# Site-Specific Spectra for the City of Mexicali, Mexico, Obtained from April 2010 Earthquake Records
## 2 Determination of the Site-Specific Spectra
### Accelerations Records  
We analyzed the seismic records of all the available public stations (the Mexican stations) that recorded the 4 April 2010, earthquake. Stations are 12 to 140 km from the epicenter, with the closest stations near the Cerro Prieto geothermal power plant. These stations were deployed over sediments (six in total). Additionally, we analyzed records of six more stations deployed over rock, with the farthest station near Ensenada city (Figure 2).  
The recording instruments used are Kinemetrics Etna (Pasadena, CA, USA) or GeoSIG GMS-18 (Schlieren, Switzerland). They recorded ground accelerations. All stations record three components of the ground motion and operate with a sampling rate of 200 m/s. Table 1 shows the site description and recording instrument of each station, and Figure 3 shows the location of each one.  
Seismic energy propagates through the earth and dissipates according to attenuation laws that depend on the distance and type of material it travels through. Hard material facilitates propagation, while soft material makes the propagation more difficult and dissipates seismic energy faster. To show this effect, we picked the maximum acceleration of the horizontal components at each station and plotted them versus the distance from the epicenter (Figure 4).  
The acceleration response of the sediments compared to that of the rock can be observed (Figure 4). It is worth saying that the closest stations are within a sedimentary valley. In contrast, the other stations are located within the peninsular ranges' batholith. The highest acceleration recorded was 0.496 g. It was picked from the EW component of the GEO station located 12 km from the epicenter (station settled over sediments), while the highest acceleration recorded over the rock was 0.074 g at the RAC station located 101 km away.  
The NEHRP (National Earthquake Hazards Reduction Program) researched seismic accelerations by considering a probabilistic criterion and a return period of 475 years. According to the results, rock acceleration values between 0.36 g and 0.46 g were determined  
\begin{table}
\begin{tabular}{l c c c c c} \hline \hline
**Station** & **Latitude** & **Longitude** &
\begin{tabular}{c} **Height** \\ **(nmm)** \\ \end{tabular} & **Site Type** & **Accelerometer** \\ \hline Geothermal plant (GEO) & 32.400 & \(-115.240\) & 30 & Sediments & Etna, Kinemetrics \\ Saltillo (SAL) & 32.422 & \(-115.130\) & 50 & Sediments & GeoSIC GMS-18 \\ M. De Ocampo (MDO) & 32.464 & \(-115.316\) & 14 & Sediments & GeoSIC GMS-18 \\ Chihuahua (CHI) & 32.488 & \(-115.242\) & 15 & Sediments & GeoSIC GMS-18 \\ Tamaulipas (TAM) & 32.549 & \(-115.235\) & 15 & Sediments & GeoSIC GMS-18 \\ RITTO (Ri) & 32.165 & \(-114.961\) & 15 & Sediments & GeoSIC GMS-18 \\ Rancho San Luis (RSL) & 32.116 & \(-115.840\) & 1490 & Solid Rock & GeoSIC GMS-18 \\ Heroes of the Ind. (HDI) & 32.615 & \(-115.882\) & 1130 & Solid Rock & Etna, Kinemetrics \\ Rancho Agua Caliente (RAC) & 32.020 & \(-116.301\) & 714 & Solid Rock & GeoSIC GMS-18 \\ Trinity Valley (VTR) & 31.398 & \(-115.714\) & 750 & Solid Rock & Etna, Kinemetrics \\ Three Brothers (TRH) & 31.690 & \(-116.190\) & 800 & Solid Rock & GeoSIC GMS-18 \\ CICESE (CIC) & 31.868 & \(-116.664\) & 60 & Solid Rock & GeoSIC GMS-18 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Characteristics of the stations (the whole stations in this table record ground accelerations).  
Figure 3: The stations’ locations are included in Table 1.  
for the area. The values recorded at rock stations are an order of magnitude below that range, while the values observed at sediment stations are higher but close to it.