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Location Performance and Detection Threshold of the Spanish National Seismic Network

机译：西班牙国家地震台网的定位性能和探测门限

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摘要

Spain is a low-to-moderate seismicity area with relatively low seismic hazard. However, several strong shallow earthquakes have shaken the country causing casualties and extensive damage. Regional seismicity is monitored and surveyed by means of the Spanish National Seismic Network, maintenance and control of which are entrusted to the Instituto Geográfico Nacional. This array currently comprises 120 seismic stations distributed throughout Spanish territory (mainland and islands). Basically, we are interested in checking the noise conditions, reliability, and seismic detection capability of the Spanish network by analyzing the background noise level affecting the array stations, errors in hypocentral location, and detection threshold, which provides knowledge about network performance. It also enables testing of the suitability of the velocity model used in the routine process of earthquake location. To perform this study we use a method that relies on P and S wave travel times, which are computed by simulation of seismic rays from virtual seismic sources placed at the nodes of a regular grid covering the study area. Given the characteristics of the seismicity of Spain, we drew maps for M _L magnitudes 2.0, 2.5, and 3.0, at a focal depth of 10 km and a confidence level 95 %. The results relate to the number of stations involved in the hypocentral location process, how these stations are distributed spatially, and the uncertainties of focal data (errors in origin time, longitude, latitude, and depth). To assess the extent to which principal seismogenic areas are well monitored by the network, we estimated the average error in the location of a seismic source from the semiaxes of the ellipsoid of confidence by calculating the radius of the equivalent sphere. Finally, the detection threshold was determined as the magnitude of the smallest seismic event detected at least by four stations. The northwest of the peninsula, the Pyrenees, especially the westernmost segment, the Betic Cordillera, and Tenerife Island are the best-monitored zones. Origin time and focal depth are data that are far from being constrained by regional events. The two Iberian areas with moderate seismicity and the highest seismic hazard, the Pyrenees and Betic Cordillera, and the northwestern quadrant of the peninsula, are the areas wherein the focus of an earthquake is determined with an approximate error of 3 km. For M _L 2.5 and M _L 3.0 this error is common for almost the whole peninsula and the Canary Islands. In general, errors in epicenter latitude and longitude are small for near-surface earthquakes, increasing gradually as the depth increases, but remaining close to 5 km even at a depth of 60 km. The hypocentral depth seems to be well constrained to a depth of 40 km beneath the zones with the highest density of stations, with an error of less than 5 km. The M _L magnitude detection threshold of the network is approximately 2.0 for most of Spain and still less, almost 1.0, for the western sector of the Pyrenean region and the Canary Islands.
机译：西班牙是地震活动程度较低的中低地震区。但是，几场强烈的浅地震震撼了整个国家，造成人员伤亡和大面积破坏。区域地震活动通过西班牙国家地震台网进行监测和勘测，其维护和控制权委托给国家地理研究所。该阵列目前包括分布在西班牙领土（内陆和岛屿）的120个地震台。基本上，我们有兴趣通过分析影响阵列站的背景噪声水平，次中心位置的误差和检测阈值来检查西班牙网络的噪声状况，可靠性和地震检测能力，从而提供有关网络性能的知识。它还可以测试在地震定位的常规过程中使用的速度模型的适用性。为了进行这项研究，我们使用了一种方法，该方法依赖于P和S波的传播时间，这些时间是通过模拟来自虚拟地震源的地震射线来计算的，该地震源位于覆盖研究区域的规则网格的节点上。考虑到西班牙地震活动的特征，我们绘制了M _L值2.0、2.5和3.0的地图，其焦深为10 km，置信度为95％。结果涉及到震中定位过程中涉及的站点数量，这些站点的空间分布方式以及焦点数据的不确定性（始发时间，经度，纬度和深度误差）。为了评估网络对主要震源区域的监测程度，我们通过计算等效球体的半径，根据置信椭圆形的半轴估算了震源位置的平均误差。最后，将检测阈值确定为至少四个站检测到的最小地震事件的大小。半岛的西北部，比利牛斯山脉，尤其是最西端的山脉，比蒂科迪勒拉（Betic Cordillera）和特内里费岛（Tenerife Island）是受到最佳监视的地区。起源时间和震源深度是不受区域事件约束的数据。比利牛斯山脉和比蒂科迪勒拉岛是两个地震活动度最高，地震危险性最高的伊比利亚地区，位于半岛的西北象限，是确定地震震源的区域，其近似误差为3 km。对于M _L 2.5和M _L 3.0，几乎整个半岛和加那利群岛都常见此错误。通常，对于近地表地震，震中纬度和经度的误差很小，随着深度的增加而逐渐增加，但即使在60 km的深度处也保持接近5 km。震中深度似乎很好地限制在站密度最高的区域下方40 km的深度，误差小于5 km。在西班牙的大部分地区，该网络的M _L量级检测阈值约为2.0，而在比利牛斯山地区和加那利群岛的西部地区，该阈值的检测阈值更低，接近1.0。

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