Shigeji Suyehiro

Foreshocks and earthquake prediction

Abstract If a group of foreshocks can be distinguished from ordinary seismic activities, it would contribute to earthquake prediction. In the case of two Japanese earthquakes, which were preceded by immediate foreshocks, the coefficient “b” in the magnitude versus frequency equation was significantly smaller than that of the aftershocks or of the ordinary seismic activity in the region. The same characteristics were also found for the Chilean earthquake of 1960. Very large earthquakes can also take place without any immediate foreshocks. The great Kanto earthquake of 1923 was not preceded by any immediate foreshocks. However, the seismic activity in the general region became abnormally high from one year prior to the earthquake. Whether large earthquakes in a specific region are preceded by foreshocks or not, or, if not, what kind of foregoing activity is noticeable may probably vary from one seismic region to another. Therefore, detailed seismic data over a long period of time will be necessary to know the regional characteristic of foreshocks or foregoing activity. To achieve earthquake prediction, the physical process of accumulation and release of seismic energy must eventually be made known as well as statistical data.

Stress redistribution and slow earthquakes

Abstract Earthquakes are usually assumed to result when the gradual stress buildup in a region eventually exceeds some initial local strength. There are observations which suggest that a number of mechanisms must have a significant role in stress redistribution in active earthquake regions. The stress build-up could be non-linear in time; there is evidence that this is so in the case of some major earthquakes. One mechanism that might explain non-linear strain buildup is a “slow” earthquake. This is similar to a normal earthquake except that the rupture and slip velocities are very low so that negligible energy is radiated in the normally observed seismic frequency band. The slow earthquakes studied here have been recorded by a network of Sacks-Evertson borehole strainmeters installed along the seismically active Pacific coast of Honshu (Japan). Larger slow events may have (slow) foreshocks and aftershocks in a manner similar to that for normal earthquakes. The aftershocks appear to have lower stress drops than the foreshocks. The static field changes associated with the Izu-Oshima earthquake ( m = 7) of January 1978 were sufficiently large that they were clearly recorded by the nearest three strainmeters. Interpretation of the records indicate that this earthquake was in essence a normal quake followed by a slow event of similar moment. This interpretation is corroborated by reports of ground vibration and faulting on the Izu peninsula.