Historical Seismograms - Evidence from the AD 2000 Izu Islands ...

284 Tatsuo Usami(2) A tsunami attacked the Pacific coast between 39’ and 40’ N and caused severalhundred deaths and several hundred houses were washed away.(3) Occurrence time is about 10 a.m.(4) Beginning noon of the same day, swarm earthquakes occurred in Tokyo andcontinued until 23 February. The total number of felt earthquakes was more than70. No damages were reported.New materials reveal the following facts:(1) Intensity (Japanese scale) distribution is shown in Figure 5. For comparison,isoseismals are drawn in Figure 6 for some other earthquakes which took place offthe Sanriku coast. The area of intensity 2 4 resembles those in Figure 5, howeverthe area of intensity 2 5 is smaller than the Kansei earthquake.(2) Aftershock series observed at various places is shown in Figure 5. They showthat the number of aftershocks decrease abruptly one week after the main shock. Atsome places, aftershocks occur more frequently than in Tokyo. The daily aftershocknumber observed at Sendai is as follows:17 February 54-5518 February 3019 February 1720 February 7-821 February 5-6No modern earthquake in Table 2 gives a persuasive explanation for the numberof aftershocks at Tokyo due to the Kansei event. Table 3 lists two large off-Sanrikuearthquakes in 1896 and 1933. The number of aftershocks for the 1896 event isgreater than for the 1933 event. Except for Tokyo, aftershock number of the Kanseiearthquake could be explained by considering that it gave rise to a few more aftershocksthan the 1896 event. The intensity distribution of the Kansei earthquakecould be reasonably explained if we make the magnitude somewhat larger than the1896 event.The distribution of areas hit by tsunami and the tsunami height estimated fromdamages resemble those of 1896 and 1933 earthquakes. The 1896 earthquake is dueto reverse faulting, and the 1933 event is due to normal fault movement.Let us try to determine the fault type of the Kansei earthquake employing theinitial motion of the tsunami wave observed at Nakamura, i.e., the ‘pull” motion.However, we do not know the initial motion at Nakamura for the 1896 and 1933earthquakes. The nearest point where the initial motion is known is Ayukawa,about 100 km north of Nakamura. At Ayukawa, the initial motion of tsunamiwave is “pull” for 1896 earthquake and “push” for 1933 earthquake. So, for thetime being, it may be possible to consider that Kansei earthquake is reverse faulttype. In Table 3, we compare the Kansei earthquake with some typical types ofearthquakes occurring off the coast of northeastern Japan. From Table 3, the typeof earthquake which most resembles the Kansei earthquake is the 1896 off-Sanrikuearthquake of reverse type.Historical large earthquakes of the types 1 to 3 (as defined in Table 3) are arrangedin Table 4. Eom plate tectonics theory, it is said that reverse fault movementsrepeat every one hundred years or so, but there is no definite repeat interval forthe normal fault movement. If the Kansei earthquake is of reverse type, we can see