on the fault locations are then used to calculate maximum earthquake magnitudes, using only the known fault dimensions. We demonstrate that these assumptions often lead to an underestimation of the magnitude. We also show that even in the best fault maps, there are still many unknowns regarding the fault location and potential fault systems. If we use fault maps to limit earthquake size, we must be constantly aware of these unknowns, and build them into our estimates of 1 Customer Success Strategist, One Concern, Inc., Palo Alto, CA 94301 2 Lead Modeler – Earthquake Model Development, Risk Management Solutions, Newark, CA 94560 3 Professor Emeritus, Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, Los Angeles, CA 90095 Weiser D, Porto, N, and Jackson D.D. Fault Dimensions and Maximum Magnitude. Proceedings of the 11 National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Los Angeles, CA. 2018. maximum magnitude. Finally, if we do not incorporate scenarios where a fault can interact with neighboring faults, we may severely underestimate the maximum magnitude. Fault Dimensions and Maximum Magnitude D. Weiser, N. Porto, and D.D. Jackson