By Charly SHELTON

With the recent rash of earthquakes to rock Southern California, not to mention around the world, many are asking what to expect next. Southern Californians live in earthquake country, and know that a big quake is on the horizon. CV Weekly had a chance to ask an expert about the recent earthquake in La Habra and repercussions that may be felt as a result of it. Robert Graves of the U.S. Geological Survey expertly sums up what happened and where we go from here.

CVW: I know that the earthquake was very shallow and that it sits on a precarious fault that runs through a highly populated area. Is this a fault that we should be watching more closely in the future because of this recent activity?

RG: Last Friday’s M5.1 La Habra earthquake occurred on a small secondary fault that lies at a depth of about four to five miles beneath the ground surface. This small fault is very close to, but not part of, two larger faults: the Whittier fault and the Puente Hills Thrust fault. Both of these larger faults are capable of generating [magnitude] 7 or larger earthquakes.

The Puente Hills fault is of particular concern because it extends to the west (from La Habra) and extends beneath downtown L.A. At this point in time it is not completely clear how these faults might interact; however, we are continuing to analyze the data to develop a better understanding of these fault structures.

CVW: Could this be a foreshock to a coming larger quake, possibly along the San Andreas Fault?

RG: It could, although the probability of that is decreasing rapidly with time and is now very small (much less than 1%). Additionally, it would not be a foreshock to a San Andreas earthquake because the San Andreas fault is too far away. However, if an earthquake were to occur in the near future on the Puente Hills Thrust or Whittier fault with an epicenter near Friday’s earthquake, then we would likely call the La Habra a foreshock to that larger earthquake.

CVW: Is there any concern of smaller things like this triggering a larger quake with plate movement, or is this releasing some of the pressure that has built up, reducing the intensity of the coming big quake?

RG: Obviously, the fact that an earthquake happens means that pressure or stress is being relieved in some way. However, earthquakes do not occur independently on completely isolated faults. In Southern California, we have a complex web of active faults, so that while movement on any one fault may reduce the stress on some faults, it may also increase the stress on others.

In addition, we know from past earthquakes that the occurrence of an earthquake tends to increase the probability of having another earthquake nearby in the near future; usually smaller (i.e., an aftershock), but about one in 20 times the subsequent earthquake is bigger. So, take the cue and be prepared!
By Charly SHELTON

With the recent rash of earthquakes to rock Southern California, not to mention around the world, many are asking what to expect next. Southern Californians live in earthquake country, and know that a big quake is on the horizon. CV Weekly had a chance to ask an expert about the recent earthquake in La Habra and repercussions that may be felt as a result of it. Robert Graves of the U.S. Geological Survey expertly sums up what happened and where we go from here.

CVW: I know that the earthquake was very shallow and that it sits on a precarious fault that runs through a highly populated area. Is this a fault that we should be watching more closely in the future because of this recent activity?

RG: Last Friday’s M5.1 La Habra earthquake occurred on a small secondary fault that lies at a depth of about four to five miles beneath the ground surface. This small fault is very close to, but not part of, two larger faults: the Whittier fault and the Puente Hills Thrust fault. Both of these larger faults are capable of generating [magnitude] 7 or larger earthquakes.

The Puente Hills fault is of particular concern because it extends to the west (from La Habra) and extends beneath downtown L.A. At this point in time it is not completely clear how these faults might interact; however, we are continuing to analyze the data to develop a better understanding of these fault structures.

CVW: Could this be a foreshock to a coming larger quake, possibly along the San Andreas Fault?

RG: It could, although the probability of that is decreasing rapidly with time and is now very small (much less than 1%). Additionally, it would not be a foreshock to a San Andreas earthquake because the San Andreas fault is too far away. However, if an earthquake were to occur in the near future on the Puente Hills Thrust or Whittier fault with an epicenter near Friday’s earthquake, then we would likely call the La Habra a foreshock to that larger earthquake.

CVW: Is there any concern of smaller things like this triggering a larger quake with plate movement, or is this releasing some of the pressure that has built up, reducing the intensity of the coming big quake?

RG: Obviously, the fact that an earthquake happens means that pressure or stress is being relieved in some way. However, earthquakes do not occur independently on completely isolated faults. In Southern California, we have a complex web of active faults, so that while movement on any one fault may reduce the stress on some faults, it may also increase the stress on others.

In addition, we know from past earthquakes that the occurrence of an earthquake tends to increase the probability of having another earthquake nearby in the near future; usually smaller (i.e., an aftershock), but about one in 20 times the subsequent earthquake is bigger. So, take the cue and be prepared!