Seismology, GPS, and Neotectonics: The Present-Day Plate Boundaries in Northeast Asia

1. Seismology, GPS, and Neotectonics: The Present-Day Plate Boundaries in Northeast Asia Kazuya Fujita Department of Geological Sciences Michigan State University Mikhail Kogan Lamont-Doherty Earth Observatory Colombia University Jeffrey Park Department of Geology and Geophysics Yale University

2. North America – EurasiaBoundary in Asia Poorly Known NORTH AMERICA EURASIA PACIFIC Kevin Mackey Traditional interpretation

3. Seismicity of Eastern Russia Improved Seismicity Database (But beware of explosion sources) MAJOR PLATES NA = North America EU = Eurasia PA = Pacific NA BE EU MICROPLATES OR BLOCKS OK PA OK = Okhotsk AM = Amur BE = Bering AM Mackey et al., 2004

4. Focal Mechanism Studies • Representative mechanisms determined • CMTs, surface wave, and synthetic seismogram studies are “best” • P-wave mechanisms for events M < 5.2 are unreliable unless there is well-distributed regional data • Small number of reliable mechanisms in Chersky Range Mb = 4.8 Riegel, 1994

5. Reliable Focal Mechanisms Moma rift Laptev Sea Region Chersky Range Region Compression in Moma

6. Focal Mechanism Summary

7. Okhotsk Plate • How is extension in the Arctic being accomodated in Northeast Russia? • Okhotsk plate is being extruded to southeast as it is compressed between North America and Eurasia • Northwest corner of plate is being deformed – numerous faults

8. Chersky Range Active Faults “Meteor” Image courtesy Yakutian Institute of Geological Sciences • Large faults visible in satellite imagery • Area has diffuse seismicity • But, largest events fall on Ulakhan and Chai-Yureya Faults

9. River Offsets: 4 – 28 km Mackey et al., in prep. Mackey, 1999 Reconstructed McLean et al., 2001; Fujita et al., 2004

10. How Does Okhotsk-North America Boundary Cross Northern Sea of Okhotsk? • Diffuse seismicity, but two possible trends across northern Sea of Okhotsk • Both link at Kamchatka-Aleutian corner

11. North America – Eurasia Pole(from seismology and GPS) Extension NA-EUPole Compression Pole has migrated with time Riegel, 1994 Transition from extension to compression near the southern end of the Laptev Sea More on GPS studies from M. Kogan to follow

12. Chukotka Seismicity • Strike-slip and transpression in Koryak Highlands • Rifting in eastern Chukchi and Seward Peninsulas Swarm near Neshkan 2002 - 2004

13. Bering Block • Neshkan and Chukotka are at the rifting edge of the Bering block

14. Crustal Structure:Travel-Time Data TBK Suvorov and Kornilova, 1986 Mackey et al., 1998 • Thin crust southwest of Moma “rift” • Surface waves affected in this area (Lander, 1984)

15. Crustal Structure:Travel-Time Studies Mackey et al., 2003 High velocity in cratons, average velocity in fold belts, low velocity in Sakhalin

16. Seismic Reflection:Laptev Sea Rifts Grabens mapped by seismic reflection profiling Bel’kov Rift Franke et al., 1998 Drachev, 1998

17. Crustal Structure: Magadan DistrictSeismic Refraction and Reflection Magadan-Kolyma Refraction Line Magadan-Wrangel reflection line 1959 Davydova et al., 1968 2001 Surkov et al., 2003

18. Major Questions – Seismology, Neotectonics, and Geodesy • How does plate boundary step around pole of rotation? • How is crustal shortening accommodated in northeast Russia as Arctic Ocean opens? • Does an Okhotsk plate exist? • If so, where are the northwestern and northeastern boundaries of the plate • Is it extruding?

19. Questions (continued) • How is strain partitioned in the Magadan district • What is the nature of the seismicity in eastern Chukotka • Does a Bering block exist? • What is the crustal structure in northeast Russia – how does this reflect its history? • What is/was the nature of the Moma rift?