Local Seismicity Research Articles

Cryoseismology of the severnaya zemlya archipelago – the first results of permanent monitoring

We present the first results of local seismicity monitoring of the Severnaya Zemlya archipelago for the end of 2016 to 2023, recorded by a single permanent seismic station installed on Bolshevik Island. 73 local seismic events with P- and S-phases have been identified. The possibility of their sorting (earthquake or icequake) by comparing their waveforms and time-frequency analysis with regional earthquakes that occurred in the archipelago area is considered. The spatial and temporal sequence and migration rate of the events show that glacial-type events can be the result of stress discharge in glaciers under the impact of shallow crustal earthquakes within a radius of ~30 km. It is shown how, given the difficulty of deploying a seismic network, even a single permanent seismic station can provide useful information on glacial and crustal earthquakes.

FEATURES OF GEOLOGICAL STRUCTURE AND PROSPECTS OF OIL-AND-GAS BEARING CAPACITY OF THE RIPHEAN AND VENDIAN DEPOSITS IN THE SOUTHEASTERN PART OF THE KAMO ARCH OF THE BAYKIT ANTICLISE

The paper presents the results of the integrated study of the features of geological structure of the southeastern Kamo arch of the Baikit anticlise. Use has been made of both already published and unpublished geological-geophysical data obtained through seismic survey using the common depth point (CDP 2D) and the side view seismic location (SVSL) methods. The conducted research made it possible to improve understanding of the geological structure and formation conditions of the terrigenous-carbonate rocks of the Lower Riphean (Madrа, Yurubchen and Dolgokit formations) and Vendian (Vanavara and Oskoba formations) Kamo Group in the southern Kamo arch of the Baykit anticlese in terms of the prospects for the discovery of hydrocarbon (HC) deposits therein. The interpretation of seismic data (CDP and SVSL) provided the possibility to specify the thickness of the Lower Riphean terrigenous-carbonate rocks, most promising for hydrocarbons, to generate the predictive geology map of the pre-Vendian erosion surface, and to map the HC-promising Early Riphean and Vendian deposit thickness. Along with the previously explored large HC deposits in the central Kamo arch of the Baykit anticlise, the new data presented here testify in favor of the structural-and tectonic conditions for the development of the Lower Riphean and Vendian rocks on the southeastern slope of the Kamo arch of the Baykit anticlise and imply their high potential.

An adaptive 6-dimensional floating-search multi-station seismic-event detector (A6-DFMSD) and its application to low-frequency earthquakes in the East Eifel Volcanic Field, Germany

We introduce a seismic event detector that applies signal analysis in the time and frequency domains. Signals are searched for with matching coincidences at neighbouring recording stations in space and time. No a priori waveform information is needed for the Adaptive 6-Dimensional Floating-search Multi-station Seismic-event Detector (A6-DFMSD). It combines a short / long time average algorithm (STA/LTA), frequency range selection, energy envelope matching, and backprojection techniques to find a robust detection model. As a challenging test example, the new detector is tuned and applied to a dataset with five months of microearthquake (ML < 2) recordings in the East Eifel Volcanic Field (EEVF), Germany. There, both magmatic and tectonic earthquakes occur in a depth range between 3 km and 43 km. A6-DFMSD detected 4.3 times as many events as were already known and it discovered a previously unknown event cluster. After manual localization and classification of the events, we show that A6-DFMSD finds events of different origins: tectonic, magmatic, atmospheric, and anthropogenic. In particular, low-frequency (LF) earthquakes of magmatic origin with a complicated waveform coda are very well identified. We suggest that seismological networks monitoring local seismicity in similar target zones would benefit from the use of A6-DFMSD to allow the detection of a wide range of different seismic signals.

Decoupled deformation between crust and mantle beneath Indo-Burmese Wedge: A new seismotectonic model

Ongoing oblique convergence at the eastern margin of the Indo-Eurasian collision zone provides a natural laboratory for studying the deformation and dynamics of subduction beneath the Indo-Burmese Wedge (IBW). Here, we conduct the first comprehensive seismological investigations to understand the mechanical coupling between the crust and mantle beneath IBW using shear-wave splitting analysis and stress modeling. The deformation patterns in the crust signify a strong E-W compressional stress regime throughout IBW, with negligible influence from the major geological structures. These observations derived from local seismicity strongly support that the eastward active subduction of the Indian plate beneath the Burmese sliver is responsible for the crustal-scale deformation. Contrary to the crust, our splitting measurements from the mantle are in line with the major N-S trending arcs created by slip-partitioning due to transpressional oblique subduction. The splitting measurements with an N-S orientated fast axes and the estimated depth of the anisotropy source obtained from the spatial coherency of splitting parameters strongly suggest the presence of trench-parallel sub-slab flow system driven by slab retreat with westward trench migration, which can be the major controlling mechanism of the mantle deformation beneath IBW. Throughout the IBW, a significant change in the orientations of stress and splitting parameters between the crust and mantle supports a decoupled deformation scenario, implying the necessity of a new seismotectonic model. Our integrative study on the present stress patterns and decoupled deformation mechanism between crust and mantle combined with anisotropy measurements beneath the IBW suggests active subduction in the present scenario.

Rotaphone-D, A New Model of Six-Degree-of-Freedom Seismic Sensor: Description and Performance

Abstract A new model of the Rotaphone-D short-period seismic sensor with six degrees of freedom is introduced. The basis of the instrument is horizontal and vertical geophones in a special paired arrangement. The instrument is designed for simultaneous and collocated measurements of rotational and translational components of ground motion. The basic principle of measurement is briefly described. Careful calibration is necessary for this type of measurement, which has two parts: a preliminary calibration based on laboratory measurements of the characteristics of the individual geophones, and a subsequent in situ calibration, which takes into account the actual physical conditions during field measurements and is performed during the processing of the measured data. The effect of the calibration is demonstrated by specific laboratory tests. The laboratory tests have confirmed the correct functioning of the instrument. The frequency range of Rotaphone-D is from 2 to 80 Hz and is therefore optimal for monitoring local seismicity. The instrument was subsequently used in field measurements during two several-month measurement campaigns in California, United States. Examples of six-component records from two sites—The Geysers and Long Valley Caldera—are presented and briefly interpreted from a seismological perspective. Basic instrument parameters are given. The instrument is lightweight, portable, easy to install, and offers several interesting applications, which are briefly discussed. Six-component measurements (translational and rotational) have become a powerful tool, especially in volcanic seismology and in monitoring induced seismicity.

Combined application of FTAN and cross-spectra analysis to ambient noise recorded by a microseismic monitoring network

A case study of seismic interferometry applied to a small microseismic monitoring network is here presented. The main objectives of this study are (i) to quantify the lateral variability of shear-wave velocities in the studied area, and (ii) to investigate the bias produced by noise directionality and non-stationarity in the velocity estimate. Despite the limited number of stations and the short-period character of the seismic sensors, the empirical Green's functions were retrieved for all station pairs using two years of passive data. Both group and phase velocities were derived, the former using the widespread frequency-time analysis, the latter through the analysis of the real part of the cross-spectra. The main advantage of combining these two methods is a more accurate identification of higher modes, resulting in a reduction of ambiguity during picking and data interpretation. Surface wave tomography was run to obtain the spatial distribution of group and phase velocities for the same wavelengths. The low standard deviation of the results suggests that the sparse character of the network does not limit the applicability of the method, for this specific case. The obtained maps highlight the presence of a lower velocity area that extends from the centre of the network towards southeast. Group and phase velocity dispersion curves have been jointly inverted to retrieve as many shear-wave velocity profiles as selected station pairs. While the average model can be used for a more accurate location of the local natural seismicity, the associated standard deviations give us an indication of the lateral heterogeneity of seismic velocities as a function of depth. Finally, the same velocity analysis was repeated for different time windows in order to quantify the error associated to variations in the noise field. Errors as large as 4% have been found, related to the unfavorable orientation of the receiver pairs with respect to strongly directional noise sources, and to the very short time widows. It was shown that using a one-year time window these errors are reduced to 0.3%.

Seismic structure beneath Ebeko Volcano and surrounding areas of Paramushir Island (Kuril Arc) inferred from local earthquake tomography

Paramushir is a large northernmost island of the Kuril Arc in the Russian Far East. Here, we study the northern part of Paramushir, which is dominated by the Vernadsky Ridge with an altitude of around 1000 m above sea level. This ridge is composed of a series of Pleistocene and Holocene volcanoes and includes the presently active Ebeko Volcano, which exhibits frequent phreatomagmatic eruptions and ongoing fumarolic activity. Studying the internal structure beneath Northern Paramushir is important to understand the interrelationship of magmatic, hydrothermal, hydrological and geological processes below Ebeko Volcano. We use seismic data of a portable network consisting of 20 broadband stations installed for one year from the summer 2021 to 2022 and one permanent station in the city of Severo-Kurilsk. We performed the local earthquake tomography inversion, which provided the distributions of Vp, Vs, Vp/Vs and local seismicity in an area of northern Paramushir with lateral extent of ∼20 km and depth of a ∼ 15 km. The results of several tests gave the information about the resolution limitations of the computed model, which were taken into account during its interpretation. In the resulting model, the coexistence of low Vp, high Vs, low Vp/Vs and a seismicity cluster down to ∼10 km below Ebeko indicates the presence of a large gas-saturated area that was formed due to the contacts of liquid fluids with hot magmatic intrusions. The western border of this area coincides with the location of Verkhne-Yurievsky hot water sources and possibly highlights the path of fluids traveling around the hot body below Ebeko. Below the upper part of the eastern slope, we observe a shallow anomaly of low Vs and high Vp/Vs indicating the presence of a near-surface aquifer, confirming the previous results of ambient noise tomography. Below the lower part of the eastern slope, a nearly vertical anomaly of low Vs and high Vp/Vs may represent a fault zone, which is also marked on the surface by a series of lineaments.

Spatially-varied crustal deformation indicating seismicity at faults intersection in the SE margin of the Tibetan Plateau: Evidence of S-wave splitting from microseismic identification

The Sanjiang Lateral Collision Zone (SLCZ) in the SE margin of the Tibetan Plateau is a special area where several strike-slip faults intersect, resulting in strong deformation and frequent earthquakes. We employ seismic waveforms recorded by a dense temporary broadband array (SJ array) and regional permanent stations to construct more complete microseismic catalogs by the microseismic identification in the SLCZ. New microseismic catalogs effectively increase the number of small earthquakes, revealing the details of the fault structures and providing many more records for S-wave splitting (SWS) analysis. It provides with an uncommon opportunity to detect the detailed upper crustal anisotropy in the fault intersection zone of SLCZ and to dissect the influence of faults, such as the Lijiang-Xiaojinhe fault and Red river fault, on crustal deformation. The spatial distribution of SWS parameters suggests multiple disturbance mechanisms to the upper crustal anisotropy in the study zone. Spatial distribution of dual dominant polarization directions of fast S waves near the block boundary faults uncovers the stress-focus range. Strong deformation from SWS data indicates frequent local seismicity. It reveals the spatial upper crustal deformation indicated by SWS parameters is closely related to not only stress, fault and local structure, but also local seismicity.

Machine-learning based location of the 2021 MW 7.4 Maduo, Qinghai, China earthquake sequence: Insight into intraplate seismogenesis

On 22 May 2021, an MW 7.4 earthquake occurred in Maduo County, Qinghai Province, China, which is located on the Kunlun Mountain Pass-Jiangcuo fault inside the Bayan Har block, providing a good opportunity to investigate seismogenesis of large intraplate earthquakes. We analyze two years of continuous seismic data from June 2021 to June 2023, which were recorded at 34 portable seismic stations of the MaduoArray deployed in the source zone by our group. The LOC-FLOW workflow of automatic detection and location is applied to construct a complete and high-precision seismic catalog for the region, which includes machine-learning phase picking (PhaseNet), earthquake phase association (REAL), velocity model updating and station correction (VELEST), absolute earthquake location (HypoInverse), and relative location (HypoDD). As a result, 78,832 Maduo aftershocks and other local earthquakes are detected and relocated precisely. Our results show that the length of the Maduo aftershock zone is ∼170 km, which is mainly distributed along the NWW-SEE oriented Kunlun Mountain Pass-Jiangcuo fault, and there is a horsetail bifurcation feature at the eastern end of the aftershock sequence. The seismogenic fault is nearly vertical, and local seismicity occurs on both sides of the fault. Our results also show that there is no seismic gap or aftershock sparse area in the region. Previous studies have revealed a low-velocity and high-conductivity anomaly below the source zone, reflecting fluids ascending from the lower crustal flow. These results provide new insights into the cause of the 2021 Maduo earthquake.

The Elusive Role of Aseismic Slip Along a Seaward Dipping Normal Fault in the Indirect Triggering of a Normal Faulting Earthquake Sequence in Northeast Japan Following the 2011 Tohoku‐Oki Megathrust

AbstractAlthough blind normal faults are common in subduction environments, their rheology, kinematics and interaction with the upper crust are poorly constrained. A month‐long shallow normal faulting sequence in the Ibaraki‐Fukushima prefectural border (IFPB), northeast Japan, which followed the Mw9.0 Tohoku‐Oki earthquake (TOE) and culminated in the Mw6.7 Iwaki earthquake, provides a window into megathrust‐to‐normal fault interaction. Stress change calculations indicate that direct triggering by the TOE co‐ and post‐seismic slip does not provide a plausible explanation for the IFPB earthquake sequence. In quest for an alternative triggering mechanism, we analyzed post‐TOE GNSS data from eastern IFPB. A key step in this analysis is the removal of the large‐scale post‐TOE displacement field, after which a distinct highly‐localized strain along the coastline becomes apparent. The accumulation of this strain was mostly aseismic, and migrated with time prior to the Iwaki earthquake in a manner that correlates well with post‐TOE local seismicity. We attribute the pre‐Iwaki earthquake strain accumulation to aseismic slip along low‐angle seaward dipping blind normal fault, activated by the TOE. Stresses transferred by this slip episode accelerated the failure along the IFPB shallow normal faults. This indirect triggering of the Iwaki earthquake sequence by the TOE highlights the complexity of stress transfers in subduction environments.

Monchique alkaline magmatic intrusion (SW Iberia): Geophysical modeling and relationship with active seismicity and hydrothermalism

Monchique is a prominent 902 m topographic high in SW Iberia, which stands out in the general flat landscape of southern Portugal. It lies to the north of the Africa-Eurasia plate boundary, which locally accommodates a slow oblique convergence (∼5 mm/yr). Monchique comprises alkaline magmatic rocks of Late Cretaceous age, intruded in a post-rift context. It hosts the most active seismic cluster in mainland Portugal and important hydrothermal activity.This work investigates the relationship between the alkaline intrusion, local seismicity and hydrothermalism.We present magnetic and gravity modeling based on new drone-borne magnetic data and ground gravity data. New magnetic mapping of Monchique shows a ∼ 15 km long dipolar anomaly with 7–8 km wavelength and 2000 nT amplitude. 3D magnetic inversion models the main Monchique intrusion as a high-susceptibility body, 15 km long and 6 km wide, located below the Monchique mountain and extending 5–7 km depth. 2D forward modeling and geological interpretation further support the existence of ENE-WSW oriented dike-like gabbroic bodies that may extend deeper, around which syenite units have later emplaced.We relocate the seismicity using NonLinLoc and a 3D regional tomographic model, and find that earthquakes align along four main lineations that radiate outwards from the intrusion. We also find that most earthquakes cluster between 8 and 18 km depth, below the magmatic intrusion. The b-value at the core of the seismic cluster is higher than at the surrounding region, possibly related to the local hydrothermalism. We present five new focal mechanisms that are compatible with the regional stress field, supporting a regional tectonic control.The emplacement of the Monchique alkaline intrusion left fractures in the lithosphere that currently act as preferred pathways for fluids. In the context of the present-day stress field, the enhanced fracturing and fluid circulation facilitate the localization of small-magnitude earthquakes.

Study on fault-slip process and seismic mechanism under dynamic loading of hard roof fracture disturbance

The dynamic stress disturbance induced by mining is a significant factor triggering fault-slip. To better understand the impact of mining-induced hard roof fracture on fault-slip, combined with the geological conditions of a longwall face in contact with the fault, a dynamic and static numerical model was established using FLAC3D. The influence of dynamic loading from hard roof fracture on fault stability was investigated. The characteristics of changes in fault shear stress, shear displacement, slip area, and seismic moment under dynamic loading were analyzed. Simultaneously, numerical results were compared and analyzed against microseismic (MS) monitoring records in the mining, validating the accuracy of the numerical results. The results indicate that dynamic loading is a critical factor in inducing fault-slip. As a geological weak surface, the fault impedes the transmission of shock waves and causes their attenuation. Under dynamic loading, when the fluctuation of fault shear stress and friction coefficient exceeds the threshold, the fault-slip. During the fault-slip process, accompanied by a sharp increase in shear displacement, the seismic moment increases. Finally, the influence of changes in seismic parameters (e.g. longwall–fault distance, seismic energy, and seismic location) on the fault-slip mechanism was analyzed. This study contributes to a better understanding of the disturbance mechanism of mining-induced dynamic loading on fault-slip and provides a theoretical basis for fault-slip rock bursts.

Reconstructing the Belbaşhanı Pumice Plinian eruption, Hasandağ Volcano, Turkey

Hasandağ volcano (Central Anatolia, Turkey) has recently underwent an increase in local seismicity and fumarolic activity since 2013. In the past, this volcano has produced multiple large explosive eruptions during the last million years. The Belbaşhanı Pumice is the product of a sub-Plinian to Plinian eruption dated at ~ 417 ± 20.5 ka (40Ar/39Ar). Here, we present a complete volcanological study including stratigraphy, glass chemistry, pumice morphology, geochronology, and eruption source parameters with the associated uncertainties, to characterize the Belbaşhanı Pumice eruption. The eruption involved a column of 18–29 km in height, with the main dispersal axis towards the northeast. A pumice layer up to ~ 17-m-thick accumulated in proximal deposits along the Belbaşhanı path, and up to 2-m-thick in medial-distal areas (~ 18 km northeast from the vent). The high and tubular vesicularity of the pumice clasts indicates that the Belbaşhanı eruption was predominantly magmatic. The bulk volume of the Belbaşhanı Pumice fallout deposit has been estimated as 0.5 and 8 km3 (with ~ 2 km3 being the mean value), which corresponds to Volcanic Explosivity Index (VEI) of at least 4 and up to 6. Both isopach and isopleth maps indicate that the volcanic vent may have been located at the intersection of the Tuz Gölü fault and Ulukışla caldera, within the Hasandağ volcanic complex. The glass composition of Belbaşhanı Pumice confirms that the eruption belongs to the Hasandağ magmatic system. The reconstruction of the Belbaşhanı Pumice eruption represents an essential baseline in providing volcanological constraints for further investigations of tephra fallout hazard assessment in Central Anatolia, especially considering that a new Plinian eruption cannot be ruled out at Hasandağ volcano in the future. The chemical and geochronological datasets presented here could aid in refining tephrochronological correlations, with the goal of synchronizing paleoenvironmental and paleoclimatic records alongside archaeological sites.

Local seismicity type of Kuwait revealing tectonic and anthropogenic characteristics

Local seismicity type of Kuwait revealing tectonic and anthropogenic characteristics

Integrating distributed acoustic sensing and computer vision for real-time seismic location of landslides and rockfalls along linear infrastructure

Integrating distributed acoustic sensing and computer vision for real-time seismic location of landslides and rockfalls along linear infrastructure

Geodetic plate coupling and seismic potential on the main Himalayan thrust in Nepal

We model interseismic plate coupling distribution on the Main Himalayan Thrust (MHT) in Nepal through the inversion of secular GNSS velocity data. To complement previously published data, we compile velocity data from ten additional continuous stations to improve spatial resolution in central and mid-western Nepal. A regional non-planar structural model is adopted to reproduce the MHT fault plane. In general, the coupling pattern seems nearly binary, indicating that transition from full coupling to decoupling is occurring sharply in very narrow zones. In eastern Nepal (> 84.5ºE), plate coupling is very strong from the surface to intermediate depths, including the source region of the 2015 Mw 7.8 Gorkha earthquake. Geodetically estimated slip deficit rates are consistent with the rupture history of great earthquakes in the east revealed by geomorphological observations. In the west (< 83.0ºE), a weakly coupled zone extends laterally at intermediate depths, whereas the coupling in the shallower part remains very strong. Although the slip deficit rate in the west is significantly smaller than that in the east, seismic moment accumulated, since the last complete rupture in 1505 may be capable of generating a future great event. In central Nepal, estimated slip deficit rates are comparable with those in the east, and no great event has been documented over the past several centuries. Thus, the seismic risk may be most urgent in central Nepal. The development of local seismicity and crustal deformation should be carefully monitored.Graphical

Imaging the Rivera and Cocos Plates Shape in Western Mexico from Local Seismicity Studies

Abstract The geometry of the Rivera and Cocos plates subduction below the North American plate has been studied using a total of 5337 hypocenters located in the region of Nayarit, Jalisco, Colima, and Michoacán states in western Mexico. Our results show that seismic features of the subduction at Jalisco block (JB), Colima rift zone (CRZ), and Michoacán block are well differentiated. Our study supports the hypothesis that the Jalisco subduction zone is composed of two fore-arc blocks, Banderas and Jalisco fore-arc blocks, separated by the Ipala canyon (Bandy fault). In this region, the crustal thickness of the JB is ∼30 km, whereas the Michoacán block is 35 km thick. We identified four crustal blocks along the coast in the JB from shallow seismicity data. Moreover, we found that the Rivera plate is segmented into three sections with different sizes and geometries evidenced by deep seismicity data. There is no evidence of a slab below the CRZ due to seismicity being scarce, except on the coast and the Colima volcano area where deep earthquakes (&amp;gt;70 km) are observed, which could be related to magmatic processes. The seismicity of the subduction process of the Cocos plate appears homogeneous, except for a seismic cluster at the mouth of Coalcomán River, where the epicentral area of the 1973 and 2021 earthquakes is located. Our results show that the Cocos plate is subducting with an inclination of 24°–30° and is slightly bent in a northwesterly direction. Therefore, our study suggests that current seismotectonic models of the region should be revised.

Detecting DC Electrical Resistivity Changes in Seismic Active Areas: State-of-the-Art and Future Directions

In this paper, a critical review of the geoelectrical monitoring activities carried out in seismically active areas is presented and discussed. The electrical resistivity of rocks is one of the geophysical parameters of greatest interest in the study of possible seismic precursors, and it is strongly influenced by the presence of highly fractured zones with high permeability and fluid levels. The analysis in the present study was carried out on results obtained over the last 50 years in seismic zones in China, Japan, the USA and Russia. These past works made it possible to classify the different monitoring strategies, analyze the theoretical models for interpreting possible correlations between anomalies in resistivity signals and local seismicity, and identify the main scientific and technological gaps in the literature. In addition, great attention has been paid to some recent works on the study of the correlations between focal mechanisms and the shapes of anomalous patterns in resistivity time series. Finally, some future scenarios for the development of new activities in this field have been identified.

Effect of Geometry and Size of Fiber Reinforced Plastic Deck Profile on Behavior of Bridges

Due to the important role that bridges play in rescue operations after an earthquake, it is necessary that these structures have a higher level of protection against seismic attacks. Earthquake identifies the weak points of the structure and causes the most damage there. Bridges are very vulnerable to these attacks due to their low degree of uncertainty. All bridges built before 1971 were designed with the elastic design method (permissible stress). In this method, the effects of plasticity, section cracking, and plastic deformation are not taken into account. The change of seismic locations based on the principles of elastic design is much less. It is because the structure experiences in a real earthquake, one of the consequences of which is the falling of the decks due to the loss of the support surface. The decision to strengthen the bridge was made when there were many bending and shear cracks on the king beams. The bridge was created. The use of FRP profiles can significantly prevent the damage caused by corrosion and is a good alternative to the traditional methods of strengthening the structure. In this study, a design for the deck of a steel bridge with I beams is presented. The shape of reinforcement using FRP fibers with vinyl ester resins has also been investigated, the effect of the geometry of FRP profiles has been investigated. The presented specifications are optimized to obtain a lasting shape and section, especially for the pultrusion process. FRP materials are light, resistant to corrosion and have high tensile strength. These materials come in different forms and range from multi-layer factory sheets to dry sheets that can be twisted on various structural forms before adding resin, is available. The durability and high tensile strength of FRP materials are among the advantages of these materials. The durability and long-term performance of FRP requires more research, which is ongoing and continues.

Forecasting the Long-Term Spatial Distribution of Earthquakes for the 2023 U.S. National Seismic Hazard Model Using Gridded Seismicity

ABSTRACT Probabilistic seismic hazard analyses such as the U.S. National Seismic Hazard Model (NSHM) typically rely on declustering and spatially smoothing an earthquake catalog to estimate a long-term time-independent (background) seismicity rate to forecast future seismicity. In support of the U.S. Geological Survey’s (USGS) 2023 update to the NSHM, we update the methods used to develop this background or gridded seismicity model component of the NSHM. As in 2018, we use a combination of fixed and adaptive kernel Gaussian smoothing. However, we implement two additional declustering methods to account for the fact that declustering is a nonunique process. These new declustering methods result in different forecasts for the locations of future seismicity, as represented by spatial probability density functions that are later combined with a rate model to produce a full gridded earthquake rate forecast. The method updates, particularly in the separation of the spatial and rate models as well as revised regional boundaries, in some places cause substantive changes to the seismic hazard forecast compared to the previous 2018 NSHM. Additional updates to catalog processing and induced seismicity zones also contribute to changes in the gridded seismicity hazard since 2018. However, these changes are well understood and reflect improvements in our modeling of gridded seismicity hazard.