# Application of Soil Moisture Active Passive (SMAP) Satellite Data in Seismic Response Assessment
## 2 Materials and Methods
### Data and Processing
#### 2.2.2 Soil Moisture and Precipitation Data  
SMAP soil moisture observations are the primary dataset used to identify earthquake-induced near surface moisture changes. Because this research aims to capture and study seismic-induced soil moisture changes, the effects of climatic variables such as precipitation and evaporation on the soil moisture variation should be separated. For this purpose, three datasets were adopted. Table 2 summarizes the spatial and temporal resolution of the data products used in this study. It should be noted that in the rest of this manuscript, the spatial resolution of the GLDAS and GPM are reported in km (i.e., approximately 25 km and 10 km, respectively). For the sake of consistency, converted from kg/m\({}^{2}\) to cm\({}^{3}\)/cm\({}^{3}\), soil moisture values reported by GLDAS are presented herein in volumetric form. SMAP is an Earth observation satellite, launched in January 2015, that carries an L-band radiometer and radar designed to enhance the scientific understanding of the interrelation between the land surface soil moisture, the freeze-thaw state, and the atmosphere for monitoring natural hazards, improving climate forecasts, assisting agricultural productivity, and tracking the water cycle [61]. The radar could have provided SMAP observations with a much higher spatial resolution, but it stopped transmitting data in July 2015. Nevertheless, the radiometer still continues to operate and provide high-accuracy data. This radiometer has the potential to record the soil moisture data over the global land area, excluding regions masked by urban areas, snow and ice, or inland open water bodies, and mountainous or highly vegetated areas, and its measurements are independent of weather conditions and solar illumination [58]. This study used enhanced level-3 passive surface soil moisture data (L3_SM_P_E), which were retrieved by the SMAP radiometer that provides the soil moisture in the top 5 cm of the soil column [57]. The L3_SM_P_E includes soil moisture records at 6:00 AM or 6:00 PM local solar time (LST). This data product has a grid resolution of 9 km on Equal Area Scalable Earth 2.0 grids and a 3-day average temporal resolution. In this paper, version 5 of this data product was used, which is freely available from the National Snow and Ice Data Center (NSIDC) [74] in hierarchical data format version 5 (HDF5). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Platform** & **Product** & **Spatial Resolution** & **Temporal Resolution** & **Unit** \\ \hline SMAP & Soil moisture & 9 km & 1–3 days & cm\({}^{3}\)/cm\({}^{3}\) \\ GLDAS & Soil moisture & 0.25\({}^{\circ}\) & 3-hourly & kg/m\({}^{2}\) \\ GPM & Precipitation & 0.1\({}^{\circ}\) & Half-hourly & mm \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of remote-sensing-derived data products. GLDAS, developed by NASA, is a robust tool to model global land surface states. GLDAS-2.1 is a generation of this system that combines satellite and ground-based observational meteorological data products, including precipitation, air temperature, surface pressure, solar radiation, and wind through land surface models to estimate near-real-time global land surface variables, such as soil moisture, soil temperature, evaporation, snowmelt, and canopy transpiration, among others. In other words, GLDAS-2.1 utilizes meteorological data as the inputs to land surface models to simulate land surface variables. Hence, it does not record the perishable seismic-induced soil moisture change, and it is an appropriate tool to separate earthquake-induced moisture changes from atmospherically induced changes. GLDAS-2.1 provides soil moisture products in four different soil layers, i.e., 0-10 cm, 10-40 cm, 40-100 cm, and 100-200 cm, with a spatial resolution of 25 km at 3-h time steps. For this study, soil moisture data in the top layer (0-10 cm) were downloaded in network common data form version 4 format (NetCDF4) [75]. GPM is a joint mission between international space agencies to observe the Earth's precipitation.