Toru Matsuzawa

Postseismic response of repeating earthquakes around the 2011 Tohoku‐oki earthquake: Moment increases due to the fast loading rate

We examined the temporal variation of the size of repeating earthquakes related to the 2011 Tohoku-oki earthquake (M9.0) in the northeastern Japan subduction zone for the period from July 1984 to the end of 2011. The repeaters (M2.5–6.1) show postseismic magnitude increases for most sequences located in the area of large postseismic slip at the downdip extension of the M9 source region. The magnitudes of the first events after the M9 earthquake increased by an average of about 0.3 for sequences having three or more earthquakes over the 9u2009months following it. We also examined the slip area in detail for Kamaishi repeaters whose magnitudes had been M4.9u2009±u20090.2 but which increased by about 1 after the M9 earthquake. Waveform modeling shows that the slip area for the post-M9 Kamaishi earthquakes overlaps with that before the Tohoku-oki earthquake but enlarged by about 6 times. Until the occurrence time of the last event (September 2011) in the analysis period, the rupture area remained larger than before but appeared to shrink over time. The enlargement of the rupture area suggests that an aseismic-to-seismic transition occurred in the region surrounding the pre-M9 repeaters and is most likely related to fast loading of the repeaters due to rapid postseismic slip estimated to have occurred in the area. The existence of conditionally stable regions around the repeating earthquakes and/or patches slightly larger than the earthquake nucleation sizes may explain such behavior. The temporal change of loading rate is an important factor in determining earthquake size in this case.

Seismic velocity structure in and around the Naruko volcano, NE Japan, and its implications for volcanic and seismic activities

Geofluids is important for understanding volcanic and earthquake generation processes. In this study, we obtained a detailed seismic velocity structure in and around the Naruko volcano, northeastern Japan, using data from a dense seismic observation network. We found a distinct seismic low-velocity zone (LVZ) beneath the Naruko volcano, with a diameter of nearly 10 to 20xa0km in the lower crust. This LVZ could correspond to a magma chamber. We also found a seismic low-velocity zone beneath the aftershock region of the 2008 Iwate-Miyagi Nairiku earthquake. This LVZ could correspond to an area with overpressurized fluid, which promoted the occurrence of the 2008 Iwate-Miyagi Nairiku earthquake.

Spatial distribution of the faulting types of small earthquakes around the 2011 Tohoku‐oki earthquake: A comprehensive search using template events

We developed a new method to classify the faulting types of small earthquakes using interevent waveform similarity and applied it to earthquakes in the northeast Japan subduction zone. In the method, we used separate time windows for P and S waves and established a relationship between waveform similarity and differences in focal mechanisms from event pairs whose focal mechanisms we know. Then we applied the relationship to many pairs of such focal-mechanism-known events (5607 from the moment tensor catalogue and 3623 events from the interplate repeating earthquakes catalogue) and focal-mechanism-unknown events for the period from 1984 to 2013. As a result, 8984 earthquakes were newly classified into interplate (Nu2009=u20095401), noninterplate thrust (Nu2009=u2009631), normal (Nu2009=u20091070), and strike-slip faulting earthquakes (Nu2009=u2009165). From the new data set, which doubles the number of mechanism types, we confirmed that there have been almost no interplate earthquakes in the area of large coseismic slip of the Tohoku-oki earthquake since that event. We also saw that this trend continued until at least the end of 2013, suggesting a nearly complete stress release and slow interplate stress recovery. The abundant interplate aftershocks also indicate the precise spatial extent of postsesimic slip, which is usually difficult to obtain from land-based geodetic data. The postseismic slip also suggests stress concentration at the asperities of the 1968 Tokachi-oki (M7.9) and 1994 Sanriku-oki (M7.6) earthquakes. The present-day faulting types offshore Tohoku correlated well with the static-stress change from the Tohoku-oki earthquake, suggesting a stress state change during the earthquake cycle of megathrust earthquakes.

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.

Guided wave observations and evidence for the low-velocity subducting crust beneath Hokkaido, northern Japan

At the western side of the Hidaka Mountain range in Hokkaido, we identify a clear later phase in seismograms for earthquakes occurring at the uppermost part of the Pacific slab beneath the eastern Hokkaido. The later phase is observed after P-wave arrivals and has a larger amplitude than the P wave. In this study, we investigate the origin of the later phase from seismic wave observations and two-dimensional numerical modeling of wave fields and interpret it as a guided P wave propagating in the low-velocity subducting crust of the Pacific plate. In addition, the results of our numerical modeling suggest that the low-velocity subducting crust is in contact with a low-velocity material beneath the Hidaka Mountain range. Based on our interpretation for the later phase, we estimate P-wave velocity in the subducting crust beneath the eastern part of Hokkaido by using the differences in the later phase travel times and obtain velocities of 6.8 to 7.5 km/s at depths of 50 to 80 km. The obtained P-wave velocity is lower than the expected value based on fully hydrated mid-ocean ridge basalt (MORB) materials, suggesting that hydrous minerals are hosted in the subducting crust and aqueous fluids may co-exist down to depths of at least 80 km.

Depth variations in seismic velocity in the subducting crust: Evidence for fluid‐related embrittlement for intermediate‐depth earthquakes

We investigated seismic wave velocity in the subducting crust of the Pacific slab beneath eastern Hokkaido, northern Japan. To detect depth-dependent properties of the seismic velocities in the crust, we analyzed guided waves that propagate in the crust and estimated P wave velocity (Vp) of 6.5–7.5u2009km/s and S wave velocity (Vs) of 3.6–4.2u2009km/s at depths of 50–100u2009km. The results show that the obtained Vp and Vs are 10–15% lower than those expected for the fully hydrated mid-ocean ridge basalt, suggesting the existence of aqueous fluids by ~1 vol % in the crust at this depth range. Our observations suggest that overpressurized fluids channeled in the subducting crust plays as a dominant factor for facilitating the genesis of crustal earthquakes at intermediate depths.

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.

Emergence and disappearance of interplate repeating earthquakes following the 2011 M9.0 Tohoku-oki earthquake: Slip behavior transition between seismic and aseismic depending on the loading rate

We investigated spatiotemporal change in the interplate seismic activity following the 2011 Tohoku-oki earthquake (M9.0) in the region where interseismic interplate coupling was relatively weak and large postseismic slip was observed. We classified earthquakes by their focal mechanisms to identify the interplate events and conducted hypocenter relocation to examine the detailed spatiotemporal distribution of interplate earthquakes in the mostly creeping area. The results show that many interplate earthquakes, including M~6 events, emerged immediately after the Tohoku-oki earthquake in areas where very few interplate earthquakes had been observed in the 88 previous years. The emergent earthquakes include repeating sequences, and the extremely long quiescence of small to moderate earthquakes before the Tohoku-oki earthquake suggests that the source areas for the post-M9 events slipped aseismically during the quiescence. The repeaters’ magnitudes decayed over time following the Tohoku-oki earthquake and some sequences disappeared within a year. The emergence of interplate earthquakes suggests that areas where aseismic slip had been dominant before the Tohoku-oki earthquake, started to cause seismic slip after the earthquake, probably due to the increased loading rate from the afterslip. The magnitude decrease and disappearance of repeaters can be interpreted as shrinkage in seismic areas around the repeaters’ sources as the loading rate decreased due to the afterslip decay over time. These observations suggest that changes in the loading rate can cause slip behavior transition between seismic and aseismic. This indicates that such loading-rate-dependent slip behavior plays an important role in the spatiotemporal distribution of earthquakes in interplate seismogenic zones.

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.

Complex microseismic activity and depth-dependent stress field changes in Wakayama, southwestern Japan

We examined the spatial relationship between seismicity and upper crustal structure in the Wakayama region, northwestern Kii Peninsula, Japan, by investigating microearthquake focal mechanisms and the local stress field. The focal mechanisms of most events studied fall into three categories: (1) normal faulting with N–S-oriented T-axes mainly occurring at shallow depths, (2) reverse faulting with E–W-oriented P-axes dominating at intermediate depths, and (3) strike-slip faulting with N–S-oriented T-axes and E–W-oriented P-axes mainly seen at greater depths. The stress field varies with depth: the shallow part is characterized by a strike-slip-type stress regime with N–S tension and E–W compression, while the deep part is characterized by an E–W compressional stress regime consistent with reverse faulting. The depth-dependent stress regime can be explained by thermal stress caused by a heat source, as expected from geophysical observations. Geologic faults, acting as weak planes, might contribute to generate shallow normal fault-type and deeper strike-slip fault-type microearthquakes.Graphical Abstract