Interview with Malcolm Johnston, US Geological Survey

MF In 1992 your group observed a slow earthquake on the San Andreas fault in the transitional area between the creeping and locked segment that lasted about a week and displaced a few centimeters. Is there any new evidence that such earthquakes might be commonplace on other segments of the fault, or any of its daughters, such as the Calaveras, San Jacinto, Palos Verdes, or Newport Inglewood?

MJ We have observed four clear examples of slow earthquakes. We have observed many other possible slow events on just one instrument. This probably results from the fact that we have just a few instruments and these are focused largely on the San Andreas fault. A number of these have occurred on the Calaveras (San Francisco Bay region). We have not seen any events on the San Jacinto, Palos Verdes, or Newport Inglewood faults. As you suggest, a reason might be because we have no instruments installed anywhere near these faults.

MF The slow earthquakes on the San Andreas were recorded to depths of as much as 8 km. One would imagine that truly slow earthquakes might be extremely common in the ductile middle crust, but that they'd be impossible to detect?

MJ I agree that these events might be common in the ductile middle crust. We have the sensitivity to detect them but have not "seen" them as yet. This may be because they are not there, or because they occur much slower than those in the upper 10 km.

MF If slow earthquakes are more commonplace than previously thought, this would suggest that earthquake hazards on many faults might be overestimated?

MJ From what we have seen so far, the total moment release from slow earthquakes in the few places where we have adequate instrumentation is still small compared with the plate accumulation rate, though it is comparable to the seismic moment release. However, I think that it is really too soon to make a judgment about this. Thus the hazard may or may not be overestimated.

MF Are there any studies underway to see how often slow earthquakes might occur on strike-slip faults?

MJ Yes, we are trying to determine just how often slow earthquakes occur on all types of faults not just strike slip types.

MF What is the ideal setting for a slow earthquake?

MJ I don't think we know yet what the ideal setting is for a slow earthquake.

Interview with Yehuda Bock, Scripps Institute of Oceanography

MF Is there possibly no difference between slow earthquakes and simple tectonic motion? Is it all semantics?

YB In my opinion, yes, it is only a matter of semantics and a way for seismologists to expand the frequency domain in which they have always operated.

MF If there is a difference, should the "creep" at Parkfield [California] be considered
a semi-continuous slow earthquake?

YB No. Creep is not an earthquake. It releases strain but in an aseismic manner. It can only be measured by geodesy.

MF Do you have any thoughts on the large "slow earthquakes" which seismologists haveseen in subduction zones - and, should one assign moment magnitudes to such "earthquakes?" (Should one assign a "magnitude" to a large episode of tectonic motion?)

YB This is an interesting question. Some strain is released aseismically. We've
seen this after the Landers earthquake, too, from continuous GPS measurements. Therefore, the magnitude of the earthquake (including all its aftershocks) is not an adequate measure, without taking into account aseismic deformation, such as postseismic deformation.

MF What might be SCIGN's role in possible proving/disproving the existence of slow earthquakes?

YB SCIGN will be able to measure aseismic deformation lasting from as short as seconds to years. As I indicated before we will also be able to measure
earthquake-induced ground motions directly. A seismometer measures ground accelerations and then doubly-integrates to compute ground motion. Should we then call earthquakes "ultra-fast" deformation? No, of course not. We need to distinguish between seismic and aseismic deformation.

MF Do you have other comments/observations?

I've always been somewhat amused by this strange nomenclature ["slow earthquakes"] which has been coined by seismologists. Earthquakes are one consequence of plate motions and interactions (other consequences include mountains and basins, for example). They are not a cause but only an effect (a secondary issue is whether earthquakes trigger other earthquakes - except for aftershocks after a main event, this is still an open question). Geodesy measures plate motions and crustal deformation directly by measuring precise 3-D positions of geodetic markers from once every few seconds to once every few years). Geodesy measures aseismic deformation (although we are getting to the point where we will be able to measure earthquake-induced ground motions directly, not accelerations that are measured by seismometers). Seismology measures seismic motions (it requires earthquakes or artificial earthquakes such as explosions).