Evidence of subducton zone segmen ta ton: Finite-diference tomography and earthquake reloca ton along the Java margin Anne M M Sirait 1 , Anne S. Meltzer 1 , Joshua C. Stachnik 1 , Mohamad Ramdhan 2 , Nova Heryandoko 2 1 Lehigh University, Bethlehem, PA, 2 Badan Meteorologi Klimatologi dan Geofsika, Jakarta, Indonesia Contact: amm616@lehigh.edu T51G-0379 1.Regional Tectonic Setng of Java • Java margin is part of 5600 km of the Sunda Arc subducton zone that extends from Sumatra to the Lesser Sunda Island. The Austral- ia plate converges to the Sunda block from N11°E of the western Java to almost normal to the trench in central and eastern Java. • Rough subductng plates consist of seamounts and Roo-Rise, accompany the incoming of Australian plate to the Java trench afectng the deformaton in trench and the forearc, and seismicity of the margin. • The deformaton includes frontal erosion, shortening and steepening of the frontal slope, uplif of forearc ridge, landward retreat of accretonary wedge create faults and features along the margin (e.g. splay faults on the outer forearc along the western and central Java) and wide forearc basin along the margin. 3. Finite-Diference Tomography FDtomo algorithm employs arrival tme data set and a 1D velocity startng model to iter- atvely create a 3D velocity model and updatng earthquakes hypocenters. • Using arrival tme data recorded by 44 statons from 2009—2018 BMKG catalog, a total of 4942 seismic events (54796 P-phase and 22269 S-phases) within 400 km depth with a range of magnitude of ML 1.9—7.5. • FD tomography process is done for 6 iteratons in fne grid spacing of 5km cube, con- sistng of 203 x 153 x 84 nodes. • A 1D P—velocity model of 0—400km depth (Kaulakov et al 2007, Kenneth and Eng- dahl, 1991, Wagner et al 2007) with constant Vp/Vs = 1.77 is used as a startng model. • Total of 4782 seismic events are successfully relocated and their distributon refnes the seismicity patern along the margin. • Few isolated clusters are observed at the trench (e.g. transect E, L and N) and this iso- laton appear to be associated to subductng seamounts or Roo-Rise south of the trench (represent as high residual bathymetry (up to 1.5 km heights)). • High residual bathymetry (1—2 km heights) associated with complex forearc struc- tures separates seismicity at the trench to seismicity at the forearc region. • Stripping patern of seismicity along the coastline (e.g. transect A, B, D,G, M) likely re- lated to the variaton of near-coastline structures caused by complex forearc defor- maton and/or southern volcanic chain in island arc. A wide less-intense seismicity ar- ea of the coast of central Java is likely associated with the southern volcanic chain in island arc. • Seismicity on the island arc outlines several faults (Rakutai-Kencana (RK) fault, an un- named fault located east of Opak (OPK) fault) and features near the Cimandiri (CMD) and Kendeng (KDG) faults. • Seismicity is distributed nearly vertcal, from the near surface to ~120 km depth (red boxes in the cross-sectons). • The seismicity collocated with feature of Vp/Vs > 1.75, where the seamounts (feature 1) and Roo-Rise (feature 2) observed. • Seismicity at trench is separated to seismicity in the forearc region. • Subducton of seamounts and Roo-Rise appears to infuence this partcular seismicity distributon. Conclusion • The relocaton results and velocity structure show seismic segmentaton along the strike. Seismic segmentaton observed between trench and forearc region, and along the coastline. • Seismicity at the trench related to the rough subductng plate (e.g. seamounts and Roo -Rise). The seismicity near the trench is separate to the forearc seismicity by com- plex forearc structure as a result of deformaton by the rough subductng plate. • Seismic distributon along the coastline caused by the variaton of near -coastline structures as the result of complex forearc deformaton and/or southern volcanic chain in island arc. • The upper plate structures separate the shallow seismicity and bands of seismicity adjacent to the plate interface and/or beneath the forearc. • Several bands of seismicity beneath the forearc are distributed nearly vertcal and seismicity deeper than 50 km depth reveals a steeper slab geometry compared to Slab 1.0 and Slab 2.0 models. • Inland seismicity outlines crustal faults and features near the faults. Acknowledgement: The head and staf of the Agency for Meteorology Climatology and Geophysics, Indonesia. References: Comte, D., D. Carrizo, S. Roecker, F. Ortega-Culaciat, S. Peyrat (2016) Three-dimensional elastc wave speeds in the northern Chile subducton zone: variatons in hydraton in the supraslab mantle , Geophys. J. Int. 207, 1080–1105 doi: 10.1093/ gji/ggw318 Kaulakov, I., M. Bohm, G. Asch, B.-G. Lühr, A. Manzanares, K. S. Brotopuspito, Pak Fauzi, M. A. Purbawinata, N. T. Puspito, A. Ratdomopurbo, H. Kopp, W. Rabbel, E. Shevkunova (2007) P and S velocity structure of the crust and the upper mantle beneath central Java from local tomography inversion, J. Geophys. Res. 112, B08310, doi:10.1029/2006JB004712 Roecker, S., C. Thurber, K. Roberts, L. Powell (2006) Refning the image of the San Andreas Fault near Parkfeld, California using a fnite diference travel tme computaton technique, Tectonophysics 426 189–205, doi:10.1016/j.tecto.2006.02.026 Wagner, D., I. Koulakov, W. Rabbel, B.-G. Lühr, A. Witwer, H. Kopp, M. Bohm, G. Asch, MERAMEX Scientsts (2007) Joint inversion of actve and passive seismic data in Central Java , Geophys. J. Int., doi: 10.1111/j.1365-246X.2007.03435.x 4d. Seismicity at or adjacent the faults 4. Result: a. Cluster of seismicity at the trench 4b. Seismicity beneath the forearc region Seismicity of Java from 1921—2018 from ISC and BMKG (local permanent network) cata- logs. Fault names refer to fault legends on the map in the upper panel. Seismicity is sized by the magnitude and colored based on depth. • Seismicity outlines a feature west of the Cimandiri (CMD) fault. Seismicity distributed from above plate interface toward shallow depth with range of magnitude ML 2.1—4.8. • It coincides with a feature of Vp/Vs < 1.70 and higher Vp than the surrounding (feature 8) which represents an upper plate structure. 2. Seismicity of Java • Seismicity along the Java margin is domi- nated by M <7 earthquakes and no Mw ≥ 8 earthquakes for the last 300 years. There are several 7.0 ≤ Mw < 8.0 earth- quakes located either shallow than 50 km depth or deeper than 75 km depth. • Seismicity along the Java margin appears to be segmented along the margin. Seis- mic events at or near the trench sepa- rates from seismicity at the forearc re- gion. • Stripping-lines seismicity patern along the coastline followed by less-intense seismicity area. A wide less-intense seis- micity area is observed of the coast of central Java. This seismic patern is likely caused by a complex structure along the trench and forearc associated with a rough subductng plate and/or southern volcanic chain in the island arc. • Seismicity on the island arc outlined crustal faults or features near the faults. • Seismicity outlines the Rakutai—Kencana fault. Seismicity distributed from near surface to ~40 km depth with range of magnitude ML 2.4—5.2. • A feature with Vp/Vs > 1.70 and lower Vp than the surrounding (feature 9) separate this seismicity to the plate interface seismicity. This feature rep- resents an upper plate structure. • Shallow seismicity outlines an unnamed fault east of Opak fault. • Seismicity distributed from near surface down to 20 km depth with range of magnitude ML 2.5—3.6 • This shallow seismicity crossing a Vp/Vs < 1.70 feature (feature 10). Relocated events are ploted in residual bathymetry map of the study area. (Basset and Wats, 2015). Fault names refer to fault legend on the map in the upper -lef panel. Seamounts and Roo- Rise locatons from Masson et al 1990 and Kopp et al 2006. • white contours mark area with a sample of 10 or more rays 4c. A wide less-intense seismicity area near the coastline • Less-intense seismicity area characterized with high Vp/Vs value, ranges between 1.75 – 1.77 (feature 6). • Seismicity starts to appear at the southern boundary, coincides with lower Vp/Vs fea- ture. • The high Vp/Vs features likely limit the seis- micity toward the inland area. This feature is likely associated with the southern vol- canic chain in the island arc which contn- ued to the northern edge of Java basin. Tectonic setng of the study area. Focal mechanisms for Mw≥ 7 are from the Global CMT catalog. Seamounts and Roo-Rise loca- tons are from Masson et al 1990 and Kopp et al 2006. • A nearly vertcal seismicity outlines a feature south of the Kendeng (thrust) fault (black arrows). It distributed from near surface down to ~70 km depth with a range of magnitude ML 2.4—4.0. • A feature of Vp/Vs > 1.70 and lower Vp than surrounding (feature 11), south of this seismicity separates shallow seismicity to seismicity at the plate interface. Regional tectonic setng of the study area. Focal mechanisms for Mw≥ 7 are from the Global CMT catalog. The area in the red box is enlarged in the next fgure. RK CMD OPK KDG 11 Transect K—K’ KDG 11 Transect J—J’ unnamed fault 10 Transect C—C’ RK 9 Transect A—A’ CMD 8 Slab 2.0 Slab 1.0 1 2 Transect E—E’ trench Vs (km/s) Transect N—N’ trench Vs (km/s) • One cluster of seismicity distributed vertcally, from 20 to 60 km depth (purple arrow). • Another cluster (black arrow) collocated with a feature of high Vp, Vs and higher Vp/Vs than sur- rounding. This feature is associated with sub- ductng slab structure. • Seismicity at 40—120 km depth reveal a steeper slab geometry compared to Slab 1.0 and Slab 2.0 models. • Feature on the upper plate (a lower Vp/Vs than surrounding—feature 3) separate seismicity at the plate interface to shallow seismicity beneath the coastline. Vs (km/s) Transect M—M’ Vs (km/s) 3 Transect B—B’ Transect D—D’ 4 Vs (km/s) • Seismicity focused beneath the plate interface mode (black arrow), from 30 to 80 km depth. • Seismicity below 80 km depth reveals a steep- er slab geometry compared to Slab 1.0 and Slab 2.0 models. • Feature with high Vs and lower Vp/Vs than surrounding (feature 4) limit the distributon of seismicity toward shallow depth. This fea- ture represents structure on the overriding plate. 3 4 • Seismicity sparsely distributed beneath the plate in- terface mode (black arrow), from 10 to 100 km depth. • Seismicity below 100 km depth reveals a steeper slab geometry compared to Slab 1.0 and Slab 2.0 models. • Feature with lower Vp/Vs and higher Vp and Vs than surrounding (feature 5) limit the distributon of seismicity toward the coastline. This feature repre- sents structure on the subductng plate. 5 5 3 4 5 6 6 6 5 3 4 5 3 4 1 2 ESSOAr | https://doi.org/10.1002/essoar.10501631.1 | Non-exclusive | First posted online: Tue, 14 Jan 2020 02:22:36 | This content has not been peer reviewed.