Two-Dimensional Stiffness Profiles from SASW Testing at Selected Sites: 1994 Northridge Earthquake

1994年1月17日发生的北岭地震震动了几个土壤和岩石地点，地表加速度异常高。项目的原位部分基本上是一维的；也就是说，在每个地点只测量以横波速度表示的小应变材料刚度的垂直剖面。没有研究横向变异性的刚度轮廓提出。采用一维（1-D）方法是因为该项目的时间框架为一年，也因为安装大量井眼的成本很高，而使用井眼地震技术开发二维（2- D）剖面是必要的。目标是开发2-D（在一种情况下，3-D）刚度剖面。钻孔地震技术（如井间测试、井下测试或悬浮测井）将不被采用。相反，采用非侵入式SASW技术，涉及瑞利型表面波。该技术是非侵入式的，因为源和接收器都放置在地面上。在现场进行了表面波色散的测量。然后在实验室中使用正演模拟来确定现场的横波速度剖面。除了非侵入性的成本优势之外，SASW测试的一个重要优点是它可以用来测量夹在硬层之间的软层，这是工程研究中潜在的关键测量方法。该项目包括在两个地点进行SASW测试（表面波频谱分析）：Tarzana站点和La Cienega和I- 10高速公路交汇处。在这两种情况下，地面站都测量到了异常强烈的地面震动。在每个站点，将使用高能移动振动器（Vibroseis）进行现场地震测试。这个震源，再加上4个或更多的1赫兹检波器的多通道记录，应该可以将横向尺寸为100到300米的剖面（取决于地点）开发到30到60米的深度。在Tarzana场地的情况下，剖面将在南北和东西方向进行，允许开发地下剪切刚度的三维图像。这项研究的结果有可能为理解1994年北岭地震期间产生的地面震动水平，以及为预测未来地震期间地面反应而开发具有成本效益的二维和三维地震剖面提供重要贡献。＊＊＊

9416433 Stokoe Several soil and rock sites shaken by the Northridge Earthquake of January 17, 1994 exhibited unusually high ground surface accelerations. The insitu portion of the project is basically one dimensional; that is, only a vertical profile of small-strain material stiffness, in terms of shear wave velocity, will be measured at each site. No investigation of the lateral variability of the stiffness profiles is proposed. The one -dimensional (1-D) approach is taken due to the one year time frame for the project, also because of the high cost involved in installing numerous boreholes that would be necessary to develop two-dimensional (2- D) profiles using borehole seismic techniques. The objective is to develop 2-D (and in one case, 3-D) stiffness profiles. Borehole seismic techniques (such as crosshole testing, downhole testing or suspension logging) will not be employed. Instead, the nonintrusive SASW technique involving Rayleigh-type surface waves is utilized. The technique is nonintrusive because both the source and receivers are placed on the ground surface. Measurement of surface wave dispersion is performed in the field. Forward modeling is then used in the laboratory to determine the shear wave velocity profile at the site. In addition to the cost benefit of being nonintrusive, an important benefit of SASW testing is that it can be used to measure softer layers sandwiched between stiffer layers, a potentially critical measurement in engineering studies. This project involves SASW testing (spectral analysis of surface waves) at two sites: The Tarzana site and the La Cienega and I- 10 highway interchange. In both cases, unusually high ground shaking was measured with surface stations. At each site, in situ seismic testing will be performed using a high-energy mobile vibrator (Vibroseis). This source, coupled with multi-channel recording of four or more 1-Hz geophones, should permit profiles on the order of 100 to 300 m in lateral dimension (depending on the site) to be developed to a depth on the order of 30 to 60 m. In the case of the Tarzana site, profiling will be performed in both the north-south and east-west directions, allowing a 3-D image of the underground shear stiffness to be developed. The results of this study have the potential to offer an important contribution to the understanding of ground shaking levels generated during the 1994 Northridge Earthquake as well as to the development of cost-effective 2-D and 3-D seismic profiling for the prediction of ground response during future earthquakes. ***