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Slow Earthquakes

By Michael R. Forrest
SCEC InstaNet News Feature Writer

The words "Slow Earthquake" conjure up an image of a fault moving silently, stealthily, slipping over the course of several minutes to a few weeks, generally avoiding the scrutiny of geologists and geophysicists by generating few - if any - waves.

Relatively little research has been conducted on slow events, mostly because of their elusive nature. This article presents two mini-interviews: one with Malcolm Johnston, a US Geological Survey scientist who was part of the team that detected the 1992 slow earthquake on the San Andreas fault, and the other with Yehuda Bock, a geodecist with Scripps Institute of Oceanography, who questions the terminology.

Introduction
Kanamori and Hauksson (1992) applied the "slow earthquake" moniker to a ML 3.5 earthquake in the Santa Maria Basin, which lasted an unusual two minutes. Ihmle and Jordan (1994) examined 107 shallow focus earthquakes and found that 20 were preceded by "episodes of slow, smooth deformation immediately before their high-frequency origin times," which they also referred to as slow earthquakes.

In Japan, Kawasaki, et al. (1995) detected a large, ultra-slow earthquake subsequent to the July 18, 1992, Sanriku-Oki, Japan earthquake (MS 6.9) which corresponded to a MW 7.3-7.7 event (quartz-tube extensometers recorded the crustal strain). Back in the United States, Linde et. al. (1996) revealed that in December of 1992 two borehole strain meters located near the San Andreas fault (in the transitional zone between the locked and creeping segments of the fault in central California) recorded a series of "strange signals." These were interpreted as the result of a slow earthquake.

Other research examined a slow earthquake occurring in Nicaragua, 1992, (Ihmle, 1996 a,b), with one paper dismissing the slow earthquake concept (Aki, 1992).

 

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).

References, Slow Earthquakes
Compiled by Michael R. Forrest
SCEC InstaNet News Feature Writer

Aki K., 1992, Higher-Order Interrelations Between Seismogenic Structures and Earthquake Processes, Tectonophysics 211: (1-4) 1-12, Sep 30.

Ihmle P.F. and T.H. Jordan, 1994, Teleseismic Search for Slow Precursors to Large Earthquakes, Science 266: (5190) 1547-1551, Dec 2.

Ihmle P.F., 1996, Frequency-Dependent Relocation of the 1992 Nicaragua Slow Earthquake: An Empirical Green's Function Approach, Geophysical Journal International, 127: (1) 75-85 Oct.

Ihmle P.F., 1996, Monte Carlo Slip Inversion in the Frequency Domain: Application to the 1992 Nicaragua Slow Earthquake, Geophysical Research Letters, 23: (9) 913-916, May.

Kanamori H. and E. Hauksson, 1992, A Slow Earthquake in the Santa-Maria Basin, California, Bull. Seis. Soc. 82: (5) 2087-2096, Oct.

Kawasaki I., Asai Y., Tamura Y., Sagiya T., Mikami N., Okada Y., Sakata M., and Kasahara M., 1995, The 1992 Sanriku-Oki, Japan, Ultra-Slow Earthquake, Journal of Physics of the Earth, 43: (2) 105-116.

Linde A.T., Gladwin M.T., Johnston M.J.S., Gwyther R.L., and R.B. Bilham., 1996, A Slow Earthquake Sequence on the San Andreas Fault, Nature, 383: (6595) 65-68 Sep 5.

 

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