Seismology, GPS, and Seismology, GPS, and Neotectonics: Neotectonics:

The Present-Day Plate Boundaries in The Present-Day Plate Boundaries in Northeast AsiaNortheast Asia

Kazuya FujitaKazuya FujitaDepartment of Geological SciencesDepartment of Geological Sciences

Michigan State UniversityMichigan State University

Mikhail KoganMikhail KoganLamont-Doherty Earth ObservatoryLamont-Doherty Earth Observatory

Colombia UniversityColombia University

Jeffrey ParkJeffrey ParkDepartment of Geology and GeophysicsDepartment of Geology and Geophysics

Yale UniversityYale University

North America – EurasiaNorth America – EurasiaBoundary in Asia Poorly KnownBoundary in Asia Poorly Known

Kevin Mackey

NORTH

AMERICA

PACIFIC

EURASIA

Traditional interpretation

Seismicity of Eastern RussiaSeismicity of Eastern RussiaImproved Seismicity Database

(But beware of explosion sources)

Mackey et al., 2004

NA

BE

OK

EU

AM

PA

MAJOR PLATESNA = North America

EU = Eurasia

PA = Pacific

MICROPLATES OR BLOCKS

OK = Okhotsk

AM = Amur

BE = Bering

Focal Mechanism StudiesFocal Mechanism Studies

Representative mechanisms determinedRepresentative mechanisms determined CMTs, surface wave, and synthetic CMTs, surface wave, and synthetic

seismogram studies are “best”seismogram studies are “best” P-wave mechanisms for eventsP-wave mechanisms for events

M < 5.2 are unreliable unlessM < 5.2 are unreliable unless

there is well-distributed there is well-distributed regionalregional

datadata Small number of reliable mechanisms in Small number of reliable mechanisms in

Chersky RangeChersky Range

Mb = 4.8

Riegel, 1994

Reliable Focal MechanismsReliable Focal Mechanisms

Laptev Sea Region Chersky Range RegionCompression in Moma

Moma rift

Focal Mechanism SummaryFocal Mechanism Summary

Okhotsk PlateOkhotsk Plate

• Northwest corner of plate is being deformed – numerous faults

• Okhotsk plate is being extruded to southeast as it is compressed between North America and Eurasia

• How is extension in the Arctic being accomodated in Northeast Russia?

Chersky Range Active FaultsChersky Range Active Faults

•Large faults visible in satellite imagery

•Area has diffuse seismicity

•But, largest events fall on Ulakhan and Chai-Yureya Faults

“Meteor” Image courtesy Yakutian Institute of Geological Sciences

River Offsets: 4 – 28 kmRiver Offsets: 4 – 28 km

McLean et al., 2001; Fujita et al., 2004

Mackey, 1999Mackey et al., in prep.

Reconstructed

How Does Okhotsk-North America Boundary How Does Okhotsk-North America Boundary Cross Northern Sea of Okhotsk?Cross Northern Sea of Okhotsk?

•Diffuse seismicity, but two possible trends across northern Sea of Okhotsk

•Both link at Kamchatka-Aleutian corner

North America – Eurasia PoleNorth America – Eurasia Pole(from seismology and GPS)(from seismology and GPS)

Riegel, 1994

Transition from extension to compression near the southern end of the Laptev Sea

NA-EU PoleExtension

Compression

More on GPS studies from M. Kogan to follow

Pole has migrated with time

Chukotka SeismicityChukotka Seismicity

• Rifting in eastern Chukchi and Seward Peninsulas Swarm near Neshkan 2002 - 2004

• Strike-slip and transpression in Koryak Highlands

Bering BlockBering Block

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

Crustal Structure:Crustal Structure:Travel-Time DataTravel-Time Data

Suvorov and Kornilova, 1986

Mackey et al., 1998

• Thin crust southwest of Moma “rift”

• Surface waves affected in this area (Lander, 1984)

TBK

Crustal Structure:Crustal Structure:Travel-Time StudiesTravel-Time Studies

Mackey et al., 2003

High velocity in cratons, average velocity in fold belts, low velocity in Sakhalin

Seismic Reflection:Seismic Reflection:Laptev Sea RiftsLaptev Sea Rifts

Grabens mapped by seismic reflection profiling

Drachev, 1998Franke et al., 1998

Bel’kov Rift

Crustal Structure: Magadan DistrictCrustal Structure: Magadan DistrictSeismic Refraction and ReflectionSeismic Refraction and Reflection

Magadan-Wrangel reflection lineMagadan-Wrangel reflection line

Surkov et al., 2003

Magadan-Kolyma Refraction Line

Davydova et al., 1968

1959

2001

Major Questions – Seismology, Major Questions – Seismology, Neotectonics, and GeodesyNeotectonics, and Geodesy

How does plate boundary step around How does plate boundary step around pole of rotation?pole of rotation?

How is crustal shortening accommodated How is crustal shortening accommodated in northeast Russia as Arctic Ocean in northeast Russia as Arctic Ocean opens?opens?

Does an Okhotsk plate exist?Does an Okhotsk plate exist? If so, where are the northwestern and If so, where are the northwestern and

northeastern boundaries of the platenortheastern boundaries of the plate Is it extruding?Is it extruding?

Questions (continued)Questions (continued)

How is strain partitioned in the Magadan How is strain partitioned in the Magadan districtdistrict

What is the nature of the seismicity in What is the nature of the seismicity in eastern Chukotka eastern Chukotka Does a Bering block exist?Does a Bering block exist?

What is the crustal structure in northeast What is the crustal structure in northeast Russia – how does this reflect its history?Russia – how does this reflect its history?

What is/was the nature of the Moma rift?What is/was the nature of the Moma rift?