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 – Eurasia Boundary in Asia Poorly Known
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-EU Pole
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

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 District Seismic Refraction and Reflection
Magadan-Kolyma Refraction Line
1959
Davydova et al., 1968
Magadan-Wrangel reflection line
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?