Near-field TEC response to the main shock of the 2008 Wenchuan earthquake
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2010
Received: 25 September 2008
Accepted: 3 July 2009
Published: 26 January 2011
We have registered near-field TEC response to the Wenchuan earthquake on 12 May 2008, for the first time. We found that an intensive N-shape shock-acoustic wave with a plane waveform and with half-period of about 200 sec propagated south-eastward with a velocity of 600 m/s for distance about 1000 km. The wavefront of N-shaped disturbance was parallel with the earthquake rupture direction (from SW to NE). The main directional lobe of shock-acoustic wave emitter is directed southeastward, i.e. transversely to the rupture. We suppose that the above properties of TEC response are determined by the geodynamics of the Wenchuan earthquake.
The method of ionosphere monitoring by GPS is widely used for detecting and studying ionospheric response to large earthquakes (EQ). Calais and Minster (1995) first observed anomalous signal in the time series of total electron content (TEC) in the period range 3–10 min after an earthquake that occurred in California, USA, on 17 January 1994. Frequency and propagation phase velocity (300–600 m/s) agreed with the results of numerical simulation for atmospheric gravity waves (AGW) caused by a rapid uplift and subsidence of the Earth’ surface during the earthquake. The velocity was determined as a ratio of distance from an earthquake’s epicenter to Sub-Ionospheric Point (SIP) to the time delay of perturbation registration in the corresponding SIP.
Afraimovich et al. (2001a) proposed a simple interferometric method for determining the angular characteristics of the wave vector and phase velocity of N-shape shock-acoustic waves (SAW), generated during an earthquake. The authors used GPS-arrays, consisted of three sites, the distances between which do not exceed about one-half the wavelength of the perturbation (D1 method). This method provided an estimation of SAW parameters without a priori information about the site and time of a EQ main shock. But D1 method doesn’t allow us to determine the form of phase front of SAW (plane or the spherical wavefront).
Afraimovich (2000) first suggested the concept of a new technology for detection of ionospheric disturbances using GPS arrays. According to this concept the TEC measurements along GPS line-of-sight (LOS) were considered as elements of nonequidistant phased antenna arrays of the acoustic detectors located at the ionospheric heights.
On the basis of this concept the different methods of space-time processing of TEC series were developed, including quasi-optimum algorithm, QOA (Afraimovich et al.., 2002, 2006; Kiryushkin and Afraimovich, 2007). These algorithms realized the coherent summation of the TEC series accounting for space-time parameters of disturbance. In order to determine parameters of ionospheric perturbation the parameters values were varied. For each combination of the estimated parameters the normalized criterion function C for the coherent sum of all TEC series and the reference signal were calculated. The largest maximum value Cmax corresponds to the best-fit perturbation parameters.
In this paper we used our D1 and QOA methods to study the near-field TEC response to very strong and disastrous Wenchuan earthquake which occurred on 12 May 2008.
2. General Information about the Earthquake
The Wenchuan earthquake of 12 May 2008 (magnitude 7.9) occurred at 06:28:01 UT as the result of rupture on a northeast striking reverse fault in the northwestern margin of the Sichuan Basin, China. The earthquake was the result of release of tectonic stresses accumulated by the convergence of slowly moving high Tibetan Plateau, to the west, against the Sichuan Basin and southeastern China (http://earthquake.usgs.gov).
The rupture started at the epicenter (30.986N, 103.364E) and over the next 50 sec travelled about 200 km toward the northeast, tearing apart the land along the front of the mountain range. The slip due to the earthquake in some places amounted as large as 9 meters (Parsons et al., 2008). The maximum vertical slippage occurred about 60 km northeast of the epicenter (Parsons et al., 2008).
Motion along the southern edge of the fault was predominantly dip slip (i.e., land on one side of the fault moving under land on the other side), leading to an increment of uplift of the mountain range and subsidence of the Sichuan basin.
Unfortunately IGS GPS sites in the near-field the source region of the Wenchuan earthquake are relatively sparse. Therefore we used GPS data at 27 GPS stations, which are belonging to the Crustal Movement Observation Network of China. Locations of all used GPS sites are marked in Fig. 1 by solid triangles.
The level of geomagnetic disturbances was quiet during the Wenchuan earthquake; the Kp index varied from 0 to 1 on 12 May 2008 (http://www.wdc.rl.ac.uk/cgibin/wdccl/secure/wdcdata).
3. GPS Data Processing and Results
Out of a set of GPS sites, first, three sites (A, B, C) are selected for D1 method. Site B is taken to be the center of a topocentric reference frame whose axis x is directed east, and the axis y is directed north. The receivers in this reference frame have the coordinates (xA, yA), (0, 0), (xC, yC). Location of the SIPs for selected GPS sites and GPS satellite numbers LUZH-14, LUZH-22 and XIAG-05 at the moment of maximum TEC deviation is shown in Fig. 1 by thick dots. In Fig. 1 we also marked trajectories of other subionospheric points located in the northwest region (DLHA-05) and northeast region (XIAA-12) from the epicenter.
The input data include series of slant TEC values IA(t), IB(t), IC(t), as well as corresponding series of elevation values θs(t) and the azimuth αs(t) of the LOS. To eliminate spatiotemporal variations of the regular ionosphere, as well as trends introduced by orbital motion of the satellite, a procedure is used to remove the trend with the selected time window (10 min) involving a preliminary smoothing of the initial series (the time window 2 min). Elevation θs(t) and azimuth αs(t) of the LOS are used to determine the location of the SIP. We use the transformation by Klobuchar (1986) for calculating of ‘vertical’ value TEC I(t).
The azimuth α of the wave vector shown in Fig. 1 by a thick gray arrow equals to 160°. One can see that the phase front of N-shaped disturbance is parallel with the rupture direction (from SW to NE). The phase velocity was found to be 583 m/s.
In the QAA method we firstly carried out the spatiotemporal processing of all TEC series for all 27 GPS stations within the approximation of a spherical wave front. We fixed the coordinates of the point source within the spherical wave model in the epicenter of earthquake. Then we find the propagation velocity of the ionospheric perturbation from the maximum value of the criterion parameter C.
We further used the model of a plane front of a shock acoustic wave. We carried out the simultaneous estimation of the arrival direction and the propagation velocity of the ionospheric perturbation from the maximum of the criterion parameter C. For the plane wave front we form the GPS-array direction diagram and direct it in the phase space [α, V], when assigning values of the arrival direction α and propagation velocity V and carrying out the coherent accumulation of TEC series dI(t).
The process of searching for the best C is shown in Fig. 3(b) in the form of a 2D criterion function C(α, V). The maximum value C(α, V)max of about 0.8 (marked by cross) corresponds to the values of velocity V = 560 m/s and azimuth α = 160° of the wave front propagation. It should be noted that these values coincide exactly with the ones obtained using the D1 method. Thin horizontal line marks the cross section of C(α, V) corresponding to the dependence C(V) for the plane waveform shown in Fig. 3(a) by solid line. The great value of criterion parameter illustrates the high degree of conformity of the model of the plane wave front and experimentally observed ionospheric perturbation.
The accumulated series dIΣ(t) for the experiment and the plane waveform model are presented in black line and gray line in Fig. 2(c), respectively. We can see full correspondence between the experiment and the model.
The distribution of amplitude dI(t)max on distance D from epicenter along the great circle arc (Fig. 4(b)) was constructed only for the SIPs located in southeast sector where the amplitude of responses is maximum. One can see that the amplitude dI(t)max decreases with distance D very quickly: in 5 times at distance 700–1000 km.
4. Discussion and Conclusion
It is very interesting to compare the obtained characteristics of the TEC response to the Wenchuan earthquake with similar parameters for other strong earthquakes (Afraimovich et al., 2001a, 2006; Ducic et al., 2003; Heki et al., 2006; Otsuka et al., 2006; Kiryushkin and Afraimovich, 2007). General information about these earthquakes is presented in left part of Table 1 (including the time of the main shock in the universal time UT, the position of the earthquake epicenter, depth, the magnitude, as well as the level of geomagnetic disturbance from the data on Dst-variations). It was found that the deviation of Dst for the selected days was quite moderate, which enabled the SAWs to be identified.
General information about TEC response parameters is presented in the right part of Table 1 (including the T—SAW period; A—the maximum amplitude of absolute dI, [TECU], and relative dI/I0, [%], TEC deviation for selected LOS; the kind of wave front (plane or spherical); the horizontal component of the phase velocity V; the azimuth α of the SAW vector; the altitude He of a secondary source of the wave perturbation registered by GPS-array, for the spherical wave front approximation). The second column shows also the references. The relative amplitude dI/I0 is determined by normalization of the dI to the background value I0, where I0 is the absolute vertical TEC obtained with two-hour time resolution from the global TEC maps in the IONEX format (so called Global Ionospheric Maps, GIM (ftp://cddisa.gsfc.nasa.gov/pub/gps/products/ionex/). The absolute dI and relative dI/I0 amplitude of TEC deviation increase with the increasing of earthquake’s magnitude.
Afraimovich et al. (2001b) compared the TEC disturbances from the shock-acoustic waves generated during earthquakes (in Turkey on August 17 and November 12, 1999, in Southern Sumatra on June 4, 2000 and off the coast of Central America on January 13, 2001), rocket launchings and explosions (New Mexico power explosion on June 6, 1993), using our GPS-arrays method (Afraimovich et al., 2001a) for determining SAW parameters (including angular characteristics of the wave vector, and the SAW phase velocity). It was shown that, whatever the type of source, the impulsive TEC disturbance has the character of a classical N-wave with a period of 180–360 s, and with its amplitude exceeding the level of background fluctuations under moderate geomagnetic conditions by a factor of 2–5 as a minimum. The phase velocity (600–1200 m/s) approaches the sound velocity at the height of the ionospheric F-region maximum. So, the parameters of ionospheric response to impulsive impact are determined not only by the source characteristics but substantially by the properties of ionospheric channel of the shock-acoustic waves propagation.
General information about strong earthquakes and TEC response parameters.
In order to estimate a degree of influence of Earth magnetic field on the directional properties of the ionospheric perturbation, generated during the 2008 Wenchuan earthquake, we determined a direction of the magnetic lines near to the earthquake epicenter using the IGRF model (Maus and Macmillan, 2005). It was found that at the height of 300 km the declination of the magnetic field vector was equal to −2.11°, and the inclination was −47.2°.We showed the projection of the magnetic field vector B to Earth’s surface by dotted grey arrow in Fig. 1. The analysis of the obtained data allows us to conclude that in this experiment the geomagnetic field did not influence on the directional properties of the ionospheric perturbation predominantly. Otherwise the greatest intensity of the ionospheric response would be observed in the northern and northwestern sectors from epicenter, i.e. where the direction of the propagation of the ionospheric perturbation coincided with the direction of the magnetic field lines.
So, using GPS TEC measurements from the Crustal Movement Observation Network of China, for the first time, we have registered near-field TEC response to the Wenchuan earthquake on 12 May 2008. We found that an intensive N-shape shock-acoustic wave with a plane waveform and with half-period of about 200 sec propagated south-eastward with a velocity of 600 m/s for distance about 1000 km. The wave front of N-shaped disturbance was parallel with the earthquake rupture direction (from SW to NE). The main directional lobe of shock-acoustic wave emitter is directed southeastward, i.e. transversely to the rupture.
We suppose that the above properties of TEC response are determined by the geodynamics of the Wenchuan earthquake. Detailed investigation of that phenomenon and corresponding modelling are the aims of our future works.
The work was supported by the Interdisciplinary integral project of SB RAS N 56 “Seismoionospheric and seismoelectromagnetic processes in Baikal Rift Zone”, the RFBR-GFEN grant N 06-05-39026 and RFBR grant 07-05-00127; by the Japanese Society for the Promotion of Science (JSPS); by the National Natural Science Foundation of China (grants 40774090 and 40636032) and the National Important basic Research Project (2006CB806306).
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