Keisuke Ariyoshi

Aftershock distribution and 3D seismic velocity structure in and around the focal area of the 2004 mid Niigata prefecture earthquake obtained by applying double-difference tomography to dense temporary seismic network data

A destructive large earthquake (the 2004 mid Niigata prefecture earthquake) sequence occurred in the central part (Chuetsu district) of Niigata prefecture, central Japan on October 23, 2004. We have deployed a temporary seismic network composed of 54 stations for aftershock observation just above and around the focal area of the earthquake for about a month. Using travel time data from the temporary seismic network and surrounding routine stations, we obtained precise aftershock distribution and 3D seismic velocity structure in and around the fault planes of the earthquake and four major (M ≥ 6) aftershocks by double-difference tomography. The results clearly show three major aftershock alignments. Two of them are almost parallel and dipping toward the WNW. The shallow and deep aftershock alignments correspond to the fault plane of the mainshock and that of the largest aftershock (M6.4), respectively. The third alignment is almost perpendicular to the WNW-ward dipping planes and perhaps corresponds to the fault plane of the M6 aftershock on October 27. General feature of the obtained velocity structure is that the hanging wall (western part of the focal area) has lower velocity and the footwall (eastern part of the focal area) has higher velocity. Major velocity boundary seems to shift westward in comparison to in northern and southern parts at a location near the central part of the focal area, where the main shock rupture started. Some parts of the fault planes were imaged as low velocity zones. This complex crustal structure would be one of possible causes of the multi-fault rupture of the 2004 mid Niigata prefecture earthquake sequence.

Migration process of very low-frequency events based on a chain-reaction model and its application to the detection of preseismic slip for megathrust earthquakes

In order to reproduce slow earthquakes with short duration such as very low frequency events (VLFs) migrating along the trench direction as swarms, we apply a 3-D subduction plate boundary model based on the slowness law of rate- and state-dependent friction, introducing close-set numerous small asperities (rate-weakening regions) at a depth of 30 km under high pore pressure condition, in addition to a large asperity. Our simulation indicates that swarms of slip events occur repeatedly at the small asperities, and these events are similar to the observed slow earthquake group, especially to VLF, on the basis of the relation between characteristic duration and seismic moment. No slip events occur there without the small asperities, which mean that the close-set numerous small asperities may be one of the necessary conditions for generating the short-duration slow earthquakes such as VLFs. In the preseismic stage of the megathrust earthquakes that occur at the large asperity, the swarms of VLFs have higher migration speeds and higher moment release rate as well as shorter recurrence interval. Thus, monitoring the migration of slow earthquakes may be useful in imaging the preseismic slip of megathrust earthquakes.

Development and application of an advanced ocean floor network system for megathrust earthquakes and tsunamis

Japan is prone to great earthquakes because of its position near two different subduction zones. The Philippine Sea plate subducts from the southeast, and the Pacific plate subducts from the east. The former was the source of a series of great earthquakes, of which the Tonankai earthquake of 1944 and the Nankaido earthquake of 1946 are the latest events. The latter was the source of the 2011 earthquake off the Pacific coast of Tohoku (Tohoku earthquake) of 11 March 2011 (M9).

Triggered non‐similar slip events on repeating earthquake asperities: Results from 3D numerical simulations based on a friction law

[1] We investigated the changes in slip rate and recurrence interval of repeating earthquakes using 3-D numerical simulations of a subduction zone involving large and small asperities and based on a rate- and state-dependent friction law. Our results reveal that the postseismic slip of a large earthquake can trigger an event at a nearby small asperity, although the event may differ to other repeating earthquakes at the asperity because the slip velocity of the event tends to be slower than that of other repeaters; the slip distribution also differs. These results probably explain the observation that the slip amount estimated from repeating earthquake analyses tends to be smaller than that estimated from GPS analyses for large amounts of postseismic slip, as observed for postseismic activity following the 2003 Tokachi-oki earthquake.

The detectability of shallow slow earthquakes by the Dense Oceanfloor Network system for Earthquakes and Tsunamis (DONET) in Tonankai district, Japan

In order to understand the characteristics of shallow very low-frequency (VLF) events as revealed by recent ocean-floor observation studies, we perform a trial simulation of earthquake cycles in the Tonankai district by taking the characteristics of the 1944 Tonankai earthquake and assuming that slow earthquakes occur on numerous small asperities. Our simulation results show that the increase of moment release rate of shallower VLF events in the pre-seismic stage of a megathrust earthquake is higher than that of deeper VLF events. This increase may make leveling change due to VLF swarms detectable at Dense Oceanfloor Network system for Earthquakes and Tsunamis (DONET). We also introduce the time series of hydraulic pressure data at DONET, comparing with the leveling change expected from our numerical simulation. Since leveling change due to shallower VLF swarms is so local as to be incoherent, removal of the moving-averaged data from the data stacked by four nearby observation points in the same node may be useful to detect the short-term local leveling change.

A trial estimation of frictional properties, focusing on aperiodicity off Kamaishi just after the 2011 Tohoku earthquake

Motivated by the fact that temporal earthquake aperiodicity was observed off Kamaishi just after the 2011 Tohoku earthquake, we performed numerical simulations of chain reactions due to the postseismic slip of large earthquakes by applying rate- and state-dependent friction laws. If the repeater is composed of single asperity, our results show that, (i) a mixture of partial and whole rupturing of a single asperity can explain some of the observed variability in timing and size of the repeating earthquakes off Kamaishi; (ii) the partial rupturing can be reproduced with moderate frictional instability with the aging-law and not the slip or Nagata laws; (iii) the perturbation of the activated earthquake hypocenters observed mostly in the ESE-WNW direction may reflect the fact that the large postseismic slip of the 2011 Tohoku earthquake propagated from ESE to WNW off Kamaishi; (iv) the observed region of repeating earthquake quiescence may reflect the strong plate coupling of megathrust earthquakes.

Oral nutritional support can shorten the duration of parenteral hydration in end-of-life cancer patients: a randomized controlled trial.

Tube feeding or hydration is often considered for end-of-life cancer patients despite the negative effects on quality of life. The efficacy of oral nutritional support in this setting is unknown. We conducted a randomized trial to compare the efficacies of an amino acid jelly, Inner Power® (IP), and a liquid enteral product, Ensure Liquid® (EL), in terminally ill cancer patients. We randomly assigned patients to 3 arms: EL, IP, and EL+IP. The primary endpoint was drip infusion in vein (DIV)-free survival, which was defined as the duration from nutritional support initiation to administration of parenteral hydration. Twenty-seven patients were enrolled in the study, of whom 21 were included in the intention-to-treat analysis. The median age of the subjects was 69 yr. There were significant differences between the arms with regard to the median DIV-free survival (0.5, 6.0, and 4.5 days in the EL, IP, and EL + IP arms, respectively; P = 0.05). The median overall survival was 7, 9, and 8 days in the EL, IP, and EL + IP arms, respectively. IP may shorten the duration of parenteral hydration in terminally ill cancer patients and does not affect their survival.

A trial derivation of seismic plate coupling by focusing on the activity of shallow slow earthquakes

To understand the effect of plate coupling on very low-frequency event (VLFE) activity resulting from megathrust earthquakes, we performed long-term multiscale earthquake cycle simulations (including a megathrust earthquake and slow earthquakes) on a 3-D subduction plate boundary model, based on a rate- and state-dependent friction law. Our simulation suggests that quiescence of shallow VLFEs off Miyagi may be explained by the location in the shallow central part of the 2011 Tohoku earthquake because of the locally strong coupling, while observed activation of VLFEs off Iwate (northern part of Tohoku district), Fukushima (southern part of Tohoku district), and Ibaraki (northern part of Kanto district) is explained by the location on the outer rim. The area and duration of the quiescence off Miyagi may be a new clue to evaluate the potential for plate coupling strong enough to cause the next megathrust earthquake.

Advanced Real Time Monitoring System and Simulation Researches for Earthquakes and Tsunamis in Japan

Mega thrust earthquakes generated large tsunamis quite often. Based on lessons learned from the 2004 Sumatra and the 2011 East Japan Earthquakes/Tsunamis, we recognized the importance of real time monitoring on the natural hazards. Monitoring systems using multi kinds of sensors such as the accelerometer, broadband seismometer, pressure gauge, difference pressure gauge, hydrophone and thermometer is indispensable not only for mitigation of damage from earthquakes and tsunamis, but also for understanding of broadband crustal activities around mega thrust earthquake seismogenic zones. Therefore, we have developed the Dense Ocean floor Network for Earthquakes and Tsunamis (DONET) to acquire the seafloor data in real time around the Nankai trough seismogenic zone, southwestern Japan. The first phase of deployment (DONET1) was completed and the second phase (DONET2) is being developed at the time of writing of the manuscript. At the 2011 East Japan Earthquake, DONET1 observatories detected offshore tsunamis 15 min earlier than onshore stations. Furthermore, DONET1 and DONET2 will be expected to monitor silent phenomena such as low frequency tremors and slow earthquakes for the estimation of seismic stage which would occur in the inter-seismic or pre-seismic stage. The recurrence cycle of mega thrust earthquakes, modeling of tsunami inundation and seismic response on buildings and cities are also important in the disaster mitigation programs and related measures. Real-time monitoring data should be integrated with the advanced simulations for precise earthquake and/or tsunami early warnings and rapid estimation of the damages.

Extraction of crustal deformations and oceanic fluctuations from ocean bottom pressures

DONET (Dense Oceanfloor Network system for Earthquakes and Tsunamis) has been developed and installed around Nankai Trough, which is motivated by the 2004 Sumatra-Andaman Earthquake. DONET contains pressure gauges as well as seismometers, which are expected to detect crustal deformations driven by peeling off subduction plate coupling process. From our simulation results, leveling changes are different sense among the DONET points even in the same science node. On the other hand, oceanic fluctuations such as melting ice masses through the global warming have so large scale as to cause ocean bottom pressure change coherently for all of DONET points especially in the same node. This difference suggests the possibility of extracting crustal deformations component from ocean bottom pressure data by differential of stacking data. However, this operation cannot be applied to local-scale fluctuations related to ocean mesoscale eddies and current fluctuations, which affect ocean bottom pressure through water density changes in the water column (from the sea surface to the bottom). Therefore, we need integral analysis by combining seismology, ocean physics and tsunami engineering so as to decompose into crustal deformation, oceanic fluctuations and instrumental drift, which will bring about high precision data enough to find geophysical phenomena.