Anomalous increase of chlorophyll concentrations associated with earthquakes

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1 Advances in Space Research 37 (2006) Anomalous increase of chlorophyll concentrations associated with earthquakes Ramesh P. Singh a,b, *, Sagnik Dey a, Sanjeeb Bhoi b, Donglian Sun b, Guido Cervone b, Menas Kafatos b a Department of Civil Engineering, Indian Institute of Technology, Kanpur , India b School of Computational Sciences, George Mason University, 4400 University Drive MS 5C3, Fairfax, VA 22030, USA Received 28 September 2004; received in revised form 11 July 2005; accepted 16 July 2005 Abstract Recent studies have shown land, ocean, atmosphere and ionospheric anomalies prior to earthquakes. The optical and microwave sensors onboard satellites are now capable of monitoring land, ocean, atmosphere and ionosphere which provide changes associated with natural hazards. In this paper, we have analyzed remote sensing data of the ocean coasts lying near the epicenters of recent four major earthquakes (Gujarat of January 26, 2001, Andaman of September 13, 2002, Algeria of May 21, 2002 and Bam, Iran earthquake of December 26, 2003), our detailed analysis shows increase of Chlorophyll-a (Chl-a) concentration associated with these recent earthquakes. The increase of Chl-a concentration is due to the change in sea surface temperature (SST) associated with the changes in stress regime in the epicentral region which is responsible for modifying the in situ thermal structure of the water and enhancing the upwelling of nutrient-rich water. The increase of Chl-a concentration also shows one to one relation with the increase of surface latent heat flux (SLHF) which is found to increase significantly prior to the earthquake events. Due to cloud cover, it has not been possible to quantify the effect of the chlorophyll concentrations associated with the earthquake events for each successive day during an event. However, the limited data from the adjacent oceanic regions provide strong evidence of the increase in Chl-a concentration. The monitoring of chlorophyll concentrations with higher spatial and temporal resolutions may provide early information about impending coastal earthquakes. Ó 2005 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Earthquake; Chlorophyll; Remote sensing; Early warning; Upwelling 1. Introduction Ocean optics and physical processes are fundamentally linked through the radiative heating component of the solar radiation absorbed by the ocean water primarily due to the phytoplankton pigments (Bissett et al., 2001). Chlorophyll-a (Chl-a) is the indicator of the primary productivity of phytoplankton biomass in * Corresponding author. Tel.: ; fax: addresses: rsingh3@gmu.edu, ramesh@iitk.ac.in (Ramesh P. Singh). the ocean, which can be monitored from space accurately by ocean color sensors (Sathyendranath et al., 1991; Yoder et al., 1993; Tang et al., 2002; Dey and Singh, 2003a). Pigment retrievals from ocean color sensors in case 1 water, where optical properties are dominated by Chl-a and associated detrital colored dissolved organic matter (CDOM) have achieved reasonable results (Carder et al., 1999). The spatial and temporal distributions of the Chl-a in the ocean are intimately linked with the sea surface temperature (Sathyendranath et al., 1991) and the vertical and horizontal mixing within the mixed layer (Nakamoto et al., 2000) in the ocean induced by the physical forcing (Kumar et al., 2000) /$30 Ó 2005 COSPAR. Published by Elsevier Ltd. All rights reserved. doi: /j.asr

2 672 R.P. Singh et al. / Advances in Space Research 37 (2006) Spatial and temporal variability of the Chl-a may occur due to the large scale disturbances in the oceanic circulation patterns, arising from sudden changes of thermal structure (Chaturvedi and Narain, 2003) of the water. The thermal structure of the ocean is maintained by the numerous controlling parameters, including sea surface temperature (SST), incoming solar radiation, precipitation and winds, which modulates the SLHF of the ocean (Bissett et al., 2001; Wang et al., 2002). The sudden change in one of parameters will result in changing the thermal structure of the ocean water, which will induce upwelling and ocean water mixing. In the present paper, we have investigated impact of earthquakes on the Chl-a concentration in ocean water. Initial stage of stress increase in the focal region of earthquakes gives rise to the strong coupling between the land ocean atmosphere as a result anomalous behavior in the land, ocean, atmospheric and ionospheric parameters are being observed prior to the earthquakes (Dey and Singh, 2003b; Dey et al., 2004; Ouzounov and Freund, 2004). We have carried out analysis of ocean parameters of about 10 coastal earthquakes, here we present the analysis of chlorophyll concentrations of four major coastal earthquakes (details are shown in Table 1 and the locations are shown in Fig. 1) occurred in the last 5 years. The four earthquakes chosen for the present study differ in proximity of the epicenters to the ocean, tectonic settings and bathymetry. The epicenters of the Andaman and Algeria earthquakes are located on the coastlines, whereas the epicenters of Gujarat and Iran earthquakes are near the coasts. The ocean near the epicenters of Gujarat and Iran earthquakes is very shallow compared to those near the Andaman and Algeria earthquakes (Fig. 1). The regional tectonics of these earthquakes are discussed in detail by many (Negishi et al., 2002; Rajendran et al., 2003; Eshghi and Zare, 2003; Semmane et al., 2005). Earlier Singh et al. (2002) have shown an increase in Chl-a concentration immediately after the Gujarat earthquake, but due to non-availability of continuous data it was not assured if the anomalous increase was associated with earthquake or a regional phenomenon. In this paper, we have looked for the possible reasons, on the basis of the associated oceanic parameters. The detailed analysis of Chl-a concentration along the Gujarat coasts near the epicenter of the Gujarat earthquake show increase prior to the Gujarat earthquake, similar results were also found to be associated with three other recent coastal earthquakes. 2. Data and methodology For Chl-a concentration and SST, Moderate Resolution Imaging Spectroradiometer (MODIS) level 3 products have been used in the present study. The algorithm is valid for case 2 coastal water. Case 2 Chl-a algorithm Table 1 Details of the earthquakes (source: No. Place Date Longitude Latitude Magnitude Focal depth (km) 1 Gujarat, India January 26, E 23.4N Andaman, India September 13, E 13.04N Algeria May 21, E 36.96N Bam, Iran December 26, E 28.99N Fig. 1. Locations of the epicenters of the four earthquakes shown in a bathymetry map of world.

3 R.P. Singh et al. / Advances in Space Research 37 (2006) is based on a semi-analytical bio-optical remote sensing algorithm and is calculated by dividing the water leaving radiance by the downwelling irradiance. The use of a semi-analytic algorithm reduces the errors from 40% to 20% when compared to the estimates using an empirical algorithm (Carder et al., 2004). We have used MODIS version 4 data for the present study. We have also analyzed surface latent heat flux (SLHF) data available from the National Center for Environmental Prediction (NCEP) to study the characteristics of SLHF prior and after the earthquake. The SLHF data is downloaded from the Scientific Computing Division of the National Center for Atmospheric Research (NCAR) ( Fig. 2a. Time series of images of Chl-a concentration retrieved from MODIS in the Arabian Sea during the Gujarat earthquake of January 26, 2001.

4 674 R.P. Singh et al. / Advances in Space Research 37 (2006) Fig. 2a (continued). SOURCES/NOAA/NCEP-NCAR/). The data set is projected into 2 -latitude by 2 -longitude grid. The reanalysis and upgrading of the database at NCEP have been discussed by Kalnay et al. (1996). The SLHF anomaly has been derived by applying wavelet transformations (Cervone et al., 2004) on the daily SLHF data. The Ekman transport can be taken as a quantitative characteristic for the upwelling intensity, the integral Ekman transport in the surface layer U can be computed using following equation U ¼ s y q 0 f ; ð1þ

5 R.P. Singh et al. / Advances in Space Research 37 (2006) Fig. 2b. Daily averaged upwelling index for Gujarat earthquake showing maximum rise three days prior to the main event. where s y is the coastline parallel wind stress on the water surface, q 0 is the density of water and is equal to 1000 kg/m 3, f is the coriolis parameter and f =2Xsinh, X is the angular velocity of the earth rotation ( ), and h is the latitude. U is actually the offshore Ekman transport, which is additionally modified by local parameters, such as coastline, bottom topography, and stratification. Wind stress s = kq a W 2, where q a is the density of air, k is the empirical drag coefficient and is a function of wind velocity W (Trenberth et al., 1990). The spatial distributions of Chl-a concentration near the epicenters of these earthquakes are analyzed for onemonth period together with the variations of SST and SLHF. The upwelling index has been averaged over the surrounding oceanic regions to understand the blooming of Chl-a. 3. Results and discussion Chl-a concentration and SST in the Gulf of Kutchchh and Cambay (Fig. 2a) near the epicenter and the entire Arabian Sea during January 1 31, 2001 are shown in Figs. 2a and 2c, respectively. In general, during the northeast monsoon, the northeastern part of the Arabian Sea has been found to be very productive (Tang et al., 2002; Dey and Singh, 2003a) due to strong upwelling of nutrient-rich water. The Chl-a concentration in the Gulfs during January has been found consistently greater than 2 mg m 3 and is higher than the corresponding open ocean values. Chl-a concentration is found to be more than 5 mg m 3 on 4 occasions, on January 8, 12, and The daily averaged upwelling index in this region (Fig. 2b) shows 4 peaks, on January 7, 11, 17 and 24, one day prior to the each bloom event confirming that Chl-a concentration increases due to upwelling of nutrient-rich water. The upwelling in the Arabian Sea shows enhanced productivity during winter season due to faster convection in high saline Arabian Sea water (Kumar et al., 2001). The upwelling is enhanced just prior to the earthquake, when it reaches its maximum value. Similarly, highest Chl-a concentration in region during the same time period are shown in Fig. 2a. SST higher than 25 C has been found in most parts of the Arabian Sea except the Gulf of Kutchchh and Cambay. In these Gulfs, SST increases by 2 C on January 25, one day prior to the earthquake. The sudden increase in SST is attributed to the increased SLHF, which has been observed to show anomalous increase three days prior to the main event on January 26, 2001 and found to continue until the next day (Fig. 2d). Similar response of Chl-a concentration has been found to be associated with the Andaman earthquake, where Chl-a concentration increases by 2 3 mg/m 3 on September 14 as compared to September 6, 2002 (figure not shown here) around the Andaman and Nicobar Islands. Due to thick cloud covers in the region it was not possible to get continuous cloud free satellite data to ascertain the start of blooming of Chl-a during the week of the earthquake. Increase in Chl-a concentration has also been found to be associated with the Algeria and the Iran earthquakes. Instead of showing continuous Chl-a concentration images for the these earthquakes, we show the variations of upwelling index during the month in which these three earthquakes have

6 676 R.P. Singh et al. / Advances in Space Research 37 (2006) occurred (Fig. 3). In each case, the maximum upwelling has been found few days prior to the main events. Only for the Andaman earthquake, another high upwelling has been observed on September 21, 2002 (Fig. 3a). All four earthquakes were found to be accompanied by series of aftershocks, the effect of the aftershocks on the upwelling is observed to be the strongest in case of the Andaman earthquake. The epicenter of the Andaman earthquake is lying in the open ocean, whereas for the other three, the epicenters are lying on the land, although they are close to the ocean. As the SLHF of the ocean is larger than that of the land, the magnitude of the increase in SST is highest for the Andaman earthquake. Fig. 2c. Time series of images of SST retrieved from MODIS in the Arabian Sea during the Gujarat earthquake of January 26, 2001.

7 R.P. Singh et al. / Advances in Space Research 37 (2006) Fig. 2c (continued). With the building of stress in the focal regime of earthquakes prior to the earthquake event, it is now confirmed that the SST increases prior to the earthquake (Gujarat earthquake, ames/images). It is likely that with the increase of SST of the land, the coastal upwelling may enhance which

8 678 R.P. Singh et al. / Advances in Space Research 37 (2006) Fig. 2d. Spatio-temporal variation in SLHF prior to the main event of Gujarat earthquake. may be responsible to bring the nutrients near the ocean surface. The efficiency of the upwelling will be maximum where the difference in temperature is maximum. The overall impact of the released thermal energy associated with an earthquake also depends on the distance of its epicenter from the ocean. This is because the transport of the heat energy to the surface is more efficient in the oceans compared to land, as the average continental crustal thickness (33 km) is much more compared to an average oceanic crustal thickness (8 km). Ouzounov and Freund (2004) have found increase in thermal infrared emission from the rock mass prior to an earthquake. Release of thermal energy prior to the earthquake is reflected in terms of increase in SLHF and SST of the region. Dey and Singh (2003b), Cervone et al. (2004) and Singh et al. (2004) have recently found consistent SLHF anomaly prior to coastal earthquakes, which can be used as precursory signals. SLHF has been found to migrate from the high-stress regions to lowstress zones prior to the main event and eventually appears in the low-stress zones on the surface. Tectonically, the gulf areas in the south and west of the epicenter of Gujarat earthquake are the low-stress zones, where maximum SLHF anomaly has been observed. Despite of the difference in bathymetry and tectonics, the pre-earthquake thermal energy affects the thermal stratification of the ocean water, but the magnitude of its effect depends on several factors, such as, distance of the epicenter from the ocean, magnitude of the impending earthquake (directly proportional to the amount of energy released) and the focal depth. Maximum upwelling has been found 9 days prior to the Iran earthquake, 6 days prior to the Algeria earthquake, 3 days prior to the Gujarat earthquake and 5 days prior to the Andaman earthquake, whereas the shallow focal depth is observed for the Iran earthquake, followed by the Algeria, the Gujarat and the Andaman earthquakes. This relation indicates that shallower is the focal depth, more advance is the upwelling with respect to the main event, considering the distance of the epicenter from ocean is almost similar. As the epicenter of the Andaman earthquake is closer to ocean compared to the Gujarat earthquake, the maximum upwelling occurs 2 days faster. On the basis of all the observed changes in the oceanic parameters linked sequentially in response to an earthquake, we propose a hypothesis to explain the phenomenon. The thermal energy released prior to the earthquake is transported to the surface altering the ambient thermal structure of the water. This results in increase in SST and SLHF and enhances upwelling, which in turns brings nutrient-rich water near the ocean surface giving rise to the Chl-a blooming. During non-productive period, an impending earthquake will cause primary production, but the increase in Chl-a concentration will be spatially limited. However, during the productive period, such as during the winter monsoon in the North Arabian Sea, the earthquake will enhance the normal production. The whole sequence will begin much earlier and occur much faster in case of higher magnitude of earthquake and hence it may give valuable information about an impending earthquake. 4. Conclusions Increase in Chl-a concentration associated with the coastal earthquakes is attributed to the upwelling of nutrient-rich water due to the redistribution of thermal structure of ocean water. Anomalous increase in SLHF simultaneously associated with the increase in SST are found due to the build up of stress in the epicentral regime of an earthquake. The most dominant controlling factors of the overall impact of the earthquakes in this process are the distance of epicenter from the ocean and focal depth. Also the zones of the Chl-a bloom are the low-stress zones, whereas the epicenter lies in

9 R.P. Singh et al. / Advances in Space Research 37 (2006) Fig. 3. Daily averaged upwelling index for: (a) Andaman; (b) Algeria; (c) Iran earthquakes. the corresponding high-stress zones in that particular tectonic regime. Detailed analysis of Chl-a concentration depending upon the cloud free data from the future earthquakes are required to ascertain the time lag between the blooming of Chl-a with an impending earthquake.

10 680 R.P. Singh et al. / Advances in Space Research 37 (2006) Acknowledgements This research was partially supported by NASAÕs office of Earth Science Enterprise under grant NAG , NAG and NAG , VAccess/ MAGIC projects. We are grateful to the two anonymous reviewers of their useful comments and suggestions which helped us to improve the original version of the manuscript. References Bissett, W.P., Schofield, O., Glenn, S., Cullen, J.J., Miller, W.L., Plueddmann, A.J. Resolving the impacts and feedback of ocean optics on upper ocean ecology. Oceanography 14, 30 53, Carder, K.L., Chen, F.R., Lee, Z.P., Hawes, S.K., Kamykowski, D. Semi-analytic moderate-resolution imaging spectrometer algorithms for chlorophyll a and absorption with bio-optical domains based on nitrate depletion temperatures. Journal of Geophysical Research 104, , Carder, K.L., Chen, F.R., Cannizzaro, J.P., Campbell, J.W., Mitchell, B.G. Performance of MODIS semi-analytic ocean color algorithm for chlorophyll-a. 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