Persistent water level changes in a well near Parkfield, California, due to local and distant earthquakes
Persistent water level changes in a well near Parkfield, California, due to local and distant earthquakes
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DOI：
10.1029/97jb02335
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发表时间：
1998-01
影响因子：
--
通讯作者：
E. Roeloffs
中科院分区：
文献类型：
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作者：
E. Roeloffs
加州帕克菲尔德附近30米深的布迪厄谷（BV）井的同震水位上升是对三次本地地震和五次远地地震的响应。静应变的同震变化不能解释这些水位上升，因为（1）该井对潮汐周期的应变不敏感;（2）对于远震，预期的同震静应变非常小;（3）水位响应对于当地地震的符号不正确。因此，这些水位变化一定是由地震波引起的，但与地震水位振荡不同的是，它们是单调的，持续数天或数周，似乎是由周期为几秒的波而不是长周期的表面波引起的。其他研究人员也报告了日本的类似现象。某些威尔斯井一致地表现出这种类型的同震水位变化，无论地震的方位角或震源机制如何，水位变化总是在同一方向上，并且与震源距离的平方成反比。迄今为止，同震水位上升在B V井从来没有超过季节性水位最大值，虽然它们的大小是相对较好地与地震震级和距离。在0.1至0.7 cpd的频带内，水井对气压的响应与频率无关，这意味着含水层的承压能力相当好。高含水层压缩性，可能是由于孔隙空间中的气相，是井不响应地球潮汐的最可能原因。季节性水位和降水周期之间的相位和振幅关系限制水平水力扩散系数在4.5的因子内，限制了假设地震引起的含水层水力特性变化。此外，预计整个含水层的水力传导率和（或）扩散率的变化不会使水位在一年中的每一个时间都朝同一方向变化。一个典型的同震水位上升的前2.5天可能是由一个小的同震流量减少在几十米的一个点，从井。或者，整个同震水位信号可以表示在井的几米范围内突然同震孔隙压力增加的扩散，由类似于液化的机制产生。BV井的同震水位变化类似于，并可能与同震水位、河流流量和其他地方的地下水温度变化共享一种机制，这些地方也报告了地震前的变化。然而，在BV井场没有观察到震前变化。
Coseismic water level rises in the 30-m deep Bourdieu Valley (BV) well near Parkfield, California, have occurred in response to three local and five distant earthquakes. Coseismic changes in static strain cannot explain these water level rises because (1) the well is insensitive to strain at tidal periods; (2) for the distant earthquakes, the expected coseismic static strain is extremely small; and (3) the water level response is of the incorrect sign for the local earthquakes. These water level changes must therefore be caused by seismic waves, but unlike seismic water level oscillations, they are monotonic, persist for days or weeks, and seem to be caused by waves with periods of several seconds rather than long-period surface waves. Other investigators have reported a similar phenomenon in Japan. Certain wells consistently exhibit this type of coseismic water level change, which is always in the same direction, regardless of the earthquake's azimuth or focal mechanism, and approximately proportional to the inverse square of hypocentral distance. To date, the coseismic water level rises in the B V well have never exceeded the seasonal water level maximum, although their sizes are relatively well correlated with earthquake magnitude and distance. The frequency independence of the well's response to barometric pressure in the frequency band 0.1 to 0.7 cpd implies that the aquifer is fairly well confined. High aquifer compressibility, probably due to a gas phase in the pore space, is the most likely reason why the well does not respond to Earth tides. The phase and amplitude relationships between the seasonal water level and precipitation cycles constrain the horizontal hydraulic diffusivity to within a factor of 4.5, bounding hypothetical earthquake-induced changes in aquifer hydraulic properties. Moreover, changes of hydraulic conductivity and/or diffusivity throughout the aquifer would not be expected to change the water level in the same direction at every time of the year. The first 2.5 days of a typical coseismic water level rise could be caused by a small coseismic discharge decrease at a point several tens of meters from the well. Alternatively, the entire coseismic water level signal could represent diffusion of an abrupt coseismic pore pressure increase within several meters of the well, produced by a mechanism akin to that of liquefaction. The coseismic water level changes in the BV well resemble, and may share a mechanism with, coseismic water level, stream discharge, and groundwater temperature changes at other locations where preearthquake changes have also been reported. No preearthquake changes have been observed at the BV well site, however.