Workshop on the Seismogenic Zone at Trenches Snowbird, Utah March 16-21, 2003

1. Workshop on the Seismogenic Zone at Trenches Snowbird, Utah March 16-21, 2003 Secular, Transient and Periodic Crustal Movements in Japanese Subduction Zones, and Dynamics Underlying Them Kosuke Heki Division of Earth Rotation, National Astronomical Observatory, Japan

2. Contents Keyword: Crustal deformation, Japanese Islands, Earthquakes Secular crustal movement Tokai Earthquake Vertical movements, long- and short-term buying real estates in Japan Transient crustal movement Afterslip and silent earthquakes Where do they occur? a natural creepmeter Periodic crustal movement Seasonal change of surface loads Seasonality of seismicity radio astrometry What should we do next?

3. Basic mode of crustal deformation in an island arc (interseismic shortening/coseismic extension)

4. I was here In 1994 December, there was an earthquake

5. My case … Surveyed in 1994 Summer North House Earthquake in 1994 December (Mw ~8 including afterslip) Front yard !

6. Global Positioning System (GPS) Dense Array

7. Events that occurred 1994-now 1994Hokkaido-Toho-Oki coseismic jump + northward migration of the postseismic transient 1994Sanriku-Haruka-Oki year-long afterslip comparable to the main rupture 1995Hyogoken-Nanbu coseismic + postseismic 1998-Mt.Iwate volcano-seismic crisis 2000Tottori 1997 Yamaguchi 1996Boso Silent Eq. 1997Kagoshima Eqs. 2001Geiyo 1995,6,7,8Seismic Swarm + Dike Injection time-dependent inversion 2000-Tokai silent event Still in progress 1996Hyuganada Eq. sequence two earthquakes, afterslip + creep event 2000Miyake-Kozu events Eruption, earthquakes, dike injection Hirose et al., GRL, 26, 3237, 1999. Normal/slow earthquake Silent earthquake Magma activity

8. Events to occur (interplate) Miyagi-Oki 201X An asperity at the Japan Trench Last rupture 1978 (80% probability in the next 20 yrs) Off-Aomori 20XX Last rupture 1968 Boso silent 2008 Similar events in 1983, 1990, 1996, 2002 Tokai 197X Seismic gap at Suruga Trough Nankai, Tonankai 202X Interplate events at the Nankai Trough Last rupture 1946,1944 Normal/slow earthquake Silent earthquake

9. 1944 1946 EU NA AM Tokai Izu microplate PH

10. slower convergence rate longer recurrenceinterval E D C B A 20XX 20XX 1946 1944 1854 1854 1707 1707 1605 1605 1498 1361 1099 1096 887 684 A B C D E(Tokai region) Year (AD) ！

11. Secular Transient periodic Vertical velocities available now Secular movement

12. Taneichi Shizugawa Reference

13. Combine horizontal and vertical velocities to estimate slip deficits (backslips) Tide Gauge

14. Slip deficit(backslip) Savage [1983] = +

15. asperity zone genic Seismo Epicenter Yamanaka and Kikuchi, 2001 Everywhere is Coupled Along-strike Difference in Coupling Strength (stronger at asperities)

16. Surface projection of down-dip edge max. uplift Uplift Subsidence Neutral Subsidence Uplift

17. Interseismic (backslip) Coseismic (forward slip) Slip deficit(backslip) model Averaged to zero ??

18. Tanesashi Coast Kitayamazaki Sanriku Coast North uplift South subsidence

19. secular Transient periodic Transient movement

20. Slow Slips Silent Earthquake X Afterslip

21. Interseismic (decades to centuries) locked Coseismic (seconds to minutes) Stable slide Postseismic (days to years) afterslip Fast rupture Stable slide asperity

22. From Heki et al., 1997 ? Slip of the region surrounding the asperity Slip of asperity

23. Kuji GPS Heki and Tamura, 1997.

24. Strainmeter Heki and Tamura, 1997.

25. Large asperity (large interplate earthquake) Small asperity (characteristic small-earthquake sequences) = Natural Creepmeter Matsuzawa et al. (2002)

26. 20 Cumulative slip (cm) 10 0 1995.0 1993.0 1997.0 1999.0 year After Igarashi et al. (2001) 1994 Dec.Sanriku Eq. Characteristic small-earthquake sequence (= a small isolated asperity) Afterslip of 1994 Sanriku Eq. Accelerated slip at depth

27. Hamamatsu w.r.t. Ohgata Secular + Transient + Periodic Transient Transient + Periodic A Silent Earthquake

28. Tokai region Dikeintrusion (2000/Jun-Sep) Slow slipMw»6.8 (2000/Sep- now)

29. 25% 75% 100% 2000Fall - Now Tokai Silent Eq. Mw=~6.8 at 2003.0 1997 Bungo-Channel Silent Eq. Mw~6.6 Nankai Trough 35 km 30 km 25 km 15 km Hyuganada 1996 Dec. 3 Mw~6.7 Hyuganada 1996 Oct.19 Mw~6.7 5 km Relationship with rupture updip

30. secular transient periodic Periodic movement

31. Tateyama (late April)

32. contraction extension extension Load Subsidence Earth Earth’s response to a surface load

33. Yamada Onagawa Extension in winter Contraction in winter Subsidence in winter Shizukuishi Gojoume Tobishima Naruko Snow belt

34. Estimated snow depth (density 0.4 g/cm3)

35. snow atmosphere Sea surface height Soil moisture Seasonal Change of Surface Loads All cause contraction in winter whole (0-10kPa) 1-2 kPa (100kPa) 1-2 kPa (5kPa) 1-2 kPa (0-1GPa)

36. AMeDAS snow depth meter The Snow AMeDAS average max. snow depth 1997-2001 Snow density: 0.4 g/cm3 Underestimated

37. The Atmosphere Winter value w.r.t. summer value 1kPa = 10 cm water

38. The Soil Moisture Winter value w.r.t. summer value Calculated using (1) potential evapotranspiration (2) precipitation

39. The Ocean negative positive after Sato et al., PEPI, 123, 45-63, 2001

40. fixed atmosphere x 2 snow ocean soil moisture

41. Tobishima-Naruko Kamitsushima-Susaki

42. secular transient Why do we care ? periodic It masks transient/secular signals Hamamatsu w.r.t. Ohgata It may influence seismicity

43. ….. ….. When crustal strain gradually accumulates and reaches a critical condition, relevant secondary factors, such as atmospheric pressure change, would surely inducean earthquake. Hence, at one hand measuring crustal strain, and investigating major secondary factors at other hand, it seems to me possible to forecast earthquakes. How far would this expectation realistic? ….. from “Modern Science”, 1916 March (in Japanese) by T. Terada (Physicist, founder of the Earthquake Research Inst., Univ. Tokyo.)

44. High seismicity in snow-covered area in spring/summer 5 6 4 2 8 3 7 5 10 6 6 High offshore seismicity in autumn/winter 7 M > 7.0 [Okada, 1982] 5 8 6 8 1 3 5 9 12 12 9 12 12 12 2 M > 7.9 [Ohtake & Nakahara, 1999] 10 8 2 12 11 8 Seasonal variation of earthquake occurrence

45. Critical stress Stress drop (~1MPa) Stress build-up (~30 Pa/day or ~10 kPa/yr) Recurrence interval (~100 yr) Probability of earthquake occurrenceµStress rate Stress time

46. Snow Load Clamps Faults NE Japan ~ Reverse Central Japan ~ Strike-slip

47. shear stress (positive in the direction of fault slip) s friction coeff. (0.2 ~0.5) t t normal stress (positive for unclamping) t Triggering of Earthquakes 10 kPa/yr 0.3 t+ m sn CFF = Coulomb Failure Function ±3 kPa

48. Pm : time-varying seismicity P0 : background seismicity Expected Seasonal Variation of Seismicity Pm»3/5 P0

49. Snow depth and Inland earthquake epicenters snowy not snowy More eq. in spring/summer in snowy area Little seasonality in snow-free area

50. Does not pass the Schuster’s statistical test Due to small number of earthquakes However Global Mode of Seasonal Crustal Deformation Correlation ambiguous for smaller earthquakes Opposite to ocean tidal case: correlation more significant for smaller earthquakes (Tanaka et al., JGR, 2002) A speculation:larger/smaller earthquakes may respond only to longer-/shorter- term disturbances