SUMMARY The southcentral Hikurangi subduction margin (North Island, New Zealand) has a wide, low-taper accretionary wedge that is frontally accreting a >3-km-thick layer of sediments, with deformation currently focused near the toe of the wedge. We use a geological model based on a depth-converted seismic section, together with physically realistic parameters for fluid pressure, and sediment and décollement friction based on laboratory experiments, to investigate the present-day force balance in the wedge. Numerical models are used to establish the range of physical parameters compatible with the present-day wedge geometry and mechanics. Our analysis shows that the accretionary wedge stability and taper angle require either high to moderate fluid pressure on the plate interface, and/or weak frictional strength along the décollement. The décollement beneath the outer wedge requires a relatively weaker effective strength than beneath the inner (consolidated) wedge. Increasing density and cohesion with depth make it easier to attain a stable taper within the inner wedge, while anything that weakens the wedge—such as high fluid pressures and weak faults—make it harder. Our results allow a near-hydrostatic wedge fluid pressure, sublithostatic fluid overpressure at the subduction interface, and friction coefficients compatible with measurements from laboratory experiments on weak clay minerals.
Abstract Because quartz veins are common in fault zones exhumed from earthquake nucleation temperatures (150°C–350°C), quartz cementation may be an important mechanism of strength recovery between earthquakes. This interpretation requires that cementation occurs within a single interseismic period. We review slip‐related processes that have been argued to allow rapid quartz precipitation in faults, including: advection of silica‐saturated fluids, coseismic pore‐fluid pressure drops, frictional heating, dissolution‐precipitation creep, precipitation of amorphous phases, and variations in fluid and mineral‐surface chemistry. We assess the rate and magnitude of quartz growth that may result from each of the examined mechanisms. We find limitations to the kinetics and mass balance of silica precipitation that emphasize two end‐member regimes. First, the mechanisms we explore, given current kinetic constraints, cannot explain mesoscale fault‐fracture vein networks developing, even incrementally, on interseismic timescales. On the other hand, some mechanisms appear capable, isolated or in combination, of cementing micrometer‐to‐millimeter thick principal slip surfaces in days to years. This does not explain extensive vein networks in fault damage zones, but allows the involvement of quartz cements in fault healing. These end‐members lead us to hypothesize that high flux scenarios, although more important for voluminous hydrothermal mineralization, may be of subsidiary importance to local, diffusive mass transport in low fluid‐flux faults when discussing the mechanical implications of quartz cements. A renewed emphasis on the controls on quartz cementation rates in fault zones will, however, be integral to developing a more complete understanding of strength recovery following earthquake rupture.
Abstract During earthquakes on low (<1–2 km) displacement faults in isotropic crust, more earthquake energy is consumed by fracturing and gouge formation than in ruptures along more mature faults. To investigate how pre‐existing weaknesses affect earthquake energy dissipation along low displacement faults, we studied fault rocks from the 110 km long, 0.4–1.2 km displacement, Bilila‐Mtakataka Fault (BMF), Malawi. Where the BMF is parallel to surface metamorphic fabrics, macroscale fractures define a narrow (5–20 m wide) damage zone relative to where the BMF is foliation‐oblique (20–80 m), and to faults with comparable displacement in isotropic crust (∼40–120 m). Enhanced microfracturing and widespread gouge formation, typically reported from comparable‐displacement faults, are not observed. Therefore, minimal evidence for earthquake energy dissipation into the BMF’s surrounding wall rock exists, despite geomorphic evidence for M W 7.5–8 earthquakes. We attribute this finding to differences in earthquake energy partitioning along incipient faults in isotropic and anisotropic crust.
Non‐volcanic tremor is a recently discovered fault slip style occurring with remarkable regularity in space near the down‐dip end of the locked zone on several subduction thrust interfaces. The physical mechanisms and the controls on the location of tremor have not yet been determined. We calculate the stable mineral assemblages and their water content in the subducting slab, and find that slab dehydration is not continuous, but rather restricted to a few reactions localised in pressure‐temperature space. Along geothermal gradients applicable to Shikoku and Cascadia ‐ where tremor has been relatively easy to detect ‐ tremor locations correlate with discontinuous and localised voluminous water release from the breakdown of lawsonite and chlorite + glaucophane respectively. The shape of the pressure‐temperature path for subducting slabs prevents fluid release at depths above and below where these dehydration reactions occur. We conclude that abundant tremor activity requires metamorphic conditions where localised dehydration occurs during subduction, and this may explain why tremor appears more abundant in some subduction zones than others.
The Malawi Seismogenic Source Model (MSSM) is a geospatial database that documents the geometry, slip rate and seismogenic properties (ie earthquake magnitude and frequency) of active faults in Malawi. Each geospatial feature represents a potential earthquake rupture of 'source' and is classified based on its geometry into one of three types: section fault multi-fault Source types are mutually exclusice, and so if incorporated into a PSHA, they should be assigned relative weightings. The MSSM is the first seismogenic source database in central and northern Malawi, and represents an update of the South Malawi Seismogenic Source Database (SMSSD; Williams et al., 2021a) because it incorporates new active fault traces (Kolawole et al., 2021; Williams et al., 2021b; 2022 - MAFD), new geodetic data (Wedmore et al., 2021) and a statistical treatment of uncertainty, within a logic tree approach. The seismogenic sources in this model are adapted from the faults in the Malawi Active Fault Database (Williams et al., 2021b; 2022). Prior to publication please cite this database using the following two references: Williams, JN, Wedmore, LNJ, Fagereng, Å, Werner, MJ, Mdala, H, Shillington, DJ, Scholz, CA, Folawole, F, Wright, LJM, Biggs, J, Dulanya, Z, Mphepo, F, Chindandali, P. 2022. Geologic and geodetic constraints on the magnitude and frequency of earthquakes along Malawi’s active faults: the Malawi Seismogenic Source Model (MSSM). Natural Hazards and Earth Systems Science, 22, 3607-3639, https://doi.org/10.5194/nhess-22-3607-2022 Williams, Jack N., Wedmore, Luke N. J., Fagereng, Åke, Werner, Maximilian J., Biggs, Juliet, Mdala, Hassan, Kolawole, Folarin, Shillington, Donna J., Dulanya, Zuze, Mphepo, Felix, Chindandali, Patrick R. N., Wright, Lachlan J. M., & Scholz, Christopher A. (2021). Malawi Seismogenic Source Model [Data set]. Zenodo. https://doi.org/10.5281/zenodo.5599616 Database Design and File Formats The MSSM is a geospatial database that consists of two separate components: A 3D geometrical model of fault seismogenic sources in Malawi The mapped trace of each source in a GIS vector format, with associated source attributes (Data Table). Each fault is associated with a source in the 3D geometrical model that is listed in a comma-separated-values (csv) file. The sections, faults and multi-faults that make up the individual seismogenic sources are described in separate geospatial files that describe the map-view geometry and metadata that control each sources earthquake magnitude and frequency for seismic hazard purposes. The sections, faults and multi-faults in this database are provided in a variety of GIS vector file formats. GeoJSON is the version of record, and any changes should be made in this version before they are converted to other file formats using the script in the repository that uses the GDAL tool ogr2ogr (the script is adapted from https://github.com/cossatot/central_am_carib_faults/blob/master/convert.sh - we thank Richard Styron for making this publicly available). The other versions available are ESRI ShapeFile, KML, GMT, and GeoPackage. List and brief description of the fault geometry, slip rate estimates and earthquake source attributes in the GIS vector format files that make up the MSSM. Attribuge Type Description Notes MSSM_ID integer Unique numerical reference ID for each seismic source ID 00-300 is section rupture ID 300-500 is fault rupture ID 600-700 is a multi-fault rupture name string Assigned based on previous mapping or local geographic feature.
For sections and faults, the name of the fault (flt_name) and larger multi-fault (mflt_name) system they are hosted on are given respectively. basin string Basin that source is located within. Used in slip rate calculations class string intrarift or border fault length (Ls) real number straight-line distance in km between fault tips; sum of Lsec for segmented faults; sum of Lfault for multi-faults measured in km to 1 decimal place. Must be greater than 5 km (except for linking sections). area integer Calculated from Ls multiplied by Eq. 1 or based on fault truncation. measured in km2 strike integer Azimuth of straigth line between the fault tips. azimuth is <180° Used as input for slip rate estimates in Eq. 2 dip_lower integer lower range of dip value When no previous measurements of dip are available, a nominal value of 45° is used. dip_int integer Intermediate dip value In the MSSM geometrical model, only the intermediate measurements is considered. When no previous measurements of are available, a nominal value of 53° is assigned.
No dip is assigned for multi-fault sources, as different participating faults may have different dips. dip_upper integer Upper range of dip value When no previous measurements of dip are availabe, a nominal value of 65° is used. dip_dir string Dip direction: compass quadrant that the fault dips in. slip_type string Source kinematics (e.g. normal, thrust etc). All sources in the MSSM are assumed to be normal faults. slip_rate real number Mean value from repeating Eq. 2 in Monte Carlo simulations (see manuscript for details). In mm yr-1. All sources in the MSSM are assumed to be normal so is equivalent to dip-slip rate.
Reported to two significant figures. s_rate_err real number Slip rate error: 1σ error from Monte Carlo slip rate simlations. mag_lower real number Lower magnitude estimate.
Calculated from Leonard (2010) scaling relationship (Eq. 4) for Ls or As, and using lower estimates of C1 and C2 constants in Leonard (2010). Reported to one decimal place. mag_med real number Mean magnitude estimate.
Calculated from Leonard (2010) scaling relationship (Eq. 4) for Ls or As, and using mean estimates of C1 and C2 constants in Leonard (2010). Reported to one decimal place. mag_upper real number Upper magnitude estimate.
Calculated from Leonard (2010) scaling relationship (Eq. 4) for Ls or As, and using upper estimates of C1 and C2 constants in Leonard (2010). Reported to one decimal place. ri_lower real number Lower recurrence interval estimate.
Calculated as 1σ below the mean of the Monte Carlo simulations (assuming a log normal distribution). Reported to two significant figures. ri_med real number Mean recurrence interval.
Mean value from log of recurrence interval Monte Carlo simulations. Reported to two significant figures. ri_upper real number Upper recurrence interval estimate.
Calculated as 1σ above the mean of the Monte Carlo simulations (assuming a log normal distribution). Reported to two significant figures. MAFD_id list List of integers of ID of equivalent structures in the Malawi Active Fault Database Multi-fault sources have multiple ID's. Version Control This version is intended to be "Live" and as such we encourage edits of the GeoJSON file and the submission of pull requests. Please contact Jack Williams jack.williams@otago.ac.nz Luke Wedmore luke.wedmore@bristol.ac.uk or Hassan Mdala mdalahassan@yahoo.com for information, other requests or if you find any errors within the database. It is the intention that future versions of this database will include fault slip rates that have been determined from direct geological methods (e.g. offset stratigraphy that has been dated) rather than the systems based approach that is currently used. References Kolawole, F., Firkins, M. C., Al Wahaibi, T. S., Atekwana, E. A., & Soreghan, M. J. (2021a). Rift Interaction Zones and the Stages of Rift Linkage in Active Segmented Continental Rift Systems. Basin Research. https://doi.org/10.1111/bre.12592 Leonard, M. (2010). Earthquake fault scaling: Self-consistent relating of rupture length, width, average displacement, and moment release. Bulletin of the Seismological Society of America, 100(5A), 1971-1988. https://doi.org/10.1785/0120090189 Wedmore, L. N. J., Biggs, J., Floyd, M., Fagereng, Å., Mdala, H., Chindandali, P. R. N., et al. (2021). Geodetic constraints on cratonic microplates and broad strain during rifting of thick Southern Africa lithosphere. Geophysical Research Letters. 48(17), e2021GL093785. https://doi.org/10.1029/2021GL093785 Williams, J. N., Mdala, H., Fagereng, Å., Wedmore, L. N. J., Biggs, J., Dulany, Z., et al. (2021). A systems-based approach to parameterise seismic hazard in regions with little historical or instrumental seismicity: Active fault and seismogenic source databases for southern Malawi. Solid Earth, 12(1), 187–217. https://doi.org/10.5194/se-12-187-2021 V1.1 Updates Updated seismic source files and model parameters. Changes are: Adding lower and upper dip estimates for sources (following a reviewer comment). This should be equivalent to Table 1 in the revised manuscript. Cleaning up the GIS files. In the old file there were some duplicate GIS features that are now removed Changing the name and acronyms from Malawi Seismogenic Source Database (MSSD) to Malawi Seismogenic Sources Model (MSSM). Included a basic Matlab script to plot the MSSM geometrical polygons V1.2 Updates Updated fault source geometry .csv file due to compiling error.
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