https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/98RG02075

Seismic anisotropy and mantle deformation: What have we learned from shear wave splitting?

The seismic body wave tomography method has been improved and extended to adapt to a general velocity structure with a number of complexly shaped seismic velocity discontinuities (SVDs) and with three-dimensional variations in the velocities in the modeling space. An efficient three dimensional ray tracing algorithm which iteratively uses the pseudobending technique and Snell's law is developed. The large and sparse system of observation equations is solved by using the LSQR algorithm. This method is applied to 18,679 arrival times from 470 shallow and intermediate-depth earthquakes in order to study P and S wave tomographic images beneath northeastern Japan. In addition to first P and S wave arrivals, clear later arrivals of SP waves converted at the Moho and PS and SP waves converted at the upper boundary of the subducted Pacific plate (UBPP) are also used in the inversion. The UBPP, Conrad and Moho are taken as three SVDs, and their depth distributions obtained by previous studies are used. High-resolution P and S wave tomographic images down to a depth of 200 km have been determined. Large velocity variations amounting to 6% for P wave and 10% for S wave are revealed in the crust and upper mantle. In the crust low-velocity (low-V) zones exist beneath active volcanoes. In the upper mantle the low-V zones dip toward the west from the volcanic front. A high-velocity (high- V) zone corresponding to the subducted Pacific plate is clearly delineated. Most earthquakes in the lower plane of the double-planed deep seismic zone are found to occur in relatively high-V areas. The obtained tomographic images are also found to explain other seismological observations well.

Tomographic imaging of P and S wave velocity structure beneath northeastern Japan

Extended-duration seismic signals occur episodically on some major faults, often in conjunction with aseismic or 'slow-slip' earthquake events. The mechanism underlying this tremor and its relationship to the aseismic slip are as yet unresolved. David Shelley et al. demonstrate that tremor beneath Shikoku, Japan can be explained as a swarm of small, low-frequency earthquakes, each of which occurs as shear faulting on the subduction zone plate interface. This suggests that tremor and slow slip are different manifestations of a single process. Tremor beneath Shikoku, Japan can be explained as a swarm of small, low-frequency earthquakes, each of which occurs as shear faulting on the subduction zone plate interface. This suggests that tremor and slow slip are different manifestations of a single process. Non-volcanic tremor is a weak, extended duration seismic signal observed episodically on some major faults, often in conjunction with slow slip events1,2,3,4. Such tremor may hold the key to understanding fundamental processes at the deep roots of faults, and could signal times of accelerated slip and hence increased seismic hazard. The mechanism underlying the generation of tremor and its relationship to aseismic slip are, however, as yet unresolved. Here we demonstrate that tremor beneath Shikoku, Japan, can be explained as a swarm of small, low-frequency earthquakes, each of which occurs as shear faulting on the subduction-zone plate interface. This suggests that tremor and slow slip are different manifestations of a single process.

Non-volcanic tremor and low-frequency earthquake swarms

Non-volcanic seismic tremor was discovered in the Nankai trough subduction zone in southwest Japan and subsequently identified in the Cascadia subduction zone. In both locations, tremor is observed to coincide temporally with large, slow slip events on the plate interface downdip of the seismogenic zone. The relationship between tremor and aseismic slip remains uncertain, however, largely owing to difficulty in constraining the source depth of tremor. In southwest Japan, a high quality borehole seismic network allows identification of coherent S-wave (and sometimes P-wave) arrivals within the tremor, whose sources are classified as low-frequency earthquakes. As low-frequency earthquakes comprise at least a portion of tremor, understanding their mechanism is critical to understanding tremor as a whole. Here, we provide strong evidence that these earthquakes occur on the plate interface, coincident with the inferred zone of slow slip. The locations and characteristics of these events suggest that they are generated by shear slip during otherwise aseismic transients, rather than by fluid flow. High pore-fluid pressure in the immediate vicinity, as implied by our estimates of seismic P- and S-wave speeds, may act to promote this transient mode of failure. Low-frequency earthquakes could potentially contribute to seismic hazard forecasting by providing a new means to monitor slow slip at depth.

https://courses.washington.edu/ess502/ShellyBerozaEtal_Nature_2006.pdf

Low-frequency earthquakes in Shikoku, Japan, and their relationship to episodic tremor and slip

After the disastrous 1995 Kobe earthquake, a new national project has started to drastically improve seismic observation system in Japan. A large number of strong-motion, high-sensitivity, and broadband seismographs were installed to construct dense and uniform networks covering the whole of Japan. The new high-sensitivity seismo-graph network consisting of 696 stations is called Hi-net, while the broadband seismograph network consisting of 71 stations is called F-net. At most of Hi-net stations strong-motion seismographs are also equipped both at depth and the ground surface. The network of these 659 stations with an uphole/downhole pair of strong-motion seismographs is called KiK-net, while another network consisting of 1034 strong-motion seismographs installed at the ground surface is called K-NET. Here, all the station numbers are as of April 2003. High-sensitivity data from Hi-net and pre-existing seismic networks operated by various institutions have been transmitted to and processed by the Japan Meteorological Agency since October 1997 to monitor the seismic activity in and around Japan. The same data are shared to university group in real time using satellite communication for their research work. The data are also archived at the National Research Institute for Earth Science and Disaster Prevention and stored in their database system for public use under a fully open policy.

/pdf/recent-progress-of-seismic-observation-networks-in-japan-hi-23tdzahkur.pdf

Recent progress of seismic observation networks in Japan —Hi-net, F-net, K-NET and KiK-net—

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.

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

We report on the very-low-frequency earthquakes occurring in the transition zone of the subducting plate interface along the Nankai subduction zone in southwest Japan. Seismic waves generated by very-low-frequency earthquakes with seismic moment magnitudes of 3.1 to 3.5 predominantly show a long period of about 20 seconds. The seismicity of very-low-frequency earthquakes accompanies and migrates with the activity of deep low-frequency tremors and slow slip events. The coincidence of these three phenomena improves the detection and characterization of slow earthquakes, which are thought to increase the stress on updip megathrust earthquake rupture zones.

/pdf/slow-earthquakes-coincident-with-episodic-tremors-and-slow-xi3moie8ix.pdf

Slow earthquakes coincident with episodic tremors and slow slip events.

[1] Episodic slow slip events have been recognized by means of tilt changes in the western Shikoku area, southwest Japan. The crustal tilt deformation was observed repeatedly with a recurrence interval of approximately six months coincident with the occurrences of major non-volcanic deep tremor activities in this area. Observed tilt changes can be explained by slow slip events occurring around the source area of tremors. In each episode, the source of the slow slip event and tremor migrate simultaneously. The spatial and temporal coincidence of tremors and slow slip events indicates that they both may be coupling phenomena reflecting the stress accumulation process at the subducting plate.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2004GL020848

Kazushige Obara

Papers

Recent progress of seismic observation networks in Japan —Hi-net, F-net, K-NET and KiK-net—

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

Connecting slow earthquakes to huge earthquakes

Slow earthquakes coincident with episodic tremors and slow slip events.

Episodic slow slip events accompanied by non‐volcanic tremors in southwest Japan subduction zone