GPS Array Research Articles

Variations of Equatorial Plasma Bubbles Behaviour in Southeast Asia: Insights from Geomagnetically Quiet Days

Abstract The present study aims to investigate the behaviour of equatorial plasma bubbles (EPBs) during quiet days in Southeast Asia (SEA). The behaviour of EPB was determined by observing their number (n) and zonal drift velocity (VE) from 2011 to 2013. The variation of the n and VE during quiet days was investigated monthly and seasonally each year. Three networks which are Malaysia Real-Time Kinematics GNSS Network (MyRTKnet), Sumatran GPS Array networks (SuGAr) and International GNSS Service (IGS) in SEA that consists of 127 receivers were utilized to collect the high-density GPS data. The data were then used to generate the zonal keogram of the rate of total electron content index (ROTI) to estimate n and VE. From the observed data, there were 376 days characterized as geomagnetically quiet according to the Kp index, with a total of 2592 EPBs observed using ROTI keogram. The highest VE was found to be ∼126 m/s during the equinox, while the lowest was ∼65 m/s in December solstice. In addition, the maximum and minimum n throughout the period were 1937 during the equinox and 163 in December solstice, respectively. While a distinct diurnal pattern emerged for n throughout 2011 until 2013, a clear diurnal pattern of VE was only evident in 2013. This observation suggests a potential link between the year with a diurnal VE pattern and the continuous presence of EPB.

Analisis Deformasi di Lampung dan Selat Sunda berdasarkan Data GNSS tahun 2018 hingga 2021

The Lampung Province and Sunda Strait have a seismic gap zone with the potential for major earthquakes in the future. This study analyzes the deformation occurring in this region using continuous Global Navigation Satellite System (GNSS) station data from Indonesia Continuously Operating Reference Station (InaCORS) and Sumatran GPS Array (SuGAr) from 2018 to 2021.5. The GNSS data was processed using the Bernese 5.2 scientific software, applying least squares for velocity changes and statistical tests to analyze significance. The data processing was carried out in two schemes: the first scheme covering 2018-2020, and the second covering 2019-2021. The results of the deformation analysis from 2018 to 2021, using two continuous GNSS data processing schemes, showed velocity changes relative to the Sundaland Plate ranging from ~2 mm/year to ~20 mm/year. In the eastern region of the Sumatra fault, the velocity changes were smaller, around ~5 mm/year, due to the minimal influence of tectonic activity. However, in the Sunda Strait region, the deformation was influenced by volcanic activity. The deformation occurring in Lampung Province and the Sunda Strait, based on GNSS velocity changes, significantly contributes to tectonic and volcanic activities.

Investigating ultra-low frequency emissions and total electron content anomalies as earthquake precursors in Sumatra (2019–2020)

Ultra-low frequency (ULF) emissions and total electron content (TEC) anomalies are potential earthquake precursors for short-term earthquake prediction. This study comprehensively examines ULF emissions and TEC anomalies associated with the Sumatra earthquakes spanning 2019–2020. Precursor ULF emissions are identified through the polarization power ratio calculations involving Z-component and H-component geomagnetic data, meticulously recorded by the Magnetik Acquisition Data System (MAGDAS). Employing the Single Station Function Transfer technique, ULF anomalies directed toward the epicenter of earthquakes are recognized as reliable precursors. Among the 29 earthquakes scrutinized, 25 were heralded by ULF emissions acting as precursors. Among these, 15 instances solely exhibited ULF emissions, unaccompanied by TEC anomalies, while the remaining ten were linked to TEC anomalies. The investigation of TEC anomalies employs advanced autocorrelation techniques rooted in the analysis of correlation coefficient deviations. TEC anomalies are meticulously acquired from the observations of the Sumatran GPS Array (SuGAr) stations. Furthermore, this study illuminates a positive correlation (R = 0.65) between the lead time of ULF emissions and TEC anomalies. Moreover, the frequency of ULF emissions and TEC anomalies also showcases a direct relationship with earthquake magnitude. Heightened frequencies of ULF emissions and TEC anomalies as earthquake precursors correspond to more significant magnitudes in ensuing earthquakes. The outcomes of this research significantly augment the understanding of employing ULF emissions and TEC anomalies as effective earthquake precursors in the context of Sumatra.

Ionospheric Disturbance in Indonesia during Solar Flare using GPS Observations

Electromagnetic waves transmitted by Global Navigation Satellite System (GNSS) are delayed when they travel through ionosphere layer due to free electrons in that layer. This delay could be used to deduce the variation in ionosphere, through the quantity known as Total Electron Content (TEC). The variability of the TEC in the ionosphere is affected by solar activity, such as solar flares. When a solar flare occurs, which is characterized by a strong and sudden increase in X-ray intensity, it will result in an increase in the electron density in the ionosphere, which is known as ionospheric disturbances. The increase of electron in the ionosphere increases the absorption of HF radio wave energy which can also turn off HF radio communications and cause significant errors in GNSS measurements. This study observed the SID above Indonesia which was disrupted by a solar flare that occurred on November 8, 2013, with class X1.1 angstroms. We use the Sumatran GPS Array (SuGAR) station with observation epochs every 30 seconds to estimate the TEC. The results showed that there was a sudden and simultaneous increase of all observed satellites namely GPS PRN 2, 4, 5, 9, 10, 17, 24, 26 and 28. SID occurred at 11:25:30 LT with a TEC amplitude of 1.05 – 3.94 TEC Unit. The detection time of the SID corresponds to the solar flare’s time observed from the GOES satellite with a delay of ~5 minutes.

Co-seismic ionospheric disturbances due to 2004 Sumatra-Andaman earthquake

The Coseismic Ionospheric Disturbances (CID) due to the 26th December 2004 earthquake of Mw 9.2, which occurred in the Sumatra-Andaman subduction zone, are analyzed using cGPS-aided Total Electron Content (TEC) measurements. For the CID analysis, data from nearby seven Sumatran GPS Array (SuGAr) and two International GNSS Stations (IGS) located to the south of the epicenter, at a distance of 500–1000km (near-field) and two IGS stations located to the north-west of the epicenter at a distance of 2000km (far-field) are considered. The CIDs with a propagation velocity of 595–694m/s arrived within 2–10min after the earthquake, depending upon the distance of a station from the epicentre. Variations in the CIDs can be prominently seen at the nearest cGPS Station SAMP immediately after the earthquake. NTUS, being the farthest station shows some small variations. The delay in the occurrence of variations at GPS sites can also be associated with rupture propagation. Because all the stations used in our analysis are located south of the epicenter and rupture of the earthquake propagated in the north, the trend of rupture propagation could not be analyzed clearly.

Detection of Surface Displacement of Mentawai Earthquake Mw7.8, October 25, 2010, using High Rate (HR) 1Hz GNSS Data

Conventional seismic sensors, may have some problems, for example, seismometers are vulnerable to clipping and going off-scale when recording in the near-field of large earthquakes, that caused magnitude saturation. High-rate GNSS measurement can avoid the problems in conventional seismic records, in determining true size of large earthquake. The Mw7.8 Mentawai earthquake on October 25, 2010, generate significant tsunami run-up reached 9.3 m, five hundred more casualties, and raise economic loss of 200 bilions rupiahs. We utilize 1 Hz high-rate GNSS data from 11 GNSS stations of the Sumatran GPS Array (SuGAr) network operated by Earth Observatory of Singapore (EOS) associated with the October 25, 2010, Mw7.8 Mentawai earthquake. We generate displacement waveform in three components (east, north, up) and it can be seen that all GNSS stations can detect surface displacement caused by October 25, 2010, Mw7.8 Mentawai earthquake. We also display the displacement waveform based on the distance from the GNSS station to the earthquake hypocenter, and found that displacement amplitude decreases as the distance between the earthquake hypocenter and the location of the GNSS station increases.

Slip Rate of Kumering Fault in Lampung Province Calculated from GPS Data from 2007 to 2021

Kumering Fault in Lampung Province is a segment of SFZ that was tectonically active region that had generates large earthquakes. The earthquake hazard around Kumering Fault can be estimated by understanding slip rate of Kumering Fault. This research aim is to estimate slip rate of Kumering Fault by using GPS data from 2007 to 2021. GPS data used in this research are 6 periodic GPS sites and 1 continuous Sumatran GPS Array site. GPS data is processed to obtain daily solution coordinates which are used to calculate velocity by using least-square method of linear regression. Fault parallel velocities are calculated to estimate slip rate and locking depth of Kumering Fault. The The process to obtain slip rate is using grid-search method by finding best fit velocities for all GPS sites. The velocities of GPS sites indicate Sundaland Plate movement. Fault parallel velocities shows the typical movement of right-lateral Kumering Fault. The slip rate of Kumering Fault estimated from this study is 18.2 ± 10 mm/year while the locking depth of the Kumering Fault is 17 ± 3 km. It proves that SFZ is rigid block. However, building more GPS sites in area study is mandatory to obtain better result.

Effects of 15th January 2010 Annular Solar Eclipse on Traveling Ionospheric Disturbances and Equatorial Plasma Bubbles over Low Latitude Regions of East Africa

The influence of the 15th January 2010 annular solar eclipse on traveling ionospheric disturbances (TIDs) and equatorial plasma bubbles (EPBs) is studied using data from six global navigation and satellite system (GNSS) receivers spread across the path of annularity over the low latitude region of East Africa. The GNSS receivers are stationed at Nairobi (RCMN), Malindi (MAL2), and Eldoret (MOIU) in Kenya; Mbarara (MBAR) in Uganda; Kigali (NURK) in Rwanda; and Mtwara (MTWA) in Tanzania. The study period ranges from 12th to 18th January 2010, three days before and after the 15th January 2010 annular solar eclipse. The year 2010 marked the beginning phase of solar cycle 24, evidently observed in low total electron content (TEC) values and the disturbed storm time index (Dst). The eclipse started at 7 : 06 LT and ended at 10 : 14 LT, with MOIU and RCMN experiencing eclipse magnitudes of 0.946 and 0.93, respectively. The maximum obscuration occurred between 8 : 21 LT and 8 : 34 LT across most of the stations. A detrending on vertical TEC (VTEC) derived from GNSS receivers across or close to the path of totality revealed a reduction of ∼2-3 TECU during the maximum phase of the eclipse. The level of reduction was highly close to the totality path and decreased smoothly away from the totality path. Using a background polynomial fitting technique on diurnal TEC, we analyzed TIDs along NURK-MBAR-MOIU and MOIU-RCMN-MAL2 GPS arrays. The results revealed a wavelike perturbation with a virtual horizontal velocity of 830m/s and ∼1 TECU amplitude propagating eastward along the MOIU-RCMN-MAL2 GPS array. The study reports a moderate scintillation activity of 0.5 ≤ ROTI ≤ 0.9 values, demonstrating the presence of few EPBs over the region. The results show a latitudinal variation in GPS-TEC scintillation activities and suggest a possible influence of the eclipse on the observed increase in average scintillation levels across East Africa.

Storm‐Time Observations of Traveling Ionospheric Disturbances and Ionospheric Irregularities in East Africa

AbstractIn this paper equatorward propagating large scale traveling ionospheric disturbance (LSTID) on 17 March 2015 were investigated using the Statistical Angle‐of‐Arrival and Doppler Method for GPS (SADM‐GPS) radio interferometry technique in data‐scarce East Africa. To apply the SADM‐GPS method, 5 GPS arrays each with 3 GPS receivers arranged in a triangular geometry were used. Our results show that during 15:00–18:00 UT on 17 March, TIDs with mean horizontal velocities between 161.9 and 464.4 m/s were observed. Using the wavelet analysis, the periods of TIDs in a range of 51–69 min that qualify to LSTIDs were revealed. The peak‐to‐peak phase shift of detrended total electron content (TEC) over latitudes in this study confirms the equatorward TID propagation, which was obtained by the SADM‐GPS technique. A pair of magnetometers were used to infer E × B drift and an adequate agreement was found with Swarm satellites derived plasma density that enabled us to explain the behaviour of ionospheric irregularity during the main phase of the geomagnetic storm. Moreover, significant TEC enhancement (reaching ∼20%–105%) were captured by GPS arrays during the period of TID propagation. Nevertheless, the rate of change of TEC (ROT) and ROT index (ROTI) show wavy structures that reflect TID effects over ionospheric modulations during the post noon to evening hours of 17 March 2015.

Comparison of Deformation Vectors Due to Earthquake in Subduction Zone and Sumatran Fault for Each Phase of Earthquake Cycle

This study compares the deformation in West Sumatra due to the earthquakes in the subduction zone and the Sumatran Fault. The Mw6.0 Mentawai earthquake 2019 with a thrust fault mechanism and the Mw5.4 South Solok earthquake 2019 with a strike-slip fault mechanism were used as case studies for the subduction zone and Sumatran Fault, respectively. The deformation was observed using 12 SuGAr (Sumatra GPS Array) and 8 InaCORS (Indonesian Continuously Operating Reference Station) stations, which were processed using GAMIT/GLOBK software. There are differences in the deformation vectors of the two earthquakes. The Mentawai earthquake experienced larger energy accumulation than the South Solok earthquake. The coseismic phase of the Mentawai earthquake experienced the largest horizontal shift at the SLBU station, which was 15.48 mm in the direction of S29.96W, while the South Solok earthquake is found to horizontally shift the CSDH station at the size of 5.75 mm towards S11.45E. The postseismic phase of the Mentawai earthquake lasted 60 days, longer than the South Solok earthquake (20 days). The difference in deformation characteristic between these two earthquakes found in this study will be valuable information in modeling earthquakes in Sumatra.

GEODETIC SLIP RATE ESTIMATES FOR THE KUMERING AND SEMANGKO SEGMENTS OF THE SUMATERA FAULT

<p>The Sumatran fault is a right lateral active inland fault in southern Sumatra, Indonesia. Although historical earthquake records have shown that magnitude 7 class earthquakes have occurred during the last century, the slip rates along the Sumatran fault have not been studied in detail. This is the first research using a new dense GPS array, in which stations are orthogonal to the fault, to analyze the fault slip rates along the Kumering and Semangko segments in southern Sumatra. In this study, we process GPS data from 14 campaign and continuous GPS points. The results show velocities of 14 mm/yr and 15 mm/yr for these two fault segments, respectively. Our estimated geodetic slip rate suggests that the Sumatran fault has a relatively homogeneous slip rate from southern to northern Sumatra.</p>

Tropospheric Delay in the Neapolitan and Vesuvius Areas (Italy) by Means of a Dense GPS Array: A Contribution for Weather Forecasting and Climate Monitoring

Studying the spatiotemporal distribution and motion of water vapour (WV), the most variable greenhouse gas in the troposphere, is pivotal, not only for meteorology and climatology, but for geodesy, too. In fact, WV variability degrades, in an unpredictable way, almost all geodetic observation based on the propagation of electromagnetic signal through the atmosphere. We use data collected on a dense GPS network, designed for the purposes of monitoring the active Neapolitan (Italy) volcanoes, to retrieve the tropospheric delay parameters and precipitable water vapour (PWV). This study has two main targets: (a) the analysis of long datasets (11 years) to extract trends of climatological meaning for the region; (b) studying the main features of the time evolution of the PWV during heavy raining events to gain knowledge on the preparatory stages of highly impacting thunderstorms. For the latter target, both differential and precise point positioning (PPP) techniques are used, and the results are compared and critically discussed. An increasing trend, amounting to about 2 mm/decades, has been recognized in the PWV time series, which is in agreement with the results achieved in previous studies for the Mediterranean area. A clear topographic effect is detected for the Vesuvius volcano sector of the network and a linear relationship between PWV and altitude is quantitatively assessed. This signature must be taken into account in any modelling for the atmospheric correction of geodetic and remote-sensing data (e.g., InSAR). Characteristic temporal evolutions were recognized in the PWV in the targeted thunderstorms (which occurred in 2019 and 2020), i.e., a sharp increase a few hours before the main rain event, followed by a rapid decrease when the thunderstorm vanished. Accounting for such a peculiar trend in the PWV could be useful for setting up possible early warning systems for those areas prone to flash flooding, thus potentially providing a tool for disaster risk reduction.

Dependence of Precipitation on Precipitable Water Vapor Over the Maritime Continent and Implications to the Madden‐Julian Oscillation

AbstractThe weakening of the Madden‐Julian Oscillation (MJO) as it propagates over the Maritime Continent (MC) is often referred to as the MC barrier. Here, we use 3‐hourly precipitable water vapor (PWV) data obtained from the Sumatran GPS Array and the ERA5 reanalysis to investigate the role played by the column moisture over the MC. Over Sumatra and the whole MC, we find a stronger dependence of precipitation on PWV over the ocean as compared to both inland and coastal regions. The MJO modulates the PWV over the ocean and over the MC by roughly the same amount, and the weaker precipitation variations over the MC between the MJO phases may be interpreted in terms of its weaker dependence on PWV over the MC. This different precipitation dependence on column moisture between the MC and the ocean may contribute to the MC barrier effect.

Pre-and Coseismic Analysis of the Mw7.6 Padang Earthquake 2009 from Geodetic Approach

On September 30, 2009, an Mw7.6 earthquake hit the west coast of Sumatra. This main shock caused the crustal deformation affecting a wide area in the western part of Sumatra. This paper presents the analysis of the deformation recorded by the Sumatran GPS Array (SuGAr) network. More than 40 GPS stations were installed to continuously monitor the deformation in Sumatra, among 12 of them is known to have been deformed by this earthquake. We processed whole data in 2009 to understand the dynamics of western part of Sumatra before, during and after the earthquake. Relative daily position data is obtained from postprocessing with GAMIT/GLOBK software, implementing the DGPS technique to estimate daily position from GPS RINEX data. GPS measurements shows that during the earthquake, most stations in Mentawai is deformed toward the southwest direction, varies from 8 – 50 mm, and the stations in mainland Sumatra is deformed toward various direction, varies from 5 – 28 mm. There is no anomaly and strange behaviour before the event as the precursor, and the deformation trend one day after the events seems to return to the preseismic pattern.

Interplate coupling along the Caribbean coast of Colombia and its implications for seismic/tsunami hazards

Northwestern Colombia is located at a convergent plate boundary between the Caribbean plate and the North Andean Block; however, no megathrust earthquakes have been reported during last 500 years. In order to evaluate seismic potential in this area, we analyze GPS data during 2007–2018 from GeoRED – the nationwide GPS array in Colombia – to obtain interseismic 3-dimensional velocities. GPS velocity data indicate the northern part of the North Andean Block is differentiated as another block and we name it Macondo Block. The velocity data are then inverted to estimate interplate coupling on the subducting Caribbean plate interface. The result shows an isolated, fully locked patch south of the city of Cartagena extending from 9.0 to 20 km in depth on the subduction interface with an area of ~11,000 km2. The estimated locked patch implies a potential of M-8 class earthquake with an average recurrence time of ~600 years, which is consistent with the absence of such an event in the available historical record. As another possibility, the observed surface deformation may be accommodated inelastically. A comparison of geological and geodetic strain rates in the convergence direction shows that the geological one is smaller by one or two orders of magnitude and does not support such an interpretation though the estimate itself still has a large uncertainty. For improving the evaluation of earthquake/tsunami potential, it is essential to conduct a careful geological investigation to identify recent evidence of crustal shortening or paleotsunami along northwestern Colombia.

Deformation analysis for Padang earthquake using GPS data

On September 30, 2009, an Mw 7.6 earthquake occurred off the coast of Sumatra. This earthquake caused deformation in the area around the center of the earthquake. Surface deformation in the area around the epicenter was Analyzed carried out using GPS data from the SuGAr (Sumatran GPS Array) network. The observed SuGAr stations were TIKU, PSKI, NGNG and MSAI. Data processing used GAMIT/GLOBK software. GPS data were observed for 100 days before and after the main earthquake occurrence. The measurement results indicate that the MSAI and NGNG stations in Mentawai deformed towards the south-weast direction, varies between 38 - 50 mm, and TIKU and PSKI stations on the mainland of Sumatra were deformed in north-west directions, varies from 5 - 12 mm. This indicates that this Earthquake deformed the area of Western Sumatra and nearby Mentawai regions, in Siberut Island.

Selected Years of Monsoon Variations and Extratropical Dry-Air Intrusions Compared with the Sumatran GPS Array Observations in Indonesia

Selected Years of Monsoon Variations and Extratropical Dry-Air Intrusions Compared with the Sumatran GPS Array Observations in Indonesia

On the Diurnal Cycle of GPS-Derived Precipitable Water Vapor over Sumatra

AbstractConvective processes in the atmosphere over the Maritime Continent and their diurnal cycles have important repercussions for the circulations in the tropics and beyond. In this work, we present a new dataset of precipitable water vapor (PWV) obtained from the Sumatran GPS Array (SuGAr), a dense network of GPS stations principally for examining seismic and tectonic activity along the western coast of Sumatra and several offshore islands. The data provide an opportunity to examine the characteristics of convection over the area in greater detail than before. In particular, our results show that the diurnal cycle of PWV on Sumatra has a single late afternoon peak, while that offshore has both a midday and a nocturnal peak. The SuGAr data are in good agreement with GPS radio occultation data from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) mission, as well as with imaging spectrometer data from the Ozone Measuring Instrument (OMI). A comparison between SuGAr and the NASA Water Vapor Project (NVAP), however, shows significant differences, most likely due to discrepancies in the temporal and spatial resolutions. To further understand the diurnal cycle contained in the SuGAr data, we explore the impact of the Madden–Julian oscillation (MJO) on the diurnal cycle with the aid of the Weather Research and Forecasting (WRF) Model. Results show that the daily mean and the amplitude of the diurnal cycle appear smaller during the suppressed phase relative to the developing/active MJO phase. Furthermore, the evening/nighttime peaks of PWV offshore appear later during the suppressed phase of the MJO compared to the active phase.

EFEK GELOMBANG TSUNAMI ACEH 2004 PADA GANGGUAN IONOSFER BERGERAK SKALA MENENGAH DARI PENGAMATAN JARINGAN GPS SUMATRA

Medium Scale Travelling Ionospheric Disturbance (MSTID), thought to be manifestation of atmospheric gravity wave (AGW) in the ionospheric altitude that propagates horizontally and effects on in the electron density structure of ionosphere. These atmospheric gravity waves sourced from lower atmospheric activities such as typhoons, volcanic eruptions and tsunamis. Wave energy by its coupling induction process can travel to the ionosphere region. It has been understood that the TID's wave structure have an impact on the propagation of radio waves in the ionosphere so that it will affect the performance of navigation satellite-based positioning measurements. Based on Aceh tsunami in December 2004, this study aimed to investigation of the induction of atmospheric gravity waves in the ionosphere using total electron content (TEC) data from the Sumatra GPS network (Sumatra GPS Array, SUGAR). The detection technique of TEC changes due to AGW induction with a filter to separate medium scale disturbance at the ionospheric pierce point at an altitude of 350 km (IPP, Ionospheric Pierce Point). The results show the horizontal wavelength of a medium-scale TID around 180 ± 12 Km with a velocities of around 376 ± 9 ms-1. Based on two-dimensional map, the TID moves to the southeast.

Coulomb stress perturbation after great earthquakes in the Sumatran subduction zone: Potential impacts in the surrounding region

The Sumatran subduction zone has been seismically very active compared with other major subduction zones in the world. Since 2004, a sequence of large earthquakes ruptured along the trench, including the 2004 Mw 9.2 Sumatra-Andaman, the 2005 Mw 8.6 Nias-Simeulue, the 2007 Mw 8.4 Bengkulu, the 2010 Mw 7.8 Mentawai tsunami earthquakes and numerous moderate and small events. These earthquakes released stress over tens of seconds to minutes, disturbing the lithosphere and asthenosphere across a broad region. As earthquake-introduced stress disturbance can trigger tremendous aftershocks and sometimes precipitate large earthquakes in their vicinity, gaining a detailed picture of spatial and temporal stress evolution on the fault and its neighboring faults is crucial for seismic forecasting. Here, we have developed spatiotemporal Coulomb stress models for the Sumatran region, which includes the Sumatran subduction zone and the Sumatran fault, using a well-studied postseismic model. This postseismic model is constrained by decade-long time series of geodetic observations from the Sumatran GPS Array (SuGAr) and from tide gauge data in Southeast Asia. We show that Coulomb stress changes imparted by co- and post-seismic processes of previous great ruptures could well explain the temporal and spatial connection between seismicity and stress evolution. Most importantly, our results reveal that the stress in the Mentawai seismic gap of the Sumatran subduction zone was loaded by more than two bars. These stress perturbations could potentially trigger the rupture of the Mentawai seismic gap, which was already overdue in the last seismic cycle. Now, the likelihood for the failure of this gap is even higher than before 2004. We also find that along the Sumatran fault stress has increased to more than one bar, which may explain the surrounding seismicity. Consequently, we highlight the considerable seismic hazard and associated tsunami threat to the neighboring regions.