Ductile Lower Crust Research Articles

Seismotectonic segmentation controlled by magmatic underplating in the central-southern segment of Tanlu fault zone, eastern China

Intracontinental earthquakes usually cause severe casualties and property damage, yet their mechanism is still unclear. The TanLu Fault Zone (TLFZ) has been a typical large intracontinental strike-slip fault in eastern China since the Quaternary, which has hosted many large earthquakes (M ≥ 6). Generation of large earthquakes is generally closely related to the heterogeneities on and adjacent to the fault plane, which may result from different composition and inclusions of fluids. In this study, we use the receiver function method to obtain detailed distributions of Poisson's ratio and crustal thickness in the central-southern segment of the TLFZ, where the M 8.5 TanCheng earthquake occurred in 1668. We found that the distribution of earthquakes is strongly correlated with the crustal structures, both of which exhibit clear segmentation. The strongly damaged zone (with earthquake intensity ≥ 10) of the 1668 TanCheng earthquake and present small-to-moderate earthquakes are generally located above a segment that has shallower Moho and particularly higher Poisson's ratio. We propose that the high Poisson's ratio may be caused by intrusive mafic rocks in the ductile lower crust, which plays a role as local stress concentrators and subsequently leads to large intracontinental earthquakes.

The Crustal and Uppermost Mantle Vs Structure of the Middle and Lower Reaches of the Yangtze River Metallogenic Belt: Implications for Metallogenic Process

AbstractThe Middle and Lower Reaches of the Yangtze River metallogenic belt (MLYMB) is one of the most important Fe‐Cu polymetallic belts in China. However, the mechanism and deep geodynamical process for the formation of this belt are still controversial. Here, we obtain the crustal and the uppermost mantle structures using ambient noise data from a dense seismic profile. A low velocity zone is revealed beneath the Moho of MLYMB, interpreted as the source of the deep mineralization materials. In addition, a low velocity layer (LVL) and a high velocity layer (HVL) are observed in the crust of the southern segment of the profile. The LVL is interpreted as a tectonic detachment layer between the upper and the lower crust, and the HVL is interpreted as the aggregation zone for mineralizing melts or crystallized magma chambers. Based on the observed velocity features, we propose a three‐stage model for the formation of ore deposits in MLYMB. Our model suggests that an upwelling of asthenosphere triggered by the delamination of a previously thickened lithosphere leads to the partial melting of upper mantle rocks, which eventually ponders under the Moho. The magma then infiltrates through the ductile lower crust and reaches a depth of ∼7–13 km, forming a minerals‐enriched magma chamber. Minerals‐rich hot fluids originating from the magma chamber continue to move upward along the pre‐existent faults and the minerals finally precipitate in dense veinlets when reaching shallow depths, forming the ore deposits in and around the MLYMB.

Viscoelastic Fault-Based Model of Crustal Deformation for the 2023 Update to the U.S. National Seismic Hazard Model

Abstract The 2023 update to the National Seismic Hazard (NSHM) model is informed by several deformation models that furnish geodetically estimated fault slip rates. Here I describe a fault-based model that permits estimation of long-term slip rates on discrete faults and the distribution of off-fault moment release. It is based on quantification of the earthquake cycle on a viscoelastic model of the seismogenic upper crust and ductile lower crust and mantle. I apply it to a large dataset of horizontal and vertical Global Positioning System (GPS) interseismic velocities in the western United States, resulting in long-term slip rates on more than 1000 active faults defined for the NSHM. A reasonable fit to the GPS dataset is achieved with a set of slip rates designed to lie strictly within a priori geologic slip rate bounds. Time-dependent effects implemented via a “ghost transient” have a profound effect on slip rate estimation and tend to raise calculated slip rates along the northern and southern San Andreas fault by up to several mm/yr.

Rotational Extension Promotes Coeval Upper Crustal Brittle Faulting and Deep‐Seated Rift‐Axis Parallel Flow: Dynamic Coupling Processes Inferred From Analog Model Experiments

AbstractThe lower parts of warm, thick continental crust can flow in a ductile fashion to accommodate thinning of the upper brittle crust during extension. Naturally occurring continental rifts with a rift‐axis parallel deformation gradient imply an underlying rotational component. In such settings, rift‐parallel crustal flow transports material perpendicular to the direction of rifting. We use analog experiments to investigate rotational rifting and coeval crustal flow. To test the effect of rift‐axis parallel flow on rift evolution, we use different gravitational loads resulting in a range of horizontal pressure gradient magnitudes which drive horizontal lower‐crustal flow. The use of (three dimensional) 3D Digital Volume Correlation techniques on X‐ray CT data combined with 3D Digital Image Correlation techniques applied to topographic stereo images provides detailed insights on the contemporaneous evolution of ductile flow patterns and brittle rift structures, respectively. Our results depict a complex flow field in the ductile lower crust during rotational rifting with: (a) extension‐parallel horizontal inward flow and vertical upward flow that compensates thinning of the brittle upper crustal layer; (b) rift‐axis parallel lateral flow, that compensates greater amounts of thinning further away from the rotation axis; and (c) different degrees of mechanical coupling between the brittle and viscous layers that change during rift propagation. Our analog experiments provide insights into ductile lower crustal flow patterns during rift evolution. The results emphasize the three dimensionality of rifting, which is an important effect that should be considered when estimating the amount of crustal extension from two dimensional (2D) cross sections.

Late Quaternary Fault Slip Rate Within the Qilian Orogen, Insight Into the Deformation Kinematics for the NE Tibetan Plateau

AbstractVarious kinematic models have been proposed to explain the growth process of wide compressive orogens, and the deformation rate within the entire range of the orogen (rather than that of the leading‐edge fault) provides valuable information for model evaluation. Within the Qilian Orogen, between the central and northern Qilian Shan mountain in China, we studied the deformation of a series of Quaternary fluvial terraces across the North Tuolai Shan Fault (NTF), including optically stimulated luminescence dating of terrace ages. The results show that the six levels of terraces (T6 to T1) were formed at 58.4 ± 4.4 ka, 53 ± 5 ka, 46 ± 5 ka, 38 ± 5 ka, 23.2 ± 1.8 ka, and 4.8 ± 0.4 ka, respectively. Field investigations revealed that these terrace surfaces are faulted and folded by the NTF. Based on the deformation and ages of the terraces across the fault, the vertical slip rate is estimated as 1.5 ± 0.1 mm/yr, and the shortening rate is 0.9 ± 0.1 mm/yr, with a fault dip of 58°–60°. This deformation rate is similar to rates determined along the leading‐edge fault in the northern Qilian Shan and along the fault in the southern Qilian Shan. The widely distributed shortening across the ∼300‐km‐wide Qilian Orogen supports the kinematics of distributed upper crustal shortening across the Qilian Orogen, with a ductile lower crust, rather than a large, gradually propagating crustal‐scale deformation wedge.

The influence of variations in crustal composition and lithospheric strength on the evolution of deformation processes in the southern Central Andes: insights from geodynamic models

Deformation in the orogen-foreland system of the southern Central Andes between 33° and 36° S varies in style, locus, and amount of shortening. The controls that determine these spatially variable characteristics have largely remained unknown, yet both the subduction of the oceanic Nazca plate and the strength of the South American plate have been invoked to play a major role. While the parameters governing the subduction processes are similar between 33° and 36° S, the lithospheric strength of the upper plate is spatially variable due to structures inherited from past geodynamic regimes and associated compositional differences in the South American plate. Regional Mesozoic crustal horizontal extension generated a < 40-km-thick crust with a more mafic composition in the lower crust south of 35°S; north of this latitude, however, a more felsic lower crust is inferred from geophysical data. To assess the influence of different structural and compositional heterogeneities on the style of deformation in the southern Central Andes, we developed a suite of geodynamic models of intraplate lithospheric shortening for two E–W transects (33° 40′ S and 36° S) across the Andes. The models are constrained by local geological and geophysical information. Our results demonstrate a decoupled shortening mode between the brittle upper crust and the ductile lower crust in those areas characterized by a mafic lower crust (36° S transect). In contrast, a more felsic lower crust, such as in the 33° 40′ S transect, results in a coupled shortening mode. Furthermore, we find that differences in lithospheric thickness and the asymmetry of the lithosphere–asthenosphere boundary may promote the formation of a crustal-scale, west-dipping detachment zone that drives the overall deformation and lateral expansion of the orogen. Our study represents the first geodynamic modeling effort in the southern Central Andes aimed at understanding the roles of heterogeneities (crustal composition and thickness) at the scale of the entire lithosphere as well as the geometry of the lithosphere–asthenosphere boundary with respect to mountain building.

Source rupture model of the 2018 MW6.7 Iburi, Hokkaido earthquake from joint inversion of strong motion and InSAR observations

The 2018 MW6.7 Iburi earthquake shocked the eastern Iburi region to the west of the Hidaka Collision Zone in Hokkaido, which is a destructive inland earthquake. We resolved the kinematic rupture process of the event by combining strong motions (SM) and synthetic aperture radar (SAR) images in a joint inversion. The results reveal that the duration of the whole rupture is about 17s, yielding a total seismic moment of 1.4×1019 N·m (MW=6.7). The main slip area is located at a depth of approximately 24 km with a peak slip of ∼0.8m above the hypocenter. The comparison with the regional velocity model shows the earthquake was initiated in the upper mantle, while the majority of slips are located in the lower crust, which is an “aseismic” domain in the typical sandwich model. The location of the major slip area is consistent with a high-conductivity volume. We proposed a mechanism of low frictional property (<0.3) produced by high pore pressure to explain the abnormal high dip angle and centroid depth located in the ductile lower-crust. Aftershocks are distributed in areas where the Coulomb frictional stress increases due to co-seismic displacement with a mechanism conjugating to the mainshock.

Low-frequency earthquakes observed in close vicinity of repeating earthquakes in the brittle upper crust of Hakodate, Hokkaido, northern Japan

SUMMARY We conducted a detailed investigation of an earthquake cluster distributed from the lower crust to the upper crust beneath Hakodate, Hokkaido, which included both low-frequency earthquakes (LFEs) and regular earthquakes. Relocated hypocentres clearly show that both the LFEs and regular earthquakes occurred close to each other in the brittle upper crust of this non-volcanic area, while only LFEs occurred in the lower crust. This observation indicates that LFEs can occur not only in the ductile lower crust, but also in the brittle upper crust, which suggests that LFEs can occur in an environment similar to that of regular earthquakes. Regular earthquakes that occur in close vicinity of LFEs have very similar waveforms and nearly overlapping source regions, which indicate that they reflect the repeated rupture of the same asperity patch on a fault. Temporally, the intervals between events in the repeating earthquake sequence were very short, thus suggesting that they were caused by a sudden increase in pore pressure. The cluster of LFEs and repeating earthquakes, which has a rod-like distribution extending from the bottom of the crust to the surface and tilted slightly eastward, might represent a pathway of aqueous fluid movement sourced from the subducting slab.

Helium and carbon isotopic signatures of thermal spring gases in southeast Yunnan, China

Late Palaeozoic and Cenozoic magmatism, including Pingbian Quaternary basaltic volcanoes, occurs in southeast Yunnan, China, where seismic tomography evidence indicates magma activity in the uppermost mantle. However, the characteristics of mantle fluid release remain poorly understood. Here, we report the chemical compositions of hot spring gases in SE Yunnan and measure the 3He/4He ratios, 4He/20Ne ratios, and δ13C-values (CO2, CH4) to determine their origins and the factors influencing their migration. Except for five samples contaminated by the atmosphere (4He/20Ne ratios, 0.39 ̶2.39), the Rc/Ra ratios (Rc is the air-corrected 3He/4He ratio, Ra is the air ratio with 1.4 × 10−6) range from 0.042 Ra to 0.470 Ra, indicating that the helium is mostly of crustal origin with minor addition of mantle-derived helium. The calculated maximum proportion of mantle helium was ~5% of total helium, indicating that mantle 3He leakage correlates with the ductile lower crust. The hot spring gases are fractionated by the partial dissolution of CO2 in groundwater, and the initial δ13CCO2 values are modelled by Rayleigh fractionation within a range of −26.5‰ to −6.14‰. The modelled CO2 in samples estimated by the HeC isotope coupling model originates from the mixing of limestone and organic sediment end-members. The isotopic signature of CH4 (−69.2‰ to 35.9‰) suggests a thermogenic source and microbial oxidation in some samples. Furthermore, the apparent isotopic temperatures are estimated using fractionation factors for CH4 and CO2, suggesting a lack of magmatism in the crust beneath the Red River fault zone (188–318 °C), but partial melting under the Bozhushan granitoid plutons (572–688 °C). The low emission of mantle-derived helium is attributed to the low fault permeability and minor active crustal deformation in present-day SE Yunnan, which restrain the upward migration of mantle fluids.

Spatial Variations in Crustal and Mantle Anisotropy Across the North American‐Caribbean Boundary on Haiti

AbstractHaiti, on the island of Hispaniola, is situated across the North American‐Caribbean plate boundary at the transition point between oblique subduction in the east and a transform plate boundary in the west. Here we use shear wave splitting measurements from S waves of local (0–50 km) and intermediate depth (50–150 km) earthquakes as well as SK(K)S phases from teleseismic earthquakes to ascertain good spatial and vertical resolution of the azimuthal anisotropic structure. This allows us to place new constraints on the pattern of deformation in the crust and mantle beneath this transitional region. SK(K)S results are dominated by plate boundary parallel (E‐W) fast directions with ~1.9 s delay times, indicating subslab trench parallel mantle flow is continuing westward along the plate boundary. Intermediate depth earthquakes originating within the subducting North American plate show a mean fast polarization direction of 065° and delay time of 0.46 s, subparallel to the relative plate motion between the Caribbean and North American plates (070°). We suggest a basal shear zone within the lower ductile crust and upper lithospheric mantle as being a potential major source of anisotropy above the subducting slab. Upper crustal anisotropy is isolated using shear wave splitting measurements on local seismicity, which show consistent delay times on the order of 0.2 s. The fast polarization directions indicate that the crustal anisotropy is controlled by the fault networks in close proximity to the major strike‐slip faults, which bisect the north and south of Haiti, and by the regional stress field where faulting is less pervasive.

A New Uniform Moment Tensor Catalog for Yunnan, China, from January 2000 through December 2014

AbstractBecause of the collision of the Indian and Eurasian tectonic plates, the Yunnan Province of southwestern China has some of the highest levels of seismic hazard in the world. In such a region, a catalog of moment tensors is important for estimating seismic hazard and helping understand the regional seismotectonics. Here, we present a new uniform catalog of moment tensor solutions for the Yunnan region. Using a grid-search technique to invert seismic waveforms recorded by the permanent regional network in Yunnan and the 2 yr ChinArray deployment, we present 1833 moment tensor solutions for small-to-moderate earthquakes that occurred between January 2000 and December 2014. Moment magnitudes in the new catalog vary from Mw 2.2 to 6.1, and the catalog is complete above Mw∼3.5–3.6. The moment tensors are constrained to be purely double-couple and show a variety of faulting mechanisms. Normal faulting events are mainly concentrated in northwest Yunnan, while farther south along the Sagaing fault the earthquakes are mostly thrust and strike slip. The remaining area includes all three styles of faulting but mostly strike slip. We invert the moment tensors for the regional stress field and find a strong correlation between spatially varying maximum horizontal stress and Global Positioning System observations of horizontal ground velocity. The stress field reveals clockwise rotation around the eastern Himalayan syntaxis, with northwest–southeast compression to the east of the Red River fault changing to northeast–southwest compression west of the fault. Almost 88% of the centroid depths are shallower than 16 km, consistent with a weak and ductile lower crust.

Crustal structure and magmatic evolution in the Pearl River Delta of the Cathaysia Block: New constraints from receiver function modeling

We utilized receiver functions to study the crustal structure beneath the Pearl River Delta in the southwestern part of the Cathaysia Block, which was characterized by widespread tectono-magmatism during the late Mesozoic. Combination of waveform modeling and H-k stacking was used to derive crustal thickness, Vp/Vs ratio and average P-wave velocity. The results show that the crustal thickness ranges from 26 km in the center of the Pearl River Delta to 32 km in the periphery; the Vp/Vs ratio is between ~1.70 and 1.74. The average P-wave velocity ranges from 6.4 to 6.8 km/s and shows higher value beneath the islands along the Lianhuashan Fault zone. A weakly positive correlation between the crustal thickness and Vp/Vs ratio is observed in southwestern Pearl River Delta, indicating depth-dependent extension of the continental crust, that is, ductile lower crust was stretched to a much higher degree in comparison with the brittle felsic upper crust. The crust in the northeastern Pearl River Delta may be more basaltic owing to underplating caused by the subduction of the Paleo-Pacific Plate, and thus a relatively high Vp/Vs ratio is observed there. The overall low crustal Vp/Vs ratio and the relatively higher crustal P-wave velocities beneath the Lianhuashan Fault zone may imply a felsic-intermediate composition and high metamorphic grade of the crust along the fault. The Lianhuashan Fault zone may provide a pathway for rapid magmatic transportation from the underplating beneath the crust to the surface.

Seismicity of the Askja and Bárðarbunga volcanic systems of Iceland, 2009–2015

A large seismic network deployed in the Icelandic highlands recorded >100,000 earthquakes from 2009 to 2015. We develop a local magnitude scale, appropriate for use in central Iceland, which is similar to the scale used by the Iceland Meteorological Office. Using this large catalogue of earthquakes, we analyze the spatial and temporal changes in seismicity rates and b-values. In microearthquakes recorded from the usually ductile lower crust we find that b-values are high, reflecting the presence of high thermal gradients and low stresses driving seismicity associated with the movement of melt. In contrast, b-values in the upper crust are variable. Low b-values, indicative of a high stress environment, are observed during seismic swarms such as those around Mt. Herðubreið and around Bárðarbunga caldera. A persistently seismically active area around a geothermal area within Askja caldera has a b-value around 1 but has a strong annual cycle of seismicity. We attribute the annual cycle to varying load from the snow cover modulating the seismicity. Seismicity driven by the intrusion of a large dyke has a b-value well above 1, driven by the high pore fluid pressures and thermal gradients around the dyke.

Right Lateral Shear and Rotation in the Northeast of the Arabian-Iranian Collision Zone

Accommodation of continental convergence by crustal thickening and lateral transport is mainly featured as strike-slip faulting along the trends roughly orthogonal to the orientation of plate convergence. This style of faulting will affect seismicity of the involving areas which can be proved in low seismic zones by determining regional stress pattern using numerical methods. Accordingly, the stress distribution and deformation pattern of the South Sanandaj-Sirjan zone in the northeastern part of the Iranian-Arabian collision zone is investigated here using a three dimensional mechanical model. The modeled area is bounded between the Zagros thrust fault on the west and Dehshir-Baft fault in the east. The model is composed of three layers: the upper two layers represent the upper brittle and lower ductile crust of the collided continent and the lowest layer represents the lithospheric mantle. The upper crust behaves as an elastic material while the lower crust is considered as a non-Newtonian viscous fluid layer. The lithospheric mantle is taken as a low-viscosity material which is not allowed to move in any direction relative to the overlying layers. The Zagros thrust fault was treated with two different dip values saying 90o and 45o but Dehshir-Baft fault was modeled as a vertical fault and allowed to have a dextral movement regarding to the existing evidence. The driving mechanism applied to the western side of the model was chosen considering two different approaches including a kinematic approach (the Arabian-Eurasian convergence velocity; 35 mm/yr) and a dynamic approach (an external boundary force equal to 3.55E+17 N). The resulted stress field indicates an orogen-parallel component of right lateral shear along the Zagros fault implying a rotational deformation pattern within the modeled region that suggests a stress partitioning in the study area. The pattern also indicates a stress accumulation towards the south which could be a reason for the regional seismic quiescence between the two seismic Zagros thrust and Dehshir-Baft faults. Based on the present modeling results, it seems that high stress localization on the boundary faults can be a support of block structure approach or quasi-rigid blocks deformation within the study area. The resultant patterns of stress and displacement fields are generally totally comparable with plate boundary shear zones and have been proven by field data.

The T‐Reflection and the Deep Crustal Structure of the Vøring Margin, Offshore mid‐Norway

AbstractSeismic reflection data along volcanic passive margins frequently provide imaging of strong and laterally continuous reflections in the middle and lower crust. We have completed a detailed 2‐D seismic interpretation of the deep crustal structure of the Vøring Margin, offshore mid‐Norway, where high‐quality seismic data allow the identification of high‐amplitude reflections, locally referred to as the T‐Reflection. Using a dense seismic grid, we have mapped the geometry of the T‐Reflection in order to compare it with filtered Bouguer gravity anomalies and seismic refraction data. The T‐Reflection is identified between 7 and 10 s. Sometimes it consists of one single smooth reflection. However, it is frequently associated with a set of rough multiple reflections displaying discontinuous segments with varying geometries, amplitudes, and contact relationships. The T‐Reflection seems to be connected to deep sill networks and is locally identified at the continuation of basement high structures or terminates over fractures and faults. The T‐Reflection presents a low magnetic signal. The spatial correlation between the filtered positive Bouguer gravity anomalies and the deep dome‐shaped reflections indicates that the latter represent a high‐impedance boundary contrast associated with a high‐density and high‐velocity body. In ~50% of the outer Vøring Margin, the depth of the mapped T‐Reflection is found to correspond to the depth of the top of the Lower Crustal Body (LCB), which is characterized by high P wave velocities (&gt;7 km/s). We present a tectonic scenario, where a large part of the deep crustal structure is composed of preserved upper continental crustal blocks and middle to lower crustal lenses of inherited high‐grade metamorphic rocks. Deep intrusions into the faulted crustal blocks are responsible for the rough character of the T‐Reflection, whereas intrusions into the ductile lower crust and detachment faults are likely responsible for its smoother character. Deep magma intrusions can be responsible for regional metamorphic processes leading to an increasing velocity of the lower crust to more than 7 km/s. The result is a heterogeneous LCB that likely represents a complex mixture of pre‐ to syn‐breakup mafic and ultramafic rocks (cumulates and sills) and old metamorphic rocks such as granulites and eclogites. An increasing degree of melting toward the breakup axis is responsible for an increasing proportion of cumulates and sill intrusions in the lower crust.

Lithospheric flexural strength and effective elastic thicknesses of the Eastern Anatolia (Turkey) and surrounding region

The Lithospheric structure of Eastern Anatolia and the surrounding region, including the northern part of the Arabian platform is investigated via the analysis and modeling of Bouguer anomalies from the Earth Gravitational Model EGM08. The effective elastic thickness of the lithosphere (EET) that corresponds to the mechanical cores of the crust and lithospheric mantle is determined from the spectral coherence between Bouguer anomalies and surface elevation data. Its average value is 18.7km. From the logarithmic amplitude spectra of Bouguer anomalies, average depths of the lithosphere-asthenosphere boundary (LAB), Moho, Conrad and basement in the study area are constrained at 84km, 39km, 16km and 7km, respectively. The geometries of the LAB and Moho are then estimated using the Parker-Oldenburg inversion algorithm. We also present a lithospheric strength map obtained from the spatial variations of EET determined by Yield Stress Envelopes (YSE). The EET varies in the range of 12–23km, which is in good agreement with the average value obtained from spectral analysis. Low EET values are interpreted as resulting from thermal and flexural lithospheric weakening. According to the lithospheric strength of the Eastern Anatolian region, the rheology model consists of a strong but brittle upper crust, a weak and ductile lower crust, and a weak lower part of the lithosphere. On the other hand, lithosphere strength corresponds to weak and ductile lower crust, a strong upper crust and a strong uppermost lithospheric mantle for the northern part of the Arabian platform.

Analysis of the seismicity in central Tibet based on the SANDWICH network and its tectonic implications

We have located a total of 232 local earthquakes using data recorded by the SANDWICH seismic network from November 2013 to October 2014 in central Tibet across the Bangong-Nujiang suture (BNS). The focal depths of all earthquakes are shallower than 30km and therefore are in the upper crust. The absence of lower crust earthquakes may imply a weak, ductile lower crust in central Tibet. Moreover, these earthquakes are dispersed throughout conjugate strike-slip fault zones, indicating that evenly distributed upper crustal deformation might predominate in central Tibet. This observation agrees with the hypothesis that conjugate fault zones accommodate coeval east-west extension and north-south contraction via continuous deformation. Moreover, the focal mechanisms show that strike-slip and normal faulting are the dominant types of deformation and that the extension in central Tibet is oriented approximately east-west. Despite some anomalies, the kinematics implied by most of the focal mechanisms correlate well with those of the surface structures.

Northern California Seismic Attenuation: 3DQPandQSModels

The geological complexity of northern California and its accreted terranes produces pronounced regional variations in the attenuation (1/ Q ) of earthquake waves. Using local earthquakes, 3D Q P and Q S models have been obtained for this region that includes the San Andreas fault system, the Great Valley, the Sierra Nevada batholith, and the Gorda subduction volcanic system. Path attenuation t * values were determined from P and S spectra of 959 spatially distributed earthquakes, magnitude 2.5–6.0, using 1254 stations from Northern California Earthquake Data Center networks and Mendocino and Sierra Nevada temporary arrays. The t * data were used in Q inversions, using existing 3D velocity models and basic 10 km node spacing. Uneven data coverage was accommodated with linking nodes into larger areas for useful Q images across the 3D volume. Results at shallow depth show very low Q in the Sacramento Delta, the Eureka area, and parts of the San Francisco Bay area. In the brittle crust, low Q tends to be associated with regions of fault zones and microearthquakes. In the ductile lower crust, low Q is observed below fault zones with large cumulative displacement and inferred grain size reduction, with apparent broad ductile shear zones across the fault system in the Coast Ranges. Underlying active arc volcanoes, low Q features are apparent below 20 km depth. Under the Long Valley caldera, two distinct partial melt features at ∼3 and ∼15 km depth are inferred. Moderately high Q is associated with Sierra Nevada and Salinian igneous rocks, whereas the Franciscan subduction complex shows moderately low Q . Prominent high Q relates to the Great Valley ophiolite. The pattern of path‐specific attenuation rate, from 3D Q , is generally similar to the pattern of psuedospectral acceleration residuals for the South Napa earthquake, indicating that it may be beneficial to incorporate 3D Q models into future ground‐motion prediction equations for northern California. Online Material: Figures of Q P and Q S resolution and station delays for t *, and table of Q P , V P , Q S , and V S .

Localized seismic deformation in the upper mantle revealed by dense seismic arrays.

Seismicity along continental transform faults is usually confined to the upper half of the crust, but the Newport-Inglewood fault (NIF), a major fault traversing the Los Angeles basin, is seismically active down to the upper mantle. We use seismic array analysis to illuminate the seismogenic root of the NIF beneath Long Beach, California, and identify seismicity in an actively deforming localized zone penetrating the lithospheric mantle. Deep earthquakes, which are spatially correlated with geochemical evidence of a fluid pathway from the mantle, as well as with a sharp vertical offset in the lithosphere-asthenosphere boundary, exhibit narrow size distribution and weak temporal clustering. We attribute these characteristics to a transition from strong to weak interaction regimes in a system of seismic asperities embedded in a ductile fault zone matrix.

Earthquakes get a more flexible source

Geophysics Earth's surface deforms in part as a result of ruptures along brittle crustal faults that generate earthquakes. Understanding rock deformation in the ductile lower crust and mantle is challenging. Using the densest seismic arrays in the world, Inbal et al. have found an unexpected