Near-field strain observations of the October 2013 Ruisui, Taiwan, earthquake: source parameters and limits of very short-term strain detection

Volumetric strain changes associated with the October 2013 M-w 6.2 Ruisui earthquake were recorded by a network made up with four borehole Sacks-Evertson dilatometers in eastern Taiwan. These instruments are located within 25-30 km of the seismic source providing also high-resolution near-field observations. Co-seismic offsets larger than a few 10(2) n epsilon were seen by most of the sensors. We relocated the 30 km x 30 km fault plane through a grid-search approach. The inferred fault parameters (217 degrees, 48 degrees, 49 degrees) are in reasonable agreement with those resulting from the inversions of long-period seismic waves (209 degrees, 59 degrees, 50 degrees) as well as from GPS data inversion (200 degrees, 45 degrees, 42 degrees). Moreover, analysis of the 100-Hz sampling data 10 s before seismic radiations indicate no pre-seismic strain change emergent from the instrumental noise level (from 10(-2) to 10(-1) n epsilon). Such an observation sets limits on any precursory change in a nucleation area, taken to have dimensions of about 250-300 m, seconds before the mainshock. Thus, the upper limit of any pre-seismic moment is about 10(-5) % of the total seismic moment of the Ruisui earthquake.