THE FOCAL MECHANISM AND TECTONIC SIGNIFICANCE OF THE MS5.6 EARTHQUAKE ON JULY 24, 2009 IN NIMA, TIBET

doi:10.3969/j.issn.0253-4967.2022.04.011

Abstract

Abstract:

The paper collects the seismic waveforms of the M_S5.6 earthquake that occurred in southern Nima, central Tibe on July 24, 2009 recorded by Tibet seismic network and the mobile seismic networks of the orresponding period, i.e. Western Tibet/Y2 and TITAN. The seismic waveform data were preprocessed by rglitches, rmean, rtrend, taper, transfer and filtering. Then we hand-picked the arrival times of the P-and S-waves(0.05~2Hz for P wave, and 0.05~0.5Hz for S wave). The Hypo2000 method was applied to accurately relocate the earthquake.

Because the earthquake occurred in the hinterland of Tibetan plateau, there are few local seismic stations available. Since the seismic stations and seismic phase information used in processing by different institutions are different, the epicenter location and focal mechanism determined by various institutions are different. Compared with the result(31.30°N, 86.10°E)relocated by Tibet seismic network, our result(31.08°N, 86.05°E)is more reliable due to the uniform distribution of stations used in our study, which is roughtly identical to the GCMT result(31.05°N, 86.10°E)inverted by the moment tensor method.

Based on the relocated result, we apply the Cut-and-Paste(CAP)inversion method to invert the focal mechanism and focal depth. The waveform is decomposed into Pn1 and surface wave to perform cross-correlation fitting of theoretical waveform and actual waveform, respectively. To suppress the noise and influence of the source region medium, the bandpass filter is selected as 0.05~0.15Hz for body wave and 0.05~0.1Hz for surface wave. We set the earthquake source time function as 5s and search for the best focal depth at the depth of 1~30km, and the search step is 1km concerning the magnitude of the earthquake. The result shows that the earthquake has a best-fitting focal depth of 19.3km from the mean sea level and is of strike-slip faulting(the nodal plane Ⅰ: 220°/82°/-17° and nodal plane Ⅱ: 314°/73°/-171°).

The shear stress and normal stress of the two nodal planes of the earthquake are calculated according to the stress field characteristics of the earthquake area. The generation of the earthquake is consistent with the stress field characteristics of NS compression and EW extension in the region. Referring to the near-EW strike-slip fault zone constrained by the EW-trending Wozang Fault and the NWW-trending Zhala Fault in the 1︰250000 regional geological survey map near the epicenter area, it is inferred that the earthquake is of EW-trending dextral strike-slip faulting.

Most of the earthquakes that occurred along the 31°N belt near this earthquake area are EW-trending strike-slip ones, even in the interior of the Tangra-Yumco Rift. Considering the physical properties beneath Tibetan plateau, the low-velocity and high-conductivity layers are widely distributed in the depth range of 20km to 30km in the thick crust. According to surface geology and deep structures revealed by regional geophysics(receiver function, magnetotellurics, and tomography)of the region, the earthquake occurred on the top of the brittle-ductile transition zone with a low seismic velocity between the middle and upper crust beneath the south boundary faults of the Seng-ge Kambab-Lhaguo Tso-Yongzhu-Jiali ophiolite mélange zone(SYMZ), 30km away from the Tangra-Yumco Rift to the west. The occurrence of the earthquake indicates that SYMZ, which formed in the Late Jurassic, was reactivated in an EW-trending strike-slip manner during the quick uplift of the plateau. This cognition is of great significance to understand the geodynamic mechanisms of the EW-trending extension within the Tibetan plateau.

Key words: Tibetan plateau, NS-trending rift, seismic location, focal mechanism, ophiolite mélange zone

摘要：

文中收集了由西藏地震台网和同期流动观测台网记录到的2009年7月24日西藏尼玛 M_S5.6 地震的波形数据, 应用Hypo2000方法对该地震开展了绝对到时精定位。因定位所使用的台站分布均匀, 与仅使用固定台站数据的定位结果(31.30°N, 86.10°E)相比, 我们得到的尼玛地震的精定位结果(31.08°N, 86.05°E)更为可靠, 该结果与通过矩张量定位方法(GCMT)获得的结果(31.05°N, 86.10°E)较为一致。在获得震中定位结果的基础上, 使用CAP方法得到该地震的震源深度(海拔高度为0m起算)为19.3km, 该地震具有走滑断裂性质。结合地震所处的应力场特征对本次地震的2个节面(节面Ⅰ, 走向为220°, 倾角为82°, 滑动角为-17°; 节面Ⅱ, 走向为314°, 倾角为73°, 滑动角为-171°)的剪应力和正应力进行计算, 发现该地震的震源机制与区域内SN向挤压、 EW向扩张的应力场特征相吻合。参考震中所在位置的1︰25万区域地质调查结果, 结合该区EW向窝藏断裂和NWW向扎拉断裂所限定的近EW向走滑断裂带的特征推断, 该地震具有EW向右旋走滑性质。根据区域地表地质及区域地球物理揭示的深部结构特征推测, 该地震发生在当惹雍错裂谷西侧约30km处的狮泉河-拉果错-永珠-嘉黎蛇绿岩混杂岩带下方中上地壳间的低速软弱层(即壳内脆韧性转换带)顶部, 暗示了晚中生代形成的狮泉河-拉果错-永珠-嘉黎蛇绿岩混杂岩带在高原快速隆升过程中以EW向走滑方式再次活化。这一解释对理解青藏高原EW向伸展作用有着重要意义。

关键词: 青藏高原, SN向裂谷, 地震定位, 震源机制, 蛇绿岩混杂带

CLC Number:

P315.2

LI Zong-xu, HE Ri-zheng, JI Zhan-bo, LI Yu-lan, NIU Xiao. THE FOCAL MECHANISM AND TECTONIC SIGNIFICANCE OF THE M_S5.6 EARTHQUAKE ON JULY 24, 2009 IN NIMA, TIBET[J]. SEISMOLOGY AND GEOLOGY, 2022, 44(4): 992-1010.

李宗旭, 贺日政, 冀战波, 李娱兰, 牛潇. 2009年7月24日西藏尼玛M_S5.6地震的震源机制及其构造意义[J]. 地震地质, 2022, 44(4): 992-1010.

Figures/Tables 9

Table 1 Parameters of the MS5.6 earthquake on July 24, 2009 in Nima, Tibet calculated by different institutions.(after SHEN Xiao-qi et al., 2015)

发震时间(UTC时间)	东经/(°)	北纬/(°)	深度/km	震级	数据来源
2009-07-24, T03:11:58	86.10	31.30	33.0	M_S5.6	中国地震台网中心
2009-07-24, T03:12:00	86.00	31.28	7.0	M_S5.7	西藏地震台网
2009-07-24, T03:12:00	86.00	31.28	7.0	M_W5.8	沈小七等, 2015
2009-07-24, T03:11:57	85.96	31.17	13.3	M_W5.8	IRIS
2009-07-24, T03:12:02	86.10	31.05	28.1	M_W5.8	GCMT
2009-07-24, T03:11:56	85.90	31.16	13.4	M_W5.8	USGS
2009-07-24, T03:11:56	85.97	31.20	10.0	M_W5.7	EMSC
2009-07-24, T03:11:58	85.90	31.10	17.0	M5.6	GFZ
2009-07-24, T03:11:55	86.05	31.08	19.3	M_W5.57	本文

Fig. 1 Map of station distribution, earthquake location result, and focal mechanism in the study area.

Fig. 2 Waveform of the MS5.6 earthquake on July 24, 2009 in Nima, Tibet and stations’ distribution used in the earthquake location.

Fig. 3 Crustal velocity model used in this study(NIU Xiao et al., 2021).

Fig. 4 CAP inversion results, and contrast of seismic locations, focal mechanism solutions and SKS wave splitting in the Tangra-Yumco rift area.

Fig. 5 Central focal mechanism(a)and its 3-D radiation pattern(b).

Table 2 Comparison of focal mechanism solution obtained by this study with that from GCMT, USGS, and the central focal mechanism solution

结果对比	节面Ⅰ			节面Ⅱ			P轴		T轴		B轴
	走向	倾向	滑动角	走向	倾向	滑动角	方位	俯角	方位	俯角	方位	俯角
本文	222°	82°	-17°	314°	73°	-171°	177°	17°	269°	6°	18°	71°
GCMT	226°	83°	-16°	318°	74°	-172°	181°	14°	273°	6°	22°	72°
USGS	231°	84°	-14°	323°	76°	-174°	186°	14°	277°	6°	28°	74°
中心解	226°	83°	-16°	318°	74°	-172°	181°	16°	273°	6°	22°	73°

Fig. 6 Relative shear stress(a)and relative normal stress(b)of regional stress field on various faults.

Fig. 7 The focal mechanism solution and its dislocation type plan of the Lhasa terrane.

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