Slow slip, a mechanism by which faults can relieve stress, was thought to be distinct from earthquakes. However, a global review of slow-slip phenomena suggests that instead there is a continuum between the two types of event.

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An integrated perspective of the continuum between earthquakes and slow-slip phenomena

▶ Gathers original articles on topics in earth and planetary sciences ▶ Coverage includes geomagnetism, aeronomy, space science, seismology, volcanology, geodesy and planetary science ▶ Official journal of the Society of Geomagnetism and Earth, Planetary and Space Sciences, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan, and The Japanese Society for Planetary Sciences

「Earth,Planets and Space」のオープンアクセス出版

Slow earthquakes are characterized by a wide spectrum of fault slip behaviors and seismic radiation patterns that differ from those of traditional earthquakes. However, slow earthquakes and huge megathrust earthquakes can have common slip mechanisms and are located in neighboring regions of the seismogenic zone. The frequent occurrence of slow earthquakes may help to reveal the physics underlying megathrust events as useful analogs. Slow earthquakes may function as stress meters because of their high sensitivity to stress changes in the seismogenic zone. Episodic stress transfer to megathrust source faults leads to an increased probability of triggering huge earthquakes if the adjacent locked region is critically loaded. Careful and precise monitoring of slow earthquakes may provide new information on the likelihood of impending huge earthquakes.

https://science.sciencemag.org/content/sci/353/6296/253.full.pdf

Connecting slow earthquakes to huge earthquakes

Nonvolcanic tremor is observed in close association with geodetically observed slow-slip events in subduction zones. Accumulating evidence points to these events as members of a family of slow earthquakes that occur as shear slip on the downdip extensions of fault zones in a regime that is transitional between a frictionally locked region above and a freely slipping region below. By virtue of their locations and their properties, slow earthquakes are certain to provide new insights into the behavior of earthquakes and faulting and into the hazard they embody.

Slow Earthquakes and Nonvolcanic Tremor

Canine distemper virus is a member of the genus Morbillivirus in the family Paramyxoviridae. Canine distemper has been recorded in domestic dogs for centuries. It is now recognized as a worldwide problem of carnivores and has the second highest fatality rate of any infectious disease, after rabies, in domestic dogs. The importance of this disease in nondomestic animals has become evident with vaccine-induced infections in a variety of species and large-scale epidemics in captive and free-ranging felids. To date, canine distemper has been reported in all families of terrestrial carnivores: Canidae, Felidae, Hyaenidae, Mustelidae, Procyonidae, Ursidae, and Viverridae. Veterinarians, including those working with nondomestic carnivores, should be familiar with the clinical signs, diagnosis, and clinical management of this disease.

/pdf/canine-distemper-in-terrestrial-carnivores-a-review-3ptob4wjkk.pdf

Canine distemper in terrestrial carnivores: a review.

Many large earthquakes are preceded by one or more foreshocks, but it is unclear how these foreshocks relate to the nucleation process of the mainshock. On the basis of an earthquake catalog created using a waveform correlation technique, we identified two distinct sequences of foreshocks migrating at rates of 2 to 10 kilometers per day along the trench axis toward the epicenter of the 2011 moment magnitude (M(w)) 9.0 Tohoku-Oki earthquake in Japan. The time history of quasi-static slip along the plate interface, based on small repeating earthquakes that were part of the migrating seismicity, suggests that two sequences involved slow-slip transients propagating toward the initial rupture point. The second sequence, which involved large slip rates, may have caused substantial stress loading, prompting the unstable dynamic rupture of the mainshock.

https://science.sciencemag.org/content/sci/335/6069/705.full.pdf

Propagation of slow slip leading up to the 2011 M(w) 9.0 Tohoku-Oki earthquake.

Swarms of non-volcanic tremor in southeastern Japan are associated with slow slip events and tend to occur with a periodicity of 12 or 24 h. This periodicity can be reproduced by a combination of stresses due to Earth tides and transient stress changes caused by slow slip events. Non-volcanic tremors may therefore be useful for understanding stress relaxation at the subduction-zone interface. Slow slip events1 are accompanied by swarms of non-volcanic tremor2,3 along the subduction zone of the Philippine Sea plate in southwest Japan: the swarms often occur with a periodicity of about 12 or 24 h (refs 4, 5). These episodic events are considered to be a manifestation of stress relaxation at the subducting plate interface1,6,7. Here, we analyse seismic data to record the locations and durations of non-volcanic tremor swarms that occurred in May 2005 and February 2006. We evaluate the magnitude of stress changes produced by slow slip events as well as the effective normal stress on the plate interface where slow slip events occur. We find that the observed periodicity in tremor occurrence originates from a combined effect of the periodic stress due to Earth tides and the transient stress due to slow slip events. Our calculations show that non-volcanic tremor is sensitive to stress change. This phenomenon can therefore be effective in monitoring the process of stress relaxation at subducting plate interfaces.

/pdf/non-volcanic-tremor-resulting-from-the-combined-effect-of-1qnl18d343.pdf

Non-volcanic tremor resulting from the combined effect of Earth tides and slow slip events

Natural infection with canine distemper virus in a Japanese monkey (Macaca fuscata).

A reliable estimate of completeness magnitude, M c , above which all earthquakes are considered to be detected by a seismic network, is vital for seismicity-related studies. We show a comprehensive analysis of M c in Japan. We use the catalog maintained by the Japan Meteorological Agency (JMA) and also available information on seismic stations that report to JMA. For computing M c , we adopt a commonly used method based on the Gutenberg–Richter frequency-magnitude law. Presently, M c =1.0 might be typical in the mainland, but to have a complete catalog, one needs to use earthquakes with magnitudes of 1.9 or larger. Comparison with the Southern California Seismic Network (SCSN) suggests that the recent event detectability in the mainland generally shows similar completeness levels to that in the authoritative region of SCSN. We argue that the current M c of Japan is due to the success of network modernization over time. Particularly, we show that the spatiotemporal change of M c closely matches the addition of the Hi-net borehole stations to enhancing seismic-station density; it started in October 1997 in southwestern Japan, continuing to northeastern Japan until 2002. As suggested from this matching, we confirm that M c inversely correlates with station density. Further, we find that irrespective of the network change after 1997, this correlation is unchanged in time, demonstrating that the influence on M c from factors beyond station density does not vary in time. Contrary to Alaska and California (Wiemer and Wyss, 2000), our results do not attribute such factors simply to anthropogenic noise. Because this is due to the borehole stations that reduce ambient noise, we conclude that in Japan the anthropogenic noise has an insignificant effect on M c .

Analysis of the completeness magnitude and seismic network coverage of Japan

Static changes in the Coulomb Failure Function (ΔCFF) forecast an increase in seismicity in and around the Tokyo metropolis after the 2011 off the Pacific coast of Tohoku Earthquake (magnitude 9.0). Among the 30,694 previous events in this region with various depth and focal mechanism, almost 19,000 indicate a significant increase of the ΔCFF, while less than 6,000 indicate a significant decrease. An increase in seismicity is predicted in southwestern Ibaraki and northern Chiba prefectures where intermediate-depth earthquakes occur, and in the shallow crust of the Izu and Hakone regions. A comparison of seismicity before and after the 2011 event reveals that the seismicity in the above regions indeed increased as predicted from the ΔCFF.

/pdf/change-in-seismicity-beneath-the-tokyo-metropolitan-area-due-25kmcttvow.pdf

Hiroshi Tsuruoka

Papers

Propagation of slow slip leading up to the 2011 M(w) 9.0 Tohoku-Oki earthquake.

Non-volcanic tremor resulting from the combined effect of Earth tides and slow slip events

Natural infection with canine distemper virus in a Japanese monkey (Macaca fuscata).

Analysis of the completeness magnitude and seismic network coverage of Japan

Change in seismicity beneath the Tokyo metropolitan area due to the 2011 off the Pacific coast of Tohoku Earthquake