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BY-NC-ND 3.0 license Open Access Published by De Gruyter Open Access October 7, 2016

Tilt offset associated with local seismicity: the Mt. Etna January 9, 2001 seismic swarm.

  • Salvatore Gambino EMAIL logo
From the journal Open Geosciences

Abstract

On the 9th of January 2001 a seismic swarm on the southeastern flank of Mt. Etna at 3.5 km beside sea level (b.s.l.), caused co-seismic variations on short and long baseline tiltmeters of the Mt. Etna permanent tilt network.

Taking account of the geometry and mechanism of the active tectonic structure obtained by seismological studies, the theoretical tilt linked to the faulting source was calculated at multiple different recording stations. It was found that the amount of measured deformation exceeded that which was generated seismically, indicating that much of the deformation along the fault was aseismic.

The 9 January 2001 episode represents a shear response to a local stress caused by a volcanic source that acted in the period preceding the 2001 eruption. Tilt data also suggest a marked slip of 70-140 cm along the fault, probably due to the presence of fluids.

1 Introduction

Fault reactivation gives rise to earthquakes and ground surface deformation caused by sudden slippage across the fault surface. Investigations into ground deformation are important to improve our knowledge of tectonic processes. Continuous high precision measurements (like tilt, strain) are considered the most appropriate to investigate the small extent of ground deformation related to fault rupturing [1].

On Mt. Etna, a tilt network measures ground angular movements using borehole and long-base instruments [2, 3]. Borehole instruments use high precision electrolytic bubble sensors; long-base devices have two orthogonal fluid-filled tubes positioned inside tunnels where fluid levels are measured at the ends of the tubes. Tiltmeters are a powerful tool for volcano monitoring and tilt changes have accompanied the rapid rise of magma and formation of dikes and eruptive fissures at Mt. Etna (e.g. [4]).

Faulting causes tilt changes with amplitudes related to source-station distance and earthquake magnitude [5]; they are generally characterized by impulsive variations with simultaneous recordings at different stations in correspondence with the earthquake. Changes are generally small for shallow (0-5 km depth) local earthquakes at Mt. Etna, comprised between 0.1 and 1.5 microradians [3]. These permanent offsets are assumed to represent the change produced by the fault dislocation in a tectonic context (e.g. [5,6]) while in an active volcanic setting, the tensile component (e.g. a dike) can dominate the deformation. Analyses of co-seismic variations in different areas have highlighted how continuous borehole tilt data may also comprise seismic shaking effects, such as movements on cracks and fractures near the instrument site, instability of instruments or translational ground acceleration caused by passing waves [1,5,7,8] that cause larger variations [9-11]. Nonetheless, these effects are not present on a long-base device [1,12,13].

Tilt changes associated with a seismic swarm (Mmax = 3.5) occurring in the eastern flank of Mt. Etna on January 9, 2001 (Fig. 1) were examined. This swarm was characterized by tens of events with similar waveforms whose high precise relocation, with focal planes, allowed accurate reconstruction of the seismogenic fault geometry [14].

Figure 1 Map of permanent tilt networks and surface faults of Mt. Etna. Lower right inset map shows the main regional fault systems: MF=Messina-Fiumefreddo line, ME=Malta Escarpment. Dashed lines define the sliding sector. The dashed box shows the area drawn in Fig. 2.
Figure 1

Map of permanent tilt networks and surface faults of Mt. Etna. Lower right inset map shows the main regional fault systems: MF=Messina-Fiumefreddo line, ME=Malta Escarpment. Dashed lines define the sliding sector. The dashed box shows the area drawn in Fig. 2.

Amplitudes and azimuths of the coseismic tilt offsets are compared with those from the dislocation theory using parameters obtained by seismology and possible reasons for their discrepancy are discussed.

2 Mt. Etna

Located between the compressive domain of WesternCentral Sicily and the tensional domain of the Calabrian Arc, Mt. Etna (Fig. 1) has formed at the intersection of two regional fault systems, which have NNW-SSE and NE-SW trends (e.g. [15,16]), respectively (Fig. 1).

The interaction between regional stress, dike-induced rifting and gravity causes a fairly continuous and roughly eastward and downward motion of Mt. Etna’s eastern flank (e.g. [17]). This sliding area (Fig. 1) is delimited to the north by the Pernicana-Provenzana Fault System (e.g. [18, 19]) and to the south by the Trecastagni and Tremestieri faults (e.g. [20, 21]) and the décollement surface is located at a depth of 3 km b.s.l. ([22] and references therein).

Earthquakes at Mt. Etna, usually defined as seismo- tectonic events, can generally be divided in two major groups: volcanic-tectonic (VT) earthquakes are generated by tectonic stress and/or by stress arising from rising magma (e.g. [23]) and long-period events most likely driven by stress changes caused by an intermittent degassing process occurring at depth [24].

VT earthquakes occur mainly in the form of swarms composed of micro or small earthquakes which seldom exceed magnitude 4. The deep (ca. 15-30 km) and intermediate (ca. 5-15 km) have high-frequency seismic waves (mostly above 4-5 Hz), the events <5 km show medium to high frequencies [25].

Genesis of this seismicity may have three main sources: regional tectonic stress, local stress induced by dike propagation or stress due to slower inflating sources [23, 25, 26]. In particular, seismic activity in the southeastern sector of volcano, located between 3 and 8 km b.s.l., has been associated with an E-W compression induced by a pressurizing source just westwards and at the same depth [22].

Following the effusive flank eruptions of December 1991- March 1993 there was a recharge phase accompanied by near constant and marked inflation of the overall volcano that lasted until 2001. This phase culminated with the July - August 2001 and the October 2002 - January 2003 effusive-explosive flank eruptions [27].

The 2001 eruption is one of the most studied events both from volcanological and geophysical viewpoint. It occurred from seven eruptive fissures, showed high levels of explosivity and produced 5-10 × 106 m3 of ash and ~25 × 106 m3 of lava [28]. The final accelerated intrusive process of the 2001 eruption and the opening of the eruptive fractures was accompanied by important seismic activity and marked ground deformation was recorded on the 12-17 of July [29, 30].

In particular, between late 2000 and July 2001, the recharge was accompanied by seismic activity, including an intense seismic swarm on January 9, 2011, that spread across a wide area mainly in the southern and eastern part of the volcano [23].

3 Tilt network

The Istituto Nazionale di Geofisica e Vulcanologia - Osservatorio Etneo began using bubble sensor tiltmeters on Mt. Etna in 1977 [2]. The Mt. Etna permanent tilt networks currently comprise 14 bi-axial instruments installed in shallow boreholes and one long-base instrument positioned inside two 80-m-long tunnels at the Volcanological Observatory of Pizzi Deneri [3].

Borehole instruments use a high precision electrolytic bubble sensor [31] to measure the angular movement (∂z/∂x) and each tiltmeter has two perpendicular axes, x and y, with the x-axis generally oriented toward the volcano summit. This sensor orientation is usually chosen to better detect ground tilt associated with pressure change in an isotropic pressure source located under the active crater (i.e., producing a symmetrical ground deformation field).

In 2001, the tilt network configuration, as reported in Fig. 1, included eight active bi-axial instruments installed in shallow boreholes at about 3 m depth and one long baseline instrument. Four stations were equipped with AGI (Applied Geomechanics) 510 tiltmeters with a precision of 0.01 microrad (DAM, MDZ, MEG, MMT), three with AGI 722 model with a precision of 0.1 microrad (CDV, MNR, MSC), while a Kinemetrics instrument with a precision of 0.025 microrad was set up at SPC station. The real accuracy is however affected by environmental noise linked to temperature, rainfall or air pressure. At a depth of a few meters, the thermoelastic effects may produce a noise both in seasonal and daily variations (e.g. [1, 32]) so that the instrumental accuracy is not less than 0.1 microradians.

In 1997, a long-baseline instrument comprising two 80 m long orthogonal tubes filled with mercury, whose levels are measured at ends of the tubes, was positioned along the tunnels at the Volcanological Observatory of Pizzi Deneri (PDN, 2850 m a.s.l.) [2]. This device has a low temperature noise and an accuracy of ca. 0.05 microradians.

Data acquisition varies from 1 sample every minutes to 1 sample every 30 minutes, including acquisition of tilt components, air and ground temperatures, air pressure and instrumental control parameters. In recent years the network has been improved with deep stations using advanced high resolution (<5 nanoradians) self-levelling instruments [33].

4 Seismicity

The seismic swarm analyzed here started on January 9, 2001 at 02:51 GMT with around one hundred VT earthquakes with M>1.0 that affected the southeastern flank of Mt Etna. This seismicity clearly showed P and S phases with evident onsets and predominant frequencies in the range of 5-15 Hz and very similar waveforms (multiplets) [14].

The most energetic event (Md=3.5) occurred at 02:51 and was preceded by 3 other events with magnitudes of 2.5, 2.7 and 2.8 occurring in the previous 45 seconds (tab. 1). The aftershock “sequence” comprised a magnitude 3.0 shock at 03:32 and one of 3.1 at 04:31 (Table 1). The epicenters were within an area of about 2 km2 near the town of Zafferana Etnea [34]; focal depths ranged between 3 and 5 km (Fig. 2). Fig. 3 reports the earthquake minute rate and related strain release from 02:00 to 05:00 GMT. The strain release was obtained as the square root of the seismic energy computed by using the Richter (1958) relationship:

Table 1

Location parameters of the analyzed earthquakes (extracted by [14]).

N

Date

Origin time

Md

Latitude

Longitude

Depth

Gap

No

RMS

ERZ

ERH

1

09/01/01

02:51:14.98

2.5

37.705

15.070

3.92

61

18

0.11

0.5

0.3

2

09/01/01

02:51:33.70

2.7

37.701

15.071

3.78

55

23

0.10

0.4

0.3

3

09/01/01

02:51:51.09

2.8

37.702

15.071

3.59

54

24

0.11

0.4

0.3

4

09/01/01

02:51:58:44

3.5

37.703

15.076

3.77

57

36

0.14

0.2

0.2

5

09/01/01

02:56:01.59

2.1

37.704

15.064

4.52

73

17

0.11

0.5

0.5

6

09/01/01

03:04:14.32

1.8

37.704

15.068

5.13

98

12

0.06

1.0

0.8

7

09/01/01

03:23:51.57

1.8

37.704

15.065

4.36

91

15

0.08

0.4

0.7

8

09/01/01

03:32:18.56

3.0

37.708

15.080

3.70

54

35

0.16

0.2

0.2

9

09/01/01

03:53:19.32

1.8

37.705

15.068

4.49

96

13

0.04

0.7

0.7

10

09/01/01

04:31:39.45

3.1

37.708

15.078

3.78

54

36

0.16

0.2

0.2

11

09/01/01

04:39:03.01

2.1

37.706

15.073

4.72

74

19

0.07

0.6

0.6

12

09/01/01

08:13:17.75

2.5

37.702

15.071

3.59

54

22

0.05

0.4

0.3

13

09/01/01

16:28:53.01

2.0

37.702

15.072

4.63

54

21

0.10

0.5

0.4

14

09/01/01

21:20:33.36

1.9

37.701

15.069

3.55

91

18

0.08

0.4

0.4

15

11/01/01

14:01:51.05

2.8

37.702

15.074

3.63

54

42

0.14

0.2

0.2

16

25/01/01

06:32:12.87

2.3

37.701

15.069

3.72

53

27

0.08

0.4

0.3

17

04/02/01

10:25:20.11

1.7

37.700

15.064

3.42

87

10

0.10

0.7

0.6

18

04/02/01

10:25:24.58

2.2

37.700

15.066

3.85

45

24

0.14

0.5

0.4

19

25/03/01

18:25:30.99

1.7

37.699

15.066

3.51

77

13

0.06

0.7

1.0

Md = duration magnitude; GAP = azimuthal gap (degrees); RMS = travel-time residual root mean square (s); No = number of P and S arrivals.

Figure 2 Location and E-W cross-section of the 9, January 2001 seismic swarm. Focal mechanisms solutions for the four stronger events (2.8≤Md≤3.5) are reported. Three focal solutions show pure strike-slip fault planes. Numbers are referred to in Table 1.
Figure 2

Location and E-W cross-section of the 9, January 2001 seismic swarm. Focal mechanisms solutions for the four stronger events (2.8≤Md≤3.5) are reported. Three focal solutions show pure strike-slip fault planes. Numbers are referred to in Table 1.

logE(erg)=9.9+1.9M0.024M2

In [14] the authors re-located a group of multiplets using a cross-spectrum method [36] and obtained accurate relative relocations with uncertainty of few tens of meters. The relocated events clearly describe the geometry of the seismogenic structure; the events lie on a 77° dipping plane, oriented ENE-WSW (N55°E) in line with one of the planes of the focal mechanisms (Fig. 2) for the four stronger events (2.8≤Md≤3.5). Fault plane solutions (FPSs) at Mt.Etna are commonly performed under the basic assumption of a double couple source. In a volcano the possibility of having tensile crack, corrige compensated- linear-vector-dipole (CLVD) and isotropic source mechanisms is high in particular for events occurring during dike intrusion and fracture propagations; In particular, [37, 38] found evidence for CLVD and isotropic source mechanisms during 1991-93 and 2001 Mt. Etna episodes.

However, the 9 January 2001 events are several kilometers away from the eruptive centers, and at 3-5 km depth with high frequency (5-15 Hz) waveforms. For these features the basic assumption of a double couple source seems to be reasonable [14, 39].

5 Tilt changes

Impulsive coseismic tilt variation (offset) events were detected by three tiltmeters. The signals recorded at PDN show more precise data since they had a higher sampling rate (1 minute) and greater sensitivity. These signals highlight that the coseismic tilt (Figure 3C) was linked to the main local earthquake at 02:51 (together with the events occurring in the first 2 minutes), and had a sudden variation of ca. 0.4 microradians on the radial component and 0.2 microradians on the tangential one. Tilting at PDN then continued for about 38 minutes, cumulating respectively 0.20 and 0.18 microradians on the two components.

Figure 3 Earthquakes rate (occurrence every minute) (a) and related strain release (b). Coseismic variations detected at PDN long-base station (c).
Figure 3

Earthquakes rate (occurrence every minute) (a) and related strain release (b). Coseismic variations detected at PDN long-base station (c).

Between 2:30 and 3:00 GMT, the CDV station showed a change in the tilt vector modulus of 0.5 microradians (Fig. 4), while a large variation of more than 4.0 microradians was recorded at the SPC station (Fig. 4). It is noteworthy that between 3:00 and 3:30 GMT, the tangential component at the CDV and SPC stations (Fig. 4) showed a positive change of 0.4-0.5 microradians. Tilt vectors related to 02:30-03-00 and 03:00-03:30 are reported in Fig. 5 and Table 2. No variations were recorded at the more distant stations on the western (MMT, MSC, MMT) and northern (DAM and MNR) flanks. MDZ station was out of order.

Figure 4 Coseismic tilt variations detected at CDV and SPC stations.
Figure 4

Coseismic tilt variations detected at CDV and SPC stations.

6 Tilt expected from seismicity

For the 9 January 2001 swarm, Alparone and Gambino (2003) determined a right-lateral strike mechanism along ca. 77° dip, N55°E oriented plane located 3 km north-west of Zafferana Etnea (Fig. 1).

The cumulative seismic moment release associated with the seismic events of the 02:51-03:00 seismic events was estimated by using the formula [40] for Mt. Etna earthquakes:

log(M0)=(17.60±0.37)+(1.12±0.10)ML

where ML is the local Magnitude of each event. After the conversion of Md in ML by using the [41] relation:

ML=1.164(±0.011)Md0.337(±0.020)

A Mo no greater than = 4.1*1022 dyne-cm was obtained. In light of this result, assuming a medium rigidity (µ) of a shear modulus of 10 GPa [42], a 2 km long and 1 km wide fault (S = 2 km2) and using the general relation of [43]:

M0=μSu¯

A displacement (ū) of 21 cm was obtained.

Considering fault geometry parameters and 21 cm of right strike-slip, I calculated the expected tilt at all the network stations (Table 2) for a tabular dislocation model striking N55°E, dip 77°, located at the earthquake hypocenter by using the [44] dislocation model.

Table 2

Recorded and calculated tilt at different stations of the network. Values in brackets are referred to 02:54-03:30 for PDN and 03:0003:30 for CDV and SPC time periods. Modules are in microradians.

RECORDED

CALCULATED

Station

module

direction

Module

Direction

PDN

0.44 (0.27)

N253°E (N274°E)

0.068

N238°E

CDV

0.5(0.56)

N277°E (N210°E)

0.155

N285°E

SPC

4.20 (0.4)

N215°E (N75°E)

0.180

N10°E

MSC

0.0

0.014

MGT

0.0

0.011

MMT

0.0

0.007

MNR

0.0

0.014

DAM

0.0

0.010

Figure 5 Comparison between observed (blue) and modeled (red and purple) tilt vectors. The numbers are referred to 02:30-03-00 GMT (1) and 03:00-onwards GMT (2) time periods. Values less than 0.1 microrads (the bore-hole sensitivity) are not reported.
Figure 5

Comparison between observed (blue) and modeled (red and purple) tilt vectors. The numbers are referred to 02:30-03-00 GMT (1) and 03:00-onwards GMT (2) time periods. Values less than 0.1 microrads (the bore-hole sensitivity) are not reported.

The tilt vector at the PDN station was 0.068 microradians, N238°E oriented, and 0.155 microradians, N285°E for the CDV station (Table 2).

Recorded and calculated values have been reported on a map as tilt vectors in Fig. 5.

Moreover, I applied the same procedure to the 3:005:00 period cumulative seismic moment, obtaining a displacement of 8 cm and very small tilt vectors (oriented as previous) of 0.026 and 0.050 microradians respectively at PDN and CDV.

7 Discussion and conclusions

A residual tilt offset associated with seismic events is assumed to represent the change produced by the fault dislocation [5]. Analyses of co-seismic variations on bore-hole instruments (e.g. [11,45-47]) have highlighted how continuous tilt measurements at the short-baseline may have, in addition to the real ground tilt, larger variations for different effects related to seismic shaking.

The recorded data at SPC during the January 9th events is an example of this; the large variation of tilt (>4.0 microradians) not observed at CDV (2.5 km away) is probably linked to seismic shaking effects (e.g. [10]). The short transients in the CDV tilt data instead suggest that it was due to real ground tilt and not instrument response to seismic shaking [6]. No shaking effects involve a long-base device such as the mercury instrument of Pizzi Deneri Observatory [13].

The January 9, 2001 seismic swarm started at 02:51 GMT and 6 VT events (Fig. 3A) were recorded in two minutes with an M=3.5 main event. Fig. 5 shows that the direction of the tilt vector as measured by the long base tiltmeter and CDV match well the direction obtained by using the [44] model between 2:30 and 3:00 GMT. However, there is a magnitude difference considering that the calculated value is roughly 30% less than the recorded value at CDV and ca. 15% less than that at PDN.

This means that most likely only a part of the stick-slip obtained by modeling is related to the co-seismic effects, along the 9 January 2001 active fault, suggesting that most of the slip over the fault must be aseismic. This observation is common in volcanic areas (e.g. [48, 49]) and also on Mt. Etna. In particular [19] concluded that only 30% of the total deformation of the Pernicana Fault is attributable to co-seismic displacements and [50] showed a seismic efficiency lower than 30% for the Fiandaca Fault (see Fig. 1 for their locations).

If this is the case for the 9 January 2001 swarm, only the 15-30% of the total deformation is attributable to co- seismic displacements. Hence, the active fault has been affected by a larger slip (70-140 cm) with respect to the 21 centimeters suggested by seismicity.

MGT, MNR, DAM, MSC, and MMT tilt stations are located at some distance from the earthquake foci (Fig. 1) and did not detect variations. Using seismological parameters (Tab. 2) at these stations, the module predictions were low (7 to 14 nanorads). It is interesting to note that if I consider a fault displacement 3.3-6.6 times larger than that obtained by seismology (as suggested by recorded data) the expected tilt is less than instrumental sensitivity (0.1 microradians) in all cases and I should not observe any change even though fault displacement occurred.

In summary the seismic source of the 9 January 2001 event was not close to Mt. Etna’s internal magma sources (e.g. [51,52])., it was more deeper than the detachment surface of the eastern flank [22] and seems to have tectonic features rather than volcanic. An analysis of tilt data help to exclude the presence of magma.

Initially, I ran tensile models using the same fault geometry, with different ranges of reasonable openings in order to obtain the deformations linked to a possible dike intrusion along the fault. Table 3 reports the expected tilt signals considering an opening of 10 cm and Fig. 5 shows the obtained tilt vectors. Vectors are quite different to those recorded and seem to exclude an intrusive process. However, the presence of fluids in this sector of the volcano is possible; recent magnetotelluric surveys (8 km depth) on the eastern flank of Mt. Etna have shown several high- conductivity zones, suggesting a diffuse presence of hydrothermal activity and fluid circulation [53]. Moreover, seismicity is characterized by multiplets that can arguably be interpreted as caused by a single persistent, and possibly non-destructive, source; this factor simply does not agree with a purely tectonic origin of these earthquakes.

Table 3

Calculated modulus and direction of tilt vectors for a tensile model with opening of 10 cm along a 77°dip, N55°E oriented plane located 3 km north-west of Zafferana Etnea. Values obtained at the other stations are <0.01 microdarians.

Modulus

Direction

PDN

0.054

N150°E

CDV

0.354

N341°E

SPC

0.150

N351°E

In conclusion, the 9th of January swarm is the result of a marked faulting episode along a pre-existing structure of the eastern flank that may have been aided by the presence of fluids. A local stress, induced by the recharge that preceded the Mt. Etna 2001 eruption [22], is the most reliable source of the dislocation. The variations recorded between 03:00 and 3:30 at the three tilt stations (Fig. 5) differ somewhat from the previous 02:51-03:00 ones showing ground deformation that may be linked to an additional source.

Acknowledgement

I’m particularly indebted to the technical staff of Tilt Group of INGV-OE for their professionalism and for ensuring the regular working of the network. I thank Nico Fournier and Karoly Nemeth for their critical reading of the manuscript and constructive comments. I also thank S. Conway for improving the English of this paper.

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Received: 2015-12-23
Accepted: 2016-6-12
Published Online: 2016-10-7
Published in Print: 2016-10-1

© 2016 S. Gambino, published by De Gruyter Open

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License.

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