Long Term Trends of Repeating Earthquakes.

Can a sequence of repeating earthquakes^a (repeaters) describe what is happening within the Earth?

I propose a study of repeaters to look for long term trends in arrival times from assumed seismic event locations at known seismic recording station locations. We need to use repeating events to validate the assumption of seismic location, otherwise every event might be useful for this kind of study. This study will require the following analysis procedure to be done on a long term record of data.

1. Choose a study area. I have the Jiayi (Chiayi,  嘉義) area in central Taiwan (add longitude and latitude description). With more understanding perhaps a study area can be purposely selected.
2. Standardise^b and analyse waveforms. Analysis will produce sets of repeaters.
3. Repeaters should show a regular, long term and stable pattern of repetition. Major events might be the cause of changes to travel times between repeaters and seismic stations or major events might produce many repeaters in a short time^c.
4. Assume an event location for a group of repeaters. Repeating events can be assumed to originate within a quarter of their dominant wavelength (Geller and Mueller, 1980).
5. Match repeater waveforms arriving at multiple seismic stations.
6. Measure repeater arrival time differences between different stations.
7. Look for long term trends in the data. A long term trend might be toward either shorter differences or longer differences. No observed trends will render this study redundant.

To summarise the analysis procedure: Select study area, Find repeaters, Measure travel times, Find trends.

Discussion, Assumptions and Predictions.


The inspiration for this study is to begin analysis of the seismic record outside of the assumption of plate tectonic theory. I do not believe that plate tectonic theory provides any useful insights into earthquake behaviour. I will undertake this study based on one assumption, that is Earthquakes are the result of gravitational settling. Gravitational settling is the action of the planet as a response to its preference to be spherical due to its own gravity. Erosion is gravitational settling. Avalanches, lahars and landslides are gravitational settling. Glacial movement is gravitational settling. I am working on the assumption that tectonic movement is the result of a large scale example of the same thing and that seismicity is one of the observations we can make from this subterranean process.

This will be a long term endeavour. I would estimate that at least 10 or 20 years of data would be required to establish statistical confidence in any trends discovered. We have a number^d of years on record in Jiayi, but it is difficult for me to estimate how much data might be necessary.

I predict that a trend will be found with 50 years worth of reliable data. I tentatively^e predict that in the Jiayi area the trend will be toward a greater difference between arrival times of repeating events at different seismic station locations. This prediction is based on the gravitational environment Taiwan is located in. Taiwan lies between the continental mass of China and Russia and the negative mass of the Pacific basin, but on the continental side of the division between the two environments, The Ryuku Trench and the Manila Trench. Gravitational settling will therefore work towards the Pacific and from the continent. This assumed motion will lessen the gravitational stress upon Taiwan, allow its mass to expand and thus decrease wave propagation velocities.

Notes.

^a Definition and citation for repeaters needed.
^b The process for the standardisation of waveforms needs to be clarified.
^c Citation needed. Corrections might be developed so that trends in such affected data might be assumed. Such corrections would rely upon the assumption that there is in fact a trend in effect.
^d Number required.
^e Tentative because there is more happening to the east and because it looks like GPS measurements would predict the opposite.

Bibliography.

Geller, R. J., and C. S. Mueller (1980). Four similar earthquakes in central California, Geophys. Res. Lett. 7, no. 10, 821–824.