SUMMARY Seismic and geodetic examinations of the Hikurangi subduction zone (HSZ) indicate a remarkably diverse and complex system. Here, we investigate the 3-D P-wave velocity structure of the HSZ by applying an iterative, nested regional-global tomographic algorithm. The new model reveals enhanced details of seismic variations along the HSZ. We also relocate over 57 000 earthquakes using this newly developed 3-D model and then further improve the relative locations for 75 per cent of the seismicity using waveform cross-correlation. Double seismic zone characteristics, including occurrence, depth distribution and thickness change along the strike of the HSZ. An aseismic but fast Vp zone separates the upper and lower planes of seismicity in the southern and northern North Island. The upper plane of seismicity correlates with low Vp zones below the slab interface, indicating fluid-rich channels formed on top and/or within a dehydrated crust. A broad low Vp zone is resolved in the lower part of the subducting slab that could indicate hydrous mineral breakdown in the slab mantle. In the northern North Island and southern North Island, the lower plane of seismicity mostly correlates with the top of these low Vp zones. The comparison between the thermal model and the lower plane of seismicity in the northern North Island supports dehydration in the lower part of the slab. The mantle wedge of the Taupo volcanic zone (TVZ) is characterized by a low velocity zone underlying the volcanic front (fluid-driven partial melting), a fast velocity anomaly in the forearc mantle (a stagnant cold nose) and an underlying low velocity zone within the slab (fluids from dehydration). These arc-related anomalies are the strongest beneath the central TVZ with known extensive volcanism. The shallow seismicity (<40 km depth) correlates with geological terranes in the overlying plate. The aseismic impermeable terranes, such as the Rakaia terrane, may affect the fluid transport at the plate interface and seismicity in the overlying plate, which is consistent with previous studies. The deep slow slip events (25–60 km depths) mapped in the Kaimanawa, Manawatu and Kapiti regions coincide with low Vp anomalies. These new insights on the structure along the HSZ highlight the change in the locus of seismicity and dehydration at depth that is governed by significant variations in spatial and probably temporal attributes of subduction zone processes.
Abstract This study utilizes Interferometric Synthetic Aperture Radar (InSAR) to examine subsidence along the coastal strip of the Miami barrier islands from 2016 to 2023. Using Sentinel‐1 data, we document vertical displacements ranging from 2 to 8 cm, affecting a total of 35 coastal buildings and their vicinity. About half of the subsiding structures are younger than 2014 and at the majority of them subsidence decays with time. This correlation suggests that the subsidence is related to construction activities. In northern and central Sunny Isles Beach, where 23% of coastal structures were built during the last decade, nearly 70% are experiencing subsidence. The majority of the older subsiding structures show sudden onset or sudden acceleration of subsidence, suggesting that this is due to construction activities in their vicinity; we have identified subsidence at distance of 200 m, possibly up to 320 m, from construction sites. We attribute the observed subsidence to load‐induced, prolonged creep deformation of the sandy layers within the limestone, which is accelerated, if not instigated, by construction activities. Distant subsidence from a construction site could indicate extended sandy deposits. Anthropogenic and natural groundwater movements could also be driving the creep deformation. This study demonstrates that high‐rise construction on karstic barrier islands can induce creep deformation in sandy layer within the limestone succession persisting for a decade or longer. It showcases the potential of InSAR technology for monitoring both building settlement and structural stability.
Abstract The Eastern Aleutian-Alaska Subduction Zone (EAASZ) manifests significant along-strike variations in structure and geometry. The limited spatial resolution in intermediate-depth earthquake locations precludes investigation of small-scale variations in seismic characteristics. In this study, we use an existing 3D seismic velocity model and waveform cross-correlation data to relocate the earthquakes in 2016 near the EAASZ. Our improved absolute and relative earthquake locations reveal complex spatial characteristics of double seismic zones (DSZs). There are significant variations in location, depth, layer separation, and length of the DSZs along the EAASZ. We also observe nonuniform layer separations along the slope of the subducting slab that may imply either rheological or crustal thickness variations. In addition, our results suggest a triple seismic zone (TSZ) beneath Kenai. The interplay among different factors, including dehydration of metamorphic facies, intraslab stress, preexisting structures, and abrupt changes in slab geometry, may explain the observed variations in seismogenesis of the DSZs and TSZs. The comparison of our relocated seismicity with the thermal model for the slab beneath Cook Inlet shows that the intermediate-depth earthquakes occur between 500°C and 900°C isotherms. The 2016 Mw 7.1 Iniskin earthquake and its aftershocks are located at ∼800°C–900°C. The intricate small-scale variations in different characteristics of the DSZs and intermediate-depth seismicity and their correlations with major geometrical and physical controls can provide insight into what governs the seismogenesis of subduction-induced earthquakes.
The focal mechanism provides seismological constraints on the geological faults that generate the earthquakes and thus is important for regional seismotectonic research. Focal mechanism calculation based on the P-wave first-motion-polarity is a widely used method, particularly helpful for small to moderate-size earthquakes. However, determining the P-wave first-motion polarity can be challenging and subjective for smaller earthquakes. Here, we propose a deep-learning method (EQpolarity) for determining the P-wave first-motion polarity using the vertical-component seismic waveforms. The proposed deep-learning method was trained using a large-scale dataset from South California and then adapted to the Texas earthquake data via a transfer learning method. The original and secondary models obtained 95.43% and 98.82% accuracy on the Texas database, respectively, indicating the effectiveness of transfer learning. We further apply the deep learning method to thousands of events on the TexNet catalog to determine the focal mechanisms. Most of the focal mechanism solutions align well with the strikes, dips, and rakes of the known faults that were explored previously using full-waveform-based methods. The generation of the large focal mechanism database offers significant insights into the seismotectonic status of West Texas. The open-source package of EQpolarity can be accessed at https://github.com/chenyk1990/eqpolarity.
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