Takeshi Mikumo

Nonvolcanic tremor observed in the Mexican subduction zone

Nonvolcanic tremor (NVT) activity is revealed as episodes of higher spectral amplitude at 1–8 Hz in daily spectrograms from the continuous seismological records in Guerrero, Mexico. The analyzed data cover a period of 2001–2007 when in 2001–2002 a large slow slip event (SSE) had occurred in the Guerrero-Oaxaca region, and then a new large SSE occurred in 2006. The tremor burst is dominated by S-waves. More than 100 strong NVT bursts were recorded in the narrow band of ~40 × 150 km^2 to the south of Iguala City and parallel to the coastline. Depths of NVT hypocenters are mostly scattered in the continental crust between 5 and 40 km depth. Tremor activity is higher during the 2001–2002 and 2006 SSE compared with that for the “quiet” period of 2003–2005. While resistivity pattern in Guerrero does not correlate directly with the NVT distribution, gravity and magnetic anomaly modeling favors a hypothesis that the NVT is apparently related to the dehydration and serpentinization processes.

Short slip duration in dynamic rupture in the presence of heterogeneous fault properties

Recent studies of strong motion data consistently show that the risetime (duration of slip at particular locations on the fault) is significantly shorter than the overall rupture duration. The physical explanation for this observation and its implications have become central issues in earthquake source studies. Two classes of mechanisms have been proposed to explain short risetimes. One explanation is that velocity-weakening frictional behavior on the fault surface causes the fault to self-heal. This possibility is suggested by rate-dependent friction observed in laboratory experiments and by some two-dimensional dynamic numerical simulations of earthquake rupture. It has recently been demonstrated, however, that the velocity dependence of friction observed in the laboratory is too weak to cause faults to self-heal. An alternative explanation for short risetimes is that spatially heterogeneous fault strength (e.g., barriers) limit the slip duration. In this paper we investigate this second explanation for short risetimes by constructing a three-dimensional dynamic rupture model for the 1984 Morgan Hill, California earthquake (Mw = 6.2) using a kinematic model previously obtained from waveform inversion of strong motion data. We assume velocity-independent friction and a critical stress fracture criterion and derive a dynamic model specified by the spatial distribution of dynamic stress drop and strength excess that reproduces the slip and rupture time of the kinematic model. The slip velocity time functions calculated from this dynamic model are then used in a subsequent inversion to fit the strong motion data. By alternating between dynamic and kinematic modeling, we obtain a dynamic model that provides an acceptable fit to the recorded waveforms. In this dynamic model the risetime is short over most of the fault, which is attributable entirely to the short scale-length slip/stress drop heterogeneity required by the strong motion data. A self-healing mechanism, such as strongly velocity-dependent friction, is not required to explain the short risetimes observed in this earthquake.

Stress-Breakdown Time and Slip-Weakening Distance Inferred from Slip-Velocity Functions on Earthquake Faults

We estimate the critical slip-weakening distance on earthquake faults by using a new approach, which is independent of the estimate of fracture energy or radiated seismic energy. The approach is to find a physically based relation between the breakdown time of shear stress Tb, the time of peak slip-velocity Tpv, and the slip-weakening distance Dc, from the time histories of shear stress, slip, and slip velocity at each point on the fault, which can be obtained from dynamic rupture calculations using a simple slip-weakening friction law. Numerical calculations are carried out for a dynamic shear crack propagating either spontaneously or at a fixed rupture velocity on a vertical fault located in a 3D half-space and a more realistic horizontally layered structure, with finite-difference schemes. The results show that Tpv is well correlated with Tb for faults even with a heterogeneous stress-drop dis- tribution, except at locations near strong barriers and the fault edges. We also inves- tigate this relation for different types of slip-weakening behavior. We have applied the method to two recent, strike-slip earthquakes in western Japan, the 2000 Tottori and the 1995 Kobe events. We integrated the slip-velocity functions on the vertical fault obtained from kinematic waveform inversion of strong- motion and teleseismic records from the arrival time of rupture Tr to the time of the peak-slip velocity Tpv, and we then corrected the slip obtained at Tpv for the errors expected from the dynamic calculations. It was found that the slip-weakening dis- tance D c estimated in the frequency window between 0.05 and 0.5 Hz ranges between 40 and 90 cm on the two earthquake faults. However, if we consider the limited frequency resolution of the observed waveforms, probable time errors in the slip- velocity functions obtained from kinematic inversion, and the uncertainty of the slip- weakening behavior, the above estimates may be those located between the minimum resolvable limit and the upper bound of their real values. The estimated Dc values do not necessarily seem to indicate larger values in the shallower part and smaller values in the deeper part of the fault, but rather a spatially heterogeneous distribution that appears to be dependent on the local maximum slip. This possible dependence might be interpreted by the frictional properties of the fault such as the degree of roughness or the thickness of gouge layers, in addition to stress heterogeneities.

Source Rupture Process of the Tecomán, Colima, Mexico Earthquake of 22 January 2003, Determined by Joint Inversion of Teleseismic Body-Wave and Near-Source Data

The spatial and temporal slip distribution of the Tecoman, Colima, Mexico earthquake is estimated from near-source strong-motion and teleseismic body-wave data. To perform a stable inversion, we applied smoothing constraints and determined their optimal relative weights on the observed data using an optimized Akaikes Bayesian information criterion (ABIC). The source parameters are as follows: (strike, dip, slip) = (300°, 20°, 93°), seismic moment M = 2.3 × 1020 N m; source duration = 30 sec; along-strike distance = 35 km; along-dip distance = 70 km. We found that the rupture process can be divided into three stages: the rupture nucleated near the hypocenter (stage I), then it broke the first asperity centering about 15 km southwest from the epicenter (stage II); and the rupture propagated to the northeast and the second asperity was broken (stage III). We also estimated the shear-stress change due to the rupture process of the mainshock on and around the major fault zone. It appears that one cluster of aftershocks for the first 5 days, which took place in and adjacent to the zones of stress, increased due to the fault rupture during the mainshock, but overall correlation between the aftershock location and the stress pattern is not clear. Manuscript received 14 May 2003.

Seismotectonics of the Hida region, central Honshu, Japan

Abstract Seismotectonic features of the northern Hida region, central Honshu, Japan have been investigated in some detail, mainly on the basis of the last 6 years of observations of seismicity and focal mechanism of a large number of earthquakes, with reference to the geological and tectonic setting and evidence of a large earthquake in the past. It was found that high seismicity is concentrated with a remarkably clear lineation, but with a relatively low activity in the central section, along the Atotsugawa fault extending for about 70 km. This is one of major Quaternary faults in this region. The high seismicity with spatially nonuniform distribution may be related to postseismic stress concentration after the 1858 Hida earthquake (M = 7.0), and to heterogeneous fault strength. Seismicity is also high with belt-like extension beneath the Hida mountain range which is the highest mountain system in the Japan Islands. The depth distribution of seismicity is clearly bounded at 15 km below the Atotsugawa fault and 8 km beneath the Hida mountains. The local variations can be interpreted as being due to the difference in the brittle-ductile transition depth which suggests higher temperature beneath the mountains, involving active volcanoes. The focal mechanism solutions indicate that the maximum compressive stress is oriented in an ESE-WNW direction, which is more consistent with the direction of motion of the Pacific plate relative to the Eurasian plate rather than the suggested relative motion between the North America and Eurasian plates. The magnitude of the shear stress working in this region is estimated to be less than 700 bar.

Estimation of the Critical Slip-Weakening Distance: Theoretical Background

It has been shown that a trade-off exists between estimates of the break- down strength drop and the critical slip-weakening distance (e.g., Guatteri and Spu- dich, 2000). For this reason, only the fracture energy, proportional to these two parameters, may be estimated from waveform modeling. However, Mikumo et al. (2003) proposed a new technique to estimate the slip-weakening distance of earth- quakes, separate from the fracture energy. For this method to be valid, the peak slip- velocity time must be close to the stress breakdown time. Here we explain the theo- retical background of this assumption and clarify the limitations of this technique using numerical simulations based on the boundary integral equation method. The theoretical analysis using the boundary integral equation and some numerical tests indicates that a rather smooth rupture process and relatively sharp change in stress at the stress breakdown time in the slip-weakening curve ensure the validity of the method.

Three-dimensional P-wave velocity structure beneath Central Japan: low-velocity bodies in the wedge portion of the upper mantle above high-velocity subducting plates

Abstract The three-dimensional (3-D) P-wave velocity structure beneath Central Japan has been investigated in detail by an inversion method. 7490 P-wave arrival times from 120 shallow and intermediate depth earthquakes that have occurred in this region are used to estimate velocity anomalies in 3-D subdivided blocks and hypocentral perturbations, simultaneously. The results reveal complex 3-D structures, with low-velocity zones in the wedge portion of the upper mantle above the high-velocity Philippine Sea and Pacific plates subducting beneath this region. Prominent low-velocity bodies exist just beneath active volcanoes, particularly in the Hida mountain range. Low-velocity bodies are spatially correlated with the low-Q zones estimated from seismic intensity data. One low-velocity body coincides with an anisotropic body detected from the study of shear-wave splitting. Dome-shaped low-velocity masses seem to represent partially melted mantle diapirs. No clear evidence on velocity contrast has been identified across the Fossa Magna, which is a tectonic boundary between Northeast and Southwest Japan.

DYNAMIC RUPTURE ANALYSIS : INVERSION FOR THE SOURCE PROCESS OF THE 1990 IZU-OSHIMA, JAPAN, EARTHQUAKE (M=6.5)

A waveform inversion has been applied to strong motion data using a dynamic shear crack model. We studied the 1990 Izu-Oshima earthquake (MJMA = 6.5), which has vertical strike-slip faulting with unilateral rupture propagation. The inversion has two steps, a waveform inversion and a crack inversion, that are applied iteratively. A waveform inversion is used to determine the distribution of rupture starting times and slip dislocations using the slip functions calculated by the initial crack model, or by previous crack inversion. A crack inversion is used to calculate dynamic crack propagation that explains the results of the above inversion. In this step, we use the estimated rupture times as a locking fracture criterion; the maximum shear stress attained before a fault segment breaks gives a lower bound estimate of the peak shear strength at each fault segment. Then the dynamic stress drop distribution is estimated from the slip distribution obtained from waveform inversion assuming a dynamic crack model. From the results, we determine the rise time distribution and the distribution of a dimensionless stress ratio S defined as (strength excess)/(stress drop). Our analysis gives the following picture of the rupture process of the 1990 Izu-Oshima earthquake: (1) An asperity-type faulting having large slip and high stress drop was detected in the region around the initiation point of rupture. (2) South of the asperity zone, barrier-type faulting characterized by incoherent propagating rupture, small slip, long rise time, and high strength excess was detected. This zone corresponds to the intersection of the fault with the 1978 earthquake (MJMA = 7.0).

Dynamical fault rupture processes in heterogeneous media

Abstract The fault rupture processes in a horizontally layered medium and also in a three-dimensionally heterogeneous structure are investigated on three-dimensional, spontaneous dynamic shear crack models. The wave equations for three-dimensional space are solved numerically by a finite difference scheme under the appropriate boundary conditions and the finite stress fracture criterion. The heterogeneous properties of the elastic medium, particularly the existence of low-velocity zones, give remarkable effects on the rupture process, yielding appreciably decelerated rupture velocities and large fault displacements in and around the zones and strong motions in the near-field. The large fault displacements are enhanced when the rupture breaks the ground surface. The static seismic moment in the case with a low-velocity zone is essentially the same as in a homogeneous half-space. An attempt is made to simulate the rupture process of a moderate-size earthquake, by applying the above shear crack model in a heterogeneous medium with depth-dependent and laterally heterogeneous stress drop. The model appears to explain the observed features to a satisfactory degree.

Three-dimensional seismic structure of subducting lithospheric plates under the Japan islands

Abstract The three-dimensional seismic structure of subducting lithospheric plates under the Japan Islands has been investigated in detail by using an inversion method developed by Aki and co-workers. The present analysis clearly reveals the subduction of the Pacific plate, with a gradual narrowing in breadth down to about 600 km beneath the Sea of Japan, and also gives some indication of the Philippine Sea plate in the uppermost mantle, less than 50 km under the southernmost part of Japan. The upper boundary of the descending Pacific plate shows sharp velocity contrasts with respect to the overlying low-velocity zone, while the lower boundary appears to have a transitional nature. Large positive Bouguer anomalies in northeastern Japan may be explained, in part, by the effects of the subducting Pacific plate, but those over the Sea of Japan cannot be accounted for by lateral heterogeneities in the upper mantle.