FIG. 1. Tilt precursor of the 1944 Tonankai earthquake (4–7). (a) Rupture zone of the 1944 Tonankai earthquake, (b) Leveling lines near Kakegawa. (c) Bench mark distribution along a leveling line near Kakegawa along which precursory and coseismic tilt were observed, (d) Elevation difference observed mainly for sectors 2 and 3 (between bench marks 5259 and 5260 shown in c) plotted as a function of time.
1960s, however, are limited so that this result is still subject to some uncertainty.
FIG. 2. A slow foreshock of the Mw=9.5 1960 Chilean earthquake (8). (a) Strain record at Pasadena (lower trace) shows slow deformation before the expected arrival time of the mainshock P wave indicated (dashed line). Upper trace is a regular seismogram showing the beginning of the foreshock. (b) Interpretation of the slow precursory source.
A recent large earthquake for which precursory slow deformation (15% of the mainshock) was suggested from the source spectrum is the 1989 Mw=8.1 Macquarie Ridge earthquake (11) (Fig. 3), although this change was not detected on the time domain record (12).
In contrast to these, many studies using close-in strain and tilt meters have concluded that precursory slip, if any, is very small, <1%, for many California earthquakes (13) such as the 1987 Whittier Narrows earthquake (14) (Mw=6.0; Fig. 4), the 1987 Superstition Hills earthquake (15) (Mw=6.6; Fig. 5), the 1989 Loma Prieta earthquake (16) (Mw=6.9;
FIG. 3. Slow precursory source of the 1989 Macquarie Ridge earthquake (11). Source time function is shown by the solid curve. Time 0 refers to time of high-frequency radiation—i.e., origin time of the earthquake.