Bend-rotation wave as a mechanism of macroseismic effects

Abstract

Instrumental measurements in epicentral zones of earthquakes and especially on outcropped faults are still relatively rare. This also concerns macroseismic observations during strong earthquakes. Issues regarding faulting kinematics, distribution of effective forces, accelerations and other parameters of near-surface seismic process, and, consequently, the focus mechanisms in the case of their outcropping, are still insufficiently studied. Macroseismic observations and measurements that are available show various effects. We put a special attention to the following macroseismic observations: eruption of soil and hard rock with overturning in air; bouncing of heavy blocks with simultaneous shifting 2-5 m aside; overturning of soil and ground blocks with 180' rotation along horizontal axis (Bolt 1978, Nikonov 1992). In Japan, movement of rectangular ledger blocks immediately near a fault formed at the graveyard was noted (Nikonov 1992). It is essential that ledger stones were of standard size, and stood vertically on pins fixed in underlying horizontal slabs; the pins remained undamaged, whereas vertical slabs have been displaced along the fault to the opposite wall of the fault. Hence, not a mere shear movement but slab tossing in air by some angle to the horizon took place. Interesting data can be found in the publication by Popova (1990). During Kudmdag earthquake (March 14, 1983) in the South-Western Turkmenia, with M = 5.4, I = 8, and focus depth h = 3 km, a 20 km long fault cropped out crossing the settlement built on a plane. Engineering -seismologic study revealed strengthening of shocking force by 0.5-0.7 (comparing to average values for the settlement) in strips along the fault, 150-400 m wide on one side, and 75-200 m wide on the other side of the fault. For earthquakes with high magnitude and intensity, the width of zones with strengthened impact along outcropping faults increases significantly. So, for earthquakes of epicentral intensity of 9-10, the width of the zone of highest intensity at the fault amounts to 2-6 km (Popova 1990). We draw special attention to the measurements which showed differences with distance in acceleration values recorded by accelerographs; a representative example is here the Rudbar earthquake in North Iran (20 June 1990, Ms = 7.7). At this event, a 80-km long fault ripped on the surface in three areas; the left-lateral shifts had the horizontal extent of 0.6 m (maximum), and the vertical extent up to 0.95 m (Berberian et al. 1992). The peak accelerations recorded at different distances from the focus and fault plane are listed in Table 17.1. It is seen that the values of vertical and horizontal accelerations increase exponentially up to 0.65g when approaching the ripped fault at a distance of 8 km, so that the value of 2g was possibly achieved at the fault's edges. Many authors have noted clear features of non-linearity of intense soil movements in the zone of tectonic faults (Aksenov et al. 1992, 1993, Aksenov and Lokajiek 1997, Li et al. 1994). The following features were observed in all reports: low-frequency oscillations (0.4-1.5 Hz), - high-amplitude monochromatic coherent oscillations, low velocity of phase range of 160-180 m/s with duration of up to 5 minutes. All these three features have been observed only in a zone adjacent to the tectonic fault. Moreover, the distance from the fault walls where the non-linear effects are observed depends on the earthquake magnitude. It is important to note that there is a rather sharp boundary of the space beyond which all non-linear effects disappear (Li et al. 1994). Presentation of models of mechanisms for all non-linear effects mentioned above, including rupture mechanism, involves many problems. Basing on theoretical works on non-linear dynamics (Lyapunov 1892, Poincare 1928, Mandelshtam 1950, Nicolis 1986), laboratory experiments and field studies, the author suggests a model based on bend-rotation waves as a mechanism of fracturing and macroseismic effects. © Springer-Verlag Berlin Heidelberg 2006.