A highly stressed area where eventual ruptures have often been observed to nucleate is characterized by low b values of earthquake frequency‐size distribution. Crustal deformation due to the ...occurrence of large earthquakes causes stress perturbation in nearby regions, so an investigation into spatiotemporal b values can play a crucial role in the distribution of postseismic hazards after the 2016 Kumamoto earthquake sequence along the Futagawa‐Hinagu fault zone, which culminated in the magnitude 7.3 mainshock. Together with an analysis of aftershock decay p value that can be used to infer stressing history, a highly stressed area with a characteristic dimension of 10 km at the southern end of the causative faults was found. Our observation is explained by postseismic deformation due to an afterslip on the causative faults and viscoelastic relaxation model. Similar to the Kumamoto mainshock rupture, which started at a low‐b‐value area, the observed highly stressed area shows a high likelihood of future earthquake ruptures.
Plain Language Summary
Earthquake frequency‐size distribution before and after the 2016 Kumamoto (Japan) earthquakes and the aftershock decay infer heterogeneity of the stress field. Slips on the causative fault zone and flow in the Earth's interior explain this stress field and increase stress near one end of the causative fault zone. Our result implies a seismic hazard in which a future earthquake rupture is more likely to start at the stress‐increasing area than in other regions.
Key Points
Earthquake frequency‐size distribution before and after the 2016 Kumamoto earthquake and the aftershock decay reveal stress heterogeneity
The stress field that showed a low‐b‐value zone near the eventual mainshock reveals a zone with decreasing b on the Hinagu fault
The northernmost of the southern Hinagu fault that remains unruptured is considered to be highly stressed with potential future ruptures
Following a lightning strike to a wind turbine in Japan, we have observed a large burst of neutrons lasting 100 ms with a ground fluence of ~1,000 n cm−2, thousands of times greater than the peak ...neutron flux associated with the largest ground level solar particle event ever observed. This is the first detection of an unequivocal signature of neutrons from a terrestrial gamma ray flash, consisting of a 2.223 MeV gamma‐ray spectral line from a neutron‐capture on hydrogen reaction occurring in our detector, and is shown to be consistent with the production of 1012–1013 photoneutrons from a downward terrestrial gamma ray flash (TGF) at 1.0 km, with a gamma ray brightness typical of upward TGFs observed by satellites.
Key Points
Gamma rays producing a neutron flash with a ground fluence of 1,000 n cm−2 and 100 ms duration was observed following a lightning strike
This observation is consistent with simulations of photoneutron production from a terrestrial gamma ray flash (TGF) with typical brightness
These measurements definitively show that low altitude atmospheric electricity can drive nuclear processes
Plain Language Summary
Using detectors to measure gamma‐rays, we have observed a cloud of neutrons following a lightning strike to a wind turbine in Japan.
Seismicity and geoelectric potential changes, possibly associated with the seismic swarm activity in 2000 in the Izu Island region, Japan, are analyzed in the framework of the natural time χ, which ...has been recently introduced as an index of the kth event χ = k/N, where N is the total number of events. The Izu 2000 swarm activity lasted for about 2 months with some 7000 shocks with magnitude M ≥ 3 and five M ≥ 6 shocks, and was preceded by a pronounced electrical activity with innumerable signals that started 2 months prior to the swarm onset. It is shown, first, that the seismicity subsequent to the electrical activity approaches to a critical stage a few days before the occurrence of the first M ≥ 6 shock and, second, that the electrical signals also have the properties characteristic to the critical stage. Despite the big differences in time scale and numbers of electric signals and earthquakes, these features are found similar to those in Greece. The present results suggest that both in Greece and Japan, the electrical activity as well as the subsequent seismicity may have a self‐similar structure and exhibits similar dynamic evolution toward critical stage.
Crustal deformation due to the 2016 earthquake sequence in Kumamoto, Japan, that culminated in a preceding earthquake of magnitude M6.5 and a subsequent M7.3 earthquake 28 h later, caused stress ...perturbation on and around the causative Futagawa-Hinagu fault zone. Monitoring changes in seismicity pattern along this zone plays a role in understanding the process before and after major earthquakes. For this purpose, stress-dependent laws in statistical seismology can be used: the Gutenberg-Richter frequency-size law and the Omori-Utsu aftershock-decay law. We review the results obtained by using these laws in previous studies to show a zone of high stress near the eventual epicenters of the M6.5 and M7.3 earthquakes before the start of the Kumamoto sequence, and after it, showing a decreasing trend in stress along the Futagawa-Hinagu fault zone. Detailed analysis suggests aseismic slips along the causative fault zone. The aseismic preslip locally reduced stress just prior to the M7.3 earthquake near its epicenter. Recently, a system was proposed by Gulia and Wiemer (2019) that utilizes the Gutenberg-Richter frequency-size law to judge, immediately after a large earthquake, whether it was the mainshock or a foreshock to a future event. Based on the reviewed results and our new results, further research that takes into account the spatial variation of frequency-size distribution, allowing the exploration of the possibility of a local preslip of a future nearby earthquake, is needed to improve this system.
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•Changes in seismicity and stress distribution due to the 2016 Kumamoto earthquakes.•Stress field revealed aseismic pre- and after-slow slips along the causative faults.•System to discriminate foreshocks and aftershocks was tested for the Kumamoto case.•Test showed that seismicity before the largest event was regarded as aftershocks.•Aseismic slips and changes in stress are keys to modify the discrimination system.
Gamma‐ray glows are observational evidence of relativistic electron acceleration due to the electric field in thunderclouds. However, it is yet to be understood whether such relativistic electrons ...contribute to the initiation of lightning discharges. To tackle this question, we started the citizen science “Thundercloud Project,” where we map radiation measurements of glows from winter thunderclouds along Japan's sea coast area. We developed and deployed 58 compact gamma‐ray monitors at the end of 2021. On 30 December 2021, five monitors simultaneously detected a glow with its radiation distribution horizontally extending for 2 km. The glow terminated coinciding with a lightning flash at 04:08:34 JST, which was recorded by the two radio‐band lightning mapping systems, FALMA and DALMA. The initial discharges during the preliminary breakdown started above the glow, that is, in vicinity of the electron acceleration site. This result provides one example of possible connections between electron acceleration and lightning initiation.
Plain Language Summary
Thunderstorms are natural particle accelerators. The strong electric field inside thunderclouds accelerates relativistic electrons, which emit gamma rays via interaction with the atmosphere. High‐energy photons generated in this process have been observed as radiation enhancements called gamma‐ray glows. Winter thunderclouds along the sea of Japan are an ideal target for monitoring glows because their altitudes are usually sufficiently low for the generated gamma‐ray photons to reach the ground. We started a new citizen science “Thundercloud Project” in this area, where we distributed radiation detectors to citizen supporters to observe glows and to reveal their relationship with the aerological condition and lightning discharges. On 30 December 2021, five of those sensors detected a glow from a single thundercloud. Two of them recorded a sudden termination of the glow coinciding with a lightning flash, which was monitored by our two radio mapping systems of FALMA and DALMA. The initial discharges of the flash started at a location about 1.6 km above the glow region with an unusually fast downward progression. This paper is the first report of our citizen science project. We discuss the possibility that accelerated electrons contribute to the initiation of lightning discharges.
Key Points
We started the citizen science “Thundercloud Project,” a multi‐point observation campaign of gamma‐ray glows from thunderstorms
On 30 December 2021, five radiation monitors detected a 2‐km‐long size gamma‐ray glow, which suddenly terminated with a lightning flash
Two radio mapping systems of lightning identified the initiation of the discharges, which started at a location above the glow region
At 17:47 UT on 20 September 1999, a large earthquake of magnitude Mw 7.6 struck the central Taiwan near a small town of Chi‐Chi. The ground‐based receivers of the global positioning system (GPS) in ...the Taiwan area detected coseismic ionospheric disturbances (CIDs) in the total electron content (TEC) triggered by the Chi‐Chi earthquake. When the CIDs travel away from the origin on the Earth surface and then propagate into the ionosphere, their amplitudes and periods generally become smaller and longer, respectively. Moreover, two global grid searches, adapting the ray‐tracing and the beam‐forming techniques, have been used to analyze the observed GPS TEC. We have not only estimated the average speed of the CIDs propagating in the atmosphere and ionosphere but also determined the location of CID origin on the Earth surface by using the two techniques. The results show that the observed CIDs result from shock‐acoustic waves triggered by sudden and large vertical motions of the Chi‐Chi earthquake.
•Topdressing urea in green-harvested sugarcane results in high ammonia (NH3) losses.•High amounts of crop residues and tropical conditions reduce the efficiency of NBPT.•Doubling current NBPT ...concentration in urea (530mgkg−1) reduces NH3 losses from urea.•NBPT concentrations ≫>1000mgkg−1 urea do not grant better results.
Nitrogen loss as ammonia (NH3) is a major problem when urea is topdressed over crop residues. The treatment of urea with N-(n-butyl) thiophosphoric triamide (NBPT) temporarily inhibits urease activity and reduces NH3 volatilization loss under many agroecological conditions. However, the amount of straw on the soil in green sugarcane trash blanketing (GCTB) systems affects the success of NBPT in reducing NH3 loss. We hypothesized that an increase in the NBPT concentration in urea above the current level (530mg NBPT kg−1 urea) in Brazil is necessary to reduce the NH3 volatilization and to improve the efficiency of urea in GCTB systems. We evaluated the NH3 loss from urea treated with NBPT under field conditions in GCTB systems. Six field trials were conducted across the State of São Paulo, the main sugarcane-growing region in Brazil. The treatments were urea, ammonium nitrate, urea treated with NBPT at concentrations of 530, 850, 1500, and 2000mgkg−1, and a control plot without N fertilizer. The amount of volatized NH3 was assessed through a closed semi-static collector containing two acid-trap foam disks, and analyzed by colorimetry using flow injection analysis. NBPT concentrations above 530mgkg−1 delayed the time of maximum rate of loss and also reduced cumulative NH3 loss. Ammonia emissions were linearly reduced with increasing NBPT concentrations until 1000mgkg−1. Any further increase in the NBPT concentration did not result in a substantial reduction of NH3 volatilization. Increasing the NBPT concentration in urea is an effective approach to optimize the adoption of urea-based fertilizers in tropical GCTB systems.
An on‐ground observation program for high‐energy atmospheric phenomena in winter thunderstorms along the Japan Sea has been performed via measurements of gamma ray radiation, atmospheric electric ...field, and low‐frequency radio band. On 11 February 2017, the radiation detectors recorded gamma ray emission lasting for 75 s, and then abruptly terminated with a nearby lightning discharge. The gamma ray spectrum extended up to 20 MeV and was reproduced by a cutoff power law model with a photon index of
1.36−0.04+0.03, being consistent with Bremsstrahlung radiation from a thundercloud (known as a gamma‐ray glow or a thunderstorm ground enhancement). The low‐frequency radio monitors, installed ∼50 km away from the gamma ray observation site recorded leader development of an intracloud/intercloud discharge spreading over ∼60 km area with a ∼300‐ms duration. The timing of the gamma ray termination coincided with the moment when the leader development of the intracloud/intercloud discharge passed 0.7 km horizontally away from the radiation monitors. The intracloud/intercloud discharge started ∼15 km away from the gamma ray observation site. Therefore, the glow was terminated by the leader development, while it did not trigger the lightning discharge in the present case.
Plain Language Summary
This study presents high‐energy radiation, atmospheric electric field and low‐frequency radio measurements of winter thunderstorms in Japan. Long‐duration gamma ray bursts, called “gamma ray glows,” are thought to originate from electrons accelerated and multiplied by strong electric fields in thunderclouds. There are unsolved questions such as electron‐acceleration mechanisms, position of the acceleration site, lifetime, and life cycle of the bursts. We observed a gamma ray burst lasting for ∼75 s from a thundercloud abruptly terminated with a lightning discharge. The gamma ray source was destroyed by a cloud‐to‐cloud discharge over the gamma ray observation site but not related to the triggering of the discharge in the present case.
Key Points
A gamma ray glow and its termination with a lightning discharge was observed in a Japanese winter thunderstorm
The glow was terminated by leader development of a horizontally long intracloud/intercloud discharge passing nearby overhead
The intracloud/intercloud discharge was not triggered by the glow in the present case because it started far from the gamma ray glow site
Lightning is an important source of nitrogen oxides (LNO
x
). The actual global production of LNO
x
is still largely uncertain. One of the reasons for this uncertainty is the limited available ...observation data. We measured the concentrations of total reactive nitrogen (NO
y
), nitric oxide (NO) and nitrogen dioxides (NO
2
) and then obtained NO
x
oxidation products (NO
z
: NO
z
= NO
y
- NO
x
) at a station at the top of Mount Fuji (3776 m a.s.l.) during the summer of 2017. Increases in NO
y
and NO
2
were observed on 22 August 2017. These peaks were unaccompanied by increases in CO, which suggested that the observed air mass did not contain emissions from combustion. The backward trajectories of the above air mass indicated that it moved across areas where lightning occurred. The NO
y
concentration was also calculated by using a chemical transport model, which did not take NO
x
produced by lightning into account. Therefore, the NO
y
concentration due to lightning can be inferred by subtracting the calculated NO
y
from the observed NO
y
concentrations. The concentration of NO
y
at 13:00 on 22 August 2017 originating from lightning was estimated to be 1.11 ± 0.02 ppbv, which comprised 97 ± 2% of the total NO
y
concentration. The fractions of NO
2
and NO
z
in the total NO
y
were 0.54 ± 0.01 and 0.46 ± 0.03, respectively. The NO concentration was below the detection limit. We firstly observed increase of concentrations of NO
y
originating from lightning by ground-based observation and demonstrated the quantitative estimates of LNO
x
using model-based calculation.
Daytime longitudinal structures of the electron density (Ne) and temperature (Te) in the topside ionosphere observed by Hinotori and DEMETER are examined under various conditions of solar flux, local ...time, and seasons. Results from both satellites show a similar longitudinal Ne structure in the morning from July to October, although the value of Ne observed by Hinotori is higher than that of DEMETER owing to higher solar flux. This result implies that the longitudinal structure of Ne may appear in any solar cycle. Further, a negative correlation between Ne and Te in the longitudinal structures appears in the morning when Ne is low, while a positive correlation appears around the magnetic equator when Ne is sufficiently enhanced during noontime in the high solar flux. A spectrum analysis performed on the DEMETER data reveals that wave numbers 1–2 for Ne and Te are dominant and nondominant. The observed wave numbers 3–4 for Ne are dominant during November–May and June–October, while they are dominant for Te during October–June and July–September. Both Ne and Te show the largest power of wave number 3 in December and wave number 4 in September. Further, observed annual variations of wave numbers 3–4 for Ne and Te also differ from wave numbers 3–4 generated by waves in the lower thermosphere. It can be interpreted as discrepancies between the longitudinal distributions of Ne and Te caused by difference in the condition of zonal winds driving E region dynamo and meridional winds modulating the ionospheric plasma structures.
Key Points
Annual variations of longitudinal structure of Ne and Te in the ionosphere
Wave‐4 and wave‐3 dominate in December and September
Zonal and medrional wind affect longitudinal structure of Ne and Te
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