An official website of the United States government
Because splay faults branch at a steep dip angle from the plate-boundary décollement in an accretionary wedge, their coseismic displacement can potentially result in larger tsunamis with distinct characteristics compared to megathrust-only fault ruptures, posing an enhanced hazard to coastal communities. Elsewhere, there is evidence of coseismic slip on splay faults during many of the largest subduction zone earthquakes, but our understanding of potentially active splay faults and their hazards at the Cascadia subduction zone remains limited. To identify the most recently active splay faults at Cascadia, we conduct stratigraphic and structural interpretations of near-surface deformation in the outer accretionary wedge for the ~400 km along-strike length of the landward vergence zone. We analyze recently acquired high-frequency sparker seismic data and crustal-scale multi-channel seismic data to examine the record of deformation in shallow slope basins and the upper ~1 km of the surrounding accreted sediments and to investigate linkages to deeper décollement structure. We present a new fault map for widest, most completely locked portion of Cascadia from 45 to 48°N latitude, which documents the distribution of faults that show clear evidence of recent late Quaternary activity. We find widespread evidence for active splay faulting up to 30 km landward of the deformation front, in what we define as the active domain, and diminished fault activity landward outside of this zone. The abundance of surface-deforming splay faults in the active outer wedge domain suggests Cascadia megathrust events may commonly host distributed shallow rupture on multiple splay faults located within 30 km of the deformation front.
more »
« less
Kinematics and Evolution of the Southern Eastern California Shear Zone, Based on Analysis of Fault Strike, Distribution, Activity, Roughness, and Secondary Deformation
Abstract The Eastern California shear zone is a complex set of dextral faults that accommodates significant plate motion and has produced large earthquakes. The evolution of this system and why it consists of closely spaced, irregular faults that fail in multi‐fault ruptures are not well understood. Here we analyze the geometry, spatial distribution, and Quaternary slip activity of right‐lateral faults in the southern Mojave block. We find these faults are oriented favorably for accommodating regional dextral plate motion and do not show evidence of replacement following counterclockwise rotation to unfavorable positions, although activity may be migrating westward as previously proposed. We also confirm that the shear zone is transpressive, with widespread restraining bends, distributed convergent deformation, and significant impact on near‐fault topography. Observations also show that faults are geometrically complex, as represented by along‐strike variability in fault strike. We document a correlation between strike variability and fault activity (slip rate or net slip), which is evident within the shear zone as well as for a control group of other faults. We suggest that strike variability represents a form of geometric roughness, which may inhibit fault slip and result in complex ruptures, slip‐strengthening behavior, and a prevalence of off‐fault deformation. Other factors, including preexisting crustal fabric, edge effects, and changes in the stress field, may further complicate kinematics. These results suggest that faults of the shear zone are still juvenile and somewhat unique, yet offer an important window into how broadly distributed shear may evolve into a through‐going continental transform system.
more »
« less
- Award ID(s):
- 1802026
- PAR ID:
- 10366976
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Tectonics
- Volume:
- 40
- Issue:
- 11
- ISSN:
- 0278-7407
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Flow connectivity between master and relay faults in the Ikertôq shear zone demonstrates that multiple ruptures during ancient earthquakes occurred during a single seismic event. The Ikertôq shear zone (ISZ) is part of the Paleoproterozoic Nagssuqtoqidian orogeny continental collision in West Greenland that includes a > 50 km pseudotachylyte system. As part of an NSF REU, this team mapped various faults throughout a 2 km transect on high-resolution UAV images of exhumed pseudotachylyte vein systems on the western end of Sarfannguit island to investigate the kinematics of multi-fault ruptures during individual seismic events. Pseudotachylyte veins exhibit a complex rupture geometry with linked kinematics between oblique reverse master faults striking approximately 240 and steep east-west relay faults dominated by strike-slip movement. Near complete exposure of veins provide a unique opportunity to document fault linkages and the partitioning of slip, including the interconnectivity of flow patterns of melt in pseudotachylyte veins, as well as angular ladders of melt. We measured the thickness of pseudotachylyte fault veins and injection veins along transects to examine slip partitioning between multiple reverse faults and strike-slip relay faults. Melt thickness is used as a proxy for earthquake slip since the pseudotachylyte melt occurred on faults that exhibit preexisting brittle displacement. The results of preliminary calculations from energy balance equations show that typical slip on some oblique reverse master faults was on the order of a meter or less, while typical slip on some east-west relay faults was cm scale. Our data clarify that most slip occurred on oblique reverse master faults with subsidiary slip on east-west relay faults.more » « less
-
Abstract On 4 and 6 July 2019, two large strike‐slip earthquakes withW‐phase moment magnitudesMWW6.5 (foreshock) andMWW7.1 (mainshock) struck the Eastern California Shear Zone, northeast of Ridgecrest. The faulting geometry and kinematic coseismic slip distribution of both events are determined by jointly inverting seismological and geodetic observations guided by aftershock and surface rupture locations. The foreshock ruptured two orthogonal faults with a prominent L‐shaped geometry with maximum slip of ~1.1 m on the NE‐SW segment. The mainshock faulting extended NW‐SE along several primary fault segments that straddle the foreshock slip. The surface rupture and slip model indicate mostly near‐horizontal strike‐slip motion with maximum slip of ~3.7 m, but there is a localized vertical dip‐slip motion. Both the foreshock and mainshock ruptures terminate in regions of complex surface offsets. High aftershock productivity and low rupture velocity may be the result of rupture of a relatively immature fault system.more » « less
-
Previous studies have described the geometry of pseudotachylyte-bearing faults in the Ikertôq shear zone (ISZ) as “paired shears.” This study found these are more complex structures formed in preexisting, wedge-shaped, imbricate damage zones. As part of an NSF REU, this project conducted outcrop-scale mapping in part of the 50-km ISZ pseudotachylyte system on Sarfannguit Island in southwestern Greenland. The pseudotachylyte system is comprised of master oblique-reverse faults concordant to strongly foliated host gneisses linked through discordant strike-slip relay faults. Within the imbricate wedges, pseudotachylytes are complexly distributed around rotated and folded gneissic blocks between stacked systems of master reverse faults. This study mapped five imbricate wedges using high resolution drone imagery in map view and hand photography on vertical outcrops. This resulted in a new geometrical three-dimensional perspective. Wedges form where foliation is platy, typically between more coherent hanging wall and footwall blocks. Preliminary calculations indicate average rotations of eight to eighteen degrees within the damage zones. Field measurements suggest the upper fault in the imbricate wedge is approximately planar, while the lower fault splays off the upper fault at a twenty to thirty degree angle, creating a concave-up cuspate geometry. Both faults have the same shear sense, with fold axes and pseudotachylyte slickenlines indicating reverse oblique offset, usually with a component of dextral shear. Initial deformation and brecciation of the blocks is interpreted as forming prior to the pseudotachylyte-forming event.more » « less
-
Abstract Active traces of the southern Fairweather fault were revealed by light detection and ranging (lidar) and show evidence for transpressional deformation between North America and the Yakutat block in southeast Alaska. We map the Holocene geomorphic expression of tectonic deformation along the southern 30 km of the Fairweather fault, which ruptured in the 1958 moment magnitude 7.8 earthquake. Digital maps of surficial geology, geomorphology, and active faults illustrate both strike-slip and dip-slip deformation styles within a 10°–30° double restraining bend where the southern Fairweather fault steps offshore to the Queen Charlotte fault. We measure offset landforms along the fault and calibrate legacy 14C data to reassess the rate of Holocene strike-slip motion (≥49 mm/yr), which corroborates published estimates that place most of the plate boundary motion on the Fairweather fault. Our slip-rate estimates allow a component of oblique-reverse motion to be accommodated by contractional structures west of the Fairweather fault consistent with geodetic block models. Stratigraphic and structural relations in hand-dug excavations across two active fault strands provide an incomplete paleoseismic record including evidence for up to six surface ruptures in the past 5600 years, and at least two to four events in the past 810 years. The incomplete record suggests an earthquake recurrence interval of ≥270 years—much longer than intervals <100 years implied by published slip rates and expected earthquake displacements. Our paleoseismic observations and map of active traces of the southern Fairweather fault illustrate the complexity of transpressional deformation and seismic potential along one of Earth's fastest strike-slip plate boundaries.more » « less
