#1Seismic Structure and Tectonic Evolution of Borneo and Sulawesi Harry Linang ([email protected])¹, A. Gilligan², J. Jenkins¹, T. Greenfield¹, N. Rawlinson¹ 1. University of Cambridge 2. University of Aberdeen D1448 EGU2020-12007#21. Seismic Stations -5° 0° 5° 105° 110° Δ ΜΕΤ ΜΥ ▲ Cambridge (Feb '19) OOBS (Aug '19) ▲ Cambridge (Nov '19) A Cambridge (Mar '20) 200 km BORNEO 115° 120° 125° 105° 110° 115° 120° SULAWESI S 125° 5° F0° -5° Map shows the seismic stations currently in place in Borneo and Sulawesi (also OBS) for which data will be ultimately be used in this study Current data availability ranges from 6 months to 2 years. Setback in data retrieval for stations in Indonesia due to the COVID19 pandemic. Methods: Receiver function| and Neighbourhood Algorithm Inversion Aim: To obtain reliable estimate of crustal thickness. To gain an improved understanding of the structure of the crust and mantle lithosphere beneath and across both island. CC + BY#32. EXAMPLE RF DATA 110° 10° A MET MY A Cambridge (Feb '19) OOBS (Aug '19) A Cambridge (Nov '19) A Cambridge (Mar '20) 5° 115° 120° 10° 5° 0° .09- 06- -30° KSM 0° 30° .06 60° -120° 120° 10 -150° 150° -5° 110° -5° 115° 120° 180° MET Malaysia Station: KSM • Station KSM is located in West Borneo as shown on the map. • Figure on the right shows 865 receiver functions (RFS) plotted as function of back-azimuth (BAZ) from this station. This also shows the back-azimuth data coverage for most seismic stations in this particular region. CC BY#43. H-k Stacking Analysis KSM 865 Vp: 6.3 Results: H - 28.5, k - 1.66 2.0 Vp/Vs 1.9 1.8 1.7 1.6- 1.5 10 15 115 -0 20 25 30 Depth (km) 35 T 40 40 50 50 45 45 0.2 H-k Stacking Method by Zhu and Kanamori (2000) Figure shows an example H-k stacking result for station KSM. A total of 865 receiver functions (RFS) passed quality control that are used in the stacking from 12 years of data. 0.1 - 0.0 -0.1 -0.2 Input: 865 RFs Assumed crust bulk Vp: 6.3 km/s Output: Moho thickness estimate: 28.5 km Crust bulk Vp/Vs ratio estimate: 1.66 Black points shows bootstrapping solutions for randomly selected and stacked RFs. The bootstrapping solutions shows that results obtained from the full stacking of RFs is decent, with small error estimates; depth= +- 2km and Vp/Vs= +-0.04. This varies from station to station depending on local or regional tectonic complexity. CC BY#54. Time to Depth Conversion 100 Depth (km) 80 60 40 20 0 KSM, BAZ bin size of 30.0 1'42 136- 1°30'- 0 100 200 Back-Azimuth (°) 118 110'06 110'12 110'181 110'245 110'30 70 60- th (km) Depth (k 20 10- 28.60 km 80 0.0 0.2 0.4 0.6 0.8 1.0 Figure on the left shows depth converted RFS of station KSM plotted against back-azimuth (BAZ) direction. This was done with time-depth relationship graph produced with information from H-k stacking result. This plot shows us that there are depth variation of estimated Moho depth with BAZ beneath the station. Inset figure shows the ray piercing-points of the estimated Moho (red circles) around station KSM. Figure on the right is the receiver function produced by the stacking of all the depth converted RFs. CC 3 BY#65. NA Inversion DEPTH (km) 0.00.0 T 1.0 MODEL: Demo S VELOCITY (km/s) 2.0 3.0 10.0 20.0 30.0 ויייי Misfit = 0.238 0.34 0.30 0.25 4.0 5.0 0.21 Amplitude 0.17 0.13 0.08 0.04 0.00 -0.04 Observed and predicted receiver functions -5.0 -2.5 Misfit 0.238 Demo 0.0 2.5 5.0 7.5 10.0 40.0 50.0 די 60.0 1 NO. OF MODELS OF THE BEST 1000 100 70.0 1.5 2.0 2.5 10 3.0 1000 Vp/Vs RATIO Neighbourhood Algorithm (NA-sampler) NA is used to produce a seismic velocity profiles by inversion of the RFS and also to show the robustness the Moho depth estimate. Figure on the left shows S velocity profiles obtained from inversion of RFs from station KSM. Red line (left) shows the Vp/Vs ratio result from inversion and red model (right) is the best fit model. The coloured section shows the best 1000 models and the red arrow shows possible Moho depth. The result obtained from the inversion estimated the Moho to be around -27 km, which is consistent with the H-k stacking method. Figure on the top right shows the fitting between observed (black) and predicted (blue) receiver function from the inversion. CC BY#76. Results 5° 10° 100 km 20 20 -5° 110° 115° 120° Estimated Moho Depth km 25 30 35 40 45 The map shows average estimated Moho depth beneath a seismic station obtained so far in Borneo, from the receiver function analysis and NA inversion. Initial results shows very thick crustal estimate in North Borneo and thinnest in West Borneo. Results in the north province aligned well with current understanding of geology while results in west section requires further interpretations. Black circles shows station locations where estimation of Moho depth are not yet determined. Outlook: • • Further interpret results obtained from the NA Inversion Continue looking at stations in Kalimantan when more data are available in the coming months, including data coming from Sulawesi. To use the Virtual Deep Seismic Sounding (VDSS) method on seismic stations in tectonically complex area (i.e. North Borneo) for better estimation of crustal thickness. CC 3 BY