The Relationship Between Oceanic Transform Fault Segmentation, Seismicity, and Thermal Structure

Mid-ocean ridge transform faults (RTFs) are typically viewed as geometrically simple, with fault lengths readily constrained by the ridge-transform intersections. This relative simplicity, combined with well-constrained slip rates, make them an ideal environment for studying strike-slip earthquake behavior. As the resolution of available bathymetric data over oceanic transform faults continues to improve, however, it is being revealed that the geometry and structure of these faults can be complex, including such features as intra-transform pull-apart basins, intra-transform spreading centers, and cross-transform ridges. To better determine the resolution of structural complexity on RTFs, as well as the prevalence of RTF segmentation, fault structure is delineated on a global scale. Segmentation breaks the fault system up into a series of subparallel fault strands separated by an extensional basin, intra-transform spreading center, or fault step. RTF segmentation occurs across the full range of spreading rates, from faults on the ultraslow portion of the Southwest Indian Ridge to faults on the ultrafast portion of the East Pacific Rise (EPR). It is most prevalent along the EPR, which hosts the fastest spreading rates in the world and has undergone multiple changes in relative plate motion over the last couple of million years. Earthquakes on RTFs are known to be small, to scale with the area above the 600°C isotherm, and to exhibit some of the most predictable behaviors in seismology. In order to determine whether segmentation affects the global RTF scaling relations, the scalings are recomputed using an updated seismic catalog and fault database in which RTF systems are broken up according to their degree of segmentation (as delineated from available bathymetric datasets). No statistically significant differences between the new computed scaling relations and the current scaling relations were found, though a few faults were identified as outliers. Finite element analysis is used to model 3-D RTF fault geometry assuming a viscoplastic rheology in order to determine how segmentation affects the underlying thermal structure of the fault. In the models, fault segment length, length and location along fault of the intra-transform spreading center, and slip rate are varied. A new scaling relation is developed for the critical fault offset length (OC) that significantly reduces the thermal area of adjacent fault segments, such that adjacent segments are fully decoupled at ∼4OC . On moderate to fast slipping RTFs, offsets ≥ 5 km are sufficient to significantly reduce the thermal influence between two adjacent transform fault segments. The relationship between fault structure and seismic behavior was directly addressed on the Discovery transform fault, located at 4°S on the East Pacific Rise. One year of microseismicity recorded on an OBS array, and 24 years of Mw ≥ 5.4 earthquakes obtained from the Global Centroid Moment Tensor catalog, were correlated with surface fault structure delineated from high-resolution multibeam bathymetry. Each of the 15 Mw ≥ 5.4 earthquakes was relocated into one of five distinct repeating rupture patches, while microseismicity was found to be reduced within these patches. While the endpoints of these patches appeared to correlate with structural features on the western segment of Discovery, small step-overs in the primary fault trace were not observed at patch boundaries. This indicates that physical segmentation of the fault is not the primary control on the size and location of large earthquakes on Discovery, and that along-strike heterogeneity in fault zone properties must play an important role

Thermal segmentation of mid-ocean ridge-transform faults

Author Posting. © American Geophysical Union, 2017. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 18 (2017): 3405–3418, doi:10.1002/2017GC006967.3-D finite element simulations are used to calculate thermal structures and mantle flow fields underlying mid-ocean ridge-transform faults (RTFs) composed of two fault segments separated by an orthogonal step over. Using fault lengths and slip rates, we derive an empirical scaling relation for the critical step over length ( inline image), which marks the transition from predominantly horizontal to predominantly vertical mantle flow at the base of the lithosphere under a step over. Using the ratio of step over length (LS) to inline image, we define three degrees of segmentation: first-degree, corresponding to type I step overs ( inline image ≥ 3); second-degree, corresponding to type II step overs (1 ≤  inline image < 3); and third-degree, corresponding to type III step overs ( inline image <1). In first-degree segmentation, thermal structures and mantle upwelling patterns under a step over are similar to those of mature ridges, where normal mid-ocean ridge basalts (MORBs) form. The seismogenic area under first-degree segmentation is characteristic of two, isolated faults. Second-degree segmentation creates pull-apart basins with subdued melt generation, and intratransform spreading centers with enriched MORBs. The seismogenic area of RTFs under second-degree segmentation is greater than that of two isolated faults, but less than that of an unsegmented RTF. Under third-degree segmentation, mantle flow is predominantly horizontal, resulting in little lithospheric thinning and little to no melt generation. The total seismogenic area under third-degree segmentation approaches that of an unsegmented RTF. Our scaling relations characterize the degree of segmentation due to step overs along transform faults and provide insight into RTF frictional processes, seismogenic behavior, and melt transport.NSF Grant Numbers: OCE-1352565, OCE-14-58201; NOAA. Grant Number: NA10NOS4000073; 2011 ExxonMobil Geosciences2018-03-1

Structure and lithology of the Japan Trench subduction plate boundary fault

The 2011 Mw9.0 Tohoku-oki earthquake ruptured to the trench with maximum coseismic slip located on the shallow portion of the plate boundary fault. To investigate the conditions and physical processes that promoted slip to the trench, Integrated Ocean Drilling Program Expedition 343/343T sailed 1 year after the earthquake and drilled into the plate boundary ∼7 km landward of the trench, in the region of maximum slip. Core analyses show that the plate boundary décollement is localized onto an interval of smectite-rich, pelagic clay. Subsidiary structures are present in both the upper and lower plates, which define a fault zone ∼5–15m thick. Fault rocks recovered from within the clay-rich interval contain a pervasive scaly fabric defined by anastomosing, polished, and lineated surfaces with two predominant orientations. The scaly fabric is crosscut in several places by discrete contacts across which the scaly fabric is truncated and rotated, or different rocks are juxtaposed. These contacts are inferred to be faults. The plate boundary décollement therefore contains structures resulting from both distributed and localized deformation. We infer that the formation of both of these types of structures is controlled by the frictional properties of the clay: the distributed scaly fabric formed at low strain rates associated with velocity-strengthening frictional behavior, and the localized faults formed at high strain rates characterized by velocity-weakening behavior. The presence of multiple discrete faults resulting from seismic slip within the décollement suggests that rupture to the trench may be characteristic of this margin

A Global Characterization of Physical Segmentation along Oceanic Transform Faults

Wesnousky (2006) found that physical fault offsets of 3 - 4 km act as barriers to rupture propagation on continental strike-slip faults. Along oceanic transform faults (OTFs), step-overs, intra-transform spreading centers, and pull-apart basins can divide the fault system into a series of parallel or sub-parallel fault segments. We have characterized the segmentation of OTFs on a global scale and are investigating the effects of this segmentation on seismic behavior. The 1-arcmin Smith and Sandwell global seafloor topography dataset (v. 12.1), comprised of satellite altimetry data blended with depth estimates from ship-borne sonar, was used to measure physical parameters of 200 OTFs. For each OTF, fault length and distance from each endpoint to the nearest ridge discontinuity were measured. The 200 individual fault segments were classified into 101 single-segment faults and 34 multi-segment fault systems, each comprised of between 2 - 7 segments with offsets ≤ 50 km. Utilizing only ridge-transform-ridge segments, we constructed scaling relations for seismic parameters. To ensure a uniform minimum resolution for our global dataset, where some regions are constrained only by satellite altimetry data, any two adjacent fault segments with an offset ≤ 20 km, were combined into a single segment. The resulting dataset included 155 fault segments. An earthquake catalog was then generated from the global Centroid Moment Tensor database for each individual fault segment. A half-space cooling model was used to calculate the thermal area of contact above the 600-degree isotherm using slip rates acquired from the GSRM plate velocity model. Following the analysis of Boettcher and Jordan (2004), maximum-likelihood estimation was used to determine the largest expected earthquake for fault segments grouped by thermal area. Scaling relations between thermal area and the largest expected earthquake, as well as the seismic coupling coefficient, were calculated. Initial results show no significant differences in the scaling relations derived for the more segmented dataset. Future analyses utilizing OTF structure delineated from higher resolution sonar data will provide additional insight into the underlying mechanics of fault slip on OTFs

Thermal Constraints on the Rheology of Segmented Oceanic Transform Fault Systems

Mid-ocean ridge transform fault (RTF) systems may be comprised of two or more fault segments that are physically offset by an extensional basin or intra-transform spreading center. These intra-transform offsets affect the thermal structure underlying the transform fault and may act as barriers to rupture propagation. The seismogenic zone of RTFs is thermally controlled and limited by the 600°C isotherm, as evidenced by earthquake hypocentral depths and laboratory friction experiments. Observations from a recent ocean bottom seismic study found that RTF earthquakes rarely occur above ~2 km depth. These findings suggest that the seismogenic zone on RTFs likely extends from ~2 km to the 600°C isotherm. Here we utilize finite element analysis to model the thermal structure of a RTF system comprised of two transform fault segments separated by an extensional offset. The mantle is assumed to have a visco-plastic rheology to simulate brittle failure at temperatures \u3c600°C. We vary offset length, spreading rate, and degree of hydrothermal circulation to examine how these parameters control the underlying thermal structure of segmented RTFs. Longer offsets and faster spreading rates result in warmer thermal structures. Enhanced hydrothermal circulation efficiently cools shallow regions, resulting in an increased area of brittle deformation, and may have a complex effect on the seismogenic zone due to the possible creation of weak, velocity-strengthening alteration phases such as serpentine and talc, and/or changes in fault zone porosity. Incorporating these processes into our model, we are able to assess the potential for an intra-transform offset to act as a barrier to rupture propagation. As a case study, we focus on the Discovery transform fault, located at 4°S on the East Pacific Rise. Discovery consists of two sub-parallel fault segments with lengths of 36 km and 27 km, separated by a 6 km intra-transform spreading center. On a number of intermediate- and fast-slipping RTFs, including Discovery, the largest earthquakes are known to re-rupture the same fault patch in relatively regular seismic cycles. The rupture patches are bounded by areas of increased microseismicity, which act as barriers to large rupture propagation. Previously, we used well-located earthquakes recorded on a NOAA hydrophone array together with a relative relocation technique to determine the absolute positions for the five rupture patches on Discovery, which host 5.4 ≤ Mw ≤ 6.0 earthquakes. In this study, we combine absolute locations of the largest earthquakes, our detailed analysis of the fault trace of Discovery, and our thermal modeling results to assess how intra-transform offsets on Discovery affect the subsurface thermal structure. Along the 6 km intra-transform spreading center we find the 600°C isotherm is shallower than 2 km, suggesting that the thermal structure of this offset creates a rupture barrier between the adjoining fault segments. By contrast, intra-transform offsets \u3c2 km identified along the surface trace of each segment only minimally affect the depth of the 600°C isotherm, resulting in a continuous seismogenic zone between the fault segments. This suggests that the thermal effect of small intra-transform offsets is not sufficient to explain the locations of rupture patches and rupture barriers on the Discovery transform fault

Oceanic transform fault seismicity and slip mode influenced by seawater infiltration

International audienceOceanic transform faults that offset mid-ocean ridges slip through earthquakes and aseismic creep. The mode of slip varies with depth and along strike, with some fault patches that rupture in large, quasi-periodic earthquakes at temperatures <600 °C, and others that slip through creep and microearthquakes at temperatures up to 1,000 °C. Rocks from both fast- and slow-slipping transforms show evidence of interactions with seawater up to temperatures of at least 900 °C. Here we present a model for the mechanical structure of oceanic transform faults based on fault thermal structure and the impacts of hydration and metamorphic reactions on mantle rheology. Deep fluid circulation is accounted for in a modified friction-effective pressure law and in ductile flow laws for olivine and serpentine. Combined with observations of grain size reduction and hydrous mineralogy from high-strain mylonites, our model shows that brittle and ductile deformation can occur over a broad temperature range, 300-1,000 °C. The ability of seawater to penetrate faults determines whether slip is accommodated at depth by seismic asperities or by aseismic creep in weak, hydrous shear zones. Our results suggest that seawater infiltration into ocean transform faults controls the extent of seismicity and spatiotemporal variations in the mode of slip

Characteristics of Oceanic Strike-Slip Earthquakes Differ Between Plate Boundary and Intraplate Settings

The April 11, 2012 Mw 8.6 earthquake that occurred off the coast of Sumatra was the largest ever recorded strike-slip event. This earthquake ruptured the oceanic lithosphere more than 100 km from the nearest plate boundary and was followed by 86 aftershocks of mb ≥ 4.8 within 200 km and 30 days of the mainshock, including a Mw 8.2 event. While the recent Great Sumatra strike-slip earthquake was extraordinary in its seismic moment release, we find that in terms of aftershock productivity it had the characteristics of an ordinary intraplate oceanic strike-slip event. Using catalog data from the International Seismological Centre (ISC) and Global Centroid Moment Tensor (CMT) Project from January 1976 through June 2012, we have investigated the rates, maximum magnitudes, and aftershock productivities of 15,519 Mw ≥ 5.5 strike-slip earthquakes in the oceanic lithosphere. We compared our results from intraplate regions to those on mid-ocean ridge transform faults (RTFs) and find that the seismic characteristics differ between the two tectonic settings. The rate of Mw ≥ 5.5 events is 35 times greater on RTFs, while the moment release rate is an order of magnitude greater in intraplate settings. Aftershock statistics of both regions can be quantified using the Epidemic Type Aftershock Sequence model with a triggering exponent, alpha, of 0.9. We find RTF earthquakes have almost an order of magnitude fewer aftershocks than do strike-slip intraplate oceanic earthquakes. Our previous work showed that the maximum magnitude of RTF earthquakes is very small (less than 40%) compared with the total fault area above the 600C isotherm, AT. The largest recorded mid-ocean ridge transform earthquake was a Mw 7.1 on the Romanche RTF. Through a statistical analysis of catalog data from 1964 through 2001 we found that the largest expected earthquake, i.e. the corner moment in a tapered Gutenberg-Richter distribution, Mc, does not grow linearly with fault area, but instead scales as AT to the 1/2 power. This scaling relation is so robust that it can be used, in combination with a scaling relation for seismic coupling, to accurately obtain the global mid-ocean ridge transform fault seismicity distribution from 2002 through the present. On a number of moderate and fast slipping RTFs, the largest earthquakes are known to re-rupture the same fault patch in relatively regular and short (5-20 year) seismic cycles. These small sections of the fault appear to be fully-coupled, such that the tectonic moment rate can be used to constrain the timing of the next maximum magnitude rupture. The much larger earthquakes and more abundant aftershocks following oceanic intraplate strike-slip earthquakes, suggest that fault zone composition alone is unlikely to be the primary cause of the small maximum magnitudes, short recurrence intervals, and few aftershocks observed on RTFs

The Relationship Between Seismicity and Fault Structure on the Discovery Transform Fault, East Pacific Rise

On the Discovery transform fault, East Pacific Rise, absolute locations of one year of microseismicity recorded during an ocean bottom seismometer (OBS) deployment and 21 years of Mw ≥ 5.4 earthquakes, obtained from the Global Centroid Moment Tensor (CMT) catalog, are correlated with fault structure using high resolution multibeam bathymetry. Fifteen Mw 5.4 - 6.0 earthquakes occurred on Discovery between 1992 - 2013, with a mean repeat time of 7 years. Absolute locations for each of these events were obtained by employing a teleseismic surface wave relative relocation technique using events from the NOAA hydroacoustic catalog as empirical Green’s functions. Each event was found to occur within one of five distinct rupture patches on Discovery. Correlation of the rupture patch locations with the structural analysis of the fault zone indicates that the 8-km long intra-transform spreading center, located approximately in the center of Discovery, acts as a barrier to large rupture propagation. Secondary structural features within the fault zone (e.g. ridges that crosscut the transform valley) may also result in barriers to rupture propagation. The 2008 OBS deployment on Quebrada, Discovery, and Gofar transform faults recorded ~25,000 0.16 \u3c mL \u3c 4.58 earthquakes on Discovery, which were relocated using HypoDD. The relocated events cluster within 3 km of the fault trace and extend ~6 km outside of the western ridge-transform intersection. Approximately 4% of the microseismicity occurred beyond the active transform, and these events appear to be dominated by aftershock sequences. The rate of microseismicity varies significantly along strike and with time. The most striking region of the fault includes a 4.5-km wide zone on the western segment that is essentially devoid of any microseismic activity during the yearlong deployment period. The microseismic gap coincides with the largest, westernmost rupture patch, which was ruptured by 3 Mw 5.9 - 6.0 earthquakes in 1996, 2001, and 2012, suggesting that this fault segment may be fully locked between large events. By contrast, the centroid location for the second largest rupture patch (4 Mw 5.5 - 5.8 earthquakes in 1996, 2001, 2007, and 2012) on the western segment coincides with the area of highest microseismicity rate. This 5-km wide zone contains approximately 25% of the relocated microseismicity on Discovery. The smallest rupture patch on Discovery (3 Mw 5.4 - 5.5 earthquakes in 1998, 2003, and 2007) is located just west of the intra-transform spreading center and coincides with a zone of low seismic productivity, suggesting increased seismic coupling. The two rupture patches on the eastern segment of Discovery were outside the OBS array, and thus the microseismic activity in those regions is not well constrained. The results of this study suggest that physical fault structure influences the location and size of large repeating rupture patches, and provides a secondary control on the location of microseismicity. Fault structure, however, cannot account for all aspects of seismic activity on Discovery, and thus we suspect that varying mechanical properties along the fault, and specifically within the large rupture patches, are the primary controls on the location and rate of microseismicity

The relationship between seismicity and fault structure on the Discovery transform fault, East Pacific Rise

There is a global seismic moment deficit on mid-ocean ridge transform faults, and the largest earthquakes on these faults do not rupture the full fault area. We explore the influence of physical fault structure, including step-overs in the fault trace, on the seismic behavior of the Discovery transform fault, 4S on the East Pacific Rise. One year of microseismicity recorded during a 2008 ocean bottom seismograph deployment (24,377 0 ML 4.6 earthquakes) and 24 years of Mw 5.4 earthquakes obtained from the Global Centroid Moment Tensor catalog, are correlated with surface fault structure delineated from high-resolution multibeam bathymetry. Each of the 15 5.4 Mw 6.0 earthquakes that occurred on Discovery between 1 January 1990 and 1 April 2014 was relocated into one of five distinct rupture patches using a teleseismic surface wave cross-correlation technique. Microseismicity was relocated using the HypoDD relocation algorithm. The western fault segment of Discovery (DW) is composed of three zones of varying structure and seismic behavior: a zone with no large events and abundant microseismicity, a fully coupled zone with large earthquakes, and a complex zone with multiple fault strands and abundant seismicity. In general, microseismicity is reduced within the patches defined by the large, repeating earthquakes. While the extent of the large rupture patches on DW correlates with physical features in the bathymetry, step-overs in the primary fault trace are not observed at patch boundaries, suggesting along-strike heterogeneity in fault zone properties controls the size and location of the large events. 1

Internal structure of the shallow Japan Trench décollement: insights into the long-term evolution of the margin and coseismic slip processes

The 2011 MW 9.0 Tohoku-oki earthquake ruptured to the Japan Trench, with largest coseismic slip occurring on the shallow part of the décollement. To better understand the controls on rupture propagation and slip, the structure and composition of the décollement near the trench were investigated during Integrated Ocean Drilling Project Expedition 343 (the JFAST project). The plate boundary décollement is localized upon a ≤4.86 m thick layer of smectite-rich pelagic clay. Stratigraphic discontinuities at the base of the hangingwall, top of the footwall and surrounding a horse of intra-décollement mudstone suggest that the fault contains multiple slip surfaces, although most of these were not recovered. The décollement damage zone is \u3c10 m wide in both the overlying frontal prism and down-going Pacific plate showing that long-term displacement on the plate boundary fault near the Japan Trench is extremely localized and in turn suggesting the fault is weak relative to the bounding sediments. A pervasive composite foliation, or scaly fabric, defined by striated, lustrous surfaces enclosing lenses of less fissile phacoids is distributed throughout the décollement clay. The asymmetry of phacoids is consistent with top-to-the-trench shear sense. Several narrow, planar discontinuities crosscut the scaly fabric, truncating or disrupting the foliation and in one case juxtaposing domains of the clay with different foliation orientation and intensity, indicating relative displacement. The scaly fabric is indicative of distributed shear across the recovered interval (~1 m), and may represent deformation at interseismic strain rates. The sharp discontinuities within the décollement clay, however, resulted from slip localization. They are similar to structures produced in friction experiments conducted at coseismic slip rates suggesting they may record earthquake deformation