Aftershock seismicity

ABSTRACT Earthquake seismicity analysis include the classical (1)G-R Law: relation of earthquake frequency and magnitude(2) Omori's Law: relation of time and aftershock (3)Båth's Law: relation of mainshock and its expected maximum aftershock. In this study, I analysis the seismicity of Nantou area, Taiwan by these empirical law and examine the abnormal earthquake behave follow by two M>6.0 earthquake in 2013.

INTRODUCTION

Follow by G-R Law: log₁₀N=a-bM (number: N, magnitude: M , coefficient: a,b), the Nantou region has the potential earthquake of magnitude >6.0.

Catalog from 2000/01/01 to 2013/01/01 shows the maximum possible earthquake could be M6.25, the 2013/03/27 M6.2 Nantou earthquakes had fully explain this prediction. However, another earthquake occurred on 2013/06/02 with a magnitude of M6.5 broke the law unexpectedly.

Our hypothesis indicates the relatively silent decade after the mega Chi-Chi M7.5 earthquake. Besides the 2000/6/10 M6.79 earthquake, between year 2000 to year 2013, none of the earthquake was occurred with a magnitude bigger than M6.0. This silent period might cause this under determine of the prediction.

AFTERSHOCK or FORESHOCK?

Before 2013/06/02 Nantou earthquake, the energy in Nantou region seems fully release by the R-G Law's consistency. However, the 2013/06/02 earthquake shows us an fully unexpected result. The Båth's Law predict that a maximum aftershock of ~M5.5 (different between mainshock and maximum aftershock is a constant of 1.1-1.2) could occur. Could another unexpected earthquake occur follow the 2013/06/02 earthquake?

Blue dots indicate the earthquakes between 2013/6/2-2013/12/31, black star denotes the maximum magnitude of these earthquakes.

Based on the Omori's Law, the aftershocks would occur between 0-day to 127-day (i.e. 06/02~10/06) after the 2013/06/02 Nantou earthquake occurs, then back to the background seismicity level. The biggest aftershock in this range is M4.24, much smaller than the predict of ~M5.5. This might explain the releasing energy of our previous assumption. Because the Nantou area had released much more energy than our prediction from the evidence of statistic, an earthquake might hard to occur in these region by the following several years even an moderate aftershock.