PRELIMINARY OPINION ON THE FEATURES AND CLASSIFICATION OF ANOMALY PATTERNS OF CRUSTAL TILT

Ill this paper, the anomaly patterns of ground tilt before and during dozens of large and moderately strong earthquakes of China had been gethered and studied. They can be divided into five kinds as follows:1. The recorded tilt curves show a gradual change before the earthquake and then return to normal before the occurrence of the earthquake, such sequences are ofter seen at stations far from the epicentre;2. The recorded curves show sudden changes (step changes) before the occurrence of earthquakes, such changes are often ,seen at stations near the epicentre;3. The recorded tilt curves show a series, of disturbance (or one-way pulses) before the earthquake and become nomal until the occurrence of the earthquake;4. The recorded curves of tilt tide show distortion until the occurrence of the earthquake;5. The recorded tilt curves display a series of small tremors so that the recorded trace becomes thicker until the earthquake occurs.The fundamental features of the records are as follows: (1) The anomaly patterns show variety; (2) The anomalies occur from a few minutes to tens of hours before the earthquakes; (3) The amplitude of anomalies is of the order of 10-8-10-7, a few amounting to 10-6, or even to 10-5; (4) The direction of anomalous tilt is often related to the azimuth of the epicenter of the earthquake with respect to the station; (5) When an anomaly due to the same earthquake is recorded by many stations, the anomaly is first detectable at stations far removed from the epicentre and then at the stations nearer to the epicenter.Finally, in this paper some problems concerned with tilt anomaly before earthquakes results of rock sample experiments and seismic models are discussed.