Preliminary study on coseismic step-like changes of water-level in the Dazhai well, Simao city, Yunnan Province

Abstract

The study on coseismic step-like changes of water-level is of theoretical and practical significance to the mitigation of secondary hazards, identification of earthquake precursors, tracking of the subsequent shocks, and the research of crustal activity (Nur and Booker, 1972;WANG et al, 1988). A lot of researches have been done by previous geoscientists. Among them, ZHANG et al (1994) inverted stress variation in the aquifers through the analysis of coseismic step-like changes of well water level. LI (1995), WANG (2000), and FU et al (2002) studied the relationship between coseismic step-like changes of water level and future seismic activity. HUANG et al (2000) investigated the water-level changes in many wells after the occurrence of one great earthquake. However, the analysis of the quantitative relation between the coseismic step-like changes of water level and earthquakes in China has not been reported so far. This paper describes the coseismic step-like changes of water level in the Dazhai well, Simao city, Yunnan Province, and tries to discuss the possible mechanism of these changes. Basic conditions of the well and principal characters of water level changesThe Dazhai observation well is located in the middle part of the Yixiangba rift basin, about 11 km in the southeast of Simao city. The rift basin is situated in the southwest part of the Jingmenkou-Shigaoqing segment of the Pu'er fault (Figure 1). The well is 112.27 m deep, installed with 150 mm diameter casing from the earth surface to 26.69 m depth. The section for water level measurement ranges from 26.69 m to 112.27 m. The diameter of water-carrying section is 108 mm. The buried depth of the top of aquifer is 7.51 m below earth surface. The well was drilled into confined aquifer in Cretaceous sandstone, with a rate of discharge of 0.3-0.6 L/s (artesian well).The hydrochemistry of the water is characterized by HCO 3 ~Ca2+-The well has been continuously monitored since January 1, 1984.Water level in the Dazhai well has been measured continuously by using mechanical float gauges, and a strip-chart analog recorder is used to record water level data. In the second half of 2001, a digital groundwater-level gauge with data-sampling interval of 1 rain has been installed. The groundwater level in the Dazhai well is primarily supplied by rainfall, and there is no withdrawal well in the vicinity. The stable observation environment, there-*

Two Mechanisms of Earthquake-Induced Hydrochemical Variations in an Observation Well

“…The Simao observation well (101.05 • E, 22.73 • N) was established for seismic monitoring and earthquake prediction (Figure 1). It is a water well with a depth of 112.27 m. The casing section extends to 26.69 m, and the sampling water is a mixture of inputs from multiple aquifers at depths between 26.69 and 110.76 m. It is fracture-confined water and the flow rate is 0.3~0.6 L/s [44]. The water yield property is high [45].…”

Characteristics of coseismic water level changes at Tangshan well for the Wenchuan MS8.0 earthquake and its larger aftershocks

“…King et al (1999) provided a relationship of M=2.5lgD for Tono well, Japan. Yang et al (2005) analyzed coseismic phenomena at Dazhai well, Simao county, Yunnan province in China, and suggested an equation as M=1.176lgD+3.104. These studies used dif- ferent earthquake catalogues and different types of magnitude scaling.…”

Relationship between Earthquake-Induced Hydrologic Changes and Faults

“…For instance, the sign of co-seismic water level changes is related to the earthquake-induced static strain in the nearfield, i.e., water level rise in the contraction zone and drop in the dilation zone [7][8][9][10][11][12]. In the intermediate-and far-field, the amplitude of co-seismic water level changes is positively related to the magnitude of the seismic energy density [3,[13][14][15][16]; the sign is repeatedly fixed in a specific well [3,13,17,18] and seems to be controlled by the hydrogeological settings, e.g., water level rise in the recharge area and water level drop in the discharge area [19,20]. In addition, some studies showed that the sensitivity of water level to dynamic strain is related to tectonic settings.…”