Structure and tectonic significance of deformed medial Ordovician flysch and melange between Albany and Saratoga Lake and in the central Hudson Valley, New York

The belt of medial Ordovician deformed flysch west of the traditionally defined Taconic boundary thrust was investigated by detailed mapping in the Capital District, from Albany to Saratoga Lake, and large scale compilation of the central Hudson Valley, between Glens Falls and Middletown. Maps and detailed sections are presented. Mapping recognized a two-fold division into a western belt of folded and faulted flysch and an eastern belt which is dominated by tectonic melange. The Folded and Faulted Flysch unit shows northward fining from greywacke-dominated to shale-dominated. Isoclinal folding and incipient melanges are characteristic in the south, whereas in the north folding is mild and deformation more localized. The boundary of this belt with the melange-dominated belt appears abrupt, but is not well exposed as it coincides in many areas with a Pleistocene/Quaternary filled bedrock valley. A flatlying and unfolded body of black shale within the Folded and Faulted Flysch near Saratoga Lake is anomalous with respect to the lithology and structure of its surroundings. The melange-dominated part could be subdivided from west to east into Western Exotic Melange, Halfmoon Greywacke Zone (HGZ) and its northern equivalent, Eastern Exotic Melange, Bedded Shale, Flysch Melange and Frontal Exotic Melange. The modifier exotic indicates assemblages of non-flysch lithologies within the melange, specifically pale green shale, sideritic mudstone and black chert. Preserved bedding and unusual lithology (abundant and thick greywackes) in the HGZ and its northern equivalent contrast distinctively with the surroundings. The structure of the southern HGZ is a large syncline, of which the eastern limb and hinge is cut by a thrust juxtaposing a complexly deformed terrane. Complex melange exposed along the Mohawk River at Cohoes Gorge is described in detail and recorded in a detailed cross-section. The Flysch Melange, comprised only of shale, siltstone and thin greywackes, has small slices (\u3c 10m) of bedded material in contrast to the Western and Eastern Exotic Melange which have virtually none. The Frontal Exotic Melange has large slices of non-flysch material (\u3c20m) and small bedded flysch slices. All units are inferred to be in thrust contact with each other. Sedimentologically, paleo-current directions and frequent sets of climbing ripples indicate linear trench topography and reworking by strong contour currents for the deposition of flysch sediments. The assemblage of non-flysch lithologies (mainly black chert, sideritic mudstone and pale green shale) was deposited on the slope/rise of a former passive margin. Coarser, more immature, exotic greywacke is probably related to the assemblage of non-flysch lithologies. A difference between exotic greywackes and normal greywackes is suggested by point counting. Phacoidal cleavage is the dominating structural element. Its average plane and other foliations dip moderately to steeply to the east. It is crosscut by late slickensided veins which are probably still associated with melange formation since they do not occur in the large bedded slices. The assemblage of non-flysch lithologies must be highly allochthonous and probably some flysch is also. An emplacement model consistent with field relations is proposed. Early in the history of emplacement of the Taconic allochthon a coherent slice of non-flysch lithologies and flysch was added in front of the detached but not completely transported Taconic allochthon. Afterwards little or no more flysch was accreted. The basal detachment essentially overrode the seafloor keeping the non-flysch lithologies and flysch slice at the thrust front. Severe disruption and melange formation resulted. The boundary between Western Exotic Melange and Folded and Faulted Flysch is the trace of the basal detachment. Only towards the final stage of shortening was flysch west of this basal detachment incipiently to mildly transported and deformed, the broad melange detachment imbricated, and younger flysch from below the main basal detachment brought up. Compilation of geological data between Saratoga Lake and Middletown finds that the belt of deformed flysch is continuous and that the basal detachment can be traced to the south into the Ellenville quadrangle. A piggy-back basin (Quassaic group) and a piece of Taconic allochthon (in the Goshen quadrangle) are tentatively identified. The Frontal Exotic Melange and the Taconic Frontal Thrust are the most continuous features. This continuity confirms suggestions of their late definition by previous workers. Between Saratoga Lake and the New York-Vermont state border, the belt of deformed flysch continues and can be traced into and correlated with the Champlain Thrust system of Vermont

Community fault model (CFM) for southern California

We present a new three-dimensional model of the major fault systems in southern California. The model describes the San Andreas fault and associated strikeslip fault systems in the eastern California shear zone and Peninsular Ranges, as well as active blind-thrust and reverse faults in the Los Angeles basin and Transverse Ranges. The model consists of triangulated surface representations (t-surfs) of more than 140 active faults that are defined based on surfaces traces, seismicity, seismic reflection profiles, wells, and geologic cross sections and models. The majority of earthquakes, and more than 95% of the regional seismic moment release, occur along faults represented in the model. This suggests that the model describes a comprehensive set of major earthquake sources in the region. The model serves the Southern California Earthquake Center (SCEC) as a unified resource for physics-based fault systems modeling, strong ground-motion prediction, and probabilistic seismic hazards assessment

Detailed 3D Fault Representations for the 2019 Ridgecrest, California, Earthquake Sequence

We present new 3D source fault representations for the 2019 M 6.4 and M 7.1 Ridgecrest earthquake sequence. These representations are based on relocated hypocenter catalogs expanded by template matching and focal mechanisms for M 4 and larger events. Following the approach of Riesner et al. (2017), we generate reproducible 3D fault geometries by integrating hypocenter, nodal plane, and surface rupture trace constraints. We used the southwest–northeast‐striking nodal plane of the 4 July 2019 M 6.4 event to constrain the initial representation of the southern Little Lake fault (SLLF), both in terms of location and orientation. The eastern Little Lake fault (ELLF) was constrained by the 5 July 2019 M 7.1 hypocenter and nodal planes of M 4 and larger aftershocks aligned with the main trend of the fault. The approach follows a defined workflow that assigns weights to a variety of geometric constraints. These main constraints have a high weight relative to that of individual hypocenters, ensuring that small aftershocks are applied as weaker constraints. The resulting fault planes can be considered averages of the hypocentral locations respecting nodal plane orientations. For the final representation we added detailed, field‐mapped rupture traces as strong constraints. The resulting fault representations are generally smooth but nonplanar and dip steeply. The SLLF and ELLF intersect at nearly right angles and cross on another. The ELLF representation is truncated at the Airport Lake fault to the north and the Garlock fault to the south, consistent with the aftershock pattern. The terminations of the SLLF representation are controlled by aftershock distribution. These new 3D fault representations are available as triangulated surface representations, and are being added to a Community Fault Model (CFM; Plesch et al., 2007, 2019; Nicholson et al., 2019) for wider use and to derived products such as a CFM trace map and viewer (Su et al., 2019)

Detailed 3D Fault Representations for the 2019 Ridgecrest, California, Earthquake Sequence

We present new 3D source fault representations for the 2019 M 6.4 and M 7.1 Ridgecrest earthquake sequence. These representations are based on relocated hypocenter catalogs expanded by template matching and focal mechanisms for M 4 and larger events. Following the approach of Riesner et al. (2017), we generate reproducible 3D fault geometries by integrating hypocenter, nodal plane, and surface rupture trace constraints. We used the southwest–northeast‐striking nodal plane of the 4 July 2019 M 6.4 event to constrain the initial representation of the southern Little Lake fault (SLLF), both in terms of location and orientation. The eastern Little Lake fault (ELLF) was constrained by the 5 July 2019 M 7.1 hypocenter and nodal planes of M 4 and larger aftershocks aligned with the main trend of the fault. The approach follows a defined workflow that assigns weights to a variety of geometric constraints. These main constraints have a high weight relative to that of individual hypocenters, ensuring that small aftershocks are applied as weaker constraints. The resulting fault planes can be considered averages of the hypocentral locations respecting nodal plane orientations. For the final representation we added detailed, field‐mapped rupture traces as strong constraints. The resulting fault representations are generally smooth but nonplanar and dip steeply. The SLLF and ELLF intersect at nearly right angles and cross on another. The ELLF representation is truncated at the Airport Lake fault to the north and the Garlock fault to the south, consistent with the aftershock pattern. The terminations of the SLLF representation are controlled by aftershock distribution. These new 3D fault representations are available as triangulated surface representations, and are being added to a Community Fault Model (CFM; Plesch et al., 2007, 2019; Nicholson et al., 2019) for wider use and to derived products such as a CFM trace map and viewer (Su et al., 2019)

The 2015 Fillmore Earthquake Swarm and Possible Crustal Deformation Mechanisms near the Bottom of the Eastern Ventura Basin, California

The 2015 Fillmore swarm occurred about 6 km west of the city of Fillmore in Ventura, California, and was located beneath the eastern part of the actively subsiding Ventura basin at depths from 11.8 to 13.8 km, similar to two previous swarms in the area. Template‐matching event detection showed that it started on 5 July 2015 at 2:21 UTC with an M ∼1.0 earthquake. The swarm exhibited unusual episodic spatial and temporal migrations and unusual diversity in the nodal planes of the focal mechanisms as compared to the simple hypocenter‐defined plane. It was also noteworthy because it consisted of >1400 events of M ≥ 0.0, with M 2.8 being the largest event. We suggest that fluids released by metamorphic dehydration processes, migration of fluids along a detachment zone, and cascading asperity failures caused this prolific earthquake swarm, but other mechanisms (such as simple mainshock–aftershock stress triggering or a regional aseismic creep event) are less likely. Dilatant strengthening may be a mechanism that causes the temporal decay of the swarm as pore‐pressure drop increased the effective normal stress, and counteracted the instability driving the swarm

The 2015 Fillmore Earthquake Swarm and Possible Crustal Deformation Mechanisms near the Bottom of the Eastern Ventura Basin, California

The 2015 Fillmore swarm occurred about 6 km west of the city of Fillmore in Ventura, California, and was located beneath the eastern part of the actively subsiding Ventura basin at depths from 11.8 to 13.8 km, similar to two previous swarms in the area. Template‐matching event detection showed that it started on 5 July 2015 at 2:21 UTC with an M ∼1.0 earthquake. The swarm exhibited unusual episodic spatial and temporal migrations and unusual diversity in the nodal planes of the focal mechanisms as compared to the simple hypocenter‐defined plane. It was also noteworthy because it consisted of >1400 events of M ≥ 0.0, with M 2.8 being the largest event. We suggest that fluids released by metamorphic dehydration processes, migration of fluids along a detachment zone, and cascading asperity failures caused this prolific earthquake swarm, but other mechanisms (such as simple mainshock–aftershock stress triggering or a regional aseismic creep event) are less likely. Dilatant strengthening may be a mechanism that causes the temporal decay of the swarm as pore‐pressure drop increased the effective normal stress, and counteracted the instability driving the swarm

Unified Structural Representation of the southern California crust and upper mantle

We present a new, 3D description of crust and upper mantle velocity structure in southern California implemented as a Unified Structural Representation (USR). The USR is comprised of detailed basin velocity descriptions that are based on tens of thousands of direct velocity (Vp, Vs) measurements and incorporates the locations and displacement of major fault zones that influence basin structure. These basin descriptions were used to developed tomographic models of crust and upper mantle velocity and density structure, which were subsequently iterated and improved using 3D waveform adjoint tomography. A geotechnical layer (GTL) based on Vs30 measurements and consistent with the underlying velocity descriptions was also developed as an optional model component. The resulting model provides a detailed description of the structure of the southern California crust and upper mantle that reflects the complex tectonic history of the region. The crust thickens eastward as Moho depth varies from 10 to 40 km reflecting the transition from oceanic to continental crust. Deep sedimentary basins and underlying areas of thin crust reflect Neogene extensional tectonics overprinted by transpressional deformation and rapid sediment deposition since the late Pliocene. To illustrate the impact of this complex structure on strong ground motion forecasting, we simulate rupture of a proposed M 7.9 earthquake source in the Western Transverse Ranges. The results show distinct basin amplification and focusing of energy that reflects crustal structure described by the USR that is not captured by simpler velocity descriptions. We anticipate that the USR will be useful for a broad range of simulation and modeling efforts, including strong ground motion forecasting, dynamic rupture simulations, and fault system modeling. The USR is available through the Southern California Earthquake Center (SCEC) website (http://www.scec.org)

Three-dimensional seismic velocity structure in the Sichuan basin, China

We present a new three‐dimensional velocity model of the crust in the eastern margin of the Tibetan Plateau. The model describes the velocity structure of the Sichuan basin and surrounding thrust belts. The model consists of 3‐D surfaces representing major geologic unit contacts and faults and is parameterized with Vp velocity‐depth functions calibrated using sonic logs. The model incorporates data from 1166 oil wells, industry isopach maps, geological maps, and a digital elevation model. The geological surfaces were modeled based on structure contour maps for various units from oil wells and seismic reflection profiles. These surfaces include base Quaternary, Mesozoic, Paleozoic, and Proterozoic horizons. The horizons locally exhibit major offsets that are compatible with the locations and displacements of important faults systems. This layered, upper crustal 3‐D model extends down to 10–15 km depth and illustrates lateral and vertical variations of velocity that reflect the complex evolution of tectonics and sedimentation in the basin. The model also incorporates 3‐D descriptions of Vs and density for sediments that are obtained from empirical relationships with Vp using direct measurements of these properties in borehole logs. To illustrate the impact of our basin model on earthquake hazards assessment, we use it to calculate ground motions and compare these with observations for the 2013 Lushan earthquake. The result demonstrates the effects of basin amplification in the western Sichuan basin. The Sichuan CVM model is intended to facilitate fault systems analysis, strong ground motion prediction, and earthquake hazards assessment for the densely populated Sichuan region.Published versio