Researchers: Shannon E. Graham* and Brendan J. Meade, Harvard University; and John P. Loveless, Smith College. *now at The College of New Jersey
Written by Linda Rowan
24 February 2020
A global model of observed plate motions on Earth’s surface is developed using 19,664 interseismic GPS velocities for 307 distinct plates. The block model, which decomposes GPS velocities as a combination of earthquake cycle deformation across locked faults and plate rotations, allowed the authors to estimate the slip deficit rates over the planet’s most active faults. The result shows that 50% of the world’s population lives within 200 kilometers of a fault with a slip rate of greater than 2 millimeters per year.
Earth’s crust can be divided into many plates that move over time and sometimes suddenly, catastrophically and perceptibly due to earthquakes that occur primarily on major faults at plate boundaries. As observations have advanced from magnetic anomalies, geologic and seismic data to geodetic data, the number of plates and complexity of global plate motion models has increased. Early models used 5 to 12 plates and in the 21st century, global models have included 25 to 56 plates. These models provide information about the movement and interaction of crustal plates, the state of stress and strain of the planet, the structure and hazards of plate boundaries, fault structure and dynamics, earthquake processes, volcanic processes and other Earth processes that are affected by the size, shape and motion of the plates. These models provide information about the structure and coupling of the crustal plates to the underlying crust and mantle, enhancing our understanding of the structure and dynamics of the rest of the crust and the mantle.
In this study, the authors use 19,664 interseismic GPS velocities measured between 1996 and 2013 to create a global plate motion model that addresses the question of how many blocks and faults are necessary to explain the GPS velocities with a resolution of about 2 millimeters per year. Some of the GPS sites and data are from UNAVCO services with the support of the National Science Foundation. The authors determine that 307 plates with 446,870 kilometers of fault traces can explain the geodetic observations with 2.2 millimeters per year of resolution.
The geodetically constrained slip deficit rates calculated by the Global Block Model were used to estimate the global interseismic moment accumulation rate. Results from this calculation indicate the earth stores seismic energy at a rate equivalent to a moment magnitude 8.66 earthquake per year, which is consistent with the average yearly earthquake magnitude based on historic seismicity from 1900 to 2013. While the model is globally consistent, there are regional differences of note. For example, some parts of the Sumatran and Chilean subduction zones have historically released more strain than the present rate of accumulation and part of the Cascadia subduction zone has stored more energy than has been released. Integrating the frequency distribution of the modeled fault slip-deficit rates over all possible slip deficit rates indicates that the geologic structures included in the global block model can account for 98 percent of Earth’s interseismic moment accumulation. This suggests that only minor faults (slip deficit rates < 1 mm/yr) are underrepresented. The model will enhance our understanding of plate tectonics and the earthquake cycle on major plate boundaries.
Graham, S. E., Loveless, J. P., & Meade, B. J. (2018). Global plate motions and earthquake cycle effects. Geochemistry, Geophysics, Geosystems, 19, 2032–2048, doi: 10.1029/2017GC007391 .
plate tectonics, earthquake cycle, moment of inertia, subduction zone
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Last modified: 2020-02-24 10:58:25 America/Denver