Precursory phenomena associated with the 1999 Chi-Chi earthquake in Taiwan as identified under the iSTEP program

Seismo-Ionospheric Precursors of the 26 December 2006 M 7.0 Pingtung Earthquake Doublet

“…Figs. 2 and 3 show that the NmF2 (or foF2) and GPS TEC anomalously decrease in the afternoon on day 4 (D-4) before the Pintung earthquakes which are in a good agreement with the previous observations for Taiwan (Liu et al 2000(Liu et al , 2001(Liu et al , 2004a(Liu et al , b, 2006. Liu et al (2001) observe the afternoon GPS TECs on day 3 and 4 before the M w 7.6 Chi-Chi earthquake decrease dramatically and the associated EIA crests anomaly moves southward.…”

“…6, 751-759, December 2008doi: 10.3319/TAO.2008 period abnormal fluctuations in the vertical motion of the reflection layer in the ionosphere. Here, it was found that the F2-peak electron density (or the associated plasma frequency foF2) and GPS TEC often decreased anomalously in the afternoon over a period of 1 -5 days before M 5.4 earthquakes in Taiwan (Liu et al 2000(Liu et al , 2001(Liu et al , 2004a(Liu et al , b, 2006. The TEC changes further illustrate that the decreased anomalies mainly appear around the fore coming epicenters and the TEC crests of equatorial ionization anomaly (EIA) tend to lessen and move toward the geomagnetic equator (Liu et al 2001(Liu et al , 2002.…”

“…before the actual occurrence of earthquakes. Predictably, after the 21 September 1999 M w 7.6 Chi-Chi earthquake, ionospheric phenomena associated with seismic activity in Taiwan have been comprehensively studied (Liu et al 2000(Liu et al , 2001(Liu et al , 2004a(Liu et al , b, 2006Chen et al 2004) and to investigate anomalous pre-earthquake signals in the lithosphere, atmosphere, and ionosphere, a project called 'integrated Search for Taiwan Earthquake Precursors ' (iSTEP, i = 1, 2002' (iSTEP, i = 1, /4 -2006i = 2, 2006/8 -2011 has been undertaken (Tsai et al , 2006. iSTEP consists of a main project and five inter-related sub-projects covering seismological variation, gravity and geomagnetic variation, radar interferometry for detection of surface deformation, ionospheric variation or seismo electromagnetic signals (SES), and statistical analysis of earthquake hazards.…”

Electromagnetic Anomalies around The Wenchuan Earthquake and Their Relationship with Earthquake Preparation

“…After a long-time stress-loading process, the micro fractures in rock will be closed completely and the tectonic activity reduced to the lowest. In order to reflect the underground stress state in meizoseismal area, Figure 5 presents the M-T diagram of earthquakes larger than M L 2.5 in the region of [30][31][32] • N and 103-104 • E. It shows that during 2006-2007, the earthquakes took place occasionally, but from September 2007 to January 2008, no earthquake occurred in this region. During this stage, nearly no newly born electromagnetic anomaly was detected.…”

“…Tsai et al [31] studied the precursory phenomena in lithosphere, atmosphere, and ionosphere which were associated with the 1999 Chi-Chi earthquake in Taiwan, and they obtained three-stage information such as increase of P wave travel time occurring 6 years before the event; surface deformation, bvalue and geomagnetic annual change rate 2-3 years before; electromagnetic precursors 2-3 months before and after the earthquake. Compared with their results, it is similar that precursors concentrated at two time segments, 1-2 years and a few months before the Wenchuan earthquake (Figure 3), which illustrate that electromagnetic precursory phenomena are sensitive in earthquake preparation process.…”