Zhengkang Shen & Huaizhong Yu

1. Slow slip events observed days before and close to the 2001 Mw7.8 Kokoxili earthquake in northern Tibet Zhengkang Shen & Huaizhong Yu

2. Outline • Tremor and slow slip events observed around circum-Pacific plate boundaries • 2001 Mw7.8 Kokoxili earthquake • Tremor events observed prior to the Kokoxili earthquake • Source location determination • Results

4. From Roger and Dragert, Science, 2003

5. Dragert et al., Science, 2001

8. Kosklodotov et al., GRL, 2003

12. Source location determination • High-pass filter the data; • Take RMS of the data; • Smooth the data with 10-sec window; • Correlate the data envelopes to find event onset differences; • Use the onset differences to determine the source

14. Kao et al., JGR, 2006

15. Source location determination • High-pass filter data • evaluate “brightness” through space ηand time τover all station records Un • Search for maximum of “brightness”

19. Dec 2000 – Dec 2003, 10 clusters of tremor events, frequency 1-10 Hz, located depth 20-40 km along SAF south of Parkfield Nadeau, Science, 2005

20. 2001/11/14 Mw7.8 Kokoxili earthquake

22. Left surface slip measured by field geology. Color code: Red, field measurements by J. Chen, Institute of Geology, CSB; Black, measurements by X. Xu, Institute of Geology, CSB; Green, measurements by G. Dang, Qinghai Seismological Bureau.

23. Yang et al., Seismol. Frontier, 2003

25. HTG花土沟 GOM格尔木 WUS乌什 WMQ乌鲁木齐 WEQ温泉 KEL库尔勒 KSH喀什 HTA和田 GTA高台 FYN富蕴 AXX安西 2001/11/10 2001/11/11 2001/11/12 2001/11/13

27. Data of 2001/11/11 after 0.1-0.2 Hz band-pass filtering

29. Envelops of amplitude-square with 10 sec smoothing

30. envelop data

32. Event Source Relocation • Assuming tremors are primarily direct S waves, select a reference station, calculate correlations of station pairs with a time window of 100 sec and a moving offset; keep the ones with correlations > 0.8 in general and > 0.7 for stations far away from each other. • Using the differential arrival times as input, invert for event hypocenters using the HYPOSAT software with a 1-D S-wave velocity model of northern Tibet (Zhang et al., 2000). • Compute travel time postfit residuals, remove the ones > 10 sec, and repeat the last step. • Retain the events with ≥8 (out of 11) stations remained in the inversion process.

34. locked depth transition creeping mantle longitude

35. HTG GOM AXX KEL WMQ GTA HTA WUS FYN WNQ KSH

36. HTG GOM KEL AXX HTA WMQ GTA WUS WNQ FYN KSH

37. HTG KEL GOM HTA AXX WMQ WUS GTA WNQ FYN KSH

38. HTG GOM KEL HTA AXX WMQ WSH GTA WNQ KSH FYN

39. HTG GOM AXX KEL GTA WMQ HTA WUS FYN WNQ KSH

40. HTG GOM KEL AXX HTA WMQ GTA WUS WNQ KSH FYN

41. HTG GOM KEL AXX HTA WMQ GTA WUS WNQ KSH FYN

44. Conclusion?Slow slip hypocenters are found to scatter in a region of 88.0-91.0oE, 34.5-37.5oN, seem to indicate that their locations are close to the epicenter of the Kokoxili earthquake. Moreover, the depths of these events fall within the range of 10 – 40 km, suggesting that slow slip events probably occurred in the brittle-ductile transition zone on or near the East Kunlun fault. The failure at the transition zone might have increased the Coulomb stress at the hypocenter and finally triggered the Mw 7.8 earthquake.

45. 格尔木GOM 喀什KSH 和田HTA 乌鲁木齐WMQ 高台GTA 花土沟HTG 乌什WUS 安西AXX 银川YCH

48. XAN

49. ARU

50. AAK