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 were seen by most of the sensors. We relocated the 30 km × 30 km fault plane through a grid-search approach. The inferred fault parameters (217°, 48°, 49°) are in reasonable agreement with those resulting from the inversions of long-period seismic waves (209°, 59°, 50°) as well as from GPS data inversion (200°, 45°, 42°). 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 ). 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.
Geodetic instruments now offer compelling sensitivity, allowing to investigate how solid Earth and surface processes interact. By combining surface air pressure data, nontidal sea level variations model, and rainfall data, we systematically analyze the volumetric deformation of the shallow crust at seven borehole strainmeters in Taiwan induced by 31 tropical cyclones (typhoons) that made landfall to the island from 2004 to 2013. The typhoon's signature consists in a ground dilatation due to air pressure drop, generally followed by a larger ground compression. We show that this compression phase can be mostly explained by the mass loading of rainwater that falls on the ground and concentrates in the valleys towards the strainmeter sensitivity zone. Further, our analysis shows that borehole strainmeters can help quantifying the amount of rainwater accumulating and flowing over a watershed during heavy rainfalls, which is a useful constraint for building hydrological models.
We analyze the high-resolution dilatation data for the October 2013 M w 6.2 Ruisui, Taiwan, earthquake, which occurred at a distance of 15-20 km away from a SacksEvertson dilatometer network. Based on well-constrained source parameters (strike = 217 • , dip = 48 • , rake = 49 • ), we propose a simple rupture model that explains the permanent static deformation and the dynamic vibrations at short period (∼3.5-4.5 s) for most of the four sites with less than 20 % of discrepancies. This study represents a first attempt of modeling simultaneously the dynamic and static crustal strain using dilatation data. The results illustrate the potential for strain recordings of high-frequency seismic waves in the near-field of an earthquake to add constraints on the properties of seismic sources.
We report evidence for frictional afterslip at shallow depths (about 5 to 7 km) during a small‐magnitude seismic sequence (with ML<5) along the Chihshang Fault, a main active structure of the Longitudinal Valley, in southeast Taiwan. The afterslip, which was recorded by a nearby borehole dilatometer, lasted about a month with a cumulative geodetic moment magnitude of 4.8 ± 0.2. The afterslip comprised two stages and controlled the aftershock sequence. The first postseismic stage, which followed a ML 4.6 earthquake, lasted about 6 h and mostly controlled the ruptures of neighboring asperities (e.g., multiplets) near the hypocenter. Then, a 4 week duration large afterslip event following a ML 4.9 earthquake controlled the rate of aftershocks during its first 2 days through brittle creep. The study presents a rare case of simultaneous seismological and geodetic observations for afterslip following earthquakes with magnitude lower than 5. Furthermore, the geodetic moment of the postseismic phase is at least equivalent to the coseismic moment of the sequence.
We report the first evidence for the detection of a slow slip event in the Longitudinal Valley, in eastern Taiwan. The slow event, which lasted about 3.5 days, has been detected by borehole strainmeters. It occurred at shallow depths (about 2 to 4 km), either on the Longitudinal Valley Fault or on the Central Range Fault. Here we investigate whether the event occurrence was influenced by transient and periodic stress perturbations, in particular by the June 2013 Mw 6.2 Nantou earthquake, which occurred about 60 km away and 6 days prior to the event. Modeled changes in Coulomb stress in the direction parallel to the geologic slip vector on the fault planes show negative static stress changes (approximately −1.5 to −1 kPa), while maximum dynamic stress changes generated by the surface waves are ranging from 5.5 to 14.5 kPa. We also observe that the slow event initiated during a maximum of Earth and ocean tidal Coulomb stress changes (about 0.8 to 1.5 kPa). Dynamic and static stress perturbations represent a few percents to tens of percents of the stress buildup through the slow rupture cycle. However, the absence of recurrent events during the 12 years of strain monitoring (2006 to 2018) prevents to estimate the recurrence interval of the slow event, which limits our ability to further interpret the link between the rupture and the perturbations. Finally, there is no large and unique load transient at the time of the initiation, therefore this single event may have likely occurred spontaneously.
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