Tohoku-oki event: Tectonic setting

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1 Tohoku-oki event: Tectonic setting This earthquake was the result of thrust faulting along or near the convergent plate boundary where the Pacific Plate subducts beneath Japan. This map also shows the rate and direction of motion of the Pacific Plate with respect to the Eurasian Plate near the Japan Trench. The rate of convergence at this plate boundary is about 100 mm/yr (9 cm/year). This is a fairly high convergence rate and this subduction zone is very seismically active. Eurasian Plate Japan Trench Pacific Plate

2 Dynamic rupture and stress transfer Coulomb stress imparted by the M=9.0 Off-Tohoku rupture and its M=7.9 aftershock to Japan Trench, Sagami Trough and Kanto Fragment Most of Sagami trough is brought farther from failure, but basal surface of Kanto fragment possible source of 1855 M~7.3 Ansei-Edo quake is stressed Increased hazard bar Remote M=6.6 aftershock (11 Mar 1946 UTC) Decreased hazard Suruga trough megathrust ( Tokai gap ) Kanto fragment Sendai Tokyo M=9.0 Unruptured megathrust Sagami trough megathrust M=7.9 Southern edge of rupture Yuji Yagi s source version 2 assumed fault friction = 0.4 Fault rake vectors are blue Animation of and reference for the Kanto fragment: Toda, Stein, Kirby & Bozkurt, Nature Geoscience, Ross Stein & Volkan Sevilgen (USGS) and Shinji Toda (DPRI, Kyoto Univ.) 18 Mar :15 PM PDT

3 Aftershocks Aftershock sequences follow predictable patterns as a group, although the individual earthquakes are themselves not predictable. The graph below shows how the number of aftershocks and the magnitude of aftershocks decay with increasing time since the main shock. The number of aftershocks also decreases with distance from the main shock. Aftershocks usually occur geographically near the main shock. The stress on the main shock's fault changes drastically during the main shock and that fault produces most of the aftershocks. Sometimes the change in stress caused by the main shock is great enough to trigger aftershocks on other, nearby faults. Image and text courtesy of the US Geological Survey

4 Historical seismicity and aftershocks Image courtesy of Charles Ammon

5 Co-seismic slip M. Simons, F. Ortega, J. Jiang, A. Sladen, and S. Minson at Caltech as part of the ARIA project. All orginal GEONET RINEX data provided to Caltech by the Geospatial Information Authority (GSI) of Japan.

6 GPS waveforms Analysis by Dr. Yokota using the GEONET data of Geographical Survey Institute

7 GPS and GM signals The figure shows the comparison between this GPS signal - twice differentiated - and the accelerometric signal, in the [0.005Hz Hz] range.

8 Long period GM Earthquake Research Institute, University of Tokyo: Dr. Furumura and Project Researcher Maeda

9 Ground motion animation: time scales... Courtesy of Takashi Furumura

10 PGD A strong ground acceleration of over 2933 cm/s/s was observed in K-NET Tsukidate observation station (Miyagi pref.) near the hypocenter, and a strong ground acceleration propagated in broad area from Ibaraki to southern Iwate. The distribution of strong ground acceleration is extending to three areas: between Iwate and Miyagi prefecture, Fukushima pref., between Tochigi and Ibaraki pref. Therefore, it is assumed that a huge fault slip have occurred on the east of these areas. The ground acceleration is decaying drastically just after the border of Itoigawa- Shizuoka Tectonic Line, and it suggests that the wave attenuated at around this area.

11 PGA A strong ground acceleration of over 2933 cm/s/s was observed in K-NET Tsukidate observation station (Miyagi pref.) near the hypocenter, and a strong ground acceleration propagated in broad area from Ibaraki to southern Iwate. The distribution of strong ground acceleration is extending to three areas: between Iwate and Miyagi prefecture, Fukushima pref., between Tochigi and Ibaraki pref. Therefore, it is assumed that a huge fault slip have occurred on the east of these areas. The ground acceleration is decaying drastically just after the border of Itoigawa- Shizuoka Tectonic Line, and it suggests that the wave attenuated at around this area.

12 Waveforms Maximum acceleration and maximum displacement of ground motion in Ishinomaki and Rikuzentakata where ground motion was strong. The arrival of 2 strong seismic wave groups is seen after about 50 seconds. They suggest that a strong seismic wave was radiated from the 2 major asperities of the Miyagi coast and Iwate coast. Two long-period pulses (40-50 second) was found in ground displacement and its amplitude is more than 50 to 100cm. The longperiod of ground motion that lasted for 100 and several tens of seconds, indicates the long time rupture process of the fault in this massive earthquake.

13 Rupture from ground motion National$Research$Institute$for$Earth$Science$ and$disaster$prevention Characteristics$of$near6source$ground$motions IWTH08''2011/03/11'14:46:48''Seismic'Intensity':' The$first$phase$is$predominant?114 IWT019''2011/03/11 14:46:41''Seismic Intensity :' ?316 IWT016''2011/03/11 14:46:39''Seismic Intensity :' ?229 IWT009''2011/03/11 14:46:32''Seismic Intensity :' Both$phases$are$distinct$?5.70 MYG003''2011/03/11'14:46:32''Seismic'Intensity':' ?570 MYG015''2011/03/11'14:46:34''Seismic'Intensity':' ?411 The$second$phase$is$predominant MYG017''2011/03/11'14:46:37''Seismic'Intensity':' ?317 FKS004''2011/03/11 14:46:38''Seismic Intensity :' ?568 FKS013''2011/03/11 14:46:45''Seismic Intensity :' Near6source$acceleration$waveforms$ display$two$remarkable$phases$of$ ground$motion$which$suggest$the$ existence$of$a$least$two$areas$of$large$slip.?296 The$first$phase$is$not$visible IBR /03/11 14:47:01''Seismic'Intensity':' In$the$northern$part$of$the$source$area$the$ first$phase$is$predominant$while$in$the$south this$phase$is$not$visible.?304 These$two$phases$of$ground$motion$suggest$ large$slip$areas$in$the$north$and$south$regions$ respectively. IBR /03/11 14:47:06''Seismic'Intensity':' The$remarkable$differences$in$grow$characteristics$of$ intensity$values$for$the$miyagi$and$ibaraki$regions$indicate$ they$originate$from$different$slip$areas. A$small$ground$motion$phase$is$observed$between$the$two$ large$distinct$phase$at$some$stations$(fks013,$fks004,$ MYG017,$MYG015).?329 CHB /03/11 14:47:07''Seismic'Intensity':' ?212 Other$secondary$phases$are$also$observed$suggesting$a$very$ complex$source$process Source: Knet-NIED 200 Time [s] 6 Strong motion distribution at Eastern Japan (5HzPGA) This PGA is commonly used as an index for the alarm to stop the train operation.

14 9N99', Tsukidate ison with 2 typical near-field motions during the 1995 Kobe, arthquake Courtesy JMA of Kazuhiko Kobe Kawashima & JR Takatori ('1*#0Y, V(02*,>&.$.<)C&., G004-Tsukidate NS G004-Tsukidate EW pe II (soil I) 2 Sp. II (G0) Response Acceleration (m/s 2 ) W L2 Type I (soil I) ) Sp.I (G0) (MYG004) Unit: gal , V+ Takatori, 1995 Kobe EQ JMA-Kobe, 1995 Kobe EQ Period (s) Comparison with Type II Design Spectra, JRA Design Specifications of Bridges Response Acceleration (m/s 2 ) Type I Type II Type III >CC/*/')61%(+?2)*E+ >CC/*/')61%(+?2)*E+ >CC/*/')61%(+?2)*E+ VVV "VVV FVVV V 8FVVV 8"VVV 8VVV VVV "VVV FVVV V 8FVVV 8"VVV 8VVV VVV "VVV FVVV V 8FVVV 8"VVV 8VVV?)E?\E?CE Period (s) V HV FVV FHV "VV "HV VV _1R/+?5E+

15 Ground motion - Worldwide

16 USGS - Finite fault model Cross-section of slip distribution. The strike direction of the fault plane is indicated by the black arrow and the hypocenter location is denoted by the red star. The slip amplitude are showed in color and motion direction of the hanging wall relative to the footwall is indicated by black arrows. Contours show the rupture initiation time in seconds.

17 Ground motion - USA & backprojection Courtesy of Dun Wang and Jim Mori

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