The sizes of the fractures have typically dimensions of several kilometers
both laterally and vertically. The number of events on the same
fracture is mostly only 6-9 and the estimated fault radii are mostly
less than 200 meters. As an sample in Figure 5 we see a view of the earthquakes
within the dominating W-E group. The events are fairly well spread over the
fracture. The sizes are the real sizes as given by the corner frequency
Even if the total number of events
during this day is 5 times more the surfaces of the events will not cover
the whole fracture area. The time order of the events over the area is
rather random, not like a domino game. Figure 6 shows the same events
as Figure 5 but now scaled according to slip size (the area of the
event circles are proportional to slip size). The conclusion is that the
size of the seismic slip during this day is very unevenly distributed
over this fracture and that the activity starts at several places
not neighbouring to each other.
Similarity to hydraulic fracturing
If the pore pressure is increased in a rock mass the pore pressure will first increase along certain fractures with highest permeability. If there remains shear stress over this fracture the pore pressure increase will trigger seismic events at locked asperities. The pore pressure will also increase in fractures crossing the main fracture and trigger slip on those fractures. This activity is initially expected to occur close to the first fracture and later spread out.
The pattern we see here with a seismic activity spread along a main
fracture and with activity on crossing fractures is very similar to what
can be observed during hydraulic fracturing of rock masses through
water injection. Thus it is possible that increase in pore pressure
is affecting the period of seismic activity.
Stable and unstable fault slip
The seismicity is the unstable slip on the fracture. Stable brittle slip is not seismicly observed but is a possible cause to seismic activity at different places over a fault area within a short-time window. The reason is simple that if a fault starts to slip stably as a whole it is likely that some asperities will remain locked, accumulate stress concentration due to surrounding fault slip, and then break seismicly when the stress is too large. The size of the slip depends then on the strength of the locked part. In Figure 6 showing the peak slips of the events on the possibly primary fault there are three events having significantly larger slip. The peak slips of those events are 3-7 mm. The first events on this fracture have all small peak slips, less than 1 mm. Then the deep 3 mm event appears while smaller slip events continue to occur at "normal" (4-7 km) depth. After 7 hours the upper 3 mm event occurs and after 10 hours the 7 mm event comes. This picture is in reasonable agreement with what one might expect if the fracture started to slip stably at the time of the main event. Possibly the deeper part has started to slip earlier or at a higher rate.
It is of course not the purpose of this simple example to reach any conclusion, the reason of presenting this discussion is to indicate what eventually will come out if more extensive and intensive analysis of this type continues in the area.
The assumption of a general stable slip process triggering seismic
events at asperities indicates that the stable slip over this single
fracture during the day is at least 7 mm which is the largest slip
of a single event on the fracture. One must, however, remember
that it may be possible that the 7 mm event was already prestressed
and was triggered by a smaller stable slip. This is, however, something
that can be analyzed, a 7 mm movement on a large fault area will
affect the stability in its close surroundings and can be expected to
be consistent with occurrences and mechanisms of the surrounding
The time developement of the slip sizes on the different fractures
Only six of the 15 fracture groups contain events having peak slips exceeding 1.5 mm. Of these 4 shows a rather strict decay of the slip sizes with the largest slips within 2 hours of the main shock. One group has a rather irregular time development with the largest slip event about 5 hours after the main shock. This is one of the N-S striking fractures. Only the largest group, 28 events on the W-E fracture, shows a rather clear increase in the slip sizes with time and with the largest slip event at the end of the day and at the east end of the fault (east of the main event which is on a N-S fracture on the north side of the W-E fracture).
The N-S group having an unclear slip development with time is far from vertical and is at the east end of the activity. It may be that this activity is affected by the possibly increasing slip on the W-E fracture where the slip increases with time.
The activity prior to this day migrated from west along a western extension of the activity of Figure 4. Together with the indications that the W-E fault started to slip at the time of the main event it seems possible that the main event was associated with some locking of the W-E fracture of the 28 events.