11:15 AM - 11:30 AM
[SSS13-03] Is the source of the 887 Nin’na earthquake Nankai trough or Osaka-Bay fault?
Keywords:887 Nin’na earthquake, Nankai trough, Osaka Bay fault, historiographical seismology, tsunami deposit
On the Nin’na earthquake of August 22, 887 (Julian calendar), which has been regarded as a great interplate earthquake along the Nankai trough off southwest Japan, Fujino and Matsu’ura (2022 SSJ fall meeting, S10-09) proposed a hypothesis that it might be the latest activity of the Osaka-Bay fault off the Kinki district. Since this hypothesis is important in that it urges us to reconsider the recurrence history of the Nankai trough earthquakes, I reexamined the source region of this event from the viewpoint of mainly historiographical seismology. The conclusion is the source of the 887 Nin’na earthquake is still considered the Nankai trough.
One basis of the Fujino-Matsu’ura (FM) hypothesis is that Nihon Sandai Jitsuroku (NSJ), a fundamental historical record of the 887 Nin’na earthquake, does not describe typical phenomena of the Nankai trough earthquakes such as temporal inactivity of specific hot springs, coseismic subsidence of the Kochi plain, and large tsunamis on the Shikoku and Kii coasts. NSJ says, however, “large tremors almost all over Japan, many official residences destroyed, sea water overflowed land causing innumerable drowned,” which can be interpreted as Nankai trough earthquake disasters. To begin with, we cannot conclude that a certain phenomenon did not occur because it is not written in NSJ. A good example is the heavy storm mentioned below which caused serious disasters to many provinces 20 days after the Nin’na earthquake, on which NSJ describes only about Kyoto.
Another basis of the FM hypothesis is that according to NSJ the 887 earthquake motion in Kyoto seems stronger than those of 1707 and 1854 Nankai trough earthquake. But, on this point, we should attach importance to the diversity of repeating Nankai trough earthquake, and we cannot exclude the 887 event from the Nankai trough series by the reason that the ground motion in Kyoto was slightly stronger than the 1707 and 1854 events.
Concerning the 887 earthquake motion in Kyoto the most important point is that strong motion lasted for at least 30 to 60 minutes according to NSJ. This is considered distinct indication that the 887 earthquake was a great Nankai trough earthquake (Ishibashi, 1999, 2000); such ground motion cannot be explained by an Osaka-Bay fault earthquake.
A crucial historical record of the 887 earthquake is Nin’na Yonen Gogatsu Mikotonori of 888, which Fujino and Matsu’ura (2022) did not refer to. This record tells that more than 30 provinces suffered from twin disasters due to the Nin’na earthquake and heavy storm (probably typhoon) 20 days after the earthquake. This means the earthquake-stricken provinces were more than 30, strongly suggesting the earthquake was not an Osaka-Bay fault event of M less than 8 but a M 8-class great Nankai trough event.
NSJ writes “tsunami was the most severe in Settsu Province (part of present Osaka Prefecture),” which Fujino and Matsu’ura (2022) considered a basis of the Osaka-Bay fault origin of the 887 event. Kawada et al. (2005), however, showed by numerical simulation that along the Settsu coast tsunami height due to the Osaka-Bay fault earthquake was less than that of the Nankai trough earthquake. Around 887, an important port, Kawajiri, on the Settsu coast flourished. This port city and surrounding areas are inferred to have been struck seriously by tsunami originated from the Nankai trough.
Fujino and Matsu’ura (2022) doubted the Nankai trough origin of the 887 earthquake because the cases had been accumulated that tsunami deposits related to this event could not be found in investigation sites while tsunami deposits related to the 684 Nankai trough earthquake were discovered. On this point, I suggest that the aforementioned heavy storm which struck many provinces 20 days after the Nin’na earthquake might have disturbed or vanished the tsunami deposits by heavy rainfall, violent waves, and large amount of sediment discharges from rivers.
One basis of the Fujino-Matsu’ura (FM) hypothesis is that Nihon Sandai Jitsuroku (NSJ), a fundamental historical record of the 887 Nin’na earthquake, does not describe typical phenomena of the Nankai trough earthquakes such as temporal inactivity of specific hot springs, coseismic subsidence of the Kochi plain, and large tsunamis on the Shikoku and Kii coasts. NSJ says, however, “large tremors almost all over Japan, many official residences destroyed, sea water overflowed land causing innumerable drowned,” which can be interpreted as Nankai trough earthquake disasters. To begin with, we cannot conclude that a certain phenomenon did not occur because it is not written in NSJ. A good example is the heavy storm mentioned below which caused serious disasters to many provinces 20 days after the Nin’na earthquake, on which NSJ describes only about Kyoto.
Another basis of the FM hypothesis is that according to NSJ the 887 earthquake motion in Kyoto seems stronger than those of 1707 and 1854 Nankai trough earthquake. But, on this point, we should attach importance to the diversity of repeating Nankai trough earthquake, and we cannot exclude the 887 event from the Nankai trough series by the reason that the ground motion in Kyoto was slightly stronger than the 1707 and 1854 events.
Concerning the 887 earthquake motion in Kyoto the most important point is that strong motion lasted for at least 30 to 60 minutes according to NSJ. This is considered distinct indication that the 887 earthquake was a great Nankai trough earthquake (Ishibashi, 1999, 2000); such ground motion cannot be explained by an Osaka-Bay fault earthquake.
A crucial historical record of the 887 earthquake is Nin’na Yonen Gogatsu Mikotonori of 888, which Fujino and Matsu’ura (2022) did not refer to. This record tells that more than 30 provinces suffered from twin disasters due to the Nin’na earthquake and heavy storm (probably typhoon) 20 days after the earthquake. This means the earthquake-stricken provinces were more than 30, strongly suggesting the earthquake was not an Osaka-Bay fault event of M less than 8 but a M 8-class great Nankai trough event.
NSJ writes “tsunami was the most severe in Settsu Province (part of present Osaka Prefecture),” which Fujino and Matsu’ura (2022) considered a basis of the Osaka-Bay fault origin of the 887 event. Kawada et al. (2005), however, showed by numerical simulation that along the Settsu coast tsunami height due to the Osaka-Bay fault earthquake was less than that of the Nankai trough earthquake. Around 887, an important port, Kawajiri, on the Settsu coast flourished. This port city and surrounding areas are inferred to have been struck seriously by tsunami originated from the Nankai trough.
Fujino and Matsu’ura (2022) doubted the Nankai trough origin of the 887 earthquake because the cases had been accumulated that tsunami deposits related to this event could not be found in investigation sites while tsunami deposits related to the 684 Nankai trough earthquake were discovered. On this point, I suggest that the aforementioned heavy storm which struck many provinces 20 days after the Nin’na earthquake might have disturbed or vanished the tsunami deposits by heavy rainfall, violent waves, and large amount of sediment discharges from rivers.