Synchronized and asynchronous modulation of seismicity by hydrological loading: A case study in Taiwan

Many distinct physical factors help trigger earthquakes, among them the loading of the earth’s crust by water. Many studies have demonstrated a seasonal modulation of seismicity linked to water forcing in volcanic areas, plate boundary zones and other regions. Such seasonal modulation of seismicity is often subtle, however, only becoming apparent after analysis of events over many years. Moreover, statistical analyses are often hampered by the limited number of events associated with a low level of background seismicity, insufficient station coverage or short catalogue durations.

In a recent paper, LML External Fellow Jiancang Zhuang and colleagues consider hydrological influence on seismicity in the context of Taiwan, a particularly suitable location for studying this link. Taiwan is located on the convergent boundary where the Philippine Sea plate collides with the Eurasian plate at a rate of 85 to 90 mm/year. This region experiences many damaging earthquakes, with about one magnitude 6+ earthquake each year and also has heavy seasonal precipitation of more than 2000 mm/year on average. The largest annual precipitation is close to 4 m, and the spatiotemporal distribution of rainfall is uneven. Indeed, some 70% of the annual precipitation during monsoons and typhoons between May and September. This leads to fluctuations of 5 to 15 m in groundwater levels and 5 to 20 mm in Global Navigation Satellite System (GNSS) vertical displacement time series, reflecting the Earth’s elastic response to the seasonal water loading. In their analysis, Zhuang and colleagues quantify the spatiotemporal relationship between the hydrological cycle and earthquake seasonality by analysing time series of seismicity, groundwater level, and the GNSS vertical time series.

Their analysis reveals that the groundwater level and GNSS vertical motions show a clear annual periodicity, with low groundwater levels in winter during the peak GNSS annual uplift. The seismicity rate often reaches its peak in winter and spring around the time of low groundwater level and peak uplift. Peak earthquake occurrences occasionally appear in summer and autumn (for instance, in 2004 and 2009). Overall, the study finds a pronounced correlation between annual seismicity rate and the hydrological water cycle in this region. The strong correlation implies that, at any given time, a large population of faults is critically stressed such that the small stress perturbations from the water cycle can lead to triggering of small events. In this way, the seasonal hydrological cycle may modulate the release of seismic energy for small-magnitude earthquakes. However, a higher background seismicity rate in winter may also increase the probability of higher-magnitude ruptures on large fault systems.

The paper is available at https://advances.sciencemag.org/content/7/16/eabf7282

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