Nodal Planes Research Articles

Precise Relocation and Source Characterization of Two Earthquake Sequences in Yellow Sea Using Regional Lg-Wave Observations

ABSTRACT Two significant earthquakes of magnitude ML 4.6 and 4.5 occurred on 18 January and 3 December 2021 in the central region of the Yellow Sea, respectively. The earthquakes occurred beneath the Gunsan sedimentary basin at about 10 km depth with a strike-slip faulting mechanism on nodal planes striking northwest–southeast (NW–SE) and north-northeast–south-southwest (NNE–SSW). Despite a lack of close-by seismographic stations, we successfully utilized regional Lg-wave observations on both coasts of the sea—the Korean peninsula on the east and eastern China on the west. For nine earthquakes in two event sequences, the Lg-wave differential travel times of the nearby event pairs at the common station are carefully measured using the waveform cross-correlation technique. The double-difference earthquake relocation method is employed to obtain precise relative epicentral locations using the Lg correlation measurements. Relocated epicenters align along the NW–SE direction, indicating that the nodal plane striking the same direction is the likely fault plane on which both sequences occurred. This is the first case reported in the literature in which the causative fault plane has been identified for earthquakes in the central Yellow Sea region. It has an important implication for current regional tectonics; it favors neither old tectonic features trending NE–SW (Qianliyan uplift) nor the north–south alignment of significant earthquakes in the region along the Amur plate boundary. The Lg waves from the earthquake sequences are dominant seismic signals on all three-component records at stations in 160–550 km and allowed us to analyze source properties of the two largest earthquakes using the empirical Green’s function approach. Azimuthal variations of the source corner frequencies suggest that earthquake rupture likely propagated toward southeast (125°) along the fault plane, supporting the aftershock relocation results.

Preliminary Observations of the 5 April 2024 Mw 4.8 New Jersey Earthquake

Abstract On 5 April 2024, 10:23 a.m. local time, a moment magnitude 4.8 earthquake struck Tewksbury Township, New Jersey, about 65 km west of New York City. Millions of people from Virginia to Maine and beyond felt the ground shaking, resulting in the largest number (>180,000) of U.S. Geological Survey (USGS) “Did You Feel It?” reports of any earthquake. A team deployed by the Geotechnical Extreme Events Reconnaissance Association and the National Institute of Standards and Technology documented structural and nonstructural damage, including substantial damage to a historic masonry building in Lebanon, New Jersey. The USGS National Earthquake Information Center reported a focal depth of about 5 km, consistent with a lack of signal in Interferometric Synthetic Aperture Radar data. The focal mechanism solution is strike slip with a substantial thrust component. Neither mechanism’s nodal plane is parallel to the primary northeast trend of geologic discontinuities and mapped faults in the region, including the Ramapo fault. However, many of the relocated aftershocks, for which locations were augmented by temporary seismic deployments, form a cluster that parallels the general northeast trend of the faults. The aftershocks lie near the Tewksbury fault, north of the Ramapo fault.

Illuminating the Hierarchical Segmentation of Faults Through an Unsupervised Learning Approach Applied to Clouds of Earthquake Hypocenters

AbstractWe propose a workflow for the recognition of the hierarchical segmentation of faults through earthquake hypocenter clustering without prior information. Our approach combines density‐based clustering algorithms (DBSCAN and OPTICS), and principal component analysis (PCA). Given a spatial distribution of earthquake hypocenters, DBSCAN identifies first‐order clusters, representing regions with the highest density of connected seismic events. Within each first‐order cluster, OPTICS further identifies nested higher‐order clusters, providing information on their number and size. PCA analysis is applied to first‐ and higher‐order clusters to evaluate eigenvalues, allowing discrimination between seismicity associated with planar features and distributed seismicity that remains uncategorized. The identified planes are then geometrically characterized in terms of their location and orientation in the space, length, and height. This automated procedure operates within two spatial scales: the largest scale corresponds to the longest pattern of approximately equally dense earthquake clouds, while the smallest scale relates to earthquake location errors. By applying PCA analysis, a planar feature outputted from a first‐order cluster can be interpreted as a fault surface while planes outputted after OPTICS can be interpreted as fault segments comprised within the fault surface. The evenness between the orientation of illuminated fault surfaces and fault segments, and that of the nodal planes of earthquake focal mechanisms calculated along the same faults, corroborates this interpretation. Our workflow has been successfully applied to earthquake hypocenter distributions from various seismically active areas (Italy, Taiwan, and California) associated with faults exhibiting diverse kinematics.

Deep‐Focus Earthquake Mechanisms at the Subducting Nazca Plate (Peru‐Brazil Border): Cold Slab Behavior in a Warm Plate

AbstractWe calculate focal mechanisms and centroid depths for deep‐focus earthquakes (DFEs) along the Peru‐Brazil border. We obtained a total of 28 focal solutions for events with magnitudes between 4.2 and 7.5 Mw and occurring between 2014 and 2022. Focal mechanisms indicate predominance of normal faulting, demonstrating a rather uniform down‐dip compression (DDC) regime within the plate. The orientations of the nodal planes suggest that earthquakes tend to occur along faults parallel to the local slab strike, although other fault types are documented. Stress orientations derived from the focal mechanisms agree with patterns expected if faulting were initiated by transformational faulting on a metastable olivine wedge (MOW) under DDC. Centroid depths range between 557 and 659 km, defining a narrow seismic zone within the lower portion of the subducting plate and an aseismic upper portion. We suggest that DFEs nucleate through transformational faulting within a narrow MOW preserved at a colder slab segment right above the lower mantle and juxtaposed to a shallower, warmer segment at around 500 km depth. This thermal complexity was possibly produced through flat subduction initiated by the subduction of the Nazca Ridge. We speculate that subduction of other aseismic ridges is possibly controlling the thermal state of the Nazca slab as a whole and, consequently, the depth distribution of DFEs along the South America subduction front.

Not trench-parallel but trench-normal source fault of the 1994 Hokkaido Toho-oki earthquake as revealed by the aftershock relocation using HypoDD

In the southwestern area of the Kurile Islands subduction zone, three large earthquakes including the Mw8.2 1969 and the Mw8.3 1994 Hokkaido Toho-oki earthquakes and the Mw7.8 1975 tsunami earthquake occurred in almost the same location. The three main shocks and their aftershocks were re-determined simultaneously using a double-difference earthquake location method, HypoDD. As a result, the 1969 and the 1975 main shocks and their aftershocks were concentrated near the upper surface of the subducting Pacific plate, indicating that both events were plate boundary megathrust earthquakes as shown by previous studies. On the other hand, the 1994 main shock and its aftershocks were distributed within the Pacific plate indicating that the 1994 event was an intraslab event as reported by previous studies. The result that interplate earthquakes and intraslab earthquakes could be appropriately distinguished is strong evidence that the accuracy of hypocenter determination is sufficiently high. The centroid moment tensor solution of the 1994 main shock has two nodal planes which are parallel or normal to the Kurile Trench. According to the high-resolution hypocenter distribution obtained in the present study, the aftershocks appear to locate on the nodal plane normal to the trench, suggesting that the source fault ruptured by the 1994 main shock was not parallel but normal to the Kurile Trench. Moreover, we found that the Coulomb failure stress change (ΔCFS) produced by the trench-normal fault plane is more likely to trigger the largest aftershock than ΔCFS produced by the trench-parallel fault plane.Graphical abstract

Quasilinear Earthquake Chains in Groups of Seismic Events Baikal Rift System

Identification of a large number of quasi-linear chains of earthquakes in the epicentral seismicity field of the Baikal region and their study showed that among these chains there may be not only chains of “migration” earthquakes, but also chains formed during a random spatio-temporal distribution of earthquakes. In this work, using a statistical analysis of the distribution of distances between epicenters, the possibility of forming chains of “migrations” within groups of seismic events is shown and their distribution is studied. It is noted that chains in groups of earthquakes are distinguished not only by the distribution of distances, but also by the time between them. The formation of chains of earthquakes was established in the areas of the following groups of earthquakes: Busiyingol and 1976 and 1991. in the same area, the South Baikal, Kyakhtinsky, Kichersky earthquakes of the Tompudinsky series, Oldongsinsky and Charudinsky groups. It is shown that these chains were formed during the implementation of these groups. In the identified areas of concentration of chains of grouping seismicity, a connection can be traced between the location and direction of the chains with the strike of fault zones, near-fault cracks and the orientation of nodal planes in earthquake sources.

Experimental investigation on aerodynamic noise and flow structures of a vibrissa-shaped cylinder

The noise mitigation effect of bio-inspired geometries has attracted growing attention from both research and industry, such as the vibrissa-shaped cylinder derived from the harbor seal. Experiments were conducted to investigate the far-field noise and the near-field wake of the flow past a vibrissa cylinder, a circular cylinder, and an elliptical cylinder at Re=3.6×104, in the subcritical flow regime. The frequency characteristic of the far-field acoustic pressure and the near-field velocities are analyzed. The mean and fluctuating velocities, dominant flow modes from proper orthogonal decomposition in both vertical and horizontal planes as well as the time-frequency behavior of the dominant flow structures from wavelet transform are also presented to better understand the wake dynamics and the direct relation of these flow structures with the far-field noise. The vibrissa cylinder reduces the overall sound pressure level by 13.2 dB and 8.3 dB compared with the circular and the elliptical cylinders, respectively, with a remarkable attenuation of the tonal peak associated with vortex shedding. From the detailed velocity measurements in multiple wake planes, it is clearly observed that vortex shedding of the vibrissa cylinder is weaker in strength and significantly less coherent in the spanwise direction than the other two cylinder cases, accompanied by more transient changes. The results also reveal the distinct flow behaviors behind the nodal and saddle planes of the vibrissa cylinder, further contributing to this three-dimensional vortex shedding. Consequently, the power spectral density of the tonal peaks associated with the vortex shedding in both near-field velocities and far-field acoustic pressure are attenuated, leading to a lower noise level. Understanding the detailed flow dynamics of the vibrissa cylinder will provide useful insights into more efficient bio-inspired cylinder designs in noise mitigation and wake control.

Kinematic and dynamic fault slip analyses: Implications from the surface rupture of the 2023 Elbistan (Kahramanmaraş) (Mw7.6) earthquake, Türkiye

On 6 February 2023, the Elbistan (Kahramanmaraş) (Mw7.6) earthquake produced a strike-slip type coseismic surface rupture zone, involving the sinistral slip of the Doğanşehir Fault (DF) and Çardak Fault (ÇF). Fault slickenlines and seismic slickenlines were included in the inversion as part of the dynamic analysis. The inversion of fault slickenlines collected from nine outcrops along the surface rupture following the earthquake, many of which developed during the event and are referred to as coseismic slickenlines, reveals a strike-slip stress regime (σ2 = σv) characterized by a NE-SW (mean azimuth: N045°E) σ1 (maximum) axis. Relict slip planes adjacent to main slip surfaces were found to host slickenlines that appear much older than the most recent earthquake, referred to as paleoslip slickenlines. Inversion of the paleoslip slickenline collected from four outcrops reveals an extensional stress regime (σ1 = σv) characterized by a NE-SW σ3 (min) axis, consistent with previously published studies. To define a significant stress tensor concerning present-day faulting within the investigated area, the inversion method was applied to the seismic slickenlines of shallow earthquakes in the surface rupture area. The inversion of sixty selected nodal planes reveals a strike-slip stress regime (σ2 = σv) characterized by an NNE–SSW (N10°E) σ1 (max) axis. Besides, the moment tensor summation analysis using displacement vectors inferred from visible field markers offset during the earthquake was applied to the seismic slickenlines as part of the kinematic analysis. The shortening and extension results for the moment tensor summation are 044/36 and 307/11 indicating NE-SW compression. Results of the moment tensor summation (weighted by fault displacement) agree broadly with the inversion of seismic slickenlines results, indicating an NE orientation for shortening and an SE orientation for extension.

Source parameters and aftershock pattern of the October 7, 2021, M5.9 Harnai earthquake, Pakistan

On October 7, 2021, a magnitude 5.9 earthquake struck the Harnai (Baluchistan) region of Pakistan, causing several fatalities and injuries within the epicentral area. First-order tectonic deformation in this region is caused by the convergence of the Indian Plate with respect to the Eurasian Plate. The Katwaz Block hinders the motion of the Indian Plate, resulting in the formation of strike-slip faults. In this study, the P-wave first-motion polarity technique was used to determine the mainshock faulting style. Cyclic scanning of the polarity solutions was applied to determine the most suitable focal mechanism solution among the available solutions generated by the FOCMEC (focal mechanism) software. The nodal planes correspond to different faulting styles (i.e., thrust and strike-slip faulting). A nodal plane oriented in the NW-SE direction corresponded to a strike-slip mechanism, which was considered to be the fault plane. Tectonically, this earthquake was associated with the Harnai-Karahi strike-slip fault zone owing to the fault strike and direction of slip. The apparent stress drop, fault length, and moment magnitude of the Harnai earthquake were 35.4 bar, 6.1 km, and 5.9, respectively. A lower b-value for the Gutenberg-Richter law was observed prior to the earthquake. Higher α- than b-values (α >b) indicate that this earthquake was governed by large events as opposed to small-magnitude events. The Harnai sequence had a decay exponent close to unity, lasted for 145 days, and produced few aftershocks. The study will help the future hazard mitigation in the region.

Synthesis, Photophysical and Electronic Properties of a D-π-A Julolidine-Like Pyrenyl-o-Carborane.

We synthesized 2-(1-1,2-dicarbadodecaboranyl(12))-6,6,12,12-tetramethyl-7,8,11,12-tetrahydro-6H,10H-phenaleno[1,9-fg]pyrido[3,2,1-ij]quinoline (4), a julolidine-like pyrenyl-o-carborane, with pyrene substituted at the 2,7-positions on the HOMO/LUMO nodal plane. Using solid state molecular structures, photophysical data, cyclic voltammetry, DFT and LR-TDDFT calculations, we compare o-carborane and B(Mes)2 (Mes=2,4,6-Me3C6H2) as acceptor groups. Whereas the π-acceptor strength of B(Mes)2 is sufficient to drop the pyrene LUMO+1 below the LUMO, the carborane does not do this. We confirm the π-donor strength of the julolidine-like moiety, however, which raises the pyrene HOMO-1 above the HOMO. In contrast to the analogous pyrene-2-yl-o-carborane, 2-(1-1,2-dicarbadodecaboranyl(12))-pyrene VI, which exhibits dual fluorescence, because the rate of internal conversion between locally-excited (LE) and charge transfer (CT) (from the pyrene to the carborane) states is faster than the radiative decay rate, leading to a thermodynamic equilibrium between the 2 states, 4 shows only single fluorescence, as the CT state involving the carborane as the acceptor moiety in not kinetically accessible, so a more localized CT emission involving the julolidine-like pyrene moiety is observed.

Imaging the Fault Plane Structure of Palu-Koro Fault from Earthquake Data Using Python

Abstract Palu-Koro Fault is one of the active faults located in Sulawesi and positioned in the middle of the Island, dividing the west and east-southeast arms. Its long history of earthquakes provides sufficient focal mechanisms data to be analyzed. This research used seven focal mechanism data of shallow magnitude earthquakes surrounding the pull-apart basin area of the Palu-Koro Fault. The focal mechanism data used is secondary data obtained containing the information needed. This research aims to create a 3D model of the pull-apart basin displaying the geometries of the fault planes and providing a closer look at the fault system surrounding the bounding fault zones. Python script was made to classify the mechanism of each earthquake based on its correlation to the trend of the Palu-Koro Fault. Nodal planes are categorized as whether it is presumed to be the fault plane or the auxiliary plane of the focal mechanism. The result found that not all earthquake data was classified through the assumed parameter set. Only five of seven earthquake data managed to be classified, while the rest showed a different focal mechanism. It is found from the python script created revealed the presence of three fault types: normal fault, normal left lateral oblique fault, and left lateral strike-slip. The 3D fault plane interpolation showed two distinct dipping directions, suggesting their association with the basin’s bounding fault, either the west or the east side. From the gathered information, previous research, and geological interpretation, the earthquake data can be concluded into three fault associations to the bounding fault: synthetic extension fault, antithetic extension fault, and sinistral strike-slip fault.

Rupture Model of the 5 April 2024 Tewksbury, New Jersey, Earthquake Based on Regional Lg-Wave Data

Abstract On 5 April 2024, an earthquake of magnitude 4.8 occurred in Tewksbury, New Jersey. It was the largest instrumentally recorded event since 1900 in New Jersey and southern New York. Millions of people around New York City, ∼65 km east-northeast of Tewksbury, felt the shaking from the mainshock, but the epicentral area experienced no known significant property damages. We determine the focal mechanism, which is oblique faulting, and retrieve the Lg-wave relative source time functions (RSTFs) from the stations at regional distances to understand rupture processes and ground motions. Our fault-slip models well explain azimuthal variations of the RSTFs. The models show the rupture propagating toward the east-northeast (∼50° to 60°), not along the fault strike. The slip distribution on the nodal plane striking north and dipping to the east shows a slip area of 1.1 km radius with the rupture propagating down-dip. The down-dip rupture may account for the observed lack of strong shaking in the epicentral area.

High-frequency hydrogen combustion dynamics driven by local flame displacement and multidimensional thermoacoustic interactions

Knowledge of the underlying physical mechanisms responsible for the triggering of high-frequency transverse combustion dynamics is of fundamental importance in the development of heavy-duty gas turbine combustors, aircraft engine afterburners, and bipropellant liquid rocket engines. Detailed information about three-dimensional thermoacoustic interactions and local flame dynamics, however, remains largely unknown and unanticipated, mainly because high-amplitude transverse mode instabilities are challenging to excite and detect in well-controlled sub-scale laboratory environments. To overcome this impasse, here we exploit a spatially tailored rectangular injector assembly consisting of ten equidistant horizontal slit nozzles to eliminate the complications of out-of-plane flame dynamics characterization. A total of 56 datasets of self-induced instabilities were acquired over a wide range of operating conditions to understand spatiotemporal phase dynamics and important mode shapes, in conjunction with 2D Rayleigh angle reconstruction and phase-resolved OH PLIF-based local flame front identification. Experimentally, we show that high-frequency transverse instabilities are excited only under high temperature and high thermal power conditions, manifested as non-evanescent pressure fluctuations at 6.50 kHz strongly coupled to the second-order tangential mode of the rectangular combustion chamber. Two vertically-oriented pressure nodal planes and the characteristic phase transition perpendicular to the horizontal slit injector direction are accurately measured and reconfirmed by Helmholtz simulations in terms of their interpositions and spatial orientation. Remarkably, the periodic formation of co-propagating coherent structures and concomitant local flame displacement/pinch-off are revealed to play an important role in driving the high-frequency hydrogen combustion dynamics.

Strong controlling effect of stress evolution on the 2022 Ms5.0 Honghe earthquake sequence

Understanding stress evolution patterns as a response to earthquake ruptures at geologically complex tectonic faults is vital because of the role of fault geometry as a source for the stress evolutionary constraints. Here, we analyze the 2022 Ms5.0 Honghe earthquake sequence and calculate the tectonic stress distribution of the two Ms. ≥ 3.5 earthquakes in this sequence. Results indicate that the focal mechanism type of the sequence obtained corresponds to strike-slip motion with an ESE-oriented strike and a steeply dipping NE-oriented nodal plane. The focal region is subjected to NNW-oriented horizontal compression and ENE-oriented horizontal tension. We find that the adjacent Ailaoshan fault constituted the major seismogenic fault of the mainshock and the regional stress field exerted a strong controlling effect on the associated postseismic events. Our results suggest that the stabilization of tectonic faults may help to enhance the effects of stress accumulation on the occurrence of medium-to-strong earthquakes.

Drop Dissolution Intensified by Acoustic Levitation.

Acoustic levitation can provide significant benefits for many fundamental research questions. However, it is important to consider that the acoustic field influences the measurement environment. This work focuses on the dissolution of immobilised drops using acoustic levitation in liquid-liquid systems. Previous work demonstrated that the acoustic field of standing waves impacts mass transfer by affecting the spread of dissolved substances in the continuous phase in two distinct ways: (I) solutes may either pass through nodal planes of the standing waves or (II) not pass. The binary systems examined for case (I) are 1-hexanol-water and 1-butanol-water, and for case (II), n-butyl acetate-water and toluene-water. This work quantifies the intensification effect of acoustic levitation on dissolution for the two types of behaviour, by comparing them with reference measurements of mechanically attached dissolving drops. The system was designed to ensure minimal intensification. The minimum intensification of mass transfer for levitating drops in the used setup of case (I) was 25%, and for case (II), it was 65%, both increasing with decreasing surface-equivalent diameter. With this understanding, acoustic levitation can be used more accurately in the field of mass transfer studies.

Micro-seismicity associated with CO2 geological storage activities in Hontomín, Spain

We present the first local micro-seismicity catalogue including focal mechanisms for the Hontomín plant, the only site of its kind in Spain and one of the very scarce CO2 storage facilities in Europe. The catalogue reveals a correlation between micro-seismicity occurrence and bottom hole pressure (BHP) at the injection well. A burst of micro-seismicity ranging from -1 to 0.4 ML, thus imperceptible to human senses, took place coeval with the longest duration test of the period studied when the BHP was the highest. Events clustered at ∼ 0.4 km below the well bottom hole. Focal mechanisms could be well resolved for 3 events with similar results, showing a strong strike-slip component and a minor reverse component, with similarly oriented nodal planes. The local stress fields inverted from the focal mechanism solutions exhibit an orientation of the maximum horizontal stress axis (SHmax) that ranges from NE to ESE and differs from the principal stress orientations obtained in previous works for the same area. The micro-seismicity locations and tensors obtained are useful inputs to models aiming to better understand the deformation effects of fluids’ injection underground.

Impact of Single-Melamine Tautomerization on the Excitation of Molecular Vibrations in Inelastic Electron Tunneling Spectroscopy.

Vibrational quanta of melamine and its tautomer are analyzed at the single-molecule level on Cu(100) with inelastic electron tunneling spectroscopy. The on-surface tautomerization gives rise to markedly different low-energy vibrational spectra of the isomers, as evidenced by a shift in mode energies and a variation in inelastic cross sections. Spatially resolved spectroscopy reveals the maximum signal strength on an orbital nodal plane, excluding resonant inelastic tunneling as the mechanism underlying the quantum excitations. Decreasing the probe-molecule separation down to the formation of a chemical bond between the melamine amino group and the Cu apex atom of the tip leads to a quenched vibrational spectrum with different excitation energies. Density functional and electron transport calculations reproduce the experimental findings and show that the shift in the quantum energies applies to internal molecular bending modes. The simulations moreover suggest that the bond formation represents an efficient manner of tautomerizing the molecule.

Shear-wave splitting associated with fluid processes beneath Styra, South Euboea: First results

The intense and persistent seismic activity on the Island of Euboea (or Evia) was a unique opportunity to study anisotropy in a region without any notable prior seismicity. The study area is located in the central-western Aegean Sea and constitutes the endpoint where the North Aegean Trough and four parallel dextral strike-slip fault branches terminate against the Greek mainland, along with their sinistral counterparts. Even though seismicity has been recorded around the island, the affected area close to the town of Styra has no known reported earthquakes. In 2022, surprising activity was initiated by two moderate earthquakes (ML 4.8 and ML 5.0) that occurred on 29 November and were felt as far as Athens. The activity continued well into the second quarter of 2023, with over 1000 microearthquakes being recorded. A pre-planned nearby temporary installation of a broadband instrument (GR27), in the context of the AdriaArray project, offered recordings of the activity. Here, we investigated the occurrence and cause of shear-wave splitting beneath Styra. Based on focal mechanisms of local earthquakes we inferred the stress axes. A fully automated process was used to analyze data for shear-wave splitting and determine the polarization direction of the Sfast (φ), the time-delay (td) and the normalized time-delay (tn). A total of 272 acceptable results showcased a unimodal distribution of φ with an average of N68°E and a mean td of 80 ms. We observed temporal variations of splitting parameters, associated with outbursts of seismicity, not with individual events. The determination of the shear-wave velocity anisotropy, jointly with observations of other splitting parameters, led us to hypothesize that anisotropy is the result of along-fault fluid processes. However, this would require a convincing selection of the NE-SW nodal plane as the preferred fault orientation. Further enrichment of the catalogue and an increase in splitting results is required to draw more robust conclusions.

Enhanced optical vector bottle beams with obscured nodal surfaces

Optical bottle beams, characterized by their unique three-dimensional dark core, have garnered substantial interest due to their potential applications across multiple domains of science and technology. This paper delves into the current methods used to create these beams and provides a method to obscure their nodal planes through coaxial non-interfering orthogonally polarized beams to generate bottle beams with enhanced uniformity. Experimental and theoretical results show the enhanced vector bottle beam maintains a smaller, more spherically uniform potential well and interesting quasi-particle polarization characteristics.

Seismic hazard identification around Surabaya City, East Java, Indonesia: a preliminary result based on the June 2023 Mojokerto earthquake

An earthquake was felt in the Mojokerto area, East Java (Indonesia) with a magnitude of M 4.4. This area has a low seismicity level according to the latest earthquake catalog. In this paper, we conduct a study to determine the direction of fault movement in the area, using data from BMKG seismic sensors with a distance of less than 250 km. The moment tensor inversion results show that the Strike/Dip/Rake values at each nodal plane are 257/88/4 and 167/86/175. To determine the direction of fault movement, we prefer a strike of 167 degrees for this earthquake, following the direction of deformation around the area from the northeast of Java Island. From the results of the moment tensor solution and the history of seismicity that has occurred in the area around Surabaya, it can be shown that the city of Surabaya is a city that is close to seismic activity, although it tends to have small activity, this needs to be considered considering that Surabaya is one of the cities with the highest population density in Indonesia. Further research needs to be done related to disaster vulnerability around the city of Surabaya. From the results of the source mechanism and seismicity in the Surabaya area it can be concluded that the Surabaya area has a disaster risk, there needs to be further research that discusses the local footprint effects of soil in the area in the Surabaya area. This research is needed for further discussion of the possibility of disasters in the Surabaya area.