arXiv subjects

K. Z. Nanjo

Publications and source records attributed to K. Z. Nanjo.

11 recordsLinked to original sources

arXiv2026

Seismic quiescence and activation prior to the 2025 M8.8 Kamchatka, Russia earthquake

The 2025 M8.8 Kamchatka earthquake in the Kamchatka-Kuril subduction system provides a unique opportunity to investigate the preparatory processes of a great subduction event. Despite Kamchatka's high seismic activity, the long-term evolution of seismicity preceding major ruptures has been poorly documented. Identifying temporal patterns-such as multiyear quiescence and short-term activation-is essential for understanding megathrust failure processes. We applied the Epidemic-Type Aftershock Sequence model and change-point analysis to earthquakes with M5+ within a 100-km radius of the 2025 mainshock epicenter, using the Advanced National Seismic System catalog spanning 1975-2025. This approach quantified temporal variations in the seismicity rate and detected statistically significant change points. We found a pronounced approximately 20-year quiescent interval beginning around mid-2003, followed by an abrupt activation that commenced with the M7.4 foreshock on 20 July 2025. Similar quiescence-to-activation sequences near the eventual hypocenter have been reported in previous studies for the 1997 M7.8 Kronotsky and 2006 M8.3 Simushirskoe earthquakes, which ruptured segments immediately north and south of the 2025 rupture, respectively. Together with these earlier studies, our findings suggest that such quiescence-to-activation transitions may represent a recurring preparatory process for megathrust earthquakes in the Kamchatka-Kuril subduction system, although this inference is based on a small number of events. Decadal-scale quiescence has also been reported off Hokkaido, Japan, overlapping the area assessed as having an elevated likelihood of a 17th-century-type M9-class earthquake. Given that Hokkaido is in the Kamchatka-Kuril subduction system, tracking a possible transition from quiescence to activation is important for situational awareness and time-dependent hazard assessment.

physics.geo-ph↗

arXiv2025

Non-randomness of Japan megaquakes implied by stress recovery and accumulation

Monitoring stress recovery and accumulation associated with megaquakes helps to assess their recurrence. Previous studies proposed a high likelihood of imminent recurrence for the 2011 Tohoku and the 17th-century Hokkaido megaquakes belonging to the magnitude-9 class, although their current stress state remains uncertain. Here we compare the occurrence of small earthquakes relative to larger ones, using b-values, showing high b-values in the source area of the Tohoku earthquake, indicating low stresses. In contrast, low b-values occurred in the source area of the 17th-century earthquake, indicating high stresses as seen before the Tohoku event. Around the low-b-value zone, phenomena that provide insight into subsequent large earthquakes are observed, and which were reported for worldwide earthquakes: seismic quiescence, a seismic gap, strong plate coupling, and slow earthquake activity avoiding megaquakes' rupture zones. Results imply that the Hokkaido and Tohoku megaquakes occur nonrandomly in time, rather their recurrence intervals are more characteristic.

physics.geo-ph↗

arXiv2025

Changes in seismicity in a volcanically active region of the Izu Peninsula, Japan

The eastern Izu Peninsula in Japan is volcanically and seismically active. Ordinary earthquakes frequently occurred at shallow depths in 2006 and 2009, when they clustered as swarms. Beneath ordinary earthquakes, low-frequency earthquakes (LFEs) were infrequently observed. To better understand the characteristics of those LFEs, we produced a LFE catalog for 2005-2020, using the matched-filter method. Timeseries analyses based on the Epidemic-Type Aftershock Sequence model showed quiescence, i.e., a change in occurrence rate, which became quiet. For comparison, the same analysis was conducted using the Japan Meteorological Agency catalog of ordinary earthquakes, and similar results were shown. The change points for both types of earthquakes fell during and after each of the swarms, revealing an earlier start of quiescence for shallow ordinary earthquakes than for deep LFEs. Surface displacement data obtained from the Geospatial Information Authority of Japan showed that the uplift was in abatement or unobservable during the study period. Our seismicity and geodetic results are interpretatively hypothesized as being associated with magmatic activity, as follows. When the absence of a supply of magma from a depth toward the crustal magma reservoir caused no significant uplift, magma intruded from this reservoir into the shallow crust, resulting in the 2006 and 2009 swarms. Due to this intrusion, stresses decreased in and around shallow parts of the reservoir and then quiescence for ordinary earthquakes began during or after each swarm. Due to this decrease, stresses in and around deep parts of it late decreased, delaying the start of quiescence of deep LFEs.

physics.geo-ph↗

arXiv2024

Earthquake detection capacity of the Dense Oceanfloor Network system for Earthquakes and Tsunamis (DONET)

We studied the earthquake detection capacity of DONET (Dense Oceanfloor Network system for Earthquakes and Tsunamis) operating in the Nankai Trough, a target region monitored for future megathrust earthquakes. The focus of this paper was to evaluate the impact on this capacity from the malfunction of parts of the network. For this purpose, the completeness magnitude, above which all earthquakes are considered to be detected by a seismic network, was used. Then, a catalog that includes events observed by DONET was used. We found spatiotemporal variability of completeness magnitude, ranging from values below 1 in one of the areas where stations are densely deployed to values above 2 at the periphery and outside of the DONET area. We conducted a simulation computation for cases of malfunction of densely distributed stations. The results showed that completeness estimates in the area near the malfunctioning stations were about 1 magnitude larger. This implies that malfunction repair and/or replacement with new stations would be desirable because they pronouncedly affect earthquake monitoring. We then demonstrated an example of how to use the information of completeness magnitude as prior knowledge to compute the b value of the Gutenberg-Richter distribution. The result indicates the b value as a proxy that can help to image stress heterogeneity when there is a magnitude-6 class slow slip event on the Nankai Trough plate boundary.

physics.geo-ph↗

arXiv2021

Changes in seismicity pattern due to the 2016 Kumamoto earthquake sequence and implications for improving the foreshock traffic-light system

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 hours 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.

physics.geo-ph↗

arXiv2021

Changes in the b value in and around the focal areas of the M6.9 and M6.8 earthquakes off the coast of Miyagi prefecture, Japan, in 2021

We investigated changes in the b value of the Gutenberg-Richter's law in and around the focal areas of earthquakes on March 20 and on May 1, 2021, with magnitude (M) 6.9 and 6.8, respectively, which occurred off the Pacific coast of Miyagi prefecture, northeastern Japan. We showed that the b value in these focal areas had been noticeably small, especially within a few years before the occurrence of the M6.9 earthquake in its vicinity, indicating that differential stress had been high in the focal areas. The coseismic slip of the 2011 Tohoku earthquake seems to have stopped just short of the east side of the focus of the M6.9 earthquake. Furthermore, the afterslip of the 2011 Tohoku earthquake was relatively small in the focal areas of the M6.9 and M6.8 earthquakes, compared to the surrounding regions. In addition, the focus of the M6.9 earthquake was situated close to the border point where the interplate slip in the period from 2012 through 2021 has been considerably larger on the northern side than on the southern side. The high-stress state inferred by the b-value analysis is concordant with those characteristics of interplate slip events. We found that the M6.8 earthquake on May 1 occurred near an area where the b value remained small, even after the M6.9 quake. The ruptured areas by the two earthquakes now seem to almost coincide with the small-b-value region that had existed before their occurrence. The b value on the east side of the focal areas of the M6.9 and M6.8 earthquakes which corresponds to the eastern part of the source region of the 1978 off-Miyagi prefecture earthquake was consistently large, while the seismicity enhanced by the two earthquakes also shows a large b value, implying that stress in the region has not been very high.

physics.geo-ph↗

arXiv2019

Capability of Tokai Strainmeter Network to Detect and Locate a Slow Slip: First Results

The Tokai Strainmeter Network (TSN), a dense network deployed in the Tokai region, which is the easternmost region of the Nankai trough, has been designed to monitor slow slips that reflect changes in the coupling state of the plate boundary. It is important to evaluate the current capability of TSN to detect and locate slow slips. For this purpose, the probability-based magnitude of completeness developed for seismic networks was modified to be applicable to the evaluation of TSN's performance. Using 35 slow slips having moment magnitudes M5.1-5.8 recorded by TSN in 2012-2016, this study shows that the probability that TSN detected and located a M5-class slow slip is high (> 0.9) when considering a region in and around the TSN. The probability has been found to depend on the slip duration, especially for M5.5 or larger, namely the longer the duration, the lower the probability. A possible use of this method to assess the network's performance for cases where virtual stations are added to the existing network was explored. The use of this application when devising a strategic plan of the TSN to extend its coverage westwards is proposed. This extension that allows TSN to cover the entire eastern half of the Nankai trough is important, because the historical records show that the eastern half of this trough tends to rupture first.

physics.geo-ph↗

arXiv2016

Anomalous decrease of relatively large shocks and increase of the p and b values preceding the April 16, 2016 M7.3 earthquake in Kumamoto, Japan

The 2016 Kumamoto earthquakes started with a magnitude (M) 6.5 quake on Apr 14 on the Hinagu fault zone (FZ), followed by active seismicity including an M6.4 quake. Eventually, an M7.3 quake occurred on Apr 16 on the Futagawa FZ. We investigated if any sign indicative of the M7.3 quake could be found in the space-time changes of seismicity after the M6.5 quake. We determined in advance the threshold magnitude, above which all earthquakes are completely recorded. We then showed that the occurrence rate of relatively large (M >= 3) earthquakes significantly decreased 1 day before the M7.3 quake. Significance of this decrease was evaluated by one standard deviation of sampled changes in the rate of occurrence. We next confirmed that seismicity with M >= 3 was well modeled by the OU law with p ~ 1.5 +/- 0.3, which indicates that the temporal decay of seismicity was significantly faster than a typical decay with p = 1. The larger p value was obtained when we used data of the longer time period in the analysis. This significance was confirmed by a bootstrapping approach. Our detailed analysis shows that the large p value was caused by the rapid decay of the seismicity in the northern area around the Futagawa FZ. Application of the slope (the b value) of the GR frequency-magnitude distribution to the spatiotemporal change in the seismicity revealed that the b value in the northern area increased significantly, the increase being db = 0.3-0.5. Significance was verified by a statistical test of db and a test using bootstrapping errors. Based on our findings, combined with the results obtained by a stress inversion analysis performed by the NIED, we suggested that stress near the Futagawa FZ had reduced just prior to the occurrence of the M7.3 quake. We proposed, with some other observations, that a reduction in stress might have been induced by growth of the slow slips on the Futagawa FZ.

physics.geo-ph↗

arXiv2016

Seismicity prior to the 2016 Kumamoto earthquakes

We report precursory seismic patterns prior to the 2016 Kumamoto earthquakes, as measured by four different methods based on changes in seismicity that can be used for earthquake forecasting: the b-value method, two methods of seismic quiescence evaluation, and an analysis of seismicity density in space and time. The spatial extent of precursory patterns differs from one method to the other and ranges from local scales (typically, asperity size) to regional scales (e.g., 2 degree x 3 degree around the source zone). The earthquakes were preceded by periods of pronounced anomalies, which lasted in yearly scales (1.5 years), or longer (> 3 years). We demonstrate that a combination of multiple methods detected different signals prior to the Kumamoto earthquakes. This indicates great potential to reduce the hazard at possible future sites of earthquakes relative to long-term seismic hazard assessment. We also found that the seismic quiescence in a regional scale area, detected by using the two methods of seismic quiescence evaluation, was a common precursor to the 2016 Kumamoto earthquakes and 2015 Off Satsuma Peninsula earthquake. The result allows us to interpret both events as the onset that occurred at a section along the tectonic line from the Okinawa Trough through the Beppu-Shimabara graben.

physics.geo-ph↗

arXiv2016

A fiber-bundle model for the continuum deformation of brittle material

The deformation of brittle material is primarily accompanied by micro-cracking and faulting. However, it has often been found that continuum fluid models, usually based on a non-Newtonian viscosity, are applicable. To explain this rheology, we use a fiber-bundle model, which is a model of damage mechanics. In our analyses, yield stress was introduced. Above this stress, we hypothesize that the fibers begin to fail and a failed fiber is replaced by a new fiber. This replacement is analogous to a micro-crack or an earthquake and its iteration is analogous to stick-slip motion. Below the yield stress, we assume that no fiber failure occurs, and the material behaves elastically. We show that deformation above yield stress under a constant strain rate for a sufficient amount of time can be modeled as an equation similar to that used for non-Newtonian viscous flow. We expand our rheological model to treat viscoelasticity and consider a stress relaxation problem. The solution can be used to understand aftershock temporal decay following an earthquake. Our results provide justification for the use of a non-Newtonian viscous flow to model the continuum deformation of brittle materials.

cond-mat.mtrl-sci↗

arXiv2005

Pattern Informatics and Its Application for Optimal Forecasting of Large Earthquakes in Japan

Pattern informatics (PI) technique can be used to detect precursory seismic activation or quiescence and make earthquake forecast. Here we apply the PI method for optimal forecasting of large earthquakes in Japan, using the data catalogue maintained by the Japan Meteorological Agency. The PI method is tested to forecast large (magnitude m >= 5) earthquakes for the time period 1995-2004 in the Kobe region. Visual inspection and statistical testing show that the optimized PI method has forecasting skill, relative to the seismic intensity data often used as a standard null hypothesis. Moreover, we find a retrospective forecast that the 1995 Kobe earthquake (m = 7.2) falls in a seismically anomalous area. Another approach to test the forecasting algorithm is to create a future potential map for large (m >= 5) earthquake events. This is illustrated using the Kobe and Tokyo regions for the forecast period 2000-2009. Based on the resulting Kobe map we point out several forecasted areas: the epicentral area of the 1995 Kobe earthquake, the Wakayama area, the Mie area, and the Aichi area. The Tokyo forecasted map was created prior to the occurrence of the Oct. 23, 2004 Niigata earthquake (m = 6.8) and the principal aftershocks with m >= 5.0. We find that these events occurred in a forecasted area in the Tokyo map. The PI technique for regional seismicity observation substantiates an example showing considerable promise as an intermediate-term earthquake forecasting in Japan.

physics.geo-ph↗