There wa sjust a M4.8 in south Texas...normal slip, slightly oblique. It is at a poorly-constrained 3 km depth. I wonnder if this is on one of the regional normal growth faults. I worked offshore Texas on the Middle Miocene trend ("big Hum", after the foraminifera bigerina humblei (?). I can't believe I remember that after 30 years; I can't remember much now. These growth faults are like mega landslides. The trend I was working on could have 20 km of slip, and be rooted at about 6 km depth, with fold and thrust belts at their "toes".

Again I'm recalling after a long time, but there are onshore trends, one if "Frio", and that may be Oligocene (>24 million years). These get younger towards the deep gulf, as sedimentation causes the shelf to prograde. So, the onshore trends should be largely inactive, but that does not preclude the occassional quake.

Alternatively, the quake is far enough from the Gulf that the geology and faulting is different.

Lots of oil and gas in sands deposited downthrown (hanging-wall) to these faults.

Chris