## 3 Results and Discussion
### Detection of P-Wave Arrival  
Earthquake monitoring and early warning systems not only depend on the accurate estimation of permanent displacements, but also rely on the capability of the sense of P-wave arrival which is employed to predict the arrival and intensity of destructive S and surface waves. Figures 2 and 3 have shown that the integrated results could get accurate permanent offsets. The following sections mainly focus on another issue. The enlarged view of the first 20 s of the integrated and GPS-only results for station 5028/P744 is shown in Figure 5, and the similar enlarged view for station AKT006/0183 is shown in Figure 6. From coseismic displacement and velocity waveforms, we can observe that the GPS-only solution is noisy and has a precision limited to several millimeters in displacement and few centimeters per second in velocity. The vertical component is much noisier as expected, due to the satellite constellation configuration and the high correlation between zenith tropospheric delay and the height component. The precision of vertical displacement is of the order of few centimeters, and vertical velocity precision is around several centimeters per second, which is not enough to detect P-wave accurately. With the aid of the seismic data, the tightly-integrated filter is capable of producing a precise integrated displacement and velocity waveform, especially in the up component. The  
Figure 4: _Cont._  
small-amplitude P-wave can be clearly observed, and the P-wave arrival can also be easily detected from the integrated waveform. This is a significant improvement over the GPS-only solution where earthquake signal is detected only after the S-wave arrival, which is generally a few seconds later than the P-wave arrival for near-field stations. The bottom sub-figures in Figures 5 and 6 are STA/LTA ratio values based on tightly-integrated results for north/east/up components, which are used to pick up the earthquake P-wave arrival. The short-term average (STA) through long-term average (LTA) picker is the most broadly used automatic algorithm in seismology [31]. It continuously calculates the average values of the absolute amplitude of a seismic signal in two consecutive moving-time windows. The short time window (STA) is sensitive to seismic events while the long time window (LTA) provides information about the temporal amplitude of seismic noise at the site [32]. When the ratio of both exceeds a pre-set threshold means the arrival of P-wave. The STA/LTA picker parameter settings are always a tradeoff between several seismological and instrumental considerations. For these two earthquake events in this paper, the STA window duration is 0.2 s, the LTA window duration is 2 s, and the pre-set threshold is set to 10. We can clearly identify P-wave arrivals in the STA/LTA ratio time series. It is noted that the P-wave  
Figure 5: An enlarged view of the first 20 s of the coseismic displacements and velocities in all three components on the collocated 5028 (seismic) and P744 (GPS) pair during the El Mayor–Cucapah earthquake. The 5 Hz GPS-only and 200 Hz tightly-integrated displacements and velocities are respectively shown by the blue dotted lines and red lines. The bottom sub-figures are STA/LTA ratio results based on tightly-integrated results, which show the first arrival time of seismic wave. The sub-figures show, from left to right, the north, east and up components. appears in vertical component first and in the horizontal components a few milliseconds later. The detected earthquake P-wave arrival time of station AKT006/0183 is 41.41 s compared with the USGS reference value 41.55 s calculated by TauP Toolkit [33], and the P-wave arrival time of station 5028/P744 is 11.49 s compared with the reference value 11.58 s. It is demonstrated that the integrated results could be used to pick up an accurate P-wave arrival time. However, it is difficult for the GPS-only solution to be accurately identified P-waves because of the significantly less precision.