Molly Gallahue ranks as an outstanding young scientist whose graduate research at Northwestern University yielded surprising new results about a fundamental challenge: to better forecast shaking from future earthquakes. This holds major implications for mitigating earthquake hazards to people and property worldwide. She was a key member of CHIMP, the California Historical Intensity Mapping Project, headed by fellow grad student Leah Salditch, a previous recipient of this award. CHIMP compiled shaking observations across California from the largest earthquakes over the past 160 years. The exciting and high-profile result showed that the U.S. National Seismic Hazard Maps predict much higher shaking than occurred. Moreover, the team found similar discrepancies for Italy, Japan, France, and Nepal.

Molly’s thesis investigated how these misfits in predictions and observations arise. She first explored how seismic hazard changes from the predicted effects of near-surface geology. This is important because cities (e.g., Los Angeles) are often in sedimentary valleys or basins. It hadn’t been addressed before because it’s hard to answer. Seismologists have learned over the years that sedimentary valleys both amplify and deamplify shaking, depending on both the severity and the frequency of shaking. Molly tackled this problem with an impressive mastery of concepts, general technical prowess, and a high level of energy and commitment. After showing that this effect on the discrepancy was small, she then looked at other possible causes, applying the same high degree of rigor. She explored whether the discrepancies between predicted and historically observed shaking arise because of incompleteness—missing earthquakes—in historical intensity catalogs, and showed that this effect contributes to, but is not a major cause of, the discrepancies. Finally, she explored the effect of the ground motion to intensity conversion equations (GMICE) used to compare ground motion amplitudes in the hazard maps and historical data and showed that this subtle issue explains much of the discrepancy.

Already a well-regarded member of the earthquake hazards community, Molly’s work is currently in use in hazards mitigation. She received an AGU Outstanding Student Presentation Award, and her work collecting oral histories from California residents who recalled the 1993 Big Pine and 1992 Joshua Tree earthquakes was funded by an AGU Centennial Grant. She brings dedication to using science to benefit society to her new job as senior catastrophe model developer with Gallagher Re, a global reinsurance company whose work includes “quantifying and interpreting the risk posed by natural or man-made hazards.”

—Donna Jurdy, Northwestern University, Evanston, Ill.

Presentation Title: CHIMP - a shaking dataset for historical large earthquakes in California

Event: 2019 Fall Meeting

Awarding Section: Natural Hazards