of other regional earthquakes (e.g. Zoback and Zoback, 1980). I also want to find out

Transcription

1 4. Focal Mechanism Solutions A way to investigate source properties of the 2001 sequence is to attempt finding well-constrained focal mechanism solutions to determine if they are consistent with those of other regional earthquakes (e.g. Zoback and Zoback, 1980). I also want to find out whether the source characteristics of the 2001 sequence have changed since the last sequence of To obtain focal mechanisms of the 2001 sequence I use the FOCMEC algorithm (Snoke et al., 1984). The algorithm accepts not only P wave but also SH wave polarities. This improves focal sphere coverage and provides better-constrained solutions. I organize the earthquakes chronologically keeping the largest events (based on peak to peak amplitude values of the S waves) of a day at the top of the list. Using P and SH wave polarities I calculated focal mechanisms mostly for the largest earthquakes from the list. I also look for earthquakes that have been recorded on as many stations as possible. The greatest number of stations that recorded a single earthquake is 5. The quality of focal mechanism solutions does not solely depend on the number of stations that recorded an event but also upon the projection of those stations onto the focal sphere for the phase being considered. Even with 4 stations yielding 8 data points for an earthquake it is sometimes not possible to obtain a solid solution. The radiation patterns of P and SH waves (Figure 31) for a quadrant might not be always interpreted with a unique source. 60

2 Figure 31. Radiation patterns for (a) P waves and (b) SH waves for a vertical strike-slip. In (a) the + and - indicate the direction of first motion of the P with respect to the source, and the arrows in (b) indicate the first-motion directions for SH. Vectors a and n are the slip direction and the normal to the fault plane, respectively. (Snoke, 1984.) I wrote a SAC script that reads a three-component seismogram, rotates the horizontal components to the great circle path, band-passes the data with a Butterworth 2 pole 1-10 Hz filter and integrates the data obtaining displacement (see Figure 32 for an illustration). P wave polarities are picked together with P wave arrival times. I use the SAC script to additionally pick SH polarities. The chosen filter rejects unwanted frequencies from the transverse seismogram leaving mainly clean SH arrivals. The first motion for P arrivals are picked from the vertical components and SH arrivals from the transverse component. While it is theoretically possible to further constrain focal mechanism solutions using amplitude ratio data as: SV/SH and/or SV/P and/or SH/P, it would be necessary to obtain free-surface amplitude corrections. The high frequency characteristics of the data (even after filtering) make this task quite cumbersome so I could not take the advantage of the amplitude ratio data. 61

3 Figure 32. A three-component accelerogram (top three traces) and displacement (lower three traces) after rotation. From the top of each: radial, transverse and vertical component. Data filtered with 1 10 Hz, 2 pole Butterworth filter. 62

4 Figure 33. The earthquakes with focal mechanism solutions. Pale red dots are earthquakes located using P & S waveform cross correlation and HYPODD. Bright red dots are earthquakes for which focal mechanism solutions are obtained. They also represent earthquake activity up to June 29 (Julian day 180). Blue dots are earthquakes that occurred on July 1 (182 Julian day), the last observed episode of increased seismicity. They also belong to the shallow NE cluster. The purple dot is an earthquake on July 30 (181 Julian day) the day before the last episode of increased seismicity. I tried to find focal mechanisms for some earthquakes using only seismograms recorded on 3 stations (data points: 3 P and 3 SH arrivals). Even though I get more or less consistent solutions for these events, I cannot accept them. 63

5 64 Figure 34. Focal mechanism solutions for the 2001 Enola sequence. The color scheme is the same as in Figure 33 (inset). The focal mechanism on the far left is the mainshock solution by Herrmann et al.

6 For example: If I reject two stations out of five for an earthquake with a wellconstrained solution I get a substantially different focal mechanism (i.e. a strike-slip goes into a thrust motion). I consider a focal mechanism to be acceptably constrained if the earthquake was recorded on at least four stations (data points: 4 P and 4 SH arrivals). Due to occasional portable station rearrangements during the data acquisition of the 2001 sequence, only three stations recorded most of the deeper SW cluster. This number of stations does not allow calculation of many constrained focal mechanisms. In order to obtain good solutions for these events it would be necessary to develop a wave propagation model and organize a detailed study of the deep cluster earthquake focal mechanisms. I here present 15 well-constrained focal mechanism solutions for 15 selected earthquakes. As mentioned earlier in the chronology discussion (see Chapter 3.4) of the 2001 Enola sequence the first recorded earthquakes took place in a dispersed zone below the future shallow NE cluster. During this time four and more stations recorded these events. They are shown as bright red dots in Figure 33 and Figure 34. This period includes events from the first recorded episode of increased seismicity on Julian day 136. When the deep SW cluster was developing (increased seismicity episode on Julian days ) there were only 3 stations recording the events. No focal mechanism solutions are available for these events at the moment. I attempted to find a composite focal mechanism solution for these closely spaced earthquakes but the stations that recorded them appeared as smeared dots on the focal sphere. The stations were far enough from the earthquakes so when plotted on the focal sphere (composite solution) they did not change their position much. It was not possible to obtain a well-constrained solution. 65

7 On Julian day 182, the last recorded episode of increased seismicity, four and more stations had recorded the earthquakes. Focal mechanisms for the shallow NE cluster are shown as blue beach balls. The magenta dot is the focal mechanism of an earthquake on Julian day 181, two hours before the last recorded episode of increased seismicity. The Enola swarm region is a part of the mid-continent stress province (Zobak and Zobak, 1980). The province is under a fairly uniform ENE compressive stress regime that would produce right-lateral motion, on the NE-SW striking faults. The focal mechanisms for the shallow NE cluster (blue dots) are all identical pure strike-slips (Figure 34). One interpretation of the solutions suggests right-lateral faults, with a ~40º NE-SW strike. The other interpretation is just the opposite: NW-SE striking faults would under this stress regime produce left-lateral motion. The mainshock solution is comparable to the focal mechanism solutions of the shallow NE cluster (blue beach balls ). Mapped faults surrounding the swarm area are parallel with the proposed compressive stress axis and are, therefore in this context, likely to be seismically inactive. The very east end of the swarm area is surrounded in part by WNW trending fault segments that could accommodate left-lateral strike. Nevertheless there are no mapped faults within the strict swarm area that would undoubtedly suggest a possibility of any type of faulting. The focal mechanisms for the earthquakes at the beginning of the data acquisition (red dots in Figure 34) are not as uniform as the shallow NE cluster solutions. These earthquakes preceding the 182 Julian day are strike-slip but 6 out of 7 show a small thrust component. 66

8 Even though the clusters together assume a NE-SW trend, the SW cluster exhibits more of an opposite - NW-SE lineation. There is only one calculated focal mechanism from the SW cluster and it agrees (left-lateral strike-slip) with the NW-SE trend. It is necessary to find focal mechanism solutions for the other earthquakes in the cluster and investigate whether this solution is a rule or merely an exception. The 1982 swarm focal mechanisms are very similar to the solutions of the 2001 sequence. Chiu et al., (1984) calculated composite focal mechanism for 6 different groups of earthquakes that could be closely related in space and time (clusters). The focal mechanisms were predominantly strike-slip with a small component of normal dip-slip or with a small component of thrusting. Saikia and Herrmann (1986) found moment-tensor solutions for three 1982 sequence earthquakes by waveform modeling. The focal mechanisms of these events were determined to be predominantly strike-slip, with E to NE trending pressure axes, that is compatible with other earthquake focal mechanisms in the swarm area based on calculations of Chiu et al., (1984) and Haar et al., (1984). The similarity of the focal mechanism solutions for both sequences would suggest that source properties on some level did not change over 20 years. It is also in agreement with the data to assume that the driving force or the ultimate cause of the sequences is the same. It is reasonable to propose that the principle axis of the regional stress field is causing increased swarm-like seismic activity in the Enola region. The focal mechanism solutions certainly do not disagree with the hypothesis. 67

9 To explain the highly localized effects of the principal stress field axis requires recognition of some specific properties of the Enola 8 km 3 crust volume. The absence of mapped faults that would accommodate some of these specific properties in the strict swarm area certainly does not help. Based on the results of Chapter 3.3 the strict swarm area could be a highly localized fractured zone. The refined earthquake relocation schemes (see Chapter 3.3.) could be the first indicators of the fractured character of the Enola crustal volume. It has been proposed that the swarm area is highly fractured (Chiu et al., 1984; Schweig et al., 1991; Booth et al., 1990) based on temporal Vp/Vs ratio changes, a seismic reflection line interpretation and analyses of shear wave splitting. Pujol et al. (1989) showed using the Joint Hypocenter Determination (JHD) technique that lower seismic velocities following a circular pattern are centered in the swarm area in the context of higher velocities of the surrounding crust. Špicák and Horálek (2001) investigated the 1997 earthquake swarm in the West Bohemia/Vogtland region and found two types of constantly occurring focal mechanisms characterizing the sequence. Because the focal mechanisms were similar to those of the earthquakes produced by artificial fluid injection at a 50 km distant borehole, they proposed a possible role of magma and fluids in the seismogenesis of the 1997 swarm. Spatial migration of the swarm activity was interpreted as more evidence for fluid influenced seismicity. They suggested that the regional tectonic stresses were insufficient alone to generate the earthquakes until modified locally by fluids. The earthquake swarms in the Southern Apennines chain in Italy provide more relevant examples. A 1997 seismic sequence demonstrated a variety of focal mechanism solutions 68

10 (Milano et al. 1999). They do not mention a fluid role in the seismogenic processes of the swarms but propose that the variety of the focal mechanism solutions reflect a local stress field controlled by variable fault orientations. Numerous small magnitude (M L < 2.5) earthquakes of the 2001 sequence suggest that local fractures occur under a low stress and a significantly larger fracture is not likely to occur. Identical focal mechanisms for the last observed high rate seismicity episode (day 182) would suggest that these local fractures ruptured following similar orientation. It must be that in the Enola zone some level of fracture similarity exists so that the regional stress field was accommodated equally, producing nearly identical focal mechanisms for the 5 largest earthquakes. These indications might be just the right specific properties needed for this confined crustal volume and no other locality to produce numerous small magnitude earthquakes under the regional stress regime. 69