Testing earthquake links in Mexico from 1978 to the 2017 M = 8.1 Chiapas and M = 7.1 Puebla Shocks

The M = 8.1 Chiapas and the M = 7.1 Puebla earthquakes occurred in the bending part of the subducting Cocos plate 11 days and ~600 km apart, a range that puts them well outside the typical aftershock zone. We find this to be a relatively common occurrence in Mexico, with 14% of M > 7.0 earthquakes since 1900 striking more than 300 km apart and within a 2 week interval, not different from a randomized catalog. We calculate the triggering potential caused by crustal stress redistribution from large subduction earthquakes over the last 40 years. There is no evidence that static stress transfer or dynamic triggering from the 8 September Chiapas earthquake promoted the 19 September earthquake. Both recent earthquakes were promoted by past thrust events instead, including delayed afterslip from the 2012 M = 7.5 Oaxaca earthquake. A repeated pattern of shallow thrust events promoting deep intraslab earthquakes is observed over the past 40 years

Automated detection and location of tectonic tremor along the entire Cascadia margin from 2005 to 2011

We have constructed an automated routine to identify prominent bursts of tectonic tremor and locate their source region during time periods of raised amplitude in the tremor passband. This approach characterizes 62 episodes of tectonic tremor between 2005 and 2011, with tremor epicenters forming a narrow band spanning the entire length of the Cascadia Subduction Zone. We find a range of along-strike lengths in individual episodes, but the length appears proportional to both duration and geodetic moment, consistent with proposed scaling laws for slow earthquake phenomena. Examination of individual episodes in detail reveals intriguing updip-downdip migration patterns, including slow updip migration during initiation and repetitive downdip migration between different episodes. The broader catalog of tremor episodes refines the inferences from earlier work that episodic tremor and slip is segmented along-strike and correlated with apparent seismogenic zone segmentation based on the distribution of fore-arc basins and geologic terranes. The overall band of tremor is offset ~50 km from the downdip edge of interseismic coupling along the central and northern parts of the subduction zone. Along the southern part of the subduction zone, it is adjacent to this boundary, suggesting that the locked and transition zones may be more closely linked in southern Cascadia

Quantification of the Temporal Evolution of Collagen Orientation in Mechanically Conditioned Engineered Cardiovascular Tissues

Load-bearing soft tissues predominantly consist of collagen and exhibit anisotropic, non-linear visco-elastic behavior, coupled to the organization of the collagen fibers. Mimicking native mechanical behavior forms a major goal in cardiovascular tissue engineering. Engineered tissues often lack properly organized collagen and consequently do not meet in vivo mechanical demands. To improve collagen architecture and mechanical properties, mechanical stimulation of the tissue during in vitro tissue growth is crucial. This study describes the evolution of collagen fiber orientation with culture time in engineered tissue constructs in response to mechanical loading. To achieve this, a novel technique for the quantification of collagen fiber orientation is used, based on 3D vital imaging using multiphoton microscopy combined with image analysis. The engineered tissue constructs consisted of cell-seeded biodegradable rectangular scaffolds, which were either constrained or intermittently strained in longitudinal direction. Collagen fiber orientation analyses revealed that mechanical loading induced collagen alignment. The alignment shifted from oblique at the surface of the construct towards parallel to the straining direction in deeper tissue layers. Most importantly, intermittent straining improved and accelerated the alignment of the collagen fibers, as compared to constraining the constructs. Both the method and the results are relevant to create and monitor load-bearing tissues with an organized anisotropic collagen network

Changes in Plate Motions and the Shape of Pacific Fracture Zones

Geosat passes, the new 2-min global gravity grid [Smith and Sandwell, 1995], and shipboard bathymetry across central Pacific fracture zones were used to identify features common to fracture zone segments that formed during times of changes in plate motions. These features are not predicted by current “locked fault” fracture zone models. During a change in spreading direction that induces tension across the transform fault, large-offset (greater than ∼500 km) transforms develop multiple parallel faults, spaced 50 to 100 km apart. The gravity signature of small-offset transform faults under tension includes a broader and more symmetric trough than observed on segments that formed during periods of steady spreading. Parts of fracture zones that form subsequent to a spreading reorientation that causes compression across the transform fault generally exhibit a single fault scarp that fits the locked fault model. Seafloor formed during a period of change usually marks a transition between structural styles, for example, between multiple fracture zone strands and a narrower single-fault fracture zone. Widening of the transform fault zone under tension and narrowing under compression are consistent with the assumption that during a change in spreading direction the new spreading ridges propagate to, but not across, the old transform fault

Evolution and Strength of Pacific Fracture Zones

Previous studies have shown that Pacific fracture zones are strong in some locations, sustaining the stresses associated with differential subsidence across a locked fault, while at other locations they show signs of weakness, acting as preferential conduits for volcanism or supporting anomalously low shear stresses. We find that half of all Geosat crossings of central Pacific fracture zones are inconsistent with a flexurally maintained scarp. We test two hypotheses for the origin of such anomalous crossings: (1) anomalous structures formed at the transform fault during times of changes in plate motions, and preserved at “strong” fracture zones; and (2) anomalous structures representing a posttransform response of the fracture zone to subsequent tectonic activity. We find that three-quarters of the anomalous crossings occur over the parts of fracture zones that formed during or immediately subsequent to times of changes in spreading directions. With the exception of several locations overprinted by hot spot volcanism, these same crossings show no obvious correlation with regional tectonic processes. These observations suggest that most anomalous fracture zone topography is inherited from the transform fault and is not a product of subsequent activity, consistent with the hypothesis that fracture zones generally remain strong throughout their lifetimes

2010), Seismic anisotropy beneath Cascadia and the Mendocino triple junction: Interaction of the subducting slab with mantle flow, Earth Planet

Mantle flow associated with the Cascadia subduction zone and the Mendocino Triple Junction is poorly characterized due to a lack of shear wave splitting studies compared to other subduction zones. To fill this gap data was obtained from the Mendocino and FACES seismic networks that cover the region with dense station spacing. Over a period of 11-18 months, 50 suitable events were identified from which shear wave splitting parameters were calculated. Here we present stacked splitting results at 63 of the stations. The splitting pattern is uniform trench normal (N67°E) throughout Cascadia with an average delay time of 1.25 s. This is consistent with subduction and our preferred interpretation is entrained mantle flow beneath the slab. The observed pattern and interpretation have implications for mantle dynamics that are unique to Cascadia compared to other subduction zones worldwide. The uniform splitting pattern seen throughout Cascadia ends at the triple junction where the fast directions rotate almost 90°. Immediately south of the triple junction the fast direction rotates from NW-SE near the coast to NE-SW in northeastern California. This rotation beneath northern California is consistent with flow around the southern edge of the subducting Gorda slab