Vol52,2,2009 117 ANNALS OF GEOPHYSICS, VOL. 52, N. 2, April 2009 Microseismic feasibility study: detection of small magnitude events (ML<0.0) for mapping active faults in the Betic Cordillera (Spain) Martin Häge and Manfred Joswig Institute for Geophysics, Universität Stuttgart, Germany Abstract We present the results of the first application of the newly developed concept «Nanoseismic Monitoring» on ac- tive faults in the region close to Murcia, Spain. The aim of this microseismic feasibility study is to test if it is pos- sible to record small magnitude events (ML<0.0) within a short period of time with surface installations and to in- vestigate if these events are related to the regional catalog in terms of amount of events. The seismic monitoring was performed with one small array called the Seismic Navigating System. It consists of one central three com- ponent and three one component seismometers arranged tripartitely around the central station. In the measurement period of two nights at two different sites we were able to detect 19 microearthquakes down to ML = -2.6. The re- sults correlate well with the frequency-magnitude distribution of the regional bulletin. This in turn will allow for estimation of monitoring rates before actual field measurements just from bulletin data. Given an activity rate of 5 to 10 events per night one may map active fault zones within just a few weeks of field campaign. Mailing address: Dr. Martin Häge, Institute for Geo- physics, Universität Stuttgart, Azenbergstrasse 16, 70174 Stuttgart, Germany; e-mail: haege@geophys.uni-stuttgart.de Key words Betic Cordillera – active faults – micro- seismicity – Gutenberg-Richer law – Nanoseismic Monitoring 1. Introduction Characterizing recent seismicity and map- ping active fault segments must be based on the compilation of seismological bulletins. The fun- damental data collection by semi-permanent seismic networks is a time-consuming and cost- ly task. Only a few studies deal with the investi- gation of small magnitude events (ML<0.0) (e.g., Abercrombie, 1995; von Seggern et al., 2003; Ruiz et al., 2006), whereby most researchers in- vestigate aftershocks and not the background seismicity or use borehole sensors for event de- tection. Butler (2003) suggests the term «na- noearthquakes» for events with ML<0.0. The concept of Nanoseismic Monitoring (Joswig, 2008), a technique developed to detect and characterize small magnitude sources, is tested as a short-term alternative for semi-per- manent seismic networks to reduce network recording time. It is successfully tested for On- Site-Inspections of the Preparatory Commis- sion for the Comprehensive Nuclear-Test-Ban Treaty Organization and was applied to detect and characterize small magnitude events trig- gered by material impacts in sinkholes along the western Dead Sea shores (Wust-Bloch and Joswig, 2006). Nanoseismic Monitoring as a kind of seismological microscope shall finally help to shed light on small earthquake trigger mechanisms. Fault weakness can be caused by increased fluid pressure that reduces the effec- tive normal stress (Hainzl et al., 2006). In this Vol52,2,2009 17-06-2009 19:02 Pagina 117 118 M. Häge and M. Joswig model, stress and pore pressure redistribution after large earthquakes come along with higher permeability for fluids causing possible new nucleation points and a characteristic migration scheme (Cox, 1995; Miller et al., 2004). The detection and location of small magnitude seis- micity may support this model of fluid transport and a shear stress behavior driven by porosity reduction (Johnson and McEvilly, 1995; Miller et al., 1996). For the study, a seismically active section of the Betic Cordillera (near Murcia in Spain) with favorable signal-to-noise conditions was select- ed. Since we started to build up the system at that time one recording unit was available for field use. The small magnitude seismicity de- tected and partly localized by the single, small- aperture tripartite array within two successive monitoring nights, at two different sites, is com- pared with the regional 1984-2003 bulletin of the Instituto Geográfico National, Madrid (IGN). Brune and Allen (1967) have shown along the San Andreas Fault system that usually a two-day measurement of seismic activity is sufficient to make an approximate estimation about the local rate of microseismicity. 2. Geological and tectonic setting The Betic Cordillera, which is situated in the southern part of Spain, is a collision zone generated by the nearby African-Eurasian plate boundary. This boundary is defined by a high seismicity which is distributed over several hundreds of kilometers (Calvert et al., 2000). The area selected for the feasibility study lies within the Subbetic Zone which, together with the Prebetic Zone, represents the External Zone of the Betic Cordillera. The thickness of the crust beneath the Betic is 25-39 km (Banda et al., 1993). Focal depths of regional events is re- stricted to the top 40 km (fig. 1) with moderate magnitudes generally less than 5.5 (Buforn et Fig. 1. Spatial distribution of 1040 earthquakes from 1984-2003 (Source: IGN, 2004). The two measuring sites (Capres and Burete) are indicated by triangles pointing up (not in scale). Our located events are marked by black dots and the two co-detected events by triangles pointing down, connected with lines. The dashed black line sketches the trace of the Crevillente Fault Zone (CFZ). Vol52,2,2009 17-06-2009 19:02 Pagina 118 119 Detection of small magnitude events (ML<0.0) for mapping active faults in the Betic Cordillera (Spain) al., 2004). The shallow seismicity is associated with the dense spread of fractures in this region (Sanz de Galdeano et al., 1995). Nanoseismic Monitoring was carried out at two different locations in the vicinity of the Crevillente Fault Zone (CFZ): one near Capres, at the fringe of the Fortuna basin; the other near Burete, in the east of the Sierra de Espuna (fig. 1). The CFZ strikes NE-SW parallel to the axis of the Subbetic Zone and extends laterally over 600 km. The main activity of the CFZ was dur- ing Late Miocene (Alfaro et al., 2002), and it is still active. Focal mechanisms indicate that the CFZ is a right-lateral strike slip fault which can also be observed on geological features (Buforn et al., 1988). Estimates for the total displace- ment along the CFZ range between 75-100 km (Nieto and Rey, 2004) and 400 km (de Smet, 1984). The Subbetic Zone consists of deposits situ- ated far from the South Iberian Margin (Ruano et al., 2004) which comprise parautochthonous to allochthonous, non-metamorphic sediments. The main geological units in these areas be- side Quaternary unconsolidated sediments and Miocene alluvial fan deposits include Triassic marls, claystones, gypsum, dolomites as well as Jurassic and Cretaceous limestones and marls. To consider potential site effects, the thickness of sediments to the top of the base- ment at each of the two monitoring sites has been estimated on the basis of available data and field evidence: near Capres the thickness of sediments reaches 100 m (IGN, 1972a), near Burete it is about 50 m (IGN, 1972b; Poisson and Lukowski, 1990). 3. Data acquisition and processing Data was acquired by one Seismic Naviga- tion System (SNS). This six-channel SNS is a portable, sparse array consisting of four short- period sensors: a central, three-component in- strument and three one-component seismome- ters arranged as a tripartite array. High resolu- tion and coherency of microseismic events are attained by utilizing a small aperture of 200 m and 400 Hz sampling rate. Nanoseismic Moni- toring was performed at night to reduce the ef- fect of anthropogenic noise sources. Event de- tection and location were carried out by Son- oDet and HypoLine modules of the SparseNet software (Joswig, 1999; 2008). Measurements were performed during four nights. Due to the first operation of the system some adaptations to the equipment had to be made in the field. Two of the four nights were successfully completed and were taken for analysis. During this period, a total of 19 seis- mic events in the magnitude range -2.6 � ML � 1.5 could be detected and discriminated from noise bursts by sonogram analysis (Joswig, 1990) (see table I). Figure 2 shows two exam- ples of table I, events nos. 5 and 11, demon- strating the usefulness of sonograms for event detection. Hypocentral locations could be esti- mated for 15 events. Four other weak events did not present clear P- and S-phase onsets. How- ever, ML magnitudes could be estimated for all 19 events as maximum amplitude and distance from ts-tp time differences could be estimated with confidence by sonogram analysis (Catalog A of table II). The apparent velocities of most of the events, derived from array analysis, were not in accordance with much faster velocities of the standard velocity model for Spain (Dãnobeitia et al., 1998). Location residuals reduced signif- icantly using a data-adapted half-space model with velocities ranging from vP = 1 to 5 km/s for 0.3 to 14 km depth. Figure 1 shows the location of the 15 events (solid black dots) on the background of the re- gional seismicity from 1984-2003 (open gray circles). Both positions of the SNS deploy- ments are marked with triangles pointing up (not in scale). Most of the events south of Bu- rete are aftershocks and were generated by the 2002 Bullas (ML = 5.0) earthquake (IGN, 2004) that occurred 607 days before our measure- ment. Event locations were calculated with one single array which results in a large location er- ror of a few kilometers. Depths were estimated with the intersection of hyperboloids by tP-tP in- formation. Two events were co-detected by the local network (nos. 3 and 12 in table I) which are displayed as triangles pointing down in fig. 1. For these two events, the mean horizontal lo- cation difference is about 20 km and the mean Vol52,2,2009 17-06-2009 19:02 Pagina 119 120 M. Häge and M. Joswig magnitude variation 0.4. Figure 3 plots magni- tudes versus distances for all 19 events and the distance-correction curve used for ML calcula- tion fitted empirically to the data (note: slant- distance instead of epicentral distance for <10 km). Additionally, the two co-detected events are shown with triangles pointing down. The observed detection threshold indicates that signal-to-noise conditions were better at Burete than at Capres. The overall sensitivity limit was about ML = -1.0 at 10 km, and ML = -2.0 at 2.5 km. 4. Characterization of regional seismicity The 1984-2003 regional seismic catalog (IGN, 2004) was used to assess the perfor- mance of our measurement. Magnitudes of both datasets, are equivalent. By fig. 3, and as will be shown later in fig. 5, our magnitude of com- pleteness is ML = -1.0 in about 10 km distance. However, the analysis of regional seismicity within a 10 km radius presents several chal- lenges. First, the local bulletin (1984-2003) in- Table I. Parameters of the recorded events. Number Date Origin [UTC] Longitude Latitude Depth [km] Magnitude [ML] 1 3-04-2004 20:45:45 -1.8359 37.9567 0.3 -2.5 2 3-04-2004 21:31:27 - - - -0.8 3 3-04-2004 23:25:30 -1.4090 37.8050 12.0 1.5 4 3-04-2004 23:54:15 -1.8369 37.9136 2.0 -1.1 5 4-04-2004 1:00:58 - - - -0.8 6 4-04-2004 1:17:28 -1.8267 37.9615 1.4 -1.9 7 4-04-2004 1:40:53 -1.8209 37.9430 0.9 -2.6 8 4-04-2004 1:48:20 -1.6941 37.9436 11.0 -0.3 9 4-04-2004 3:38:51 -1.8248 37.9560 1.8 -2.0 10 4-04-2004 5:05:15 -1.8135 37.9614 2.0 -2.1 11 4-04-2004 21:33:18 -1.1682 38.2121 2.4 -1.7 12 5-04-2004 0:41:02 -1.0790 38.0490 11.0 0.6 13 5-04-2004 1:46:05 -1.1147 38.1096 7.5 0.0 14 5-04-2004 2:20:08 - - - -1.4 15 5-04-2004 2:34:35 -1.1714 38.2146 1.2 -1.4 16 5-04-2004 2:42:33 -1.1264 38.2077 2.7 -1.6 17 5-04-2004 2:50:22 -1.1213 38.2133 1.2 -1.5 18 5-04-2004 3:18:59 -1.1718 38.2375 2.7 -0.9 19 5-04-2004 5:34:59 - - - -0.9 Vol52,2,2009 17-06-2009 19:02 Pagina 120 121 Detection of small magnitude events (ML<0.0) for mapping active faults in the Betic Cordillera (Spain) cludes very few events whose source is located within 10 km radius of the two recording sites (Catalog B of table II). Consequently, a much larger reference area (about 13,620 km2) with 1040 events was selected for increased statisti- cal stability (Catalog C of table II). Second, homogeneous seismicity in time and space is a prerequisite when comparing source regions of different size. Artifacts like modifications in network geometry and density, station hardware and processing software result in a rather heteroge- neous distribution of seismicity, both in space (fig. 1), and in time. Figure 4 shows Catalog C of table II with 1040 events as open gray circles (right vertical axis) and the annual event frequency by the black curve (left vertical axis). The annual event frequency and the detec- tion level increase with time. Fig. 2. Seismograms and the corresponding sonograms of the four vertical components for two events. Seismo- grams are filtered between 3 and 30 Hz (optimized filter setting). P and S onsets are indicated with arrows. The event in a) corresponds to no. 5, the event in b) to no. 11 in table I. Table II. Data parameters of seismic Catalogs A, B and C. Catalog A includes 19 events recorded during this measurement campaign. Catalog B includes events within a 10 km radius of our two measurement sites in the pe- riod 1984-2003. Catalog C covers a larger area for a statistical robust b-value calculation in the period 1984-2003. Catalog A B C Description Nanoseismic Local Regional Monitoring Capres Burete Size of area [km2] 630 300 300 13620 Number of events 19 34 175 1040 Max ML 1.5 3.4 5 5 Min ML -2.6 1 0.4 0.4 Vol52,2,2009 17-06-2009 19:02 Pagina 121 122 Numerous investigations deal with catalog completeness and its statistical fluctuations (e.g., Rydelek and Sacks, 1989; Zúñiga and Wiemer, 1999; Woessner and Wiemer, 2005). The first change took place in 1997 when a digital recording system was installed; the sec- ond jump occurred after 2000 when another two stations (ETOB and EMUR) were added. An additional increase in number of events is caused by the aftershock activity of the 2002 Bullas ML = 5.0 earthquake. Between 1984 and 1998, there is a constant magnitude of com- pleteness (MC) of 2.6, calculated with the en- tire-magnitude-range method (Woessner and Wiemer, 2005). It has decreased since 1998. Further investigations have shown that there is no catalog contamination by quarry blasts. 5. Gutenberg-Richter relationship: regional seismicity and Nanoseismic Monitoring We make two assumptions in order to com- pare the Gutenberg-Richter relationship esti- mated from the regional seismicity (Catalog C of table II) with the same relationship based on nanoseismic data (Catalog A of table II). First, Catalog C characterizes a representative b-val- ue for the whole region. This assumption is rooted in fig. 4 which shows an average con- stant seismicity without any high seismicity cy- cles above MC = 2.6. Second, Catalog A can be obtained by concatenating the data recorded over two nights near Capres and near Burete without loss of statistical significance. Fig. 3. Magnitude-distance relationship for the 19 events detected at Burete and at Capres as well as the dis- tance correction for ML. The two co-detected events are shown with triangles pointing down, linked to the cor- responding events by gray lines. M. Häge and M. Joswig Vol52,2,2009 17-06-2009 19:02 Pagina 122 123 Detection of small magnitude events (ML<0.0) for mapping active faults in the Betic Cordillera (Spain) Fig. 5. Frequency-magnitude distributions of Catalogs A, B and C of table II. The cumulative distributions are marked in black, the normal distributions in grey. Note that Catalogs B and C are normalized to the measure- ment period of two nights. The solid black line shows the b-value for Catalog C that is applied to Catalog B (dashed black line). Fig. 4. Time-event (left vertical axis) and time-magnitude distribution (right vertical axis) of Catalog C of table II. Vol52,2,2009 17-06-2009 19:02 Pagina 123 124 M. Häge and M. Joswig Figure 5 compares the frequency-magnitude relationships for the different catalogs of table II. Note that Catalogs B and C of table II are downscaled to the measurement period of this feasibility study of two nights. Due to the small number of events of Catalogs A and B the b- value for the larger area, Catalog C, was deter- mined and applied to our study area, Catalog A. Analyzing Catalog C (black dots) a b-value (solid black line) of 1.16±0.07 was derived ac- cording to the formula of Aki (1965) with MC =2.6. This result corroborates the b-value of 1.1±0.1 estimated by López Casado et al. (1995) for magnitudes larger than 3.5 in the Murcia region between 1930-1992. In hazard assessment, the Gutenberg-Richter relation, log N = a – bM, (5.1) with N the cumulative number of earthquakes of magnitude M or greater and a and b con- stants (Ishimoto and Iida, 1939; Gutenberg and Richter, 1944), is used to predict the frequency of occurrence of large earthquakes on the basis of smaller events. Inversely, few studies investi- gate the extrapolations made from the stronger to the weaker events. Studies concerned with the detection of very small earthquakes (e.g., Iio, 1991; Piccinini et al., 2003) or with the constancy of the b-value to lower magnitudes and self-similarity of seismic events (von Seg- gern et al., 2003) failed to show a b-value de- crease towards small magnitudes. Abercrombie and Brune (1994) verified that there was no sig- nificant decrease down to magnitude 0.0 on three major fault zones in California. Further- more there is good agreement between b-val- ues extrapolated from regional bulletins and those of microseismic activity (Abercrombie, 1995; 1996). Although an investigation of in- duced seismicity (Trifu et al., 1993) showed a non-similar frequency-magnitude distribution between magnitudes -0.5 and 0.0, there is no reason why a decrease in b-value should be ex- pected for natural seismicity at local distance. In conclusion, the b-value of 1.16 from Cat- alog B was extrapolated to small magnitudes. Although it is not possible to calculate a b-val- ue for our events and the existence of high un- certainties in the statistical estimation, i.e. the normalization of the frequency of events to the measurement period of two nights, the extrapo- lated b-value fits remarkably well with Catalog A. There might also be a slight shift in magni- tudes due to the different applied velocity mod- els which are used for location and the resulting variation in epicentral distances. However, the agreement of Catalog B with A is obvious, even with an assumed maximum magnitude devia- tion of 0.4. This result suggests that it might be possible to infer on the basis of bulletin data on the expected amount of events in a certain area prior to a field campaign. A similar observation was made by Brune and Allen (1967) who found out that the amount of microseismicity could be approximately predicted by extrapola- tion of frequency-magnitude curves from 29- year records of larger earthquakes. 6. Conclusions In this feasibility study, the new concept of Nanoseismic Monitoring was applied to char- acterize small magnitude natural seismicity in the Betic Cordillera (Spain). A total of 19 events (-2.6 � ML � 1.5) were recorded and detected within an observation period of two nights, indicating the high sensitivity of Nano- seismic Monitoring. The analysis of the fre- quency-magnitude distributions shows a good approximation between the amounts of record- ed events with those extracted from local cata- logs. However, it must be further proven by in- vestigations in other geological and tectonic settings if the amount of small magnitude seis- micity can be anticipated from existing catalogs by linear extrapolation. The performance of Nanoseismic Monitor- ing of about ML = -1.0 at 10 km and ML = -2.0 at 2.5 km demonstrates its potential for a cost- effective technique for active fault mapping. Fault mapping could be realized at high resolu- tion within weeks instead of years. Due to the use of only one small array, there is a high loca- tion error of a few kilometers. A further chal- lenge was the discrepancy between the standard velocity model for Spain and our observations. Therefore it was not possible to identify specif- ic fault segments. At least two small arrays Vol52,2,2009 17-06-2009 19:02 Pagina 124 125 Detection of small magnitude events (ML<0.0) for mapping active faults in the Betic Cordillera (Spain) must be deployed to reach this aim. Both small arrays can be combined as a kind of network to reduce the azimuthal gap and hence to increase the location accuracy. Cross bearing and the in- tersection of two ts-tp circles provide additional location constraints and support the determina- tion of an appropriate velocity model. It can then be tested if relative location methods might be applicable for further improvement of the location results. Acknowledgments The authors are grateful to the Instituto Ge- ográfico National, Madrid, Spain, especially to Arancha Izquierdo Álvarez and Resurrección Antón for the allocation of the regional bulletin data used in this work and for their kindly sup- port. Hillel Wust-Bloch suggested many im- provements of the manuscript. 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