Anomalous Tremor Waves in Tidal Observations

Abstract

In observations of the earth tide, there are often some anomalous microseisms in tidal signals. These microseisms are observed by broadband digital seismographs at the same time. Some of these microseisms are generated by tropical cyclones in the western Pacific, and others are mostly accompanied by major earthquakes, called anomalous tremor waves. This article outlines the earthquake and tide observation station of the Huazhong University of Science and Technology (HUST), and reports the anomalous tremor waves recorded by the DZW gravimeter and VS‐1 tiltmeter at this station. A lot of observations show that the anomalous tremor waves appeared in the records of the low‐pass 1 channel (LP1) of the DZW gravimeter and VS‐1 tiltmeter, but were rare in that of the low‐pass 2 channel (LP2) of the instruments and other tidal instruments. The envelope of the anomalous tremor waves is mostly like a spindle or tail, with a duration of 1 to 3 days. The research of the anomalous tremor waves and the tidal instruments suggests that these anomalous tremor waves are actually micro‐tremor signals originated from complex sources, which have a wide response range, and can be recorded by broadband digital seismographs and tidal instruments. The main periods of the anomalous tremor waves aroused by the tropical cyclone in the western Pacific are about 3∼7 s, and the tremor waves before earthquakes have not only a main period of 3∼7 s but also 10∼60 s or longer periods. The differences of the responses of tidal instruments to anomalous tremor waves are mainly decided by the transfer function and property of the instruments. The DZW gravimeter and VS‐1 tiltmeter take samples per minute, so the anomalous tremor waves are the aliasing or mapping of the real signals. Whether the anomalous tremor waves before earthquakes are related to the earthquakes, or a seismic precursor needs more detailed and deeper analysis.