Geodetic measurements using a two-color Electronic Distance Meter made in California, 1975 to 2005

Measurements of changes in the distance (or length) between monuments are provided. These measurements were made using a two-color Electronic Distance Meter (EDM) that can measure distances between 1 and 10 KM. Nominal precision of these data range from 0.3 mm to 1.0 mm dependent upon the baseline’s length. These measurements were made between mid-1975 to mid-2006. Data in this archive are from eight networks, each consisting of more the 9 baselines. The locations of these networks in California include far northwestern California, Hollister, CA., Long Valley Caldera in eastern California, Parkfield, Pearblossom, and Anza; the last two located in Southern California. For four of the networks (Hollister, Parkfield, Long Valley, and Pearblossom), measurements were made roughly once per day with the goal of documenting transient deformation associated with earthquake and/or volcanic areas. Measurements of the other networks were less frequent from a few times each year to annual measurements. In many respects, these data, especially those made daily, are a forerunner of the current method of using GNSS receivers being operated continuously to measure deformation. Some highlights from these data include the following: 1) episodes of inflation of the Long Valley caldera including the energetic episode in late 1997; 2) A 20 year record with high temporal resolution of deformation that proceeded 2004 M6 Parkfield earthquake which includes at least one episode of transient deformation; and 3) A 20-year record of steady rate of strain accumulation over a 10 km long segment of the San Andreas fault in Southern California. The two color EDM is unique amongst EDMs. By ranging on two wavelengths (red and blue), it was able to resolve the correction to the optical travel-time to within 0.1 parts per million (ppm) of the length of the baseline. In contrast, the common single-color EDM require measurements of temperature and pressure at the end points of the baseline and a critical assumption that the temperature and pressure are constant along the baseline. At best, the precision of single color EDMs is 1 ppm. Refined-single color EDM measurements employed an aircraft equipped with temperature and pressure sensors to obtain the profile of these quantities along the baseline.

本数据集提供了标志物间距离（或长度）的变化测量数据。该类测量采用双色电子测距仪（two-color Electronic Distance Meter, EDM）完成，其可测量1至10千米范围内的距离。本数据集的标称精度范围为0.3毫米至1.0毫米，具体数值取决于基线的长度。本次测量的实施时段为1975年中期至2006年中期。本档案库收录的数据来自8个监测网，每个监测网包含至少9条基线。这些监测网均位于美国加利福尼亚州，具体分布包括加州极西北区域、加利福尼亚州霍利斯特（Hollister, CA.）、加州东部的长谷火山口（Long Valley Caldera）、帕克菲尔德（Parkfield）、皮尔布洛瑟姆（Pearblossom）以及安扎（Anza）；后两处监测网位于南加州。其中4个监测网（霍利斯特、帕克菲尔德、长谷火山口以及皮尔布洛瑟姆）的测量频率约为每日一次，旨在记录与地震及/或火山活动区域相关的瞬态形变。其余监测网的测量频率较低，观测间隔从每年数次至每年一次不等。从诸多层面来看，本数据集（尤其是每日获取的观测数据）是当前采用连续运行全球导航卫星系统（Global Navigation Satellite System, GNSS）接收机开展形变测量方法的先驱。本数据集的核心研究亮点包括：1）长谷火山口的膨胀活动记录，其中包含1997年末的剧烈膨胀事件；2）针对2004年帕克菲尔德M6级地震前的形变过程，获取了时长20年的高时间分辨率观测序列，其中至少包含一次瞬态形变事件；3）针对南加州圣安德烈亚斯断层（San Andreas fault）10千米长的断层段，获取了时长20年的应变累积稳定速率观测记录。该双色电子测距仪在同类设备中独具特色：通过对红光与蓝光两个波长进行测距，该设备可将光学传播时间的校正精度控制在基线长度的0.1ppm以内。与之相对，常规单色电子测距仪需要在基线两端测量温度与气压，且需做出关键假设：基线沿线的温度与气压保持恒定。单色电子测距仪的最高精度仅为1ppm。为提升测量精度，改进型单色电子测距仪会搭载配备温度与气压传感器的航空器，以获取基线沿线的温度与气压分布剖面。