Four significant earthquakes in the Gulf of California are embedded in the seismogram above. The first was a Magnitude 5.8 event occuring at 17:55:24 UTC followed by the main Magnitude 6.9 event at 17:59:57 UTC. Most of the signal on the left side of the image is due to the larger event. Subsequent events with magnitude 5.0 at 18:33:34 and magnitude 5.9 at 18:40:51 are also included in the signal. The large surface waves of the 5.9 magnitude can be seen at the lower right of the image.
The USGS link above describes the area of the earthquakes as being at the boundary of the Pacific and North American tectonic plates. The Pacific plate is moving northwestward relative to the North American plate at the rate of 45 mm/yr (1.77 inches/yr). There are multiple faults in the crust in the region caused by that motion. Some of these are located in small spreading basin areas and give rise to what are called normal and reverse thrust faults where land on one side of fault is displaced upward or downward relative to the land on the other side. There are also faults in which the land on one side of the fault moves sideways relative to other side, moving in the direction or “strike” of the fault. The motion detected by the USGS seismometry indicates that the magnitude 6.9 earthquake was of the latter type.