Investigating slow earthquake activity in subduction zones provides insight into the slip behavior of megathrusts, which can provide important clues about the rupture extent of future great earthquakes. Using the S-net ocean-bottom seismograph network along the Japan Trench, we mapped a detailed distribution of tectonic tremors, which coincided with very-low-frequency earthquakes and a slow slip event. Compiling these and other related observations, including repeating earthquakes and earthquake swarms, we found that the slow earthquake distribution is complementary to the Tohoku-Oki earthquake rupture. We used our observations to divide the megathrust in the Japan Trench into three along-strike segments characterized by different slip behaviors. We found that the rupture of the Tohoku-Oki earthquake, which nucleated in the central segment, was terminated by the two adjacent segments.

The occurrence of subduction zone earthquakes is primarily controlled by the state of stress on the interface between the subducting and overriding plates. This stress state is influenced by tectonic properties, such as the age of the subducting plate and the rate of plate motion 1-4 . It is di cult to directly measure stress on a plate interface. However, the stress state can be inferred using the Gutenberg-Richter relationship's 5 b-value, which characterizes the relative number of small compared to large earthquakes and correlates negatively with di erential stress 6-13 . That is, a subduction zone characterized by relatively frequent large earthquakes has a low b-value and a high stress state. The b-value for subduction zones worldwide varies significantly 14,15 , but the source of this variance is unclear. Here we use the Advanced National Seismic System earthquake catalogue to estimate b-values for 88 sections in di erent subduction zones globally and compare the b-values with the age of the subducting plate and plate motions. The b-value correlates positively with subducting plate age, so that large earthquakes occur more frequently in subduction zones with younger slabs, but there is no correlation between b-value and plate motion. Given that younger slabs are warmer and more buoyant, we suggest that slab buoyancy is the primary control on the stress state and earthquake size distribution in subduction zones.It is important to identify the factors that control earthquake occurrence. Pioneering studies over the past three decades 2,3,16 have suggested that the plate convergence rate and age of the subducting plate determine the maximum earthquake size in subduction zones. A young, light plate subducting at a high rate produces a compressive stress field and great earthquakes (Chilean-type), whereas an old heavy slab subducting slowly yields an extensional stress field with low seismicity (Marianas-type). However, this model is inconsistent with the 2004 Sumatra and 2011 Tohoku-Oki earthquakes, and recent studies have proposed different interpretations. For example, trench sediment thickness and tectonic stresses applied on the plate interface may control the maximum earthquake size 17 , and the seismicity of medium to large earthquakes is directly controlled by the convergence rate in Marianas-type subduction zones 18 .The stress state is obviously an important control on earthquake occurrence. Although the direct measurement of stress state is difficult, a relationship between stress state and some parameters from earthquake statistics have been proposed. Among several candidates, a parameter that is strongly related to shear stress is the b-value, which is the slope of a power-law frequencysize distribution of earthquakes 5 . A negative correlation between b-values and ambient shear stress levels has been suggested from laboratory experiments 6-8 , and by comparison with earthquake depths 9 , focal mechanisms 10 and slip patterns 11,12 . Decreases in b-values before great earthquakes can be explained by ...

Slow slip events (SSEs) on the plate interface are closely related to the occurrence of earthquakes and often trigger earthquake swarms in subduction zones. Moreover, some SSEs, accompanied by intensive foreshocks, precede large interplate earthquakes. Therefore, detecting and monitoring SSEs is important for assessing the potential of future large earthquakes. However, there are many SSEs not followed by large earthquakes, and it is unclear whether these can be distinguished from SSEs preceding large earthquakes. Here we use the epidemic‐type aftershock sequence (ETAS) model and matched‐filter technique to examine the spatial‐temporal distribution of earthquake swarms and foreshocks at Ibaraki‐Oki in the Japan Trench. We found that 19 swarm sequences repeatedly occurred during the period 1982–2008 at the same location as the foreshock sequences of the 1982 and 2008 M 7 Ibaraki‐Oki earthquakes. Both the foreshock and swarm sequences contain repeating earthquakes and have anomalously high seismicity rates inexplicable by the ETAS model, suggesting the recurrence of SSEs. The foreshock sequences in 1982 and 2008 contain more events inexplicable by the ETAS model than the swarm sequences. The fault slip of repeating earthquakes in the 2008 foreshock sequence was also larger than those of the swarm sequences, and the slip rate showed an abrupt increase 12 hr before the 2008 M 7 event. Our results imply that the SSEs that preceded the M 7 events had larger seismic moments than the other SSEs. These large SSEs might be related to the nucleation phase of the M 7 earthquakes.

Earthquake swarms are characterized by an increase in seismicity rate that lacks a distinguished main shock and does not obey Omori's law. At subduction zones, they are thought to be related to slow‐slip events (SSEs) on the plate interface. Earthquake swarms in subduction zones can therefore be used as potential indicators of slow‐slip events. However, the global distribution of earthquake swarms at subduction zones remains unclear. Here we present a method for detecting such earthquake sequences using the space‐time epidemic‐type aftershock‐sequence model. We applied this method to seismicity (M ≥ 4.5) recorded in the Advanced National Seismic System catalog at subduction zones during the period of 1995–2009. We detected 453 swarms, which is about 6.7 times the number observed in a previous catalog. Foreshocks of some large earthquakes are also detected as earthquake swarms. In some subduction zones, such as at Ibaraki‐Oki, Japan, swarm‐like foreshocks and ordinary swarms repeatedly occur at the same location. Given that both foreshocks and swarms are related to SSEs on the plate interface, these regions may have experienced recurring SSEs. We then compare the swarm activity and tectonic properties of subduction zones, finding that swarm activity is positively correlated with curvature of the incoming plate before subduction. This result implies that swarm activity is controlled either by hydration of the incoming plate or by heterogeneity on the plate interface due to fracturing related to slab bending.

Earthquake swarms, which are anomalous increases in the seismicity rate without a distinguishable mainshock, often accompany transient aseismic processes, such as fluid migration and episodic aseismic slip along faults. Investigations of earthquake swarm activity can provide insights into the causal relationship between aseismic processes and seismicity. Slow slip events (SSEs) along the plate interface in the Hikurangi Trench, New Zealand, are often accompanied by intensive earthquake swarms. However, the detailed spatiotemporal distribution of these earthquake swarms is still unclear. Here, we use the epidemic‐type aftershock‐sequence (ETAS) model to detect earthquake swarms (M ≥ 3) and create a new earthquake swarm catalog (1997–2015) along the Hikurangi Trench. We compare the earthquake swarm catalog with Global Navigation Satellite System (GNSS) time series data, and existing SSE and tectonic tremor catalogs. Most of the detected (119) earthquake swarm sequences were intraplate events, and their epicenters were mainly concentrated along the east coast of the North Island, whereas many tectonic tremors were located inland. Twenty‐five of the detected earthquake swarms occurred within 25 days before and after transient eastward GNSS displacements due to known or newly detected SSEs. We find that the earthquake swarms sometimes preceded the GNSS displacements by more than several days. SSE‐induced stress loading is therefore not a plausible triggering mechanism for these pre‐SSE earthquake swarms. We propose that high fluid pressure within the slab, which accumulated before the SSEs, may have caused intraplate fluid migration, which in turn triggered the pre‐SSE earthquake swarms.

Nuclear reprogramming of differentiated cells can be induced by oocyte factors. Despite numerous attempts, these factors and mechanisms responsible for successful reprogramming remain elusive. Here, we identify one such factor, necessary for the development of nuclear transfer embryos, using porcine oocyte extracts in which some reprogramming events are recapitulated. After incubating somatic nuclei in oocyte extracts from the metaphase II stage, the oocyte proteins that were specifically and abundantly incorporated into the nuclei were identified by mass spectrometry. Among 25 identified proteins, we especially focused on a multifunctional protein, DJ-1. DJ-1 is present at a high concentration in oocytes from the germinal vesicle stage until embryos at the fourcell stage. Inhibition of DJ-1 function compromises the development of nuclear transfer embryos but not that of fertilized embryos. Microarray analysis of nuclear transfer embryos in which DJ-1 function is inhibited shows perturbed expression of P53 pathway components. In addition, embryonic arrest of nuclear transfer embryos injected with anti-DJ-1 antibody is rescued by P53 inhibition. We conclude that DJ-1 is an oocyte factor that is required for development of nuclear transfer embryos. This study presents a means for identifying natural reprogramming factors in mammalian oocytes and a unique insight into the mechanisms underlying reprogramming by nuclear transfer.oocyte extract and proteomics | reprogramming in mammalian oocytes E mbryonic cells differentiate into specific types of cells as development progresses. Once differentiated, the reversion of a differentiated cell state to an original undifferentiated state is strictly inhibited in normal development. However, it has been experimentally shown that differentiated nuclei can be returned to an undifferentiated embryonic state after nuclear transfer (NT) to enucleated eggs or oocytes (1, 2). Such experiments provide an opportunity to reprogram somatic cells as a means to prepare undifferentiated cells, which may be differentiated into any kinds of cells for cell-replacement therapy. Recently, nuclear reprogramming technology has been expanded by the production of induced pluripotent stem (iPS) cells (3). iPS cells can be obtained by overexpressing specific sets of transcription factors such as Oct4, Sox2, Klf4, and c-myc in cultured cells. The processes leading to establishment of iPS cell lines are being carefully examined and we are begining to understand how somatic cells acquire pluripotency by this method (4-6). The mechanisms leading to pluripotency may be different between iPS cells and NT embryos because somatic nuclei transferred into unfertilized metaphase II (MII) oocytes must undergo early embryonic development before the inner cell mass (ICM) can give rise to pluripotent embryonic stem (ES) cells. In addition, the molecules and mechanisms that induce somatic cell reprogramming are expected to be different between iPS cells and NT embryos (7,8). A recent study has shown that nuclear t...

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to improve our understanding of the interactions between regular and slow earthquakes along the Japan trench, we investigated the spatial relationship of slow-earthquake activity with the preseismic, coseismic, and postseismic fault ruptures of interplate earthquakes off the Iwate and Ibaraki coasts, Japan, including two large interplate aftershocks of the 2011 Tohoku earthquake: the 2011 off Iwate earthquake (M JMA 7.4) and the 2011 off Ibaraki earthquake (M JMA 7.6). We found that the coseismic ruptures of these earthquakes did not overlap with the active areas of slow earthquakes, while their foreshocks and aftershocks occurred in slow-earthquake-prone areas. Moreover, the 2011 off Iwate earthquake and the previous M7-class events shared common fault rupture characteristics: coseismic rupture occurred in a common asperity area, and afterslip with many aftershocks was triggered in the active area of slow earthquakes. Off the Ibaraki coast, tremor activity on a subducting seamount located updip of the coseismic rupture of the 2011 off Ibaraki earthquake implies that the seamount acted as a soft barrier to the coseismic rupture of the 2011 off Ibaraki earthquake. This study demonstrates that large earthquakes off the Iwate and Ibaraki coasts feature similar rupture behaviors, spatially complementary distributions of coseismic ruptures with slow-earthquake activity and foreshock and aftershock activities within and around slow-earthquake-prone areas. This information is useful in considering future large earthquakes along the Japan Trench. Over the past few decades, a considerable number of studies have been carried out on "slow earthquakes", such as tectonic tremors, very-low-frequency earthquakes (VLFs), and slow slip events (SSEs) 1-5. These studies have shown that slow earthquakes frequently occur in regions neighboring megathrust seismogenic zone and sometimes trigger large interplate earthquakes. Therefore, the investigation of slow-earthquake activity can provide insight into the earthquake-rupture mechanism in subduction zones. To improve our understanding of the interactions between regular and slow earthquakes in subduction zones, it is important to investigate the relationship between the fault ruptures of large interplate earthquakes and the slow-earthquake activity. Nishikawa et al. 5 revealed the comprehensive spatial distribution of slow earthquakes along the Japan Trench and compared the resulting distribution with the coseismic rupture extents of the 2011 Tohoku earthquake and large interplate earthquakes that occurred before the 2011 Tohoku earthquake. The authors concluded that slow-earthquake activity spatially complements the coseismic ruptures of large interplate earthquakes and discussed the variation in fault slip behavior along the Japan Trench based on the regional characteristics of slow-earthquake activity. To expand our understanding of the variation in fault slip behavior along the Japan Trench, we probe the relationship between the preseismic, coseismic, and postseismic fault rupture...