Yasuhiro Umeda

High-amplitude seismic waves radiated from the bright spot of an earthquake

Abstract In the source region of a large shallow earthquake, there is likely to be a small spot characterized by the production of high accelerations and a gap in the aftershocks. On the seismograms of most large shallow earthquakes, and on some seismograms of moderate main shocks, two types of P-waves may be detected: the relatively low-amplitude waves following phase P 1 and the predominant high-frequency waves following phase P 2 . The duration of the seismic waves following phase P 1 (the P 1 −P 2 time) increases proportionately with the earthquake magnitude. This fact strongly suggests that the preliminary rupture period is closely related to the radiation of the predominant high-frequency seismic waves. To explain the above observation, the shear strain waves radiated from a moving rupture are calculated. Because the seismic wave velocity is generally higher than the rupture propagation velocity, maximum shear strains by seismic waves usually precede the rupture front. If the total shear strain, which is equal to the static pre-earthquake shear strain plus dynamic shear strain excited by seismic waves, exceeds the critical shear strain (~ 10 −4 ), the secondary ruptures break out in the forward direction from the initial rupture. The rupture processes of large or moderate earthquakes occur in two stages: one is the initiation of rupture corresponding to phase P 1 , and the other is the development of secondary ruptures triggered by seismic shear strain waves. The latter process generates an earthquake bright spot which radiates high-amplitude seismic waves.

The bright spot of an earthquake

Abstract Umeda, Y., 1992. The bright spot of an earthquake. In: T. Mikumo, K. Aki, M. Ohnaka. L.J. Ruff and P.K.P. Spudich (Editors). Earthquake Source Physics and Earthquake Precursors. Tectonophysics, 211: 13–22. Field surveys in the source region for some large shallow earthquakes reveal that there is a special spot characterized by high acceleration, aftershock gap and parallel faults system. This spot is called an “earthquake bright spot” which occupies a relatively small region compared to the final rupture region. At least two kinds of phases, P1 and P2, which are responsible for the rupture initiation and the bright spot formation can be identified on the broad-band seismograms for large earthquakes. Duration times of preliminary waves (P1−P2) are proportional to the earthquake magnitudes. By using this evidence from the field and seismograms, a model having three rupture stages is proposed for a growth process of large earthquakes. The 1st stage is an initiation and propagation of rupture. The 2nd stage is formation of the bright spot which produces the high-frequency seismic waves. In the 3rd stage, smooth dislocations again propagate in bi-laterally and radiate the long-period seismic waves.

Earthquake rupture complexity due to dynamic nucleation and interaction of subsidiary faults

We numerically study the dynamic interaction of propagating cracks. It is assumed that propagating cracks can nucleate and drive subsidiary cracks because of shear strain enhancement near the propagating crack tips. The critical strain fracture criterion is assumed in the analysis. Intense interaction is expected to occur among the cracks. All the cracks are assumed to be parallel and antiplane strain deformation is assumed in the computation.In the interaction of two non-coplanar cracks, a strain shadow is formed in the neighborhood of each crack because of the strain release by the introduction of the crack. The growth of each crack is accelerated when the propagating tips of each crack are outside of the strain shadow of the other crack. In general, the crack tips enter the strain shadow, and the crack tips decelerate. The calculation shows that only one of the two cracks can continue to grow, and the others growth is decelerated and arrested. If we can assume that the suite of cracks interact in a pairwise manner only, then this may suggest that only a limited number of cracks can continue to grow during the final stage of the rupture process. Hence the crack interaction causes complexity in dynamic earthquake faulting. The concepts of barrier and asperity have been employed by many researchers for the interpretation of complex seismic wave data. However, the physical realities of such concepts are obscure. Our calculations show that dynamic crack interactions can produce barriers and asperities in some cases; the crack tip deceleration or arrest due to the interactions among non-coplanar cracks can be interpreted as being due to a barrier. The dynamic coalescence among the coplanar cracks can be regarded as an asperity.Umeda found a localized area that strongly radiates high-frequency seismic waves in the epicentral areas of some large shallow earthquakes. He defined this as an “earthquake bright spot.” Our analysis implies that only a limited number of cracks continue to grow when many interactive cracks nucleate, and that all other cracks stop extending soon after nucleation. Hence, if the nucleation and termination of several cracks occur in a localized area, it will be observed seismologically as an earthquake bright spot. This is because it is theoretically known that the sudden termination of crack growth and dynamic crack coalescence efficiently emits high-frequency elastic waves.

High accelerations produced by the Western Nagano Prefecture, Japan, earthquake of 1984

Many boulders were thrown out of their former sockets by the Western Nagano Prefecture, Japan, earthquake of 1984 (Mjma = 6.8). The anomalous high accelerations of 4–16 g were estimated from the displacement of thrown-out boulders, assuming that the seismic waves had a frequency range of 5–10 Hz. Almost all of the thrown-out boulders were found on the mountain-tops, ridges and saddles. The topographic amplifications of seismic waves were estimated using five aftershocks recorded on the mountain-top and at the foot. Average amplitude ratios (mountain-top: foot) of seismic waves are 2–7 in the frequency range concerned. The high acceleration area defined by the distribution of thrown-out boulders is very small (1× 3 km) compared with the length (12 km) of the assumed main fault. Many cracks were also found in this limited small area, which is characterized by extremely low activity of aftershocks and relatively large dislocation.

Possible mechanisms of dynamic nucleation and arresting of shallow earthquake faulting

Abstract The nucleation and arresting mechanisms of large shallow earthquakes are investigated from both observational and theoretical viewpoints. We show that two distinct phases are commonly observed at the initial part of seismograms of large shallow earthquakes. The first phase denotes the onset of the P wave, which shows a very gradual increase in amplitude with time. This gradual change is interrupted by the arrival of the second phase, which causes an abrupt change in the amplitude. Our seismological observation shows that the time interval between the onset of the two phases is strongly correlated with the magnitude of the earthquake. It is probable that the above two phases are related to some aspect of inhomogeneities in the earths crust. One of the important sources of such inhomogeneities is known to be preexisting cracks and their interactions. We theoretically show in this paper that a rupture occurring in a zone of densely distributed cracks radiates elastic waves that can simulate the features of the above two phases; note that such a rupture is considerably affected by crack interactions. Theoretical calculation is also carried out to investigate the effect of crack interactions on the arresting of rupture propagation. A propagating crack can excite subsidiary cracks ahead of its crack tip, which gives rise to crack interactions. We show that these interactions sometimes facilitate the arresting of rupture propagation.

The 2000 western Tottori earthquake

1. The 2000 Western Tottori Earthquake—Seismic activity revealed by the regional seismic networks— . . . . 819 2. Swarm-like seismic activity in 1989, 1990 and 1997 preceding the 2000 Western Tottori Earthquake . . . . . 831 3. Spatial analysis of the frequency-magnitude distribution and decay rate of aftershock activity of the 2000 Western Tottori earthquake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 847 4. Local site amplification and damage to wooden houses in Shimoenoki, Tottori, Japan, by the 2000 Western Tottori Earthquake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 861

Earthquake core inferred from near field observations

Abstract The source processes of large shallow earthquakes are investigated based on the various field phenomena and on the seismograms recorded at short focal distances. The results from coseismic and postseismic field surveys in some source regions strongly show that there must be a particular region characterized by a large dislocation, large acceleration and extremely low aftershock activity. This specific region seems to have a relatively small dimension compared with the length of the main fault. The predominant short-period waves on the strong-motion seismograms are concentrated within the short intervals at the initial parts of P and S waves. This fact also suggests that the rupture elements generating the predominant short-period waves are not distributed over the entire surface of a single main fault but are concentrated in a small region. We call this confined small region in the source area “earthquake core”. The earthquake core is formed a little later than the start of smoothing dislocation and it may be located at some distance from the starting point of rupture.

Implications of Thrown-Out Boulders for Earthquake Shaking

In the source areas of some large shallow earthquakes, we have found many dislodged boulders struck by severe ground shaking. Some boulders were located at quite distances from the former sockets, which remained undisturbed with surrounding clear edges. This fact indicates the possibility that vertically upward seismic acceleration exceeded the earths gravity (1g). This phenomenon of upthrown boulders is investigated herein by examining the effects of waves, which emanate deep in the ground due to an earthquake, propagate through the ground and boulder, and reflect back to the ground, involving a variety of their interaction. An elastic dynamic analysis is carried out on the basis of a one-dimensional continuum model consisting of the ground and boulder. It is subjected to the input of the Ricker wave, which is intended to simulate an earthquake-generated wave, emanating from the bottom of the model ground. The upthrow of a boulder is taken to occur when the dynamic response at the bottom of the boulder satisfies certain conditions. It turns out that the possibility of upthrow occurrence is high when the period of the Ricker wave coincides with the fundamental period of the ground vibration. It leads to the conclusion that the upthrow takes place due to resonance in the response of the system of the ground and boulder to the external wave input. The upthrow possibility increases as the input acceleration increases. Trial is made of predicting the maximum acceleration and velocity of an earthquake, based on this consideration of the up throw phenomenon.