○ E Seismic Sensor Misorientation Measurement Using P-Wave Particle Motion: An Application to the NECsaids Array by Xin Wang, Qi-Fu Chen, Juan Li, and Shengji Wei ABSTRACT Seismic sensor orientation is one of the most critical parame- ters for modern three-component seismological observation. However, this parameter is easily subject to error imposed by strong magnetic anomalies near the station or by human error in declination calibration. It is therefore very important to in- spect and correct for sensor misorientation before utilizing three-component waveform data. In this study, we measured the epoch-dependent sensor misorientation for our temporary seismic array (NorthEast China Seismic Array to Investigate Deep Subduction, or NECsaids) by analyzing P-wave particle motions. We applied principal component analysis and the minimizing transverse energy method to study earthquakes with epicentral distance between 5° and 90° to estimate the sensor misorientation. Our results show high consistency with the direct gyrocompass measurements, with a correlation coef- ficient of 0.95. Our statistical analysis suggests that we can es- timate robust sensor misorientation utilizing 10 earthquakes with high signal-to-noise ratio records and highly linear P-wave polarizations. We also find that the influence of anisotropy or a dipping interface produces a periodical pattern with back azi- muth and is relatively small for our misorientation estimation. By analyzing the amplitude change of synthetic seismograms due to misorientation and taking into account the influence of anisotropy and dipping interfaces, as well as the measure- ment errors, we expect engineers to be able to orient seismic sensors with error smaller than 3°. Online Material: Table of the NECsaids Array sensor misor- ientation estimation. INTRODUCTION Many modern seismological studies rely on precise three- component waveform observations, such as focal-mechanism inversions, shear-wave splitting, receiver function, normal mode and surface-wave analysis. The recorded horizontal com- ponents are often rotated to a radial–transverse (R-T) coordi- nate to isolate the P–SV and SH energy. However, the back azimuth, which is required for R-T rotation, is easily subject to misalignment between the station sensor’s north component (such as BHN) and the true north direction (Fig. 1). If there is a sensor misorientation, the resulting R-T rotation will deviate from the proper rotated system, which will affect the accuracy and reliability of the following seismological studies. Typically, a field engineer determines the sensor orienta- tion during installation by finding true north, transferring a reference line, and measuring the orientation of the sensor relative to the reference line (Ringler et al., 2013). Using a fiber-optic gyrocompass to find true north and a sunshot (such as theodolite or Global Positioning System) to transfer the reference line is one of the most accurate methods to assess the sensor orientation. Unfortunately, the cost and weight of such equipment restricts their availability for many temporary seis- mic experiments. The most economic and common way to de- termine north is to use a magnetic compass with a site-specific declination correction and then align the instrument to north using an orientation rod or a ruler. However, this method has potentially large errors due to the influence of magnetic material near the station (e.g., ore deposit, steel, or the sensor itself ) or an incorrect declination. Significant errors may also occur when transferring the north line to the instrument with an orientation rod or ruler, particularly for the number of turns required to transfer north from the surface to the sensor loca- tion. In addition, the sensors are often reoriented during main- tenance visits, reinstallation, or seismometer replacements, which could reintroduce misorientation. Considerable error may be introduced in the particular case in which no physical marks are present near the sensor that may be used as a refer- ence during reinstallation. The inspection and correction of sensor misorientation has been a basic and fundamental project for seismologists, making use of methods such as the analysis of the polarizations of long-period Rayleigh waves (Laske, 1995; Zha et al., 2013; Rueda and Mezcua, 2015) and P waves (Schulte-Pelkum et al., 2001; Niu and Li, 2011), as well as calculating the correlation doi: 10.1785/0220160005 Seismological Research Letters Volume 87, Number 4 July/August 2016 901 Downloaded from https://pubs.geoscienceworld.org/ssa/srl/article-pdf/87/4/901/2743488/901.pdf by Nanyang Technological University user on 30 August 2019