takeo ishibe | The University of Tokyo (original) (raw)

Papers by takeo ishibe

Large earthquakes around Japan occur not only in the Pacific Ocean but also in the Sea of Japan, ... more Large earthquakes around Japan occur not only in the Pacific Ocean but also in the Sea of Japan, and cause both damage from the earthquake itself and from the ensuing tsunami to the coastal areas. Recently, offshore active fault surveys were conducted in the Sea of Japan by the Integrated Research Project on Seismic and Tsunami Hazards around the Sea of Japan (JSPJ), and their fault models (length, width, strike, dip, and slip angles) have been obtained. We examined the causative faults of M7 or larger earthquakes in the Sea of Japan during the 20th century using seismic and tsunami data. The 1940 off Shakotan Peninsula earthquake (MJMA 7.5) appears to have been caused by the offshore active faults MS01, MS02, ST01, and ST02 as modelled by the JSPJ. The 1993 off the southwest coast of Hokkaido earthquake (MJMA 7.8) likely occurred on the offshore active faults OK03a, OK03b, and OK05, while the 1983 Central Sea of Japan earthquake (MJMA 7.7) probably related to MMS01, MMS04, and MGM0...

Seismological Research Letters

Tsunami potential from high dip-angle splay faults is an understudied topic, although such splay ... more Tsunami potential from high dip-angle splay faults is an understudied topic, although such splay faults can significantly amplify coastal tsunami heights as compared with ordinary thrust faults. Here, we identify a hotspot for tsunamis from splay faulting in the Molucca Sea arc–arc collision zone in eastern Indonesia, which accommodates one of the world’s most complicated tectonic settings. The November 2019 Mw 7.2 earthquake and tsunami are studied through teleseismic inversions assuming rupture velocities in the range 1.5–4.0 km/s followed by tsunami simulations. The normalized root mean square error index was applied and revealed that the best model has a rupture velocity of 2.0 km/s from the steeply dipping plane. The recent high dip-angle reverse 2019 Mw 7.2 and 2014 Mw 7.1 earthquakes combined with numerous similar seismic events may indicate that this region is prone to splay faulting. This study highlights the need for understanding tsunamis from splay faulting in other su...

Seismological Research Letters

Long-term seismicity in a certain region can be estimated from the number of felt earthquakes obt... more Long-term seismicity in a certain region can be estimated from the number of felt earthquakes obtained from seismic intensity measurements or historical documents. To obtain a homogeneous estimate, continuous records with a uniform detection threshold are required. Seismic intensity data in Tokyo, which are measured by professional observers and archived by the Japan Meteorological Agency (JMA) since 1885, indicate that the mean annual number of felt earthquakes with intensities ≥1 and ≥2 on the JMA scale is 45 and 14, respectively. The database of felt earthquakes can be extended back to more than 200 yr using historical daily records. During the Edo period from 1603 to 1867, numerous historical documents in Edo (former Tokyo) recorded information about damaging earthquakes, such as the 1703 Kanto earthquake and the 1855 Edo earthquake. Several feudal clans documented daily weather and felt earthquakes in Edo. Among them, daily records documented by the Tsugaru and Sakakibara clans...

Earth, Planets and Space

We analyze the regional tsunami hazard along the Sea of Japan coast associated with 60 active fau... more We analyze the regional tsunami hazard along the Sea of Japan coast associated with 60 active faults beneath the eastern margin of the Sea of Japan. We generate stochastic slip distribution using a Monte Carlo approach at each fault, and the total number of required earthquake samples is determined based on convergence analysis of maximum coastal tsunami heights. The earthquake recurrence interval on each fault is estimated from observed seismicity. The variance parameter representing aleatory uncertainty for probabilistic tsunami hazard analysis is determined from comparison with the four historical tsunamis, and a logic-tree is used for the choice of the values. Using nearshore tsunami heights at the 50 m isobath and an amplification factor by the Green’s law, hazard curves are constructed at 154 locations for coastal municipalities along the Sea of Japan coast. The highest maximum coastal tsunamis are expected to be approximately 3.7, 7.7, and 11.5 m for the return periods of 100...

Seismological Research Letters

To archive precious analog seismograms of major earthquakes that occurred in and around Japan, we... more To archive precious analog seismograms of major earthquakes that occurred in and around Japan, we have scanned selected analog seismograms from 113 stations deployed by the Japan Meteorological Agency (JMA) and are constructing a database of the scanned digital images accessible through the website of the Headquarters for Earthquake Research Promotion (HERP) of the Japanese government. This project started in 2006 and was conducted by the Association for the Development of Earthquake Prediction. The database of HERP now contains more than 185,000 digital images of seismograms recorded during the period from 1884 to 1995. It includes seismograms of earthquakes that occurred in and around Japan and of some major teleseismic earthquakes. The oldest analog seismogram was recorded on 2 January 1884 at a station in Tokyo. The seismograms are the intellectual property of mankind, and the database of these digital images will be an important material for studies of past destructive earthqua...

Ocean Engineering

The 2016 Mw 7.8 Kaikoura earthquake and consequent tsunami have been controversial because of unc... more The 2016 Mw 7.8 Kaikoura earthquake and consequent tsunami have been controversial because of uncertainty over whether and where the plate interface ruptured and the incapability of the proposed source models to reproduce the near-field runup of 7 m. Existing models identify a wide range of locations for the interface rupture, from on land to offshore, and fail to reproduce runup of 7 m near Kaikoura. To generate the large tsunami peak in Kaikoura tide gauge record and the observed runup height, offshore seafloor movement is necessary, but the offshore extension of the plate-interface rupture and its type, either seismic rupture or a landslide, is uncertain. Here, we propose a submarine landslide in addition to the earthquake source, with the landslide delayed 10-20 min after the earthquake rupture. The landslide volume is 4.5-5.2 km 3 , located within 173.7-174.3 o E (longitude) and 42.6-42.15 o S (latitude). Our proposed dual tsunami source successfully reproduces near-field tide gauge records as well as observed near-field runup height of 7 m. We showed that more accurate source models of earthquakes can be achieved by considering observed runup data through runup inversions in addition to waveform inversions.

Geoscience Letters

Various Tsunami Service Providers (TSPs) within the Mediterranean Basin supply tsunami warnings i... more Various Tsunami Service Providers (TSPs) within the Mediterranean Basin supply tsunami warnings including CAT-INGV (Italy), KOERI-RETMC (Turkey), and NOA/HL-NTWC (Greece). The 20 July 2017 Bodrum-Kos (Turkey-Greece) earthquake (Mw 6.6) and tsunami provided an opportunity to assess the response from these TSPs. Although the Bodrum-Kos tsunami was moderate (e.g., runup of 1.9 m) with little damage to properties, it was the first noticeable tsunami in the Mediterranean Basin since the 21 May 2003 western Mediterranean tsunami. Tsunami waveform analysis revealed that the trough-to-crest height was 34.1 cm at the near-field tide gauge station of Bodrum (Turkey). Tsunami period band was 2-30 min with peak periods at 7-13 min. We proposed a source fault model for this tsunami with the length and width of 25 and 15 km and uniform slip of 0.4 m. Tsunami simulations using both nodal planes produced almost same results in terms of agreement between tsunami observations and simulations. Different TSPs provided tsunami warnings at 10 min (CAT-INGV), 19 min (KOERI-RETMC), and 18 min (NOA/HL-NTWC) after the earthquake origin time. Apart from CAT-INGV, whose initial Mw estimation differed 0.2 units with respect to the final value, the response from the other two TSPs came relatively late compared to the desired warning time of ~ 10 min, given the difficulties for timely and accurate calculation of earthquake magnitude and tsunami impact assessment. It is argued that even if a warning time of ~ 10 min was achieved, it might not have been sufficient for addressing near-field tsunami hazards. Despite considerable progress and achievements made within the upstream components of NEAMTWS (North East Atlantic, Mediterranean and Connected seas Tsunami Warning System), the experience from this moderate tsunami may highlight the need for improving operational capabilities of TSPs, but more importantly for effectively integrating civil protection authorities into NEAMTWS and strengthening tsunami education programs.

Pure and Applied Geophysics

The September 2017 Chiapas (Mexico) normalfaulting intraplate earthquake (M w 8.1) occurred withi... more The September 2017 Chiapas (Mexico) normalfaulting intraplate earthquake (M w 8.1) occurred within the Tehuantepec seismic gap offshore Mexico. We constrained the finite-fault slip model of this great earthquake using teleseismic and tsunami observations. First, teleseismic body-wave inversions were conducted for both steep (NP-1) and low-angle (NP-2) nodal planes for rupture velocities (V r) of 1.5-4.0 km/s. Teleseismic inversion guided us to NP-1 as the actual fault plane, but was not conclusive about the best V r. Tsunami simulations also confirmed that NP-1 is favored over NP-2 and guided the V r = 2.5 km/s as the best source model. Our model has a maximum and average slips of 13.1 and 3.7 m, respectively, over a 130 km 9 80 km fault plane. Coulomb stress transfer analysis revealed that the probability for the occurrence of a future large thrust interplate earthquake at offshore of the Tehuantepec seismic gap had been increased following the 2017 Chiapas normal-faulting intraplate earthquake.

Geophysical Journal International

Kanto earthquake (JMA magnitude; M JMA 7.9) caused approximately 105 000 casualties, mostly due t... more Kanto earthquake (JMA magnitude; M JMA 7.9) caused approximately 105 000 casualties, mostly due to a massive fire that was caused by the main shock (Moroi & Takemura 2004), and the 1703 Kanto earthquake with the magnitude (M) of 8.2 also caused much destruction and approximately 7000 casualties. A recent palaeoseismological study indicated that the antepenultimate Kanto earthquake occurred in 1293 (Shimazaki et al. 2011). The Earthquake Research Committee (of the government of Japan) (2014) estimated the recurrence interval of the M8-class Kanto earthquakes to be 200-400 yr, and calculated the probability of another such earthquake in the next 30 yr to be approximately 0-5 per cent. The probability of a large (M ∼ 7) earthquake other than the M8class Kanto earthquake is much higher (e.g. Grunewald & Stein 2006; Stein et al. 2006; Nanjo et al. 2013; Somerville 2014). The Earthquake Research Committee (2014) calculated the probability of occurrence of such an earthquake during the next 30 yr to be approximately 70 per cent; this is based on the history of M ∼ 7 earthquakes since 1703. The Central Disaster Prevention Council (government of Japan, under the Cabinet office) estimates 112

Geophysical Research Letters, 2016

Geophysical Research Letters, 2015

Agu Fall Meeting Abstracts, Dec 1, 2010

ABSTRACT Along the Nankai Trough, recurrence of at least nine great (M~8) interplate earthquakes ... more ABSTRACT Along the Nankai Trough, recurrence of at least nine great (M~8) interplate earthquakes have been inferred from historical documents in Japan. Historical documents also record timing of ground shaking or arrival time of seismic waves. Iida (1985) and Usami (2003) compiled arrival times of seismic waves due to the 1707 Hoei earthquake and concluded that the Tokai, Tonankai and Nankai segments simultaneously ruptured or separately ruptured within 2 hours. We statistically estimate the origin time of the Hoei earthquake based on only reliable historical documents including additional ones found after their studies, and suggest a possibility of delayed rupture on the earthquake segments. In addition, tsunami heights are computed on the basis of the result, and compared with tsunami trace heights of the 1707 Hoei earthquake. We selected primary and reliable historical documents from Kyushu to Tohoku region written within 30 years after the Hoei earthquake. The arrival times of the Hoei earthquake at each location were then retrieved from these documents. The arrival times were spatially averaged to estimate the origin time of the Hoei earthquake. The time count interval in 1707 is different from the current system: day and night were divided into 6 ``koku'', hence each ``koku'' corresponds to about 2 hours. However, timing is often indicated with early, middle or late ``koku'', making the temporal resolution as small as 40 minutes. Because the timing was measured from sunrise to sunset, the local time varies with longitude, hence a correction is applied to convert the local time to the standard time. Our likelihood estimate shows that the origin time of the Hoei earthquake was 13:47 with the standard error of 1.02 hours. In addition, we statistically tested whether the Hoei earthquake had simultaneously ruptured all the segments or separately ruptured, and evaluated the best segmentation using the Akaike's Information Criterion (AIC; Akaike, 1974). The result indicates statistically significant differences in the origin times of each segment; the rupture started on the Tonankai and Nankai segments, and the rupture on the Tokai segment was delayed by about 19 min. Tsunami computation including delayed rupture of Tokai segment was carried out to verify the result from historical documents. For the tsunami source, we adopt the 1707 Hoei earthquake model by Annaka et al. (2003), which consists of four fault segments (N1 to N4) to describe Tokai, Tonankai, and Nankai earthquakes. The rupture delay time of N1 (Tokai) segment relative to N2 (Tonankai) through N4 (Nankai) segments is assumed to be 0 to 79 min (5 min interval for 4 to 39 min and 10 min interval after 49 min). To compare the computed tsunami heights and the tsunami trace heights along the Enshu and Suruga Bay coasts (Hatori, 1977), we calculated geometric standard deviation kappa (Aida, 1978). The kappa value took the minimum value for the 14 min delay case, which is consistent with the statistical analysis from historical documents. These results suggest a possibility that N1 segment rupture was delayed relative to the N2 to N4 segment rupture.

Marine Geology, 2015

ABSTRACT Nine event deposits in a small alluvial valley along the Sanriku coast, Japan, were corr... more ABSTRACT Nine event deposits in a small alluvial valley along the Sanriku coast, Japan, were correlated with historical tsunamis and storms that have been recorded in this region since the 15th century. We identified nine sandy layers in 15 geo-slices collected at distances ranging from 140 to 260 m from the coast in a lowland back marsh protected from the sea by a high sandy ridge. Based on their sedimentary characteristics, grain-size distribution, and marine microfossil assemblages, namely Coccoliths, these event layers, which were well preserved in their order of deposition, were probably either tsunami or storm deposits. Diatom analysis revealed four stages of paleo-environmental change from tidal marsh to wetland. The uppermost event layer covering the ground surface is probably the deposit from the 2011 Tohoku-Oki tsunami. Ages for other event deposits were radiometrically dated using 14C, 137Cs and 210Pb with Bayesian estimation. The second event layers can be correlated with the 1960 Chilean or 1968 Aomori-Oki earthquake tsunamis. The candidates for the third event layer are the 1947 Catherine and 1948 Ione typhoons as well as the candidates for the second event layer. The fourth and fifth event layers are likely associated with the 1933 and 1896 Sanriku-Oki earthquake tsunamis. Four lower event layers can be correlated with historical tsunamis and storms that occurred during the 15th to 18th centuries, such as the 1611 Sanriku-Oki earthquake tsunami.

High-resolution shallow-water profiling survey revealed recent activity of submarine fault which ... more High-resolution shallow-water profiling survey revealed recent activity of submarine fault which was newly found to fill a gap between two subaerial faults, the Asamigawa and the Funai faults running on the southern coast of Beppu Bay. Historical records suggest that this activity on the north-dipping normal fault corresponds to the M7.3 Keicho-Bungo earthquake of September 1, 1596. Our previous profiling survey and correlation study of core samples show that the event caused vertical offset on south-dipping normal faults in the central and northern parts of the bay. Thus it is most likely that the Keicho-Bungo event simultaneously ruptured the faults on the both sides of the Beppu Graben. Furthermore core samples obtained from the both sides of the newly found submarine fault suggest repeated occurrence of this simultaneous rupturing. A comparison of vertical offsets of two prominent ash layers on Beppu Bay faults whose total length exceeds 230km, shows almost linear relationship b...

Large earthquakes around Japan occur not only in the Pacific Ocean but also in the Sea of Japan, ... more Large earthquakes around Japan occur not only in the Pacific Ocean but also in the Sea of Japan, and cause both damage from the earthquake itself and from the ensuing tsunami to the coastal areas. Recently, offshore active fault surveys were conducted in the Sea of Japan by the Integrated Research Project on Seismic and Tsunami Hazards around the Sea of Japan (JSPJ), and their fault models (length, width, strike, dip, and slip angles) have been obtained. We examined the causative faults of M7 or larger earthquakes in the Sea of Japan during the 20th century using seismic and tsunami data. The 1940 off Shakotan Peninsula earthquake (MJMA 7.5) appears to have been caused by the offshore active faults MS01, MS02, ST01, and ST02 as modelled by the JSPJ. The 1993 off the southwest coast of Hokkaido earthquake (MJMA 7.8) likely occurred on the offshore active faults OK03a, OK03b, and OK05, while the 1983 Central Sea of Japan earthquake (MJMA 7.7) probably related to MMS01, MMS04, and MGM0...

Seismological Research Letters

Tsunami potential from high dip-angle splay faults is an understudied topic, although such splay ... more Tsunami potential from high dip-angle splay faults is an understudied topic, although such splay faults can significantly amplify coastal tsunami heights as compared with ordinary thrust faults. Here, we identify a hotspot for tsunamis from splay faulting in the Molucca Sea arc–arc collision zone in eastern Indonesia, which accommodates one of the world’s most complicated tectonic settings. The November 2019 Mw 7.2 earthquake and tsunami are studied through teleseismic inversions assuming rupture velocities in the range 1.5–4.0 km/s followed by tsunami simulations. The normalized root mean square error index was applied and revealed that the best model has a rupture velocity of 2.0 km/s from the steeply dipping plane. The recent high dip-angle reverse 2019 Mw 7.2 and 2014 Mw 7.1 earthquakes combined with numerous similar seismic events may indicate that this region is prone to splay faulting. This study highlights the need for understanding tsunamis from splay faulting in other su...

Seismological Research Letters

Long-term seismicity in a certain region can be estimated from the number of felt earthquakes obt... more Long-term seismicity in a certain region can be estimated from the number of felt earthquakes obtained from seismic intensity measurements or historical documents. To obtain a homogeneous estimate, continuous records with a uniform detection threshold are required. Seismic intensity data in Tokyo, which are measured by professional observers and archived by the Japan Meteorological Agency (JMA) since 1885, indicate that the mean annual number of felt earthquakes with intensities ≥1 and ≥2 on the JMA scale is 45 and 14, respectively. The database of felt earthquakes can be extended back to more than 200 yr using historical daily records. During the Edo period from 1603 to 1867, numerous historical documents in Edo (former Tokyo) recorded information about damaging earthquakes, such as the 1703 Kanto earthquake and the 1855 Edo earthquake. Several feudal clans documented daily weather and felt earthquakes in Edo. Among them, daily records documented by the Tsugaru and Sakakibara clans...

Earth, Planets and Space

We analyze the regional tsunami hazard along the Sea of Japan coast associated with 60 active fau... more We analyze the regional tsunami hazard along the Sea of Japan coast associated with 60 active faults beneath the eastern margin of the Sea of Japan. We generate stochastic slip distribution using a Monte Carlo approach at each fault, and the total number of required earthquake samples is determined based on convergence analysis of maximum coastal tsunami heights. The earthquake recurrence interval on each fault is estimated from observed seismicity. The variance parameter representing aleatory uncertainty for probabilistic tsunami hazard analysis is determined from comparison with the four historical tsunamis, and a logic-tree is used for the choice of the values. Using nearshore tsunami heights at the 50 m isobath and an amplification factor by the Green’s law, hazard curves are constructed at 154 locations for coastal municipalities along the Sea of Japan coast. The highest maximum coastal tsunamis are expected to be approximately 3.7, 7.7, and 11.5 m for the return periods of 100...

Seismological Research Letters

To archive precious analog seismograms of major earthquakes that occurred in and around Japan, we... more To archive precious analog seismograms of major earthquakes that occurred in and around Japan, we have scanned selected analog seismograms from 113 stations deployed by the Japan Meteorological Agency (JMA) and are constructing a database of the scanned digital images accessible through the website of the Headquarters for Earthquake Research Promotion (HERP) of the Japanese government. This project started in 2006 and was conducted by the Association for the Development of Earthquake Prediction. The database of HERP now contains more than 185,000 digital images of seismograms recorded during the period from 1884 to 1995. It includes seismograms of earthquakes that occurred in and around Japan and of some major teleseismic earthquakes. The oldest analog seismogram was recorded on 2 January 1884 at a station in Tokyo. The seismograms are the intellectual property of mankind, and the database of these digital images will be an important material for studies of past destructive earthqua...

Ocean Engineering

The 2016 Mw 7.8 Kaikoura earthquake and consequent tsunami have been controversial because of unc... more The 2016 Mw 7.8 Kaikoura earthquake and consequent tsunami have been controversial because of uncertainty over whether and where the plate interface ruptured and the incapability of the proposed source models to reproduce the near-field runup of 7 m. Existing models identify a wide range of locations for the interface rupture, from on land to offshore, and fail to reproduce runup of 7 m near Kaikoura. To generate the large tsunami peak in Kaikoura tide gauge record and the observed runup height, offshore seafloor movement is necessary, but the offshore extension of the plate-interface rupture and its type, either seismic rupture or a landslide, is uncertain. Here, we propose a submarine landslide in addition to the earthquake source, with the landslide delayed 10-20 min after the earthquake rupture. The landslide volume is 4.5-5.2 km 3 , located within 173.7-174.3 o E (longitude) and 42.6-42.15 o S (latitude). Our proposed dual tsunami source successfully reproduces near-field tide gauge records as well as observed near-field runup height of 7 m. We showed that more accurate source models of earthquakes can be achieved by considering observed runup data through runup inversions in addition to waveform inversions.

Geoscience Letters

Various Tsunami Service Providers (TSPs) within the Mediterranean Basin supply tsunami warnings i... more Various Tsunami Service Providers (TSPs) within the Mediterranean Basin supply tsunami warnings including CAT-INGV (Italy), KOERI-RETMC (Turkey), and NOA/HL-NTWC (Greece). The 20 July 2017 Bodrum-Kos (Turkey-Greece) earthquake (Mw 6.6) and tsunami provided an opportunity to assess the response from these TSPs. Although the Bodrum-Kos tsunami was moderate (e.g., runup of 1.9 m) with little damage to properties, it was the first noticeable tsunami in the Mediterranean Basin since the 21 May 2003 western Mediterranean tsunami. Tsunami waveform analysis revealed that the trough-to-crest height was 34.1 cm at the near-field tide gauge station of Bodrum (Turkey). Tsunami period band was 2-30 min with peak periods at 7-13 min. We proposed a source fault model for this tsunami with the length and width of 25 and 15 km and uniform slip of 0.4 m. Tsunami simulations using both nodal planes produced almost same results in terms of agreement between tsunami observations and simulations. Different TSPs provided tsunami warnings at 10 min (CAT-INGV), 19 min (KOERI-RETMC), and 18 min (NOA/HL-NTWC) after the earthquake origin time. Apart from CAT-INGV, whose initial Mw estimation differed 0.2 units with respect to the final value, the response from the other two TSPs came relatively late compared to the desired warning time of ~ 10 min, given the difficulties for timely and accurate calculation of earthquake magnitude and tsunami impact assessment. It is argued that even if a warning time of ~ 10 min was achieved, it might not have been sufficient for addressing near-field tsunami hazards. Despite considerable progress and achievements made within the upstream components of NEAMTWS (North East Atlantic, Mediterranean and Connected seas Tsunami Warning System), the experience from this moderate tsunami may highlight the need for improving operational capabilities of TSPs, but more importantly for effectively integrating civil protection authorities into NEAMTWS and strengthening tsunami education programs.

Pure and Applied Geophysics

The September 2017 Chiapas (Mexico) normalfaulting intraplate earthquake (M w 8.1) occurred withi... more The September 2017 Chiapas (Mexico) normalfaulting intraplate earthquake (M w 8.1) occurred within the Tehuantepec seismic gap offshore Mexico. We constrained the finite-fault slip model of this great earthquake using teleseismic and tsunami observations. First, teleseismic body-wave inversions were conducted for both steep (NP-1) and low-angle (NP-2) nodal planes for rupture velocities (V r) of 1.5-4.0 km/s. Teleseismic inversion guided us to NP-1 as the actual fault plane, but was not conclusive about the best V r. Tsunami simulations also confirmed that NP-1 is favored over NP-2 and guided the V r = 2.5 km/s as the best source model. Our model has a maximum and average slips of 13.1 and 3.7 m, respectively, over a 130 km 9 80 km fault plane. Coulomb stress transfer analysis revealed that the probability for the occurrence of a future large thrust interplate earthquake at offshore of the Tehuantepec seismic gap had been increased following the 2017 Chiapas normal-faulting intraplate earthquake.

Geophysical Journal International

Kanto earthquake (JMA magnitude; M JMA 7.9) caused approximately 105 000 casualties, mostly due t... more Kanto earthquake (JMA magnitude; M JMA 7.9) caused approximately 105 000 casualties, mostly due to a massive fire that was caused by the main shock (Moroi & Takemura 2004), and the 1703 Kanto earthquake with the magnitude (M) of 8.2 also caused much destruction and approximately 7000 casualties. A recent palaeoseismological study indicated that the antepenultimate Kanto earthquake occurred in 1293 (Shimazaki et al. 2011). The Earthquake Research Committee (of the government of Japan) (2014) estimated the recurrence interval of the M8-class Kanto earthquakes to be 200-400 yr, and calculated the probability of another such earthquake in the next 30 yr to be approximately 0-5 per cent. The probability of a large (M ∼ 7) earthquake other than the M8class Kanto earthquake is much higher (e.g. Grunewald & Stein 2006; Stein et al. 2006; Nanjo et al. 2013; Somerville 2014). The Earthquake Research Committee (2014) calculated the probability of occurrence of such an earthquake during the next 30 yr to be approximately 70 per cent; this is based on the history of M ∼ 7 earthquakes since 1703. The Central Disaster Prevention Council (government of Japan, under the Cabinet office) estimates 112

Geophysical Research Letters, 2016

Geophysical Research Letters, 2015

Agu Fall Meeting Abstracts, Dec 1, 2010

ABSTRACT Along the Nankai Trough, recurrence of at least nine great (M~8) interplate earthquakes ... more ABSTRACT Along the Nankai Trough, recurrence of at least nine great (M~8) interplate earthquakes have been inferred from historical documents in Japan. Historical documents also record timing of ground shaking or arrival time of seismic waves. Iida (1985) and Usami (2003) compiled arrival times of seismic waves due to the 1707 Hoei earthquake and concluded that the Tokai, Tonankai and Nankai segments simultaneously ruptured or separately ruptured within 2 hours. We statistically estimate the origin time of the Hoei earthquake based on only reliable historical documents including additional ones found after their studies, and suggest a possibility of delayed rupture on the earthquake segments. In addition, tsunami heights are computed on the basis of the result, and compared with tsunami trace heights of the 1707 Hoei earthquake. We selected primary and reliable historical documents from Kyushu to Tohoku region written within 30 years after the Hoei earthquake. The arrival times of the Hoei earthquake at each location were then retrieved from these documents. The arrival times were spatially averaged to estimate the origin time of the Hoei earthquake. The time count interval in 1707 is different from the current system: day and night were divided into 6 ``koku'', hence each ``koku'' corresponds to about 2 hours. However, timing is often indicated with early, middle or late ``koku'', making the temporal resolution as small as 40 minutes. Because the timing was measured from sunrise to sunset, the local time varies with longitude, hence a correction is applied to convert the local time to the standard time. Our likelihood estimate shows that the origin time of the Hoei earthquake was 13:47 with the standard error of 1.02 hours. In addition, we statistically tested whether the Hoei earthquake had simultaneously ruptured all the segments or separately ruptured, and evaluated the best segmentation using the Akaike's Information Criterion (AIC; Akaike, 1974). The result indicates statistically significant differences in the origin times of each segment; the rupture started on the Tonankai and Nankai segments, and the rupture on the Tokai segment was delayed by about 19 min. Tsunami computation including delayed rupture of Tokai segment was carried out to verify the result from historical documents. For the tsunami source, we adopt the 1707 Hoei earthquake model by Annaka et al. (2003), which consists of four fault segments (N1 to N4) to describe Tokai, Tonankai, and Nankai earthquakes. The rupture delay time of N1 (Tokai) segment relative to N2 (Tonankai) through N4 (Nankai) segments is assumed to be 0 to 79 min (5 min interval for 4 to 39 min and 10 min interval after 49 min). To compare the computed tsunami heights and the tsunami trace heights along the Enshu and Suruga Bay coasts (Hatori, 1977), we calculated geometric standard deviation kappa (Aida, 1978). The kappa value took the minimum value for the 14 min delay case, which is consistent with the statistical analysis from historical documents. These results suggest a possibility that N1 segment rupture was delayed relative to the N2 to N4 segment rupture.

Marine Geology, 2015

ABSTRACT Nine event deposits in a small alluvial valley along the Sanriku coast, Japan, were corr... more ABSTRACT Nine event deposits in a small alluvial valley along the Sanriku coast, Japan, were correlated with historical tsunamis and storms that have been recorded in this region since the 15th century. We identified nine sandy layers in 15 geo-slices collected at distances ranging from 140 to 260 m from the coast in a lowland back marsh protected from the sea by a high sandy ridge. Based on their sedimentary characteristics, grain-size distribution, and marine microfossil assemblages, namely Coccoliths, these event layers, which were well preserved in their order of deposition, were probably either tsunami or storm deposits. Diatom analysis revealed four stages of paleo-environmental change from tidal marsh to wetland. The uppermost event layer covering the ground surface is probably the deposit from the 2011 Tohoku-Oki tsunami. Ages for other event deposits were radiometrically dated using 14C, 137Cs and 210Pb with Bayesian estimation. The second event layers can be correlated with the 1960 Chilean or 1968 Aomori-Oki earthquake tsunamis. The candidates for the third event layer are the 1947 Catherine and 1948 Ione typhoons as well as the candidates for the second event layer. The fourth and fifth event layers are likely associated with the 1933 and 1896 Sanriku-Oki earthquake tsunamis. Four lower event layers can be correlated with historical tsunamis and storms that occurred during the 15th to 18th centuries, such as the 1611 Sanriku-Oki earthquake tsunami.

High-resolution shallow-water profiling survey revealed recent activity of submarine fault which ... more High-resolution shallow-water profiling survey revealed recent activity of submarine fault which was newly found to fill a gap between two subaerial faults, the Asamigawa and the Funai faults running on the southern coast of Beppu Bay. Historical records suggest that this activity on the north-dipping normal fault corresponds to the M7.3 Keicho-Bungo earthquake of September 1, 1596. Our previous profiling survey and correlation study of core samples show that the event caused vertical offset on south-dipping normal faults in the central and northern parts of the bay. Thus it is most likely that the Keicho-Bungo event simultaneously ruptured the faults on the both sides of the Beppu Graben. Furthermore core samples obtained from the both sides of the newly found submarine fault suggest repeated occurrence of this simultaneous rupturing. A comparison of vertical offsets of two prominent ash layers on Beppu Bay faults whose total length exceeds 230km, shows almost linear relationship b...