Stress Pattern within the Pacific Plate around the Focal Area of the 2003 Tokachi-oki Earthquake Revealed by a Stress Tensor Inversion Method

To improve modeling of deep-earth dynamics it is important to understand changes in the arrangements of plate boundaries, especially trenches accommodating subduction, and major changes in tectonic plate motion. Here we focus on the sequence of key surface events in Eocene time that likely coincide with changes in deep-earth dynamics. In particular, we develop methods of analysis of seamount locations and age dates using a small number of adjustable parameters (10 per chain) on the Pacific plate with a focus on the timing of the Hawaiian-Emperor bend relative to the timing of other major Eocene tectonic changes. We find that motion between hotspots differs insignificantly from zero with rates of 2&#177;4 mm/a (&#177;2&#963;) for 0-48 Ma and 26&#177;34 mm/a (&#177;2&#963;) for 48-80 Ma. Relative to a mean Pacific hotspot reference frame, nominal rates of motion of the Hawaii, Louisville, and Rurutu hotspots are ~5 mm/a and differ insignificantly from zero. We conclude that plumes underlying these Pacific hotspots are more stable in a convecting mantle than previously inferred. We estimate the locations and ages (with uncertainties) of bends in Pacific hotspot chains using a novel inversion method. The location of the ~60&#176; change in trend at the Hawaiian-Emperor bend is well constrained within ~50-80 km (=2&#963;), but the location of the bends in the Louisville and Rurutu hotspots are more uncertain. If the uncertainty in the location of the bend in the Louisville chain is included, we find no significant difference in age between the bends of different Pacific hotspot chains. The best-fitting assumed-coeval age for the bends is 47.4&#177;1.0 Ma (&#177;2&#963;), which is indistinguishable from the age of the C21o geomagnetic reversal. The age of the bend is younger than the initiation of subduction in the Western Pacific, but approximately coeval with changes in Pacific and circum-Pacific relative plate motion. Changes to the tectonic system near the age of the bend are not limited to the Pacific basin. The smooth-rough transition flanking the Carlsberg Ridge records a threshold in the decreasing spreading rate between India and Africa, thought to record the onset of the collision of India with Eurasia, and is constrained to be between C21y and C20o (46 Ma and 43 Ma) in age. Nearly simultaneously, South America and Australia began to diverge more rapidly from Antarctica. The Eocene bend in Pacific hotspot chains may be the most evident feature recording a global re-organization of plate motions and mantle circulation possibly caused by the earlier collision of India and Eurasia or initiation of western Pacific subduction.

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Driving stress and seismotectonic implications of the 2013 Mw5.8 Awaji Island earthquake, southwestern Japan, based on earthquake focal mechanisms before and after the mainshock

Earth Planets and Space ◽

10.1186/s40623-020-01292-1 ◽

2020 ◽

Vol 72 (1) ◽

Author(s):

Kazutoshi Imanishi ◽

Makiko Ohtani ◽

Takahiko Uchide

Keyword(s):

Stress Field ◽

Stress Tensor ◽

Nankai Trough ◽

Source Region ◽

Local Scale ◽

Southwest Japan ◽

Stress Tensor Inversion ◽

Earthquake Fault ◽

Reverse Faulting ◽

Before And After

Abstract A driving stress of the Mw5.8 reverse-faulting Awaji Island earthquake (2013), southwest Japan, was investigated using focal mechanism solutions of earthquakes before and after the mainshock. The seismic records from regional high-sensitivity seismic stations were used. Further, the stress tensor inversion method was applied to infer the stress fields in the source region. The results of the stress tensor inversion and the slip tendency analysis revealed that the stress field within the source region deviates from the surrounding area, in which the stress field locally contains a reverse-faulting component with ENE–WSW compression. This local fluctuation in the stress field is key to producing reverse-faulting earthquakes. The existing knowledge on regional-scale stress (tens to hundreds of km) cannot predict the occurrence of the Awaji Island earthquake, emphasizing the importance of estimating local-scale (< tens of km) stress information. It is possible that the local-scale stress heterogeneity has been formed by local tectonic movement, i.e., the formation of flexures in combination with recurring deep aseismic slips. The coseismic Coulomb stress change, induced by the disastrous 1995 Mw6.9 Kobe earthquake, increased along the fault plane of the Awaji Island earthquake; however, the postseismic stress change was negative. We concluded that the gradual stress build-up, due to the interseismic plate locking along the Nankai trough, overcame the postseismic stress reduction in a few years, pushing the Awaji Island earthquake fault over its failure threshold in 2013. The observation that the earthquake occurred in response to the interseismic plate locking has an important implication in terms of seismotectonics in southwest Japan, facilitating further research on the causal relationship between the inland earthquake activity and the Nankai trough earthquake. Furthermore, this study highlighted that the dataset before the mainshock may not have sufficient information to reflect the stress field in the source region due to the lack of earthquakes in that region. This is because the earthquake fault is generally locked prior to the mainshock. Further research is needed for estimating the stress field in the vicinity of an earthquake fault via seismicity before the mainshock alone.

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Small-scale mantle convection system and stress field under the Pacific plate

Physics of The Earth and Planetary Interiors ◽

10.1016/0031-9201(76)90095-9 ◽

1976 ◽

Vol 13 (3) ◽

pp. 212-217 ◽

Cited By ~ 16

Author(s):

Han-Shou Liu ◽

Edward S. Chang ◽

George H. Wyatt

Keyword(s):

Stress Field ◽

Mantle Convection ◽

Pacific Plate ◽

Small Scale ◽

The Pacific

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Unusual earthquakes in the Gulf of Alaska and fragmentation of the Pacific Plate

Geophysical Research Letters ◽

10.1029/gl015i013p01483 ◽

1988 ◽

Vol 15 (13) ◽

pp. 1483-1486 ◽

Cited By ~ 44

Author(s):

J. C. Lahr ◽

R. A. Page ◽

C. D. Stephens ◽

D. H. Christensen

Keyword(s):

Gulf Of Alaska ◽

Pacific Plate ◽

The Pacific

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On the enigmatic birth of the Pacific Plate within the Panthalassa Ocean

Science Advances ◽

10.1126/sciadv.1600022 ◽

2016 ◽

Vol 2 (7) ◽

pp. e1600022 ◽

Cited By ~ 16

Author(s):

Lydian M. Boschman ◽

Douwe J. J. van Hinsbergen

Keyword(s):

Triple Junction ◽

Tectonic Evolution ◽

Plate Boundary ◽

Pacific Plate ◽

Plate Tectonic ◽

Point Location ◽

Marine Magnetic Anomalies ◽

The Pacific ◽

History Of ◽

Ridge System

The oceanic Pacific Plate started forming in Early Jurassic time within the vast Panthalassa Ocean that surrounded the supercontinent Pangea, and contains the oldest lithosphere that can directly constrain the geodynamic history of the circum-Pangean Earth. We show that the geometry of the oldest marine magnetic anomalies of the Pacific Plate attests to a unique plate kinematic event that sparked the plate’s birth at virtually a point location, surrounded by the Izanagi, Farallon, and Phoenix Plates. We reconstruct the unstable triple junction that caused the plate reorganization, which led to the birth of the Pacific Plate, and present a model of the plate tectonic configuration that preconditioned this event. We show that a stable but migrating triple junction involving the gradual cessation of intraoceanic Panthalassa subduction culminated in the formation of an unstable transform-transform-transform triple junction. The consequent plate boundary reorganization resulted in the formation of a stable triangular three-ridge system from which the nascent Pacific Plate expanded. We link the birth of the Pacific Plate to the regional termination of intra-Panthalassa subduction. Remnants thereof have been identified in the deep lower mantle of which the locations may provide paleolongitudinal control on the absolute location of the early Pacific Plate. Our results constitute an essential step in unraveling the plate tectonic evolution of “Thalassa Incognita” that comprises the comprehensive Panthalassa Ocean surrounding Pangea.

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Late Eocene and Maastrichtian paleomagnetic poles for the Pacific plate: implications for the validity of seamount paleomagnetic data

Tectonophysics ◽

10.1016/0040-1951(87)90298-8 ◽

1987 ◽

Vol 144 (4) ◽

pp. 301-314 ◽

Cited By ~ 30

Author(s):

William W. Sager

Keyword(s):

Late Eocene ◽

Pacific Plate ◽

Paleomagnetic Data ◽

The Pacific ◽

Paleomagnetic Poles

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Dynamics of the extension in the Fonualei Rift in the northern Lau Basin at 16 °S

10.5194/egusphere-egu21-7605 ◽

2021 ◽

Author(s):

Anna Jegen ◽

Anke Dannowski ◽

Heidrun Kopp ◽

Udo Barckhausen ◽

Ingo Heyde ◽

...

Keyword(s):

Velocity Gradient ◽

Lower Crust ◽

Pacific Plate ◽

P Wave ◽

Upper Crust ◽

Lau Basin ◽

Australian Plate ◽

The Pacific ◽

Refraction Seismic ◽

Roll Back

The Lau Basin is a young back-arc basin steadily forming at the Indo-Australian-Pacific plate boundary, where the Pacific plate is subducting underneath the Australian plate along the Tonga-Kermadec island arc. Roughly 25 Ma ago, roll-back of the Kermadec-Tonga subduction zone commenced, which lead to break up of the overriding plate and thus the formation of the western Lau Ridge and the eastern Tonga Ridge separated by the emerging Lau Basin.As an analogue to the asymmetric roll back of the Pacific plate, the divergence rates decline southwards hence dictating an asymmetric, V-shaped basin opening. Further, the decentralisation of the extensional motion over 11 distinct spreading centres and zones of active rifting has led to the formation of a composite crust formed of a microplate mosaic. A simplified three plate model of the Lau Basin comprises the Tonga plate, the Australian plate and the Niuafo'ou microplate. The northeastern boundary of the Niuafo'ou microplate is given by two overlapping spreading centres (OLSC), the southern tip of the eastern axis of the Mangatolu Triple Junction (MTJ-S) and the northern tip of the Fonualei Rift spreading centre (FRSC) on the eastern side. Slow to ultraslow divergence rates were identified along the FRSC (8-32 mm/a) and slow divergence at the MTJ (27-32 mm/a), both decreasing southwards. However, the manner of divergence has not yet been identified. Additional regional geophysical data are necessary to overcome this gap of knowledge.Research vessel RV Sonne (cruise SO267) set out to conduct seismic refraction and wide-angle reflection data along a 185 km long transect crossing the Lau Basin at ~16 &#176;S from the Tonga arc in the east, the overlapping spreading centres, FRSC1 and MTJ-S2, and extending as far as a volcanic ridge in the west. The refraction seismic profile consisted of 30 ocean bottom seismometers. Additionally, 2D MCS reflection seismic data as well as magnetic and gravimetric data were acquired.The results of our P-wave traveltime tomography show a crust that varies between 4.5-6 km in thickness. Underneath the OLSC the upper crust is 2-2.5 km thick and the lower crust 2-2.5 km thick. The velocity gradients of the upper and lower crust differ significantly from tomographic models of magmatically dominated oceanic ridges. Compared to such magmatically dominated ridges, our final P-wave velocity model displays a decreased velocity gradient in the upper crust and an increased velocity gradient in the lower crust more comparable to tectonically dominated rifts with a sparse magmatic budget.The dominance of crustal stretching in the regional rifting process leads to a tectonical stretching, thus thinning of the crust under the OLSC and therefore increasing the lower crust&#8217;s velocity gradient. Due to the limited magmatic budget of the area, neither the magnetic anomaly nor the gravity data indicate a magmatically dominated spreading centre. We conclude that extension in the Lau Basin at the OLSC at 16 &#176;S is dominated by extensional processes with little magmatism, which is supported by the distribution of seismic events concentrated at the northern tip of the FRSC.

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