Abstract
Abstract. We examine the association between earthquakes and Pc1 pulsations observed at a low-latitude station in Parkfield, California. The period under examination is ~7.5 years in total, from February 1999 to July 2006, and we use an automatic identification algorithm to extract information on Pc1 pulsations from the magnetometer data. These pulsations are then statistically correlated to earthquakes from the USGS NEIC catalog within a radius of 200 km around the magnetometer, and M>3.0. Results indicate that there is an enhanced occurrence probability of Pc1 pulsations ~5–15 days in advance of the earthquakes, during the daytime. We quantify the statistical significance and show that such an enhancement is unlikely to have occurred due to chance alone. We then examine the effect of declustering our earthquake catalog, and show that even though significance decreases, there is still a statistically significant daytime enhancement prior to the earthquakes. Finally, we select only daytime Pc1 pulsations as the fiducial time of our analysis, and show that earthquakes are ~3–5 times more likely to occur in the week following these pulsations, than normal. Comparing these results to other events, it is preliminarily shown that the normal earthquake probability is unaffected by geomagnetic activity, or a random event sequence.
Highlights
Non-seismic precursors to earthquakes have been reported in the literature as early as the 1950s (Kalashnikov, 1954), and take on a variety of forms
Bortnik reported precursors can occupy almost any frequency range spanning from quasi-DC to visible light (e.g. Derr, 1973; Johnston, 1989; Parrot and Johnston, 1989; Park et al, 1993), occur over a range of timescales preceding the main shock spanning from minutes (Parrot and Lefeuvre, 1985) to years (Zhao and Qian, 1994), and exhibit a range of morphologies which includes signal enhancement, attenuation, modulation, and spikes, in a bewildering variety of signals including electric, magnetic, and electromagnetic signals, resistivity changes, anomalous ionospheric variations, radon gas release, and others (e.g. Hayakawa et al, 2006, for a special issue dealing with recent progress in this area)
In this paper we examined the association between earthquakes and Pc1 pulsations observed at a low-latitude station in Parkfield, California
Summary
Non-seismic precursors to earthquakes have been reported in the literature as early as the 1950s (Kalashnikov, 1954), and take on a variety of forms. The advantage of (i) over (ii), is that a specific region can be observed continuously over a long time-period, and the natural background signal environment can be carefully assessed, and anomalous activity (potentially associated with earthquakes) more readily recognized Another advantage is that the earthquakes in a given region would, presumably, all behave in a roughly similar manner (in terms of precursors), and some sort of repeatable precursory behavior might be identified. Given the capricious nature of earthquakes, there might not ever be a large earthquake sufficiently close to the instrument to be in any way observable, or there may only be very few events if the instrument is operated for a long time This naturally forces ground-based observationalists to conduct more eventbased studies, which, while certainly suggestive, lack the assurance of statistically significant repeatability. We invert our superposed epoch analysis to calculate earthquake probability relative to Pc1 occurrence (Sect. 8), and summarize our conclusions (Sect. 9)
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