INTRAPLATE SEISMICITY IN NORTH AMERICA: A NEW VIEW

Our hypothesis is that higher integrative lithospheric strength correlates with lower rates of continental crustal seismicity and with lower earthquake magnitudes, also known as Mmax. Integrative lithospheric strength is controlled by lithospheric composition and the geotherm. Mantle S-wave velocity anomalies are indicative of lithospheric composition and temperature, and are available on a global scale from seismic surface-wave tomography. In contrast, heat flow measurements, which also reflect the mantle geotherm, are lacking in many continental intraplate regions. We have created new global maps of lithospheric S-wave velocity anomalies (delta Vs) at a depth of 175 km and quantitative compared the values of these mantle anomalies with the locations and moment magnitudes of earthquakes in the overlying crust. These S-wave anomalies are with respect to a global average velocity model. As is well known, positive delta Vs anomalies correlate with Precambrian cratonic lithosphere, and we have selected a depth of 175 km to ensure than we are within the deep cratonic lithospheric root. We seek to demonstrate a correlation of cold, strong mantle lithosphere with lower seismicity rates and lower moment magnitudes. Our earthquake catalog is described by Schulte and Mooney (Geophy. J. Int., 2005) and contains about 1,300 crustal events. This catalog contains 740 continental intraplate earthquakes with moment magnitude in the range 4 to 5. We find only 7% (50) crustal earthquakes occur above mantle lithosphere with delta Vs greater than 4.5. The number of earthquakes is nearly constsant for values of delta Vs between 0.5 to 4.5. There are 460 continental intraplate earthquakes with moment magnitude in the range 5 to 6. We find that only 10% of all earthquakes occur above mantle lithosphere with delta Vs greater than 3.5. Most earthquakes occur above mantle lithosphere with delta Vs values between 0.5 and 3.5. There are 110 earthquakes with moment magnitude in the range 6 to 7. We find that 14% (16) earthquakes occur above mantle lithosphere with values of delta Vs greater than 3.5%. There are no earthquakes above mantle lithosphere with delta Vs greater than 4.5%. In summary no earthquakes with moment magnitude greater than 7 occur above mantle lithosphere with delta Vs values greater than 3.5%.