A Mechanism for Anomalous Decline in Radon Precursory to an Earthquake

Abstract

Mechanisms for interpreting anomalous decreases in radon in ground water prior to earthquakes are examined with the help of a case study to show that radon potentially is a sensitive tracer of strain changes in the crust preceding an earthquake. The 2003 Chengkung earthquake of magnitude (M) 6.8 on December 10, 2003, was the strongest earthquake near the Chengkung area in eastern Taiwan since 1951. The Antung radon-monitoring station was located 20 km from the epicenter. Approximately 65 d prior to the 2003 Chengkung earthquake, precursory changes in radon concentration in ground water were observed. Specifically, radon decreased from a background level of 780 pCi/L to a minimum of 330 pCi/L. The Antung hot spring is situated in a fractured block of tuffaceous sandstone surrounded by ductile mudstone. Given these geological conditions, we hypothesized that the dilation of brittle rock mass occurred at a rate faster than the recharge of pore water and gas saturation developed in newly created cracks preceding the earthquake. Radon partitioning into the gas phase may explain the anomalous decrease of radon precursory to the 2003 Chengkung earthquake. To support the hypothesis, vapor-liquid, two-phase radon-partitioning experiments were conducted at formation temperature (60°C) using formation brine from the Antung hot spring. Experimental data indicated that the decrease in radon required a gas saturation of 10% developed in rock cracks. The observed decline in radon can be correlated with the increase in gas saturation and then with the volumetric strain change for a given fracture porosity.

Estimation of aseismic crustal-strain using radon precursors of the 2003 M 6.8, 2006 M 6.1, and 2008 M 5.0 earthquakes in eastern Taiwan

“…For example, the distance from the epicenter to the radon monitoring station for the 2003 Mw 6.8 Chengkung earthquake was about 20 km; this distance is quite similar to the epicenterstation distance of 25 km for the 1978 (M) 7.0 Izu-Oshima-kinkai earthquake of Japan. The observed radon anomaly precursory to the 2003 Chengkung earthquake (Kuo et al 2006b) also behaved similarly to that observed precursory to the 1978 Izu-Oshima-kinkai earthquake (Wakita et al 1980). Eighty-six days before the 2003 Chengkung earthquake (Fig.…”

Application of Recurrent Radon Precursors for Forecasting Local Large and Moderate Earthquakes

“…The observed v-shaped pattern prior to the three main shocks clearly progresses in a sequence of three stages (Kuo et al 2006 (Kuo et al 2006(Kuo et al , 2010a. During Stage 1, the radon concentration in ground water was fairly stable; there was an accumulation of tectonic strain and a slow, steady increase of regional stress.…”

“…Radon partitioning into the gas phase can explain the anomalous decreases of radon precursory to the earthquakes (Kuo et al, 2006). To support the hypothesis of radon volatilization from ground water into the gas phase, radon-partitioning experiments were conducted to determine the variation of the radon concentration remaining in ground water with the gas saturation at formation temperature (60 ℃) using formation brine from the Antung hot spring.…”

“…(2) will be improved by taking into account the initial stable radon concentration in groundwater precursory to each radon anomaly. Based on radon phase behavior and rock dilatancy, Kuo et al (2006Kuo et al ( , 2010a) developed a mechanistic model correlating the observed decline in radon with the volumetric strain change. The model consists of two parts, i.e., the radon-volatilization model and the rockdilatancy model.…”

“…Measurement of radon-222 in groundwater has been frequently used in earthquake prediction (Igarashi et al 1995;Liu et al 1985;Noguchi & Wakita 1977;Teng, 1980;Wakita et al 1980;Kuo et al 2006Kuo et al , 2010aKuo et al , 2010b. According to a worldwide survey (Hauksson 1981;Toutain & Baubron 1999), more than 80 % of radon anomalies associated with earthquakes show increases in radon concentration precursor to a rupture while a few anomalies manifested decreases in radon.…”