# Application of Soil Moisture Active Passive (SMAP) Satellite Data in Seismic Response Assessment
## 3 Results
### Moisture Differences  
The difference between the two sets of soil moisture change data, i.e., \(\Delta SM_{SMAP}\) and \(\Delta SM_{GLDAS}\), was used to identify the grids experiencing earthquake-induced soil moisture increases during seismic events. The moisture difference indicator (\(MDI\)) for each grid was expressed as follows:  
\[MDI=\Delta SM_{SMAP}-\Delta SM_{GLDAS} \tag{2}\]  
Figure 6 shows the \(MDI\) maps for all the study zones in the corresponding SMAP event window. In the Petrinja, Croatia 2020 earthquake, the \(MDI\) is positive for most grids, meaning that the soil moisture change from SMAP (\(\Delta SM_{SMAP}\)) is higher than that from GLDAS (\(\Delta SM_{GLDAS}\)). Figure 6: Difference map of SMAP-based and GLDAS-based soil moisture change for all target earthquakes. Because of the fundamental disparities between the methods used in SMAP and GLDAS, a difference between \(\Delta SM_{SMAP}\) and \(\Delta SM_{GLDAS}\) might have existed even in the absence of the earthquake. For instance, for the vegetated areas with vegetation water contents more than 5 kg/m\({}^{2}\), such as forests and wetlands, SMAP can overestimate the soil moisture content [78]. Moreover, SMAP and GLDAS estimate the soil moisture for two different soil depths. Although previous studies showed strong correlations between the soil moisture of the soil columns, whether selected shallow or deep [82], this difference between shallow and deep soil moisture is still an uncertainty. GLDAS provides soil moisture values for a deeper soil layer, and it may show lower variability (\(\Delta SM\)) in response to the atmospheric conditions [83]. Nevertheless, GLDAS helps to understand whether the SMAP increased soil moisture during the event window might be due to precipitation. To isolate the grids with a significant earthquake-induced soil moisture increase, a moisture difference threshold (MDT) was defined. In the grids for which the _MDI_ exceeds the MDT, the earthquake had a notable impact resulting in soil moisture increase. Before choosing GLDAS for this study, GLDAS and SMAP were compared for longer periods in normal conditions (i.e., without an earthquake). For example, while grids with an \(MDI>0.08\) cm\({}^{3}\)/cm\({}^{3}\) can be seen in the SMAP event window, the absolute values of the _MDI_ in the absence of an earthquake in the central Croatia region under normal conditions are generally lower than 0.04 cm\({}^{3}\)/cm\({}^{3}\). Moreover, the average absolute values of the _MDI_ for grids of this region for the time period from January 2019 to December 2020 was approximately 0.028 cm\({}^{3}\)/cm\({}^{3}\). However, because no verified or average value of MDT in the absence of earthquake has been reported in the literature, in this paper, several thresholds were evaluated. Threshold values, i.e., 0, 0.02, 0.04, and 0.08 cm\({}^{3}\)/cm\({}^{3}\), were used to identify the grids with a notable earthquake-induced soil moisture increase. Grids having an _MDI_ less than zero were neglected. This is a conservative assumption because in many cases the climate-induced soil moisture changes recorded by SMAP are less than that by GLDAS [78], i.e., in some grids, despite the \(MDI<0\), there might have been a seismic-induced soil moisture increase masked by a climate-induced soil moisture change. The evaluation of the differences between the values of \(\Delta SM_{SMAP}\) and \(\Delta SM_{GLDAS}\) (_MDI_) confirms that SMAP can be used to detect a seismic-induced soil moisture increase. In five out of the eleven target earthquakes--Petrija in Croatia, Samos in Greece, Palu in Indonesia, Kaikoura in New Zealand, and Gorkha in Nepal--some grids show _MDI_ values higher than 0.08 cm\({}^{3}\)/cm\({}^{3}\). Given that the land surface soil moisture content has a small range of values, from 0.05 to 0.6 cm\({}^{3}\)/cm\({}^{3}\), an 0.08 cm\({}^{3}\)/cm\({}^{3}\) difference between \(\Delta SM_{SMAP}\) and \(\Delta SM_{GLDAS}\) strongly supports that SMAP can capture the soil moisture increases caused by seismic events.