Yoshihisa Iio

Water-weakened lower crust and its role in the concentrated deformation in the Japanese Islands

Abstract The nature and origin of the concentrated deformation zone along the Japan Sea coast (NKTZ: Niigata–Kobe tectonic zone) was investigated by carefully analyzing the GPS data and qualitatively modeling the lower crust in NKTZ. It was concluded that this deformation zone is not a plate boundary between the Amurian plate (AMU) and the North America plate but is rather an internal deformation zone near the eastern margin of AMU. The data previously obtained on the conductivity anomalies in the lower crust and the 3 He/ 4 He ratios suggest that the concentrated deformation in NKTZ results from the lower crust in NKTZ being weakened by a high water content. The high water content is thought to result from the dehydration of subducting slabs. NKTZ has a higher water content in the lower crust than other regions do because there is no Philippine Sea plate (PHS) seismic slab beneath NKTZ. In other regions, it is estimated that the mantle wedge above the seismic Philippine Sea slab prevents the water dehydrated from the slab from rising to the lower crust, and that the lithosphere within PHS itself prevents the water dehydrated from the Pacific plate from rising up through it.

A physical understanding of large intraplate earthquakes

We examined several previously proposed models of the process by which large intraplate earthquakes are generated, and found the most plausible to be the ‘localized shear model,’ in which seismogenic faults have downward extensions in the lower crust and the localized shear deformation on the downward extensions accumulate stress on the seismogenic faults. Further, localized shear deformation may accelerate before a large intraplate earthquake occurs. This model can explain various phenomena related to crustal deformation in the Japanese Islands: preslips, seismicity, distribution of active faults, and stress and strain state. It will play an important role in the forecasting and prediction of large intraplate earthquakes.

Crustal deformation around the northern and central Itoigawa-Shizuoka Tectonic Line

We established a dense permanent GPS array around the northern and central Itoigawa-Shizuoka Tectonic Line (ISTL) in order to study loading processes of inland active faults. 1–4 years observation has revealed concentrated deformation (about 0.3 ppm/yr) around the East Matsumoto-Basin Fault, an active fault along the ISTL, consistent with historic triangulation data. The deformation pattern is explained by dislocation models incorporating horizontal detachment fault with steady slip in the upper crust. However, deformation data alone cannot determine the model and there exist different possibilities. The vicinity of the Gofukuji fault is being deformed less in spite of the large long-term slip rate. These observations indicate that the deformation pattern is laterally heterogeneous along the ISTL. The two faults are considered to be loaded by different mechanisms.

Large earthquakes initiate by the acceleration of slips on the downward extensions of seismogenic faults

Abstract In order to examine the relationship between anomalous crustal deformations and subsequent large earthquakes, we compiled and examined data on preseismic anomalous crustal deformations for two earthquakes that occurred in and around the Japanese Islands in recent years. We found that the slip on a downward extension of a seismogenic fault accelerates before the main shock and that the average slip velocity is inversely proportional to the time remaining until the main shock occurs.

Is the plastic flow uniformly distributed below the seismogenic region

We compared the cutoff depth of seismicity in and around the Nojima fault broken by the 1995 Kobe earthquake occurring in intraplate Japan with the brittle–ductile transition depth of the widely accepted strength profile model of the crust. We successfully determined the temperature profile from borehole measurements, since almost the same geothermal gradients were observed at two boreholes located about 4 km apart from each other, and the thermal conductivity and heat production were also measured by taking numerous core samples. We found that the cutoff depth was much deeper than the transition depth under the assumption that wet granite is deformed at a strain rate of 3×10−15 s−1. This small strain rate implies, however, that plastic flow is uniformly distributed below the seismogenic region. When the strain rate is assumed to be greater than 10−13 s−1, the cutoff depth can be attributed to the transition depth. This suggests that deformation is localized in a narrow fault zone below the seismogenic region, even in the intraplate region.

Acceleration of slip motion in deep extensions of seismogenic faults in and below the seismogenic region

This paper addresses concepts presented in Session 7 of the conference on “Slip and Flow Processes Near the Base of the Seismogenic Zone” held at Sendai, Japan in November, 2001. The questions raised in this session were associated with the downward extensions of seismic faults into the lower crust. The important issue is whether asiesmic slip accelerates on downward extensions prior to large earthquakes on the upper, seismic part. If this is the case, then such movement may be measurable as a precursor to large seismic events as accelerated tilt and/or distortion at the surface. Associated issues involve the geometry of downward extensions of faults, the mechanisms of localisation in the lower crust, and the mechanisms for earthquake generation near the base of the upper crust. The outcomes from this session are that aseismic slip in the lower crust could be generated by several mechanisms of localisation including yield of an elastic-viscous-plastic material, softening (including, in particular, thermal softening) of an elastic-viscous material and ductile fracture. Fine scale modelling of localisation in the lower crust is still required to resolve the issue whether accelerated motion precedes seismic slip in the upper crust. Such modelling also demands a better understanding of crustal rheology than we have at present.

Spatial and temporal characteristics of low-magnitude seismicity from a dense array in western Nagano Prefecture, Japan

We analyze an earthquake catalog and waveform data from a dense seismic array in western Nagano Prefecture in order to investigate seismic properties and source parameters. The catalog used here consists of about 30,000 smaller magnitude earthquakes that were located within a source volume of order (10 km)3. Moving window analysis shows that the seismic rate is steady over time and the temporal change in the size distribution of earthquakes shows only statistical fluctuations. Spatial analyses, however, reveals that certain features of seismicity are depth-dependent. At shallow depths (<3 km), we find an increase in average stress drop with depth that may be related to the characteristic depth of seismicity (the depth where the frequency of earthquakes is maximum). The b-value is also found to be elevated throughout depths where the seismicity is most intense but then the b-value slowly decreases as depth increases. Another result from our analysis of waveform data is a clear relation between earthquake magnitude and stress-drop in the magnitude range −0.5 to 2.5.