[SSS13-P04] Case study of succeeding earthquakes in the Hyuga-nada Sea, Japan
1.Introduction
In the Hyuga-nada Sea, Japan, two M5.4 earthquakes occurred on March 27, 2019, every few hours. On May 10, 2019, an M5.6 earthquake and an M6.3 earthquake occurred at hourly intervals. In 2016, the Headquarters for Earthquake Research Promotion compiled a report entitled Information on Prospects for Seismic Activity after a Large Earthquake." In this report, succeeding earthquakes all over Japan have been researched, but none of them has been reported in the Hyuga-nada Sea. This is because the results differ depending on the conditions to be extracted. In this report, only large earthquakes were investigated. We researched succeeding earthquakes in the Hyuga-nada Sea including middle class earthquakes.
The Hyuga-nada Sea is the region with the highest seismic activity within the hypocenter area of the Nankai Trough earthquake. In the Hyuga-nada Sea, many earthquakes occur at the plate boundary. This is why it is important to understand the succeeding earthquakes in the Hyuga-nada Sea in detail to monitor seismic activity within the hypocenter area of the Nankai Trough earthquake.
2.Methods
2.1 Data
Hypocenter data: JMA catalog
Period: April 1, 1984 to September 30, 2019
Magnitude: 3.0 or more (M≧3.0)
2.2 Methods
(1)Remove aftershocks
The earthquakes that meet the following conditions is considered aftershocks.And they were removed.
Epicenter distance: within Lkm calculated from Log(L)=0.5M-1.8(Utsu,1961).L means length of fault(km),M means magnitude.
But,if the length of the fault is less than 10km, epicenter distance is 10km.
Period:within 10 days for M≦6.0,within 30 days for M>6.0
Difference of magnitude:0.3 or more
(2)Extract succeeding earthquakes
The earthquakes that meet the following conditions is considered succeeding earthquakes.
Epicenter distance: same condition as for aftershocks
Period: within 1 day
Difference of magnitude: second earthquake magnitude - first earthquake magnitude ≧ -0.2
3.Result
Succeeding earthquakes do not occur evenly throughout the Hyuga-nada Sea, but tend to occur in specific areas of activity.The earthquakes on March 27, 2019 and May 10, 2019 also occurred in areas with a high incidence rate.In areas where these earthquakes occurred, succeeding earthquakes on August 29, 2014(Largest earthquake M6.0)and October 19, 1996(Largest earthquake M6.9)occurred(Fig1).70% of all succeeding earthquakes have the same focal mechanism as earthquakes which occur at the plate boundary in the Hyuga-nada Sea.Many of succeeding earthquakes in the Hyuga-nada Sea may have occurred at the plate boundary.
4.Use in Japan Meteorological Agency
We consider about using result of this research for operation in JMA.We are considering the following two points.
(1)Use for real time monitoring of seismic activity
(2)Use for explanatory materials about earthquake
We report about"(1)Use for real time monitoring of seismic activity".We detected that many succeeding earthquakes in the Hyuga-nada Sea have occurred within 1 day.So, if we could use this result for real time monitoring of seismic activity, we need to analyze about succeeding earthquakes automatically and immediately.We have developed system which analyze and let us know about succeeding earthquakes automatically and immediately when an earthquake occurs.If we had this system when the earthquakes on March 27, 2019 and May 10, 2019 occurred, we could recognize that these earthquakes occurred in areas with a high incidence rate.
In the Hyuga-nada Sea, Japan, two M5.4 earthquakes occurred on March 27, 2019, every few hours. On May 10, 2019, an M5.6 earthquake and an M6.3 earthquake occurred at hourly intervals. In 2016, the Headquarters for Earthquake Research Promotion compiled a report entitled Information on Prospects for Seismic Activity after a Large Earthquake." In this report, succeeding earthquakes all over Japan have been researched, but none of them has been reported in the Hyuga-nada Sea. This is because the results differ depending on the conditions to be extracted. In this report, only large earthquakes were investigated. We researched succeeding earthquakes in the Hyuga-nada Sea including middle class earthquakes.
The Hyuga-nada Sea is the region with the highest seismic activity within the hypocenter area of the Nankai Trough earthquake. In the Hyuga-nada Sea, many earthquakes occur at the plate boundary. This is why it is important to understand the succeeding earthquakes in the Hyuga-nada Sea in detail to monitor seismic activity within the hypocenter area of the Nankai Trough earthquake.
2.Methods
2.1 Data
Hypocenter data: JMA catalog
Period: April 1, 1984 to September 30, 2019
Magnitude: 3.0 or more (M≧3.0)
2.2 Methods
(1)Remove aftershocks
The earthquakes that meet the following conditions is considered aftershocks.And they were removed.
Epicenter distance: within Lkm calculated from Log(L)=0.5M-1.8(Utsu,1961).L means length of fault(km),M means magnitude.
But,if the length of the fault is less than 10km, epicenter distance is 10km.
Period:within 10 days for M≦6.0,within 30 days for M>6.0
Difference of magnitude:0.3 or more
(2)Extract succeeding earthquakes
The earthquakes that meet the following conditions is considered succeeding earthquakes.
Epicenter distance: same condition as for aftershocks
Period: within 1 day
Difference of magnitude: second earthquake magnitude - first earthquake magnitude ≧ -0.2
3.Result
Succeeding earthquakes do not occur evenly throughout the Hyuga-nada Sea, but tend to occur in specific areas of activity.The earthquakes on March 27, 2019 and May 10, 2019 also occurred in areas with a high incidence rate.In areas where these earthquakes occurred, succeeding earthquakes on August 29, 2014(Largest earthquake M6.0)and October 19, 1996(Largest earthquake M6.9)occurred(Fig1).70% of all succeeding earthquakes have the same focal mechanism as earthquakes which occur at the plate boundary in the Hyuga-nada Sea.Many of succeeding earthquakes in the Hyuga-nada Sea may have occurred at the plate boundary.
4.Use in Japan Meteorological Agency
We consider about using result of this research for operation in JMA.We are considering the following two points.
(1)Use for real time monitoring of seismic activity
(2)Use for explanatory materials about earthquake
We report about"(1)Use for real time monitoring of seismic activity".We detected that many succeeding earthquakes in the Hyuga-nada Sea have occurred within 1 day.So, if we could use this result for real time monitoring of seismic activity, we need to analyze about succeeding earthquakes automatically and immediately.We have developed system which analyze and let us know about succeeding earthquakes automatically and immediately when an earthquake occurs.If we had this system when the earthquakes on March 27, 2019 and May 10, 2019 occurred, we could recognize that these earthquakes occurred in areas with a high incidence rate.