Regional Stresses Along the Eurasia-Africa Plate Boundary Derived from Focal Mechanisms of Large Earthquakes

1991 ◽  
pp. 433-448 ◽  
Author(s):  
Agustín Udías ◽  
Elisa Buforn
Keyword(s):  
Plate Boundary ◽  
Focal Mechanisms ◽  
Regional Stresses ◽  
Large Earthquakes

scholarly journals A Microearthquake Study of the Plate Boundary, North Island, New Zealand

2021 ◽  
Author(s):  
◽  
Martin Everardus Reyners
Keyword(s):  
Plate Boundary ◽  
Pacific Plate ◽  
Focal Mechanisms ◽  
Phase Changes ◽  
Indian Plate ◽  
Depth Range ◽  
The North ◽  
The Pacific ◽  
Deep Activity ◽  
Set Up

<p>The seismicity, structure and tectonics of the North Island plate boundary have been studied by means of a microearthquake traverse oriented in the direction of dip of the subducted Pacific plate and stretching from southern Hawke's Bay to northern Taranaki. The geometry of the top of the Pacific plate is inferred from a band of concentrated microearthquake activity which can be identified with the crust of the plate. The Pacific plate appears to have two knee-like bends, one between the east coast and the Ruahine Range, where the top of the plate is about 25 km deep, the other below the volcanic front, where it is about 70 km deep. The shallower bend and subsequent restraightening of the plate can be related to phase changes in the plate, while the deeper bend can be related to volcanism. Composite focal mechanisms indicate that seaward of its shallower bend the Pacific plate is being loaded by the Indian plate, whereas landward of this bend the Pacific plate is sinking under its own weight. Both composite focal mechanisms and the distribution of microseismicity in the Pacific plate suggest the existence of a major discontinuity striking down the dip of the plate and passing beneath the Tongariro volcanic centre. A conspicuous lack of microseismicity in the Indian plate in the eastern North Island revealed in this study can be related to the plates being unlocked in this region. A feature of the seismicity of the Indian plate in the region of the Wanganui Basin is the concentration of activity in the 25-42 km depth range, shallower activity being largely confined to the northeast edge of the basin, near Mt Ruapehu and Waiouru. Composite focal mechanisms suggest the 25-42 km deep activity reflects stresses set up by locking and unlocking of the plates, while the shallower activity reflects local stresses related to volcanic phenomena.</p>

scholarly journals Seismic Vulnerability in the Himalayan Region

2011 ◽  
Vol 1 ◽  
pp. 14-17 ◽  
Author(s):  
Harihar Paudyal ◽  
Ananta Panthi
Keyword(s):  
Seismic Hazard ◽  
Seismic Vulnerability ◽  
Plate Boundary ◽  
High Potential ◽  
Himalayan Region ◽  
Indian Plate ◽  
Potential Zone ◽  
Central Himalaya ◽  
Strong Earthquakes ◽  
Large Earthquakes

The frequently occurring strong earthquakes in the Himalayan region signify the seismic vulnerability in the region. The continued northward movement of Indian plate is generating large amount of stress at the plate boundary which is being released in form of large and great earthquakes (M≥7). Absence of such great events in the Himalayan front for last six decades and in some segments for last two centuries envisages the region as a high potential zone for future seismic hazard. In this paper we studied the larger events in the central Himalayan region.Key words: Central Himalaya; Large earthquakes; Seismic hazardsThe Himalayan Physics Vol.1, No.1, May, 2010Page: 14-17Uploaded Date: 28 July, 2011

scholarly journals A Microearthquake Study of the Plate Boundary, North Island, New Zealand

2021 ◽  
Author(s):  
◽  
Martin Everardus Reyners
Keyword(s):  
Plate Boundary ◽  
Pacific Plate ◽  
Focal Mechanisms ◽  
Phase Changes ◽  
Indian Plate ◽  
Depth Range ◽  
The North ◽  
The Pacific ◽  
Deep Activity ◽  
Set Up

<p>The seismicity, structure and tectonics of the North Island plate boundary have been studied by means of a microearthquake traverse oriented in the direction of dip of the subducted Pacific plate and stretching from southern Hawke's Bay to northern Taranaki. The geometry of the top of the Pacific plate is inferred from a band of concentrated microearthquake activity which can be identified with the crust of the plate. The Pacific plate appears to have two knee-like bends, one between the east coast and the Ruahine Range, where the top of the plate is about 25 km deep, the other below the volcanic front, where it is about 70 km deep. The shallower bend and subsequent restraightening of the plate can be related to phase changes in the plate, while the deeper bend can be related to volcanism. Composite focal mechanisms indicate that seaward of its shallower bend the Pacific plate is being loaded by the Indian plate, whereas landward of this bend the Pacific plate is sinking under its own weight. Both composite focal mechanisms and the distribution of microseismicity in the Pacific plate suggest the existence of a major discontinuity striking down the dip of the plate and passing beneath the Tongariro volcanic centre. A conspicuous lack of microseismicity in the Indian plate in the eastern North Island revealed in this study can be related to the plates being unlocked in this region. A feature of the seismicity of the Indian plate in the region of the Wanganui Basin is the concentration of activity in the 25-42 km depth range, shallower activity being largely confined to the northeast edge of the basin, near Mt Ruapehu and Waiouru. Composite focal mechanisms suggest the 25-42 km deep activity reflects stresses set up by locking and unlocking of the plates, while the shallower activity reflects local stresses related to volcanic phenomena.</p>

scholarly journals Microseismic Evidence for Bookshelf Faulting in Western Montana

2021 ◽  
Author(s):  
Ellen M. Smith ◽  
Hilary R. Martens ◽  
Michael C. Stickney
Keyword(s):  
Plate Boundary ◽  
The United States ◽  
Aftershock Sequence ◽  
Strike Slip ◽  
Focal Mechanisms ◽  
Seismic Network ◽  
Double Difference ◽  
S Wave ◽  
Lateral Strike Slip ◽  
Bookshelf Faulting

Abstract One of the most seismically active regions in the United States, located hundreds of kilometers inland from the nearest plate boundary, is the Intermountain Seismic Belt (ISB). The 6 July 2017 M 5.8 earthquake occurred 11 km southeast of Lincoln, Montana, within the ISB. This was the largest earthquake to rupture in the state of Montana since the 1959 M 7.3 Hebgen Lake earthquake. We use continuous seismic data from the University of Montana Seismic Network, the Montana Regional Seismic Network, and the U.S. Geological Survey to investigate the Lincoln aftershock sequence and to evaluate crustal stress conditions. We manually picked P- and S-wave arrival times, computed 4110 hypocenter locations and 2336 double-difference relocations, and generated focal mechanisms for 414 aftershocks (12+ polarities) in the 2 yr following the mainshock. Based on the alignment of aftershocks, we infer that the mainshock occurred on a north–northeast-trending left-lateral strike-slip fault. The orientation of the fault is unexpected, given that it strikes nearly perpendicular to the prominent Lewis and Clark line (LCL) faults in the area. Although most aftershocks concentrate near the mainshock, several distinct clusters of microseismic activity emerge along subparallel faults located primarily to the west of the mainshock. The subparallel faults also exhibit left-lateral strike-slip motion oblique to the LCL. We postulate that the aftershocks reveal the clockwise rotation of local-scale crustal blocks about vertical axes within a larger, right-lateral shear zone. The inferred block rotations are consistent with a bookshelf-faulting mechanism, which likely accommodates differential crustal motion to the north and south of the LCL region. The tension axes of well-constrained focal mechanisms indicate local northeast–southwest extension with a mean direction of N60°E.

Seismicity and focal mechanisms of the Arctic region and the North American plate boundary in Asia

2015 ◽  
pp. 79-118 ◽  
Author(s):  
Kazuya Fujita ◽  
David B. Cook ◽  
Henry Hasegawa ◽  
David Forsyth ◽  
Robert Wetmiller
Keyword(s):  
North American ◽  
Plate Boundary ◽  
Arctic Region ◽  
Focal Mechanisms ◽  
North American Plate ◽  
The Arctic ◽  
The North ◽  
The Arctic Region
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