C.-H. Chen

Ionospheric disturbances triggered by the 11 March 2011 M9.0 Tohoku earthquake

[1]xa0An earthquake of magnitude 9.0 occurred near the east coast of Honshu (Tohoku area), Japan, producing overwhelming Earth surface motions and inducing devastating tsunamis, which then traveled into the ionosphere and significantly disturbed the electron density within it (hereafter referred to as seismotraveling ionospheric disturbances (STIDs)). The total electron content (TEC) derived from nationwide GPS receiving networks in Japan and Taiwan is employed to monitor STIDs triggered by seismic and tsunami waves of the Tohoku earthquake. The STIDs first appear as a disk-shaped TEC increase about 7 min after the earthquake occurrence centered at about 200 km east of the epicenter, near the west edge of the Japan Trench. Fast propagating disturbances related to Rayleigh waves quickly travel away from the epicenter along the main island of Japan with a speed of 2.3–3.3 km/s, accompanied by sequences of concentric circular TEC wavefronts and followed by circular ripples (close to a tsunami speed of about 720–800 km/h) that travel away from the STID center. These are the most remarkable STIDs ever observed where signatures of Rayleigh waves, tsunami waves, etc., simultaneously appear in the ionosphere.

The vertical propagation of disturbances triggered by seismic waves of the 11 March 2011 M9.0 Tohoku earthquake over Taiwan

In this paper, concurrent/colocated measurements of seismometers, infrasonic systems, magnetometers, HF-CW (high frequency-continuous wave) Doppler sounding systems, and GPS receivers are employed to detect disturbances triggered by seismic waves of the 11 March 2011u2009M9.0 Tohoku earthquake. No time delay between colocated infrasonic (i.e., super long acoustic) waves and seismic waves indicates that the triggered acoustic and/or gravity waves in the atmosphere (or seismo-traveling atmospheric disturbances, STADs) near the Earths surface can be immediately activated by vertical ground motions. The circle method is used to find the origin and compute the observed horizontal traveling speed of the triggered infrasonic waves. The speed of about 3.3u2009km/s computed from the arrival time versus the epicentral distance suggests that the infrasonic waves (i.e., STADs) are mainly induced by the Rayleigh waves. The agreements in the travel time at various heights between the observation and theoretical calculation suggest that the STADs triggered by the vertical motion of ground surface caused by the Tohoku earthquake traveled vertically from the ground to the ionosphere with speed of the sound in the atmosphere over Taiwan.

A Statistical Study of Lightning Activities and M ≥ 5.0 Earthquakes in Taiwan During 1993–2004

AbstractnIn this study, to see whether or not lightning activities are related to earthquakes, we statistically examine lightning activities 30xa0days before and after 78 land and 230 sea Mxa0≥xa05.0 earthquakes in Taiwan during the 12-year period of 1993–2004. Lightning activities versus the location, depth, and magnitude of earthquakes are investigated. Results show that lightning activities tend to appear around the forthcoming epicenter and are significantly enhanced a few, especially 17–19, days before the Mxa0≥xa06.0 shallow (depth Dxa0≤xa020xa0km) land earthquakes. Moreover, the size of the area around the epicenter with the statistical significance of lightning activity enhancement is proportional to the earthquake magnitude.

Typhoon-induced magnetic disturbance: Cases in the western Pacific

Three typhoons occurred on the western Pacific in September 2008 and were enhanced beyond category 3 when they approached Taiwan. Geomagnetic total intensity field recorded at 2 local monitor stations in Taiwan and 1 remote station in Japan are utilized to examine magnetic disturbances induced by these typhoons. Analytical results show that changes in the amplitudes in the frequency domain at the monitor and remote stations are consistent even though magnetic storms intensely affect magnetic fields. However, obvious discrepancy can be repeatedly found from the amplitude at the frequency band between 0.0025 and 0.007 Hz, when the typhoons with category > 3 were the closest to the monitor stations. The frequency band is different with those of induction fields from either oceanic stormy waves or swells, and is consistent with that of magnetic pulsations triggered by acoustic waves from upward air motion during typhoons.

Potential relationship between seismo-crustal displacement and EM anomalies

Summary form only given. Seismo-crustal displacements, which are derived from surface displacement data recorded by ground-based GPS stations through a band-pass filter to remove effects of the long-term plate movements, frequency-dependent variations and noise via the Hilbert-Huang transform, can be related with fault parameters and utilized to expose earthquakes in preparation periods that have been reported. Seismo-crustal displacements can be classified into three distinct characteristics from earthquakes in Taiwan and Japan. The first type shows that a region with relatively low-pressure surrounded by areas with relatively high-pressure can be formed in earthquake preparation periods. The second type indicates that earthquake-related stress accumulation is resisted in a region, where will be an epicenter of a forthcoming earthquake. The third type reveals that the crust persistently dislocates in a region before earthquake occurrences. Seismo-EM anomalies are often related with the first type of the seismo-crustal displacements because free electric charges would be trapped within the region forming high-conductivity materials. In contrast, no significant EM anomaly can be found in the second and third types. The results shed light on exposing the causal mechanisms of the seimso-EM anomalies and/or explaining indistinct EM anomalies in earthquake preparation periods.

Simultaneous observations of ionospheric disturbances from VLF transmitter signals and surface displacements from GPS related to inland earthquakes over Japan

Ionospheric anomalies have been reported for years before major seismic activities by using VLF transmitter signals. Although possible generation mechanisms of the precursory perturbations have been proposed such as anomalous electric field and atmospheric waves originated around the future epicenter, the experimental evidence indicating the coupling between the lithospheric activities and overlaying ionosphere before seismic events has not been confirmed. In this paper we analyze the VLF transmitter amplitude data from UECs VLF observation network identifying the ionospheric perturbations together with frequency dependent filtered surface displacement data from high density GPS network for inland earthquakes in Japan during several consecutive years to study the LAIC mechanism. We found that both ionospheric perturbations and surface displacements occurred almost simultaneously about one week before for some of the shallow earthquakes indicative of coupling between the precursory ground movement and relevant ionospheric perturbations. Either ionospheric or GPS anomalies were also found for other earthquakes. We are also going to discuss the significance of types of earthquakes for LAIC mechanism.