Project Report Recording of local earthquakes in Simeulue

Project Report Recording of local earthquakes in Simeulue Frederik Tilmann September 25, 2006 Abstract From December 12, 2005 to March 22, 2006 we operated a temporary network of 8 stations (Guralp CMG-6T short-period 3-component stations) on Simeulue island as part of the seismological network of the SEACAUSE experiment. Seismic noise was dominated by surf on the south coast and cultural noise on the north coast and inland stations. For location, the data from this network were combined with the GITEWS/SEACAUSE ocean bottom network. Typically 7-8 locatable events are registered per day but this rate increases dramatically after several events with magnitudes larger than 5.5 within the array. Events occur on or near the plate interface, in the overriding plate and probably in the downgoing plate. The largest concentration of events is found just south of Simeulue island and its extension along strike, near the presumed updip limit of the co-seismic rupture. These events define a nearly horizontal plane and activity exhibits strong alongstrike variability. Deeper events define a plane with a dip of approximately 15, presumably the top of the downgoing plate. The data from this network are archived at SeisUK and the IRIS-DMC with network code ZB (valid for years 2005-06). Background and Survey Procedure Based on coseismic rupture models the southern termination of the Aceh earthquake (December 26, 2004) and northern termination of the Nias earthquake (March 28, 2005) appear to be near Simeulue island (Ammon et al., 2005; Subarya et al., 2006; Briggs et al., 2006). Indications from the historical record indicate that this boundary might have been the same in previous earthquake cycles (Natawidjaja et al., 2004). The primary objective of the deployment is the characterisation of the local seismicity in this presumed segmentation boundary; a secondary objective is the determination of the velocity structure in the forearc. 8 CMG-6T stations were deployed in December 2005 (2 stations were not deployed due to instrumental problems) (Figure 1 and Table 1). Accessibility and security issues limited potential sites to locations near the coastal road. At the time of deployment, the northern part of the island (stations LFAK and LEWK) was only accessible by boat due to tsunami and earthquake related damage to the transport infrastructure, particularly bridges. All stations were deployed on private land. We followed the recommended SeisUK procedure for non-bedrock deployments. First, a sensor pit was dug out to about 60 cm depth. The bottom of the pit was covered with sand, and the sensor, which was protected by a plastic bag, is placed on the sand layer, oriented, and levelled. The pit was filled with sand to about half the height of the instrument, and the sensor pit then backfilled such that the top of the instrument was covered 15-20 cm of soil. For all sensor pits, drainage channels were dug. All cables were buried or protected by flexible orange tubing. Solar panels and GPS Bullard Laboratories, University of Cambridge, Madingley Road, Cambridge CB3 0EZ, UK, email:tilmann@esc.cam.ac.uk 1

95 20' 95 40' 96 00' 96 20' 96 40' 97 00' 3 00' 3 00' LEWK LFAK ob17 ob18 2 40' ob22 PUTR DEHI LUAN 2 40' 2 20' 200512180423A ob21 MAUD LABU BATU 2 20' ob24 2 00' 95 20' 95 40' 96 00' 96 20' 96 40' 2 00' 97 00' Figure 1: Station map of Simeulue network stations: Contemporaneous ocean bottom stations are indicated by dark grey circles. The locations of the December 18, 2005, 4:23 M=5.7 Simeulue event as determined by Harvard (focal mechanism), NEIC (star), and the temporary network (circle) differ significantly. antennae were mounted on poles or tree trunks (except LEWK where we use the power from the Caltech continuous GPS station). Where deemed necessary, livestock deterrents (fences or stakes) were erected. Stations were equipped with one or two solar panels. The sampling rate was set to 50 Hz for all stations. Data quality We calculated noise probability density functions and their diurnal variation using the method of McNamara and Buland (2004) (Figure 2 and appendix). For periods above 10 s, the noise shows little variation suggesting that instrumental noise dominates in this band. Intermediate noise levels are found in the microseismic band (3-10 s). Given the island location of the station, this is slightly surprising and probably related to the lower microseismic generation efficiency of the Indian ocean compared to the Atlantic or Pacific. Relatively high noise levels are found at 1 Hz and above, particularly at stations on the south coast. The high noise levels likely result from a combination of cultural noise and surf (most of the stations were located less than a kilometre from the coast). The network recorded both local and teleseismic earthquakes (Figure 3). Two larger earthquakes with moment magnitudes of 5.7 and 5.9 in the immediate vicinity of the land array caused nearby stations to saturate. The number of clear earthquake arrivals recorded at a station depends on the distance to the event as well as the noise level and near-station response. We attempt to quantify the relative performance of ocean bottom and land seismometers by comparing the number of P and S picks made on the land instruments with that made on nearby ocean bottom instruments that were active during the same period (Table 2). The number of P picks is similar at all stations and does not differ significantly between ocean bottom and land instruments. Differences between individual stations are much more pronounced for S picks and do not seem to relate in a simple manner to noise levels; for example, noise levels at LUAN 2

Table 1: Station list Site Lat ( N) a Lon ( E) Elevation (m) Sensor-ID Start-of-data End-of-data LEWK 2.92358 95.80408 37.3 6197 12/12/05 24/02/06 b LFAK 2.82164 95.93090 35.9 6351 12/12/05 01/01/06 c LUAN 2.62840 96.20482 19.3 6118 13/12/05 20/03/06 PUTR 2.58091 96.04877 15.3 6111 14/12/05 21/03/06 DEHI 2.54797 96.11155 11.6 6091 15/12/05 21/03/06 MAUD 2.46740 96.21812 20.3 6043 15/12/05 12/02/06 d BATU 2.34443 96.40871 32.7 6041 16/12/05 22/03/06 LABU 2.42227 96.36328 15.3 6119 16/12/05 22/03/06 a Station locations were determined by averaging the values reported by the internal GPS. Note that GPS antennas were placed up to 5 m away from the sensor. b Flash card out of memory due to high intermittent noise c Recording terminates for unknown reason; later, a solar panel is stolen from station d Power failure due to water penetration into the solar regulator are generally lower than at MAUD above 1 Hz yet the latter station recorded more than three times as many clear S picks. On average, the land stations appear to be more sensitive to S waves than the ocean bottom stations but with overlap, such that the best ocean bottom stations easily outperform the poorest land stations. Processing First, the land data of the Simeulue network were combined with the data of the ocean bottom network, which was deployed and recovered during different legs of Sonne cruise SO186 (Flueh et al., 2006; Kopp and Flueh, 2006). The results presented in the following section are based on both marine and land data. Processing included the following steps: 1. STA/LTA triggering and cutting of prospective events (defined by at least 8 nearly coincident single-station triggers); 2. manual picking; 3. prelimary location; 4. joint inversion for minimum 1D-model, station statics and hypocentres; 5. relocation of events and calculation of probability density function using direct search method (Lomax et al., 2000, NonLinLoc oct-tree method). Event locations, picks and associated waveforms are stored in a database in SEISAN format. A total of 2458 potential events triggered, 1934 of which occured after the land network was installed. For the days which have been processed so far and during which both the marine and land network were active, 46% of the triggers turned out to be unique locatable events. So far, a total of 639 events have been located. Preliminary results As expected, the whole survey region was still highly active, with two larger earthquakes within the array (M W = 5.7, 5.9) and three additional ones nearby (M W = 5.6, 5.7, 6.5). Most of the seismicity is concentrated in a relatively narrow band (<20 km wide in most places) just south of Simeulue island and its extension along strike towards the north west coast of Nias (Figure 4), approximately near the updip limit of most coseismic rupture models (Subarya et al., 2006; Briggs et al., 2006). These updip events nearly all occur at a depth of 15-20 km with no measurable dip. Seismic activity varies strongly along strike. Whereas some of the areas of particularly strong seismic activity include aftershocks of the aforementioned larger earthquakes, inspection shows that strong activity is sustained in these areas. The 3

A marked difference between day and night noise levels indicates a dominance of cultural noise for frequencies above 1 Hz (North coast, distance from coast: 1.2 km) Noise levels are noticeably higher compared to LUAN above but only slight variations exist in average noise levels as a function of time of day, indicating a dominance of surf generated noise. (South coast, distance from coast: 0.3 km) Extremely strong high frequency noise from 5pm 9pm local time. Due to its time- and band-limited nature this noise only marginally affected overall data quality at the station but increased storage requirements such that internal storage of this station was filled prematurely. (North end of island, distance from coast: 0.1 km). Noise levels in quiet period are comparable to north coast stations but high noise levels for frequencies above 1 Hz from approximately 6am 6pm local time, presumably caused by a nearby road. (Inland, distance from coast: 4.6 km) Figure 2: Vertical acceleration noise probability density functions for selected stations and diurnal variation for selected stations. Times are in GMT; local time is GMT+6 hours. LNM, HNM: Peterson low and high noise models. Equivalent plots for further stations and horizontal components are given in the appendix. 4

5 02/02/06 12:48, MW =6.7 Fiji Region. Vertical components. Epicentral distance 86. P and S arrival at LEWK, 3 components Figure 3: Examples of earthquake recordings 18/12/05 04:23, MW =5.7 Simeulue Island. Vertical components. Most stations saturate. 27/12/05 01:53, local event. Vertical components. NERC Geophysical Equipment Facility - View more reports on our website at http://gef.nerc.ac.uk/reports.php

Table 2: Comparison of number of picks at ocean bottom and land seismometers. All listed stations were operational for the test period (January 2006, odd days only). Only events near the array with at least 5 picks were taken into account. Percentages are relative to the maximum possible: one pick per event. Station #P picks #S picks ob17 89 (60%) 7 ( 4%) ob18 90 (60%) 7 ( 4%) ob21 92 (62%) 38 (25%) ob22 77 (52%) 46 (31%) ob24 95 (64%) 67 (45%) LEWK 91 (61%) 78 (52%) LUAN 110 (74%) 34 (22%) PUTR 85 (57%) 82 (55%) DEHI 77 (52%) 61 (41%) MAUD 94 (63%) 110 (74%) BATU 70 (47%) 29 (19%) LABU 87 (58%) 39 (26%) updip band abruptly cuts off 20-30 km west of Simeulue; this cutoff is not coincident with the boundary between the Sumatra-Andaman and Nias great earthquakes but is close to an inferred inflection of the estimated isoslip contours of the Sumatra-Andaman earthquake (Subarya et al., 2006). At several points fingers of increased seismic activitity extend in the dip direction and the main band appears to step landward in central Simeulue. Seismicity is sparser landward of the updip band, with events falling into at least two classes. The deeper events delineate a plane with an approximate dip of 15, presumably defining the top of the oceanic plate (cf. dip of CMT solution of the Nias and Aceh events: 8 ). The shallower group is well separated from these deeper events and locates undoubtedly within the overriding plate. Although some earthquakes are located at depths that would put them into the oceanic plate, none of these depth determinations were classed as reliable. Conclusion and Outlook The temporary network on Simeulue island has recorded a large number of earthquakes in spite of relatively high noise levels at periods of 1 s and above, with a P detection efficiency comparable to nearby ocean bottom seismometers, and better S wave detection performance. The land stations provided additional coverage close to the most active region, and thus significantly improved the accuracy of depth determinination, which generally require a station with an epicentral distance less than the focal depth for a reliable estimate. Most seismicity occured in a narrow band approximately updip of the peak of coseismic rupture. The remaining earthquakes are located along a dipping plane with 15 dip and in the overriding plate. Activity in the downgoing plate is possible but cannot be confirmed at this stage. We will continue picking the data to complete the catalogue, and determine magnitudes and focal mechanism solutions where possible. Cross-correlation of traces from closely-located events will be used to determine precise relative travel times for nearby events. We will invert travel-time residuals for a three-dimensional velocity model. Both the relative travel times and the three-dimensional model will lead to further improvements in location accuracy. Further incorporating the results of active source profiling, we will investigate whether the variabilility of seismic behaviour along strike can be correlated with structural differences. 6

a NonLinLoc minimum 4 00' ob14 3 30' km 50 45 40 35 30 25 20 15 10 5 E ar t h q u a k e D ep t h ob53 ob51 ob50 ob22 ob11 ob38 ob49 ob37 ob12 ob10 ob21 ob48 ob47 ob34 ob08 ob46 ob45 ob23 LEWK LFAK ob07 ob41 ob17 ob03 ob24 ob15 ob16 ob18 LUAN PUTR DEHI MAUD LABU BATU ob43 ob30 ob04 ob33 ob32 ob05 ob42 ob31 ob29 ob27 ob28 ob26 ob19 ob20 ob02 ob01 3 00' 2 30' 2 00' 1 30' 0 94 30' 95 00' 95 30' 96 00' 96 30' 97 00' 97 30' 1 00' b 8000 6000 4000 2000 0 2000 4000 6000 8000 Bathymetry (m) 0 50 100 150 200 0 10 20 30 40 50 0 50 100 150 200 0 10 20 30 40 50 Figure 4: a. Epicentral distribution of events. Depth below sealevel is colour-coded for events with good depth-constraint only. Circles show events with good epicentral control. Black dots represent low-quality events. b. Cross section. Circles are colored red for events with robust depth-constraints and white otherwise. Red stars show NEIC locations for 5 selected earthquakes with magnitudes larger than 5.6. Where a local-network based location also exists, both locations are connected by a red line. 7

We also plan to apply master-event relocation methods to aftershocks recorded globally before the deployment of the array, using events recorded both locally and globally (the NEIC catalogue lists over a 100 events within the area of Figure 4 during the total deployment period, i.e. when either the ocean bottom or the land array were operational). This will allow us to constrain the temporal evolution of seismicity after the great earthquakes in much more detail than previously possible. Acknowledgements We thank the Bundesministerium for Wissenschaft und Forschung for funding; the SeisUK NERC instrument facility for a loan of the seismic instruments and their staff for technical support; H. Harjono and L. Handayani of Geotek LIPI and W. Triyoso of Institut Teknologi Bandung for facilitating the experiment, B. Suwargadi for help with import and export of the instruments; B. Suwargadi, A. Gunung, Y. Firmans-Yah and M. Shaw-Champion for invaluable assistance in the field; the owners of the sites for looking after our stations; R. Kahle and M. Heintz for picking the land and part of the marine data. References Ammon, C. J., et al. (2005), Rupture process of the 2004 Sumatra-Andaman Earthquake, Science, 308, 1133 1139. Briggs, R. W., et al. (2006), Deformation and slip along the Sunda megathrust in the great 2005 Nias-Simeulue earthquake, Science, 311, 1897 1901. Flueh, E. R., T. Schoene, and W. Weinrebe (Eds.) (2006), FS Sonne Cruise Report SO186 B,C&D GITEWS, IFM-GEOMAR Report, vol. 5, Leibniz-Institut f. Meereswissenschaften an d. Christian- Albrechts-Universität Kiel. Kopp, H., and E. R. Flueh (Eds.) (2006), FS Sonne Cruise Report SO186 Leg 3 SeaCause II, IFM-GEOMAR Report, vol. 6, Leibniz-Institut f. Meereswissenschaften an d. Christian-Albrechts- Universität Kiel. Lomax, A., A. J. Virieux, P. Volant, and C. Berge (2000), Probabilistic earthquake location in 3D and layered models: Introduction of a Metropolis-Gibbs method and comparison with linear locations, in Advances in Seismic Event Location, edited by C. H. Thurber and N. Rabinowitz, pp. 101 134, Kluwer, Amsterdam. McNamara, D. E., and R. P. Buland (2004), Ambient noise levels in the continental United States, Bul. Seism. Soc. Am., 94 (4), 1517 1527. Natawidjaja, D., K. Sieh, S. N. Ward, H. Cheng, R. L. Edwards, J. Galetzka, and B. W. Suwargadi (2004), Paleogeodetic records of seismic and aseismic subduction from central Sumatran microatolls, Indonesia, J. Geophys. Res., 109, B04,306, doi:10.1029/2003jb002398. Subarya, C., M. Chlieh, L. Prawirodirdjo, J.-P. Avouac, Y. Bock, K. Sieh, A. Meltzner, D. H. Natawidjaja, and R. McCaffrey (2006), Plate-boundary deformation associated with the great Sumatra- Andaman earthquake, Nature, 440, 46 51, doi:10.1038/nature04522. Publications to date Presentation F. Tilmann, E. Flueh, I. Grevemeyer and B. Suwargadi, The short term seismological network on Simeulue Island, December 05 - March 06, Workshop Offshore studies of the Andaman-Sumatran Earthquakes, Hannover, 22-23 May 2006. 8

Presentation I. Grevemeyer, F. Tilmann, A. Krabbenhöft, E. Flueh, C. Papenberg, W. Weinrebe, M. Zillmer, H. Kopp and the SO186-3 shipboard scientific party, Seismological study of aftershocks from the 2004 M W =9.1 and 2005 M W =8.6 Sumatran earthquakes from an offshore/onshore network at Simeulue island, Indonesia, Workshop Offshore studies of the Andaman-Sumatran Earthquakes, Hannover, 22-23 May 2006. Presentation F. Tilmann, E. Flueh, I. Grevemeyer, L. Handayani, H. Kopp, B. Suwargadi, W. Triyoso and M. Heintz, First results from a combined marine and land passive seismic network near Simeulue island, AGU Fall Meeting, 11-15 Dec 2006. 9

A Additional noise probability density functions 10

(North coast, distance from coast: 0.6 km) coast: 0.9 km) coast: 0.3 km). coast: 0.2 km) Figure A1: Vertical acceleration noise probability density functions. Figure format follows Figure 2. 11

NERC Geophysical Equipment Facility - View more reports on our website at http://gef.nerc.ac.uk/reports.php (North coast, distance from coast: 1.2 km) coast: 0.3 km) (North end of island, distance from coast: 0.1 km). (Inland, distance coast: 4.6 km) from Figure A2: Horizontal acceleration noise probability density functions. See Figure 2 for further explanations 12

NERC Geophysical Equipment Facility - View more reports on our website at http://gef.nerc.ac.uk/reports.php (North coast, distance from coast: 0.6 km) coast: 0.9 km) coast: 0.3 km). coast: 0.2 km) Figure A3: Horizontal acceleration noise probability density functions. Figure format follows Figure 2. 13