279 research outputs found
Recommended from our members
Source Parameter Scaling and the Cascadia Paleoseismic Record
Several approaches to interpreting the Cascadia paleoseismic record are used to derive relationships between fault area, slip, and moment and to compare the results with the scaling relationships determined by Somerville et al. (2015) for recent subduction-zone events. In two models (CA12a and CA12b), taken from Goldfinger et al. (2012), paleoevents are classified into five characteristic areas (CA), with the slip during each event estimated based on the time between the event and either the following or the previous event. In model CA14, taken from Scholz (2014), slip on four characteristic segments is determined from the plate tectonic convergence rate, assuming a constant stress drop. In model CL, introduced in this article, the fault length for paleoevents is defined by the along-strike length over which the observations have been correlated; width and slip are interpolated from model CA14. CA12a and CA12b show large scatter compared with the global compilation because of large variations in slip for a given area. Models CA14 and CL reproduce the relationship derived for asperities (defined as patches in finite-fault models with slip >1:5 times the average slip). These models can be reconciled with the total area and average slip from Somerville et al. (2015) by increasing the fault area and decreasing the slip using scaling factors derived from the analysis of recent earthquakes (CLmod1) or by reducing the slip by a factor of âŒ8 (CLmod2). CLmod1 implies that the paleoearthquake observations are controlled by high-slip patches, whereas CLmod2 implies that much of the plate tectonic convergence is accommodated aseismically. A scenario intermediate between CLmod1 and CLmod2 is considered most likely. This study demonstrates the value of using scaling relationships based on modern earthquakes as a tool for evaluating earthquake histories derived from paleoseismic data
Recommended from our members
A seasonally modulated earthquake swarm near Maupin, Oregon
From December 2006 to November 2011, the Pacific Northwest Seismic Network (PNSN)
reported 467 earthquakes in a swarm 60 km east of Mt Hood near the town of Maupin, Oregon.
The swarm included 20 M[subscript D] â„ 3.0 events, which account for over 80 per cent of the cumulative
seismic moment release of the sequence. Relocation of 45 M[subscript D] â„ 2.5 earthquakes and moment
tensor analysis of nine 3.3 †M[subscript w] †3.9 earthquakes reveals right-lateral strike-slip motion
on a north-northwest trending, 70° west dipping, 1 kmÂČ active fault patch at about 17 km
depth. The swarm started at the southern end of the patch and migrated to the northwest at an
average rate of 1â2 m dâ»Âč during the first 18 months. Event migration was interrupted briefly
in late 2007 when the swarm encountered a 10° fault bend acting as geometrical barrier. The
slow migration rate suggests a pore pressure diffusion process. We speculate that the swarm
was triggered by flow into the fault zone from upwards-migrating, subduction-derived fluids.
Superimposed on the swarm is seasonal modulation of seismicity, with the highest rates in
spring, which coincides with the maximum snow load in the nearby Cascade Mountains. The
resulting surface load variation of about 4 Ă 10ÂčÂč N kmâ»Âč arc length causes 1 cm annual vertical
displacements at GPS sites in the Cascades and appears sufficient to modulate seismicity by
varying normal stresses at the fault and fluid flow rates into the fault zone.This is the publisherâs final pdf. The published article is copyrighted by the author(s) and published by Oxford University Press on behalf of The Royal Astronomical Society. The published article can be found at: http://gji.oxfordjournals.org/.Keywords: Earthquake dynamics, Fractures and faults, Earthquake source observations, Continental tectonics: strike-slip and transform, Dynamics and mechanics of faultingKeywords: Earthquake dynamics, Fractures and faults, Earthquake source observations, Continental tectonics: strike-slip and transform, Dynamics and mechanics of faultin
Recommended from our members
Transpression between two warm mafic plates: The Queen Charlotte Fault revisited
The Queen Charlotte Fault is a transpressive transform plate boundary between the Pacific and North American plates offshore western Canada. Previous models for the accommodation of transpression include internal deformation of both plates adjacent to the plate boundary or oblique subduction of the oceanic plate; the latter has been the preferred model. Both plates are warm and mafic and have similar mechanical structures. New multichannel seismic reflection data show a near-vertical Queen Charlotte Fault down to the first water bottom multiple, significant subsidence east of the Queen Charlotte Fault, a large melange where the fault is in a compressive left step, and faulting which involves oceanic basement. Gravity modeling of profiles indicates that the Pacific plate is flexed downward adjacent to the Queen Charlotte Fault. Upward flexure of North America along with crust thickened relative to crust in the adjacent basin creates topography known as the Queen Charlotte Islands. Combined with other regional studies, these observations suggest that the plate boundary is a vertical strike-slip fault and that transpression is taken up within each plate
Recommended from our members
Subducted seamounts and recent earthquakes beneath the central Cascadia forearc
Bathymetry and magnetic anomalies indicate that a seamount on the Juan de Fuca plate has been subducted beneath the central Cascadia accretionary complex and is now located similar to 45 km landward of the deformation front. Passage of this seamount through the accretionary complex has resulted in a pattern of uplift followed by subsidence that has had a profound influence on slope morphology, gas hydrate stability, and sedimentation. Based on potential-field data and a new three-dimensional seismic velocity model, we infer that this is the most recent of several seamounts subducted over the past several million years beneath this segment of Cascadia. More deeply subducted seamounts may be responsible for recent earthquake activity on the plate boundary in this region and for along-strike variations in the thickness of the subduction channel, which may affect coupling across the plate boundary.Keywords: Central Oregon,
Tectonics,
Nicoya,
Costa Rica,
Segmentation,
Plate,
Crustal structure,
Zone,
Convergent margin,
Ruptur
Relationship between subduction erosion and the upâdip limit of the 2014 Mw 8.1 Iquique earthquake
The aftershock distribution of the 2014 Mw 8.1 Iquique earthquake offshore northern Chile, identified from a longâterm deployment of ocean bottom seismometers installed eight months after the mainshock, in conjunction with seismic reflection imaging, provides insights into the processes regulating the upâdip limit of coseismic rupture propagation. Aftershocks upâdip of the mainshock hypocenter frequently occur in the upper plate and are associated with normal faults identified from seismic reflection data. We propose that aftershock seismicity near the plate boundary documents subduction erosion that removes mass from the base of the wedge and results in normal faulting in the upper plate. The combination of very little or no sediment accretion and subduction erosion over millions of years has resulted in a very weak and aseismic frontal wedge. Our observations thus link the shallow subduction zone seismicity to subduction erosion processes that control the evolution of the overriding plate.
Key Points:
- We investigate structure and seismicity at the up-dip end of the 2014 Iquique earthquake
rupture using amphibious seismic data.
- Seismicity up-dip of the 2014 Iquique earthquake occurs over a broad range likely
interpreted to be related to the basal erosion processes.
- Coseismic stress changes and aftershocks activate extensional faulting of the upper plate
and subduction erosion
A Plan for a Long-Term, Automated, Broadband Seismic Monitoring Network on the Global Seafloor
Establishing an extensive and highly durable, longâterm, seafloor network of autonomous broadband seismic stations to complement the landâbased Global Seismographic Network has been a goal of seismologists for decades. Seismic signals, chiefly the vibrations from earthquakes but also signals generated by storms and other environmental processes, have been processed from landâbased seismic stations to build intriguing but incomplete images of the Earthâs interior. Seismologists have mapped structures such as tectonic plates and other crustal remnants sinking deep into the mantle to obtain information on their chemical composition and physical state; but resolution of these structures from land stations is not globally uniform. Because the global surface is twoâthirds ocean, increasing the number of seismic stations located in the oceans is critical for better resolution of the Earthâs interior and tectonic structures. A recommendation for a longâterm seafloor seismic station pilot experiment is presented here. The overarching instrumentation goal of a pilot experiment is performance that will lead to the installation of a large number of longâterm autonomous oceanâbottom seismic stations. The payoff of a network of stations separated from one another by a few hundred kilometers under the global oceans would be greatly refined resolution of the Earthâs interior at all depths. A second prime result would be enriched understanding of largeâearthquake rupture processes in both oceanic and continental plates. The experiment would take advantage of newly available technologies such as robotic wave gliders that put an affordable autonomous prototype within reach. These technologies would allow data to be relayed to satellites from seismometers that are deployed on the seafloor with longâlasting, rechargeable batteries. Two regions are presented as promising arenas for such a prototype seafloor seismic station. One site is the central North Atlantic Ocean, and the other highâinterest locale is the central South Pacific Ocean
Recommended from our members
Relationship of pore water freshening to accretionary processes in the Cascadia margin: Fluid sources and gas hydrate abundance
Drilling in the Cascadia accretionary complex enable us to evaluate the contribution of dehydration reactions and gas hydrate dissociation to pore water freshening. The observed freshening with depth and distance from the prism toe is consistent with enhanced conversion of smectite to illite, driven by increase in temperature and age of accreted sediments. Although they contain gas hydrate -as evidenced by discrete low chloride spikes- the westernmost sites drilled on Hydrate Ridge show no freshening trend with depth. Strontium data reveal that all the meÂŽlange samples contain deep fluids modified by reaction with the subducting oceanic crust. Thus we infer that, at the westernmost sites, accretion is too recent for the sediments to have undergone significant illitization. Our data demonstrate that a smooth decrease in dissolved chloride with depth cannot generally be used to infer the presence or to estimate the amount of gas hydrate in accretionary margins
Recommended from our members
Basement and Regional Structure Along Strike of the Queen Charlotte Fault in the Context of Modern and Historical Earthquake Ruptures
The Queen Charlotte fault (QCF) is a dextral transform system located offshore of southeastern Alaska and western Canada, accommodating âŒ4.4 cm/yr of relative motion between the Pacific and North American plates. Oblique convergence along the fault increases southward, and how this convergence is accommodated is still debated. Using seismic reflection data, we interpret offshore basement structure, faulting, and stratigraphy to provide a geological context for two recent earthquakes, an M[subscript w] 7.5 strike-slip event near Craig, Alaska, and an M[subscript w] 7.8 thrust event near Haida Gwaii, Canada. We map downwarped Pacific oceanic crust near 54° N, between the two rupture zones. Observed downwarping decreases north and south of 54° N, parallel to the strike of the QCF. Bending of the Pacific plate here may have initiated with increased convergence rates due to a plate motion change at âŒ6 Ma. Tectonic reconstruction implies convergence-driven Pacific plate flexure, beginning at 6 Ma south of a 10° bend the QCF (which is currently at 53.2° N) and lasting until the plate translated past the bend by âŒ2 Ma. Normal-faulted approximately late Miocene sediment above the deep flexural depression at 54° N, topped by relatively undeformed Pleistocene and younger sediment, supports this model. Aftershocks of the Haida Gwaii event indicate a normal-faulting stress regime, suggesting present-day plate flexure and underthrusting, which is also consistent with reconstruction of past conditions. We thus favor a Pacific plate underthrusting model to initiate flexure and accommodation space for sediment loading. In addition, mapped structures indicate two possible fault segment boundaries along the QCF at 53.2° N and at 56° N
Recommended from our members
North-south variability in the history of deformation and fluid venting across Hydrate Ridge, Cascadia margin
Hydrate Ridge is an accretionary thrust ridge located on the lower slope of the central Cascadia convergent margin. Structural mapping based on two-dimensional and three-dimensional multichannel seismic reflection profiles and gridded bathymetry coupled with deep-towed sidescan sonar data and Ocean Drilling Program (ODP) biostratigraphy suggests that seafloor fluid venting patterns are likely controlled by the seaward-vergent (SV) structural style at northern Hydrate Ridge (NHR) and by the dominantly landward-vergent (LV) structural style at southern Hydrate Ridge (SHR). North-south structural variability across Hydrate Ridge is coincident with the seafloor authigenic carbonate distribution, which varies from aerially extensive authigenic carbonate crusts at NHR to a minor focused occurrence of authigenic carbonate at SHR. The older stratigraphy exposed at the seafloor at NHR (>1.6â1.7 Ma) has likely been subjected to a longer history of sediment compaction, dewatering, and deformation than the younger slope basin strata preserved at SHR (1.7 Ma to recent), suggesting the extent of carbonates at NHR may result from a longer history of fluid flow and/or more intense venting through a more uplifted, lithified, and fractured NHR sequence. Furthermore, recent work at SHR shows that the major seafloor fluid venting site there is fed by fluid flow through a volcanic ashâbearing turbidite sequence, suggesting stratigraphic conduits for fluid flow may be important in less uplifted, LV-dominated portions of Hydrate Ridge. In addition, the variability in structural style observed at Hydrate Ridge may have implications for the distributions and concentrations of fluids and gas hydrates in other accretionary settings and play a role in the susceptibility of accretionary ridges to slope failure
- âŠ