EXPERIMENTAL STUDY ON THE P-WAVE VELOCITY OF THE CRUST IN THE LAOHUSHAN FAULT ZONE IN GANSU PROVINCE

doi:10.3969/j.issn.0253-4967.2022.05.012

Abstract

Abstract:

The previous research shows that dense small earthquakes may occur in the Laohushan fault zone in the coming decades. Micro-earthquakes have obvious near-field characteristics due to low earthquake magnitude, high main frequency and fast energy attenuation of signal. Near-field crustal wave velocity structure, especially the shallow velocity structure, has a great impact on the precise positioning of high-frequency micro-earthquakes. To this end, the P-wave velocity (V_P) of representative rocks in this area was systematically measured at room temperature and ambient pressure of 50~500MPa. The experiment result shows that:
(1)V_P shows a logarithmic increase with pressure in the low pressure range and a linear trend increase with the pressure when exceeding the critical pressure P_c. P_c is in the range of 200~450MPa, and is 262.5MPa on average. The average P_c of crystalline rock is 250MPa and the average P_c of sandstones is 271.4MPa. The P_c of sandstones is significantly higher than that of crystalline rocks. This is attributed to the low pressure range, where the cracks and pores are closing with increasing pressure, and the cracks and pores are almost completely closed in the high pressure range.
(2)Under ambient temperature, the velocity pressure derivative in nonlinear domain of velocity-pressure curve is much larger than that in linear domain.
(3)The intrinsic velocity of intrusive rocks is greater than that of sandstone and metamorphic sandstone.
Combined with previous geothermal survey data, seismic tomography and field geological survey results, the following conclusions are obtained:
(1)Under the condition that temperature varies with depth, the influence of temperature on P-wave velocity is less than that of pressure on P-wave velocity in the depth corresponding to the pressure below the critical pressure P_c at the 500MPa pressure interval deduced by simple fitting. Temperature has a strong effect on P-wave velocity at depths where the pressure is higher than the critical pressure P_c. The experimental measurements are higher than the actual formation velocity, the difference is about 0.2142km/s.
(2)The lower limit and upper limit of mean P-wave velocities obtained by experimental measurements are smaller than the lower limit and greater than the upper limit of P-wave velocities obtained by previous inversions, respectively. It can be attributed to the high wave velocity of intrusive rocks and metamorphic rocks outcropped in the shallow part of the stratum, and the low wave velocity of sedimentary rocks in the deep part of the stratum. This is consistent with the geological survey results in this area.
(3)The combination of velocity in nonlinear domain and velocity in linear domain can provide a more real reference of P-wave velocity structure model in this region.

Key words: P-wave velocity, high-pressure experiment, micro seismic positioning, Laohushan fault zone, Tianzhu seismic gap in Gansu Province

摘要：

前人研究表明, 老虎山断裂带区域未来几十年内可能会密集发生微小地震。微小地震因震级低、信号主频率高、能量衰减快而具有明显的近场特点。近场地壳波速结构, 尤其是浅部速度结构对高频微小地震的精确定位有极大的影响。为此, 在室温和围压介于50~500MPa的条件下, 系统地测量了该地区代表性岩石的P波速度(V_P)。实验表明, 在低压范围内, V_P随着压力的增加呈对数增加; 在压力高于临界压力P_c时, V_P随着压力的增加呈线性趋势增加。P_c介于200~450MPa, 平均值为262.5MPa。结晶岩的P_c平均值为250MPa, 砂岩的P_c平均值为271.4MPa; 经对比发现, 砂岩的P_c明显高于结晶岩。这可归因于在低压范围内裂纹孔隙随着压力的增加而闭合, 而在高压范围内裂纹孔隙基本完全闭合。因此, 提高近场微小地震的定位精度应考虑弹性波波速随压力的非线性增加。此外, 实验结果显示此区域岩石弹性波速度是深度的函数。

关键词: P波速度, 高压实验, 微小地震定位, 老虎山断裂带, 甘肃省天祝地区

SONG Gang, YANG Xiao-song. EXPERIMENTAL STUDY ON THE P-WAVE VELOCITY OF THE CRUST IN THE LAOHUSHAN FAULT ZONE IN GANSU PROVINCE[J]. SEISMOLOGY AND GEOLOGY, 2022, 44(5): 1273-1289.

宋刚, 杨晓松. 甘肃省老虎山断裂带区域地壳纵波速度的实验[J]. 地震地质, 2022, 44(5): 1273-1289.

Figures/Tables 14

Fig. 1 The geotectonic location of Laohushan fault zone(after Tapponnier et al., 2001)(a), and the regional geological map of Laohushan fault zone and sample collection points in the experiment(b).

Table1 Stratigraphy and corresponding lithology of the study area

界	系	统	厚度	岩性
新生界	第四系	全新统	>5m	疏松沙砾夹亚砂土。
		上更新统	>30m	疏松黄土(新黄土)夹砂砾层。
		下更新统	>100m	灰色块状砾岩夹透镜状砂岩。
	古近-新近系	上新统	>800m	浅黄色及橘红色砂质泥岩、中细粒砂岩夹含砾砂岩。
中生界	三叠系	上统	595~3 914m	灰绿色、浅灰绿色细砂岩夹碳质页岩及页岩。有时含薄煤层。底部为含砾粗砂岩。
		中下统	296~445m	浅紫灰色厚层砂岩, 偶夹含砾砂岩、泥质粉砂岩, 底部含砂质结核。
古生界	二叠系	上统	376m	紫色中粗粒砂岩为主, 夹同色泥岩、粉砂岩。
		下统	327m	灰绿、黄灰及紫杂色砂岩、泥质粉砂岩互层, 上部常夹不稳定砾岩。
	石炭系	上统	222m	深灰色砂岩、页岩夹灰岩、粉砂岩, 含薄煤层。
		中统	67m	灰色及深灰色中细粒砂岩夹泥岩及页岩, 含煤层。
		下统	39m	深灰色厚层灰岩, 底部含透镜状石膏, 上部为砂页岩。
	泥盆系	上统	44~104m	紫色中粗粒砂岩及泥质粉砂岩、粉砂岩互层, 底部为不稳定砾岩。
	志留系	下统	>3 861m	灰绿色、黄灰色变质石英长石砂岩、长石砂岩、千枚岩, 其次为凝灰质砂岩, 千枚状粉砂岩及板岩。在北部(布台区域)夹少量变玄武岩、变安山岩、凝灰岩、片岩及结晶灰岩。
	奥陶系	上中统	2 807m	灰绿色为主的安山凝灰岩、英安凝灰岩、安山玢岩、硅质岩、板岩、灰岩及细砂岩、千枚岩。北部(布台区域)夹中酸性及中基性的火山岩。
		下统	>2 000m	灰色、灰绿色变质长石石英砂岩、千枚岩, 偶夹不稳定火山岩及硅质灰岩。

Fig. 2 Representative rock in Laohushan fault zone area.

Fig. 3 Microtextures of representative rocks in the Laohushan fault zone area.

Table2 Name of sampling site, latitude and longitude, initial length, initial diameter, initial density and lithology of the representative rock samples in the Laohushan fault zone area

样品编号	采样地点	坐标	L₀ /mm	R /mm	ρ /g·cm^-3	岩性
GJT29-1-//	正路乡红岘村	37.02°N, 103.70°E	39.94	20.01	2.555 3	砂岩
GJT29-1-⊥	正路乡红岘村	37.02°N, 103.70°E	39.92	20.00	2.559 1	砂岩
GJT43-1	喜泉乡沙塘子村	37.01°N, 103.81°E	40.01	20.03	2.622 7	中粒砂岩
GJT48-1	中泉乡金坪村	36.98°N, 104.16°E	40.02	20.01	2.384 2	砂岩
GTZW04-2	中泉乡白水村	36.96°N, 104.10°E	40.00	20.03	2.544 4	砂岩
GYF01-1	寺滩乡官草村	37.13°N, 103.80°E	40.00	20.04	2.578 2	砂岩
GYF03-GYF02-1	老虎沟	37.11°N, 103.74°E	39.92	20.05	2.679 0	砂岩
GJT32-1	正路乡黄羊淌村	36.97°N, 103.84°E	40.04	20.05	2.835 9	变辉长岩
GYF08-2	寺滩乡大庄村	37.81°N, 103.87°E	39.95	20.04	2.950 4	变辉长岩
GYF08-4	寺滩乡大庄村	37.81°N, 103.87°E	39.98	20.04	2.952 1	闪长岩
GTZW17-3	正路乡红岘村	36.99°N, 103.74°E	40.00	20.01	2.787 9	英云闪长岩
GJT09-1	寺滩乡大庄村	37.09°N, 103.82°E	39.94	20.04	2.679 0	变质砂岩

Fig. 4 The sample assembly drawing and the layout drawing of the ultrasonic transducer.

Fig. 5 VP(P)-P relationship of representative rock samples of the Laohushan fault zone area.

Table3 The average VP of sandstone, intrusive rock, metamorphic sandstone and all samples

P/MPa	0.1	50	75	100	125	150	175	200	250	300	350	400	450	500
砂岩	4.25	4.88	5.09	5.22	5.33	5.40	5.47	5.51	5.56	5.64	5.67	5.71	5.74	5.78
侵入岩	4.87	5.90	6.05	6.26	6.34	6.39	6.47	6.52	6.60	6.67	6.70	6.75	6.78	6.81
变质砂岩	4.83		5.62	5.71	5.77	5.83	5.85	5.87	5.91	5.95	5.96	5.98	6.01	6.02

Table4 Anisotropy of Upper Triassic sandstone GJT29-1

P/MPa	0.1	50	75	100	125	150	175	200	250	300	350	400	450	500
V_m	3.80	4.72	5.02	5.17	5.30	5.35	5.43	5.47	5.53	5.60	5.63	5.69	5.71	5.75
V_max-V_min	0.58	0.41	0.09	0.05	0.04	0.05	0.06	0.05	0.05	0.05	0.07	0.05	0.07	0.04
A	15.12	8.76	1.80	1.01	0.83	0.88	1.08	0.93	0.92	0.91	1.19	0.89	1.25	0.64

Fig. 6 The P-wave anisotropy of the GJT29-1 sandstone samples.

Fig. 7 The P-wave velocity and the associated fitting curve of the test rock samples.

Table5 Fitting parameters of rock P-wave velocity in Laohushan fault zone area

样品	P_c/MPa	$V P 0$ /km·s^-1	D_v/km·s^-1·MPa^-1	R²	a_v/km·s^-1·MPa^-1	c_v/km·s^-1	R²(非线性)
GJT29-1-//	200	5.332	9.156×10^-4	0.97	0.431 2	3.213	0.98
GJT29-1-⊥	200	5.278	9.162×10^-4	0.98	0.636 9	2.146	0.93
GJT43-1	450	5.805	2.821×10^-4	1.00	0.124 6	5.153	0.97
GJT48-1	300	4.935	7.894×10^-4	1.00	0.557 9	2.035	1.00
GTZW04-2	250	5.204	11.293×10^-4	0.98	0.301 3	3.810	0.99
GYF01-1	200	5.610	6.746×10^-4	0.82	0.603 1	2.633	0.94
GYF03-GYF02-1	300	5.609	7.391×10^-4	0.97	0.520 0	2.870	0.99
GJT32-1	250	6.375	6.771×10^-4	0.97	0.285 1	4.964	1.00
GYF08-2	300	6.431	8.422×10^-4	0.99	0.465 9	4.008	0.99
GYF08-4	200	6.550	5.739×10^-4	0.99	0.340 3	4.860	0.99
GTZW17-3	250	6.389	9.390×10^-4	0.94	0.621 3	3.159	0.98
GJT09-1	250	5.809	4.290×10^-4	0.97	0.238 9	4.608	0.97

Table5 Fitting parameters of rock P-wave velocity in Laohushan fault zone area

样品	P_c/MPa	$V P 0$ /km·s^-1	D_v/km·s^-1·MPa^-1	R²	a_v/km·s^-1·MPa^-1	c_v/km·s^-1	R²(非线性)
GJT29-1-//	200	5.332	9.156×10^-4	0.97	0.431 2	3.213	0.98
GJT29-1-⊥	200	5.278	9.162×10^-4	0.98	0.636 9	2.146	0.93
GJT43-1	450	5.805	2.821×10^-4	1.00	0.124 6	5.153	0.97
GJT48-1	300	4.935	7.894×10^-4	1.00	0.557 9	2.035	1.00
GTZW04-2	250	5.204	11.293×10^-4	0.98	0.301 3	3.810	0.99
GYF01-1	200	5.610	6.746×10^-4	0.82	0.603 1	2.633	0.94
GYF03-GYF02-1	300	5.609	7.391×10^-4	0.97	0.520 0	2.870	0.99
GJT32-1	250	6.375	6.771×10^-4	0.97	0.285 1	4.964	1.00
GYF08-2	300	6.431	8.422×10^-4	0.99	0.465 9	4.008	0.99
GYF08-4	200	6.550	5.739×10^-4	0.99	0.340 3	4.860	0.99
GTZW17-3	250	6.389	9.390×10^-4	0.94	0.621 3	3.159	0.98
GJT09-1	250	5.809	4.290×10^-4	0.97	0.238 9	4.608	0.97

Fig. 8 The pressure derivative(V'P) of the rock P-wave velocity in the Laohushan fault zone area.

Fig. 9 Calculated P-wave velocity-depth curve of the Laohushan fault zone area.

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