scholarly journals Geologic controls on up-dip and along-strike propagation of slip during subduction zone earthquakes from a high-resolution seismic reflection survey across the northern limit of slip during the 2010 Mw 8.8 Maule earthquake, offshore Chile

Geosphere ◽  
2019 ◽  
Vol 15 (6) ◽  
pp. 1751-1773 ◽  
Author(s):  
Anne M. Tréhu ◽  
Bridget Hass ◽  
Alexander de Moor ◽  
Andrei Maksymowicz ◽  
Eduardo Contreras-Reyes ◽  
...  
Keyword(s):  
High Resolution ◽  
Subduction Zone ◽  
Seismic Reflection ◽  
Plate Boundary ◽  
Accretionary Prism ◽  
Structural Heterogeneity ◽  
Deformation Front ◽  
Maule Earthquake ◽  
Subduction Zone Earthquakes ◽  
Trench Sediments

Abstract A grid of closely spaced, high-resolution multichannel seismic (MCS) reflection profiles was acquired in May 2012 over the outer accretionary prism up dip from the patch of greatest slip during the 2010 Mw 8.8 Maule earthquake (offshore Chile) to complement a natural-source seismic experiment designed to monitor the post-earthquake response of the outer accretionary prism. We describe the MCS data and discuss the implications for the response of the accretionary prism during the earthquake and for the long-term evolution of the margin. The most notable observation from the seismic reflection survey is a rapid north-to-south shift over a short distance from nearly total frontal accretion of the trench sediments to nearly total underthrusting of undeformed trench sediments that occurs near the northern edge of slip in the 2010 earthquake. Integrating our structural observations with other geological and geophysical observations, we conclude that sediment subduction beneath a shallow décollement is associated with propagation of slip to the trench during great earthquakes in this region. The lack of resolvable compressive deformation in the trench sediment along this segment of the margin indicates that the plate boundary here is very weak, which allowed the outer prism to shift seaward during the earthquake, driven by large slip down dip. The abrupt shift from sediment subduction to frontal accretion indicates a stepdown in the plate boundary fault, similar to the stepovers that commonly arrest slip propagation in strike-slip faults. We do not detect any variation along strike in the thickness or reflective character of the trench sediments adjacent to the change in deformation front structure. This change, however, is correlated with variations in the morphology and structure of the accretionary prism that extend as far as 40 km landward of the deformation front. We speculate that forearc structural heterogeneity is the result of subduction of an anomalously shallow or rough portion of plate that interacted with and deformed the overlying plate and is now deeply buried. This study highlights need for three-dimensional structural images to understand the interaction between geology and slip during subduction zone earthquakes.

scholarly journals Evidence for a large strike-slip component during the 1960 Chilean earthquake

2019 ◽  
Vol 218 (1) ◽  
pp. 1-32 ◽  
Author(s):  
Hiroo Kanamori ◽  
Luis Rivera ◽  
Sophie Lambotte
Keyword(s):  
Subduction Zone ◽  
Plate Boundary ◽  
Strike Slip ◽  
Lateral Strain ◽  
Long Period ◽  
Great Earthquakes ◽  
Oblique Convergence ◽  
Maule Earthquake ◽  
Subduction Zone Earthquakes ◽  
Chilean Earthquake

SUMMARY The strainmeter record observed at Isabella (ISA), California, for the 1960 Chilean earthquake (Mw = 9.5) is one of the most important historical records in seismology because it was one of the three records that provided the opportunity for the first definitive observations of free oscillations of the Earth. Because of the orientation of the strainmeter rod with respect to the back azimuth to Chile, the ISA strainmeter is relatively insensitive to G (Love) waves and higher order (order ≥ 6) toroidal modes, yet long-period G waves and toroidal modes were recorded with large amplitude on this record. This observation cannot be explained with the conventional low-angle thrust mechanism typical of great subduction-zone earthquakes and requires an oblique mechanism with half strike-slip and half thrust. The strain record at Ogdenburg, New Jersey, the Press–Ewing seismograms at Berkeley, California, and the ultra-long period displacement record at Pasadena, California, also support the oblique mechanism. We tested the performance of the ISA strainmeter using other events including the 1964 Alaskan earthquake and found no instrumental problems. Thus, the ISA observation of large G/R and toroidal/spheroidal ratios most likely reflects the real characteristics of the 1960 Chilean earthquake, rather than an observational artefact. The interpretation of the large strike-slip component is not unique, but it may represent release of the strike-slip strain that has accumulated along the plate boundary as a result of oblique convergence at the Nazca–South American plate boundary. The slip direction of the 2010 Chilean (Maule) earthquake ( Mw = 8.8) is rotated by about 10° clockwise from the plate convergence direction suggesting that right-lateral strain comparable to that of an Mw = 8.3 earthquake remained unreleased and accumulates near the plate boundary. One possible scenario is that the strike-slip strain accumulated over several great earthquakes like the 2010 Maule earthquake was released during the 1960 Chilean earthquake. If this is the case, we cannot always expect a similar behaviour for all the great earthquakes occurring in the same subduction zone and such variability needs to be considered in long-term hazard assessment of subduction-zone earthquakes.

scholarly journals Along-strike variations in protothrust zone characteristics at the Nankai Trough subduction margin

Geosphere ◽  
2021 ◽  
Author(s):  
Hannah L. Tilley ◽  
Gregory F. Moore ◽  
Mikiya Yamashita ◽  
Shuichi Kodaira
Keyword(s):  
High Resolution ◽  
Strain Localization ◽  
Seismic Reflection ◽  
Deformation Band ◽  
Nankai Trough ◽  
Accretionary Prism ◽  
Fault Propagation ◽  
Seismic Reflection Data ◽  
Reflection Data ◽  
Surface Slopes

Significant along-strike changes in the protothrust zone at the toe of the Nankai Trough accretionary prism were imaged in new high-resolution seismic reflection data. The width of the protothrust zone varies greatly along strike; two spatially discrete segments have a wide protothrust zone (∼3.3–7.8 km, ∼50–110 protothrusts), and two segments have almost no protothrust zone (∼0.5–2.8 km, <20 protothrusts). The widest protothrust zone occurs in the region with the widest and thickest sediment wedge and subducting turbidite package, both of which are influenced by basement topography. The trench wedge size and lithology, the lithology of the subducting section, and the basement topography all influence the rate of consolidation in the trench wedge, which we hypothesize is an important control over the presence and width of the protothrust zone. We conclude that protothrusts are fractures that form from shear surfaces in deformation band clusters as the trench fill sediment is consolidated. Strain localization occurs at sites with a high density of protothrusts, which become the probable locations of future frontal thrust propagation. The frontal thrust may propagate forward with a lower buildup of strain where it is adjacent to a wide protothrust zone than at areas with a narrow or no protothrust zone. This is reflected in the accretionary prism geometry, where wide protothrust zones occur adjacent to fault-propagation folds with shallow prism toe surface slopes.

scholarly journals Post-seismic response of the outer accretionary prism after the 2010 Maule earthquake, Chile

Geosphere ◽  
2019 ◽  
Vol 16 (1) ◽  
pp. 13-32 ◽  
Author(s):  
Anne M. Tréhu ◽  
Alexander de Moor ◽  
José Mieres Madrid ◽  
Miguel Sáez ◽  
C. David Chadwell ◽  
...  
Keyword(s):  
Fluid Flow ◽  
Plate Boundary ◽  
Low Frequency ◽  
Accretionary Prism ◽  
Absolute Pressure ◽  
Ocean Bottom ◽  
Intermediate Band ◽  
Outer Rise ◽  
Maule Earthquake ◽  
Seafloor Pressure

Abstract To investigate the dynamic response of the outer accretionary prism updip from the patch of greatest slip during the 2010 Mw 8.8 Maule earthquake (Chile), 10 ocean-bottom seismometers (OBSs) were deployed from May 2012 to March 2013 in a small network with an inter-instrument spacing of 7–10 km. Nine were recovered, with four recording data from intermediate-band three-component seismometers and differential pressure gauges, and five recording data from absolute pressure gauges (APGs). All instruments were also equipped with fluid flow meters designed to detect very low rates of flow into or out of the seafloor. We present hypocenters for local earthquakes that have S-P times <17 s (i.e., within ∼125 km of the network), with a focus on events located beneath or near the network. Most of the seismicity occurred either near the boundary between the active accretionary prism and continental basement or in the outer rise seaward of the trench. For many outer-rise earthquakes, the P and S arrivals are followed by a distinctive T-phase arrival. Very few earthquakes, and none located with hypocenters deemed “reliable,” were located within the active accretionary prism or on the underlying plate boundary. Nonvolcanic tremor-like pulses and seafloor pressure transients (but no very-low-frequency earthquakes or fluid flow) were also detected. Many of the tremor observations are likely T-phases or reverberations due to soft seafloor sediments, although at least one episode may have originated within the accretionary prism south of the network. The transient seafloor pressure changes were observed simultaneously on three APGs located over the transition from the active prism to the continental basement and show polarity changes over short distances, suggesting a shallow source. Their duration of several hours to days is shorter than most geodetic transients observed using onshore GPS networks. The results demonstrate the need for densely spaced and large-aperture OBS networks equipped with APGs for understanding subduction zone behavior.

High-Resolution Seismic-Reflection and Marine Magnetic Data Along the Hosgri Fault Zone, Central California

2009 ◽  
Author(s):  
Ray W. Sliter ◽  
Peter J. Triezenberg ◽  
Patrick E. Hart ◽  
Janet T. Watt ◽  
Samuel Y. Johnson ◽  
...  
Keyword(s):  
High Resolution ◽  
Fault Zone ◽  
Seismic Reflection ◽  
Magnetic Data ◽  
Central California
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