J. Earth Syst. Sci. (2021) 130:23 Ó Indian Academy of Sciences https://doi.org/10.1007/s12040-020-01530-w (0123456789().,-volV)( 0123456789().,-vol V) Seismic tomographic imaging of P wave velocity perturbation beneath Sumatra, Java, Malacca Strait, Peninsular Malaysia and Singapore 1, 2 ABEL UYIMWEN OSAGIE * and ISMAIL AHMAD ABIR 1Department of Physics, University of Abuja, P.M.B 117, Abuja, Nigeria. 2School of Physics, Universiti Sains Malaysia, 11800, Pulau Penang, Malaysia. *Corresponding author. e-mail: [email protected] MS received 6 March 2020; revised 20 July 2020; accepted 24 October 2020 P wave tomographic imaging of the crust down to a depth of 90 km is performed beneath the region encompassing Sumatra, Java, Malacca Strait, peninsular Malaysia and Singapore. Inversion is performed with 99,741 Brst-arrival p waves from 16,196 local and regional earthquakes occurred around the Sumatra Subduction Zone (SSZ) between 1964 and 2018. Tomographic results show low-velocity (low-V) anomalies that reCect both accretion and possibly, asthenospheric upwelling associated with subduction of the Australian Plate beneath Eurasia around the SSZ. The prominent low-V anomaly is thickest around the Conrad, extending beneath Straits of Malacca and parts of peninsular Malaysia, but disap- pears around the Moho in the region. Below the Moho, the subducting slab, represented by a high-velocity (high-V) anomaly, trends in the orientation of Sumatra. At these depths, the eastern shorelines of Sumatra, most parts of Malacca Strait and the west coast of peninsular Malaysia show varying degrees of positive velocity anomalies. We consider that asthenospheric upwelling around the SSZ may provide heat source for the 40 or more hot springs distributed north–south in peninsular Malaysia. Different east–west and north–south cross-sections reveal the subsurface anomalies at various parts of the region. The predominant low-V anomaly is less than 35 km in depth, but other low-V anomalies are deeper. Keywords. Seismic tomography; regional; p waves; Brst-arrivals; Sumatra Subduction Zone. 1. Introduction apparent around Sumatra, but seldom destructive around countries like Malaysia and Singapore The Sumatra Subduction Zone (SSZ) is located which are located on the stable Indochina–Sunda- along where the Indo-Australia Plate subducts land. As a result, there is little appetite for deep beneath the Eurasian Plate (Bgure 2, green and structural investigations. Although located on the brown lines). The Brst record of an earthquake in stable Sunda continental shelf, seismic activities the archive of the bulletin of International Seis- around the SSZ have historically aAected these mological Center (ISC) dates back to 1907. The other countries. absence of seismic stations at that time is evident Since the 1970s, many large earthquakes have in the scanty dataset prior to the mid-1970s. occurred around the SSZ. Some earthquakes, e.g., However, increased global seismic networks in the the 1976 (mb 6.4), 2004 (Mw 9.15), 2005 (Mw 8.6) 1970s and 1980s provided more datasets for the and 2007 (Mw 8.4) aAected areas far beyond their region. The devastating eAects of earthquakes is epicenters. For example, the 26th December 2004 23 Page 2 of 13 J. Earth Syst. Sci. (2021) 130:23 (Mw 9.15) earthquake triggered a tsunami that tomographic inversion requires four major aAected not only Indonesia, but also Sri Lanka, computational stages – model parameterization, Thailand and Malaysia which killed thousands of forward calculation, inversion and analysis of people in its wake (Komoo and Othman 2005). the model resolution (e.g., Rawlinson and The event set the scene for a large-scale eAort by Sambridge 2003;Zhao2015). In other words, a the Malaysian Government to set up the Malay- tomographic inversion algorithm is a suite of sian National Tsunami Early Warning System different routines that usually contains the (MMD 2012). By causing deformation of the above components or more. For example, Sundaland core as a result of intraplate stress tomog3D (Zhao 2004) uses pseudo-bending ray build-up after the event, previously inactive faults tracing technique for the forward calculation have been reactivated within peninsular Malaysia stage and the LSQR algorithm (Paige and (Shuib 2009). Saunders 1982) in the inversion stage. The Global one-dimensional (1D) reference velocity painstaking task of understanding the data models are good approximations for the purpose of preparation stage for use in tomog3D is com- locating earthquakes, but insufBcient to satisfy pounded by the need to develop a technique to velocity variations at local and regional distances. visualize the Bnal output. This challenge Three-dimensional (3D) seismic investigation of informed the decision to develop an inversion the crust and upper mantle will not only provide scheme. Afterall, every inversion is validated by better understanding about the regional p wave a test of model resolution. velocity distribution within the SSZ, but also for In this work, a ray tracing scheme in 3D is the surrounding regions. Seismic tomography pro- developed using the idea behind an earlier 1D vides the ability to obtain cross-sectional images of scheme (Kim and Baag 2002) which has been used the subsurface due to variations in rock properties. by some authors (e.g., Kang et al. 2013; Osagie At crustal dimensions, variations in p wave veloc- and Woohan 2013; Hong et al. 2017; Osagie et al. ities can be mapped to variations in rock properties 2017). Accuracy and speed (fast convergence rate) from which subsurface stratigraphy or anomalies in the 3D ray tracing scheme is germane to suc- can be deduced. At shallow depths, information cessful inversion. The ray tracing scheme con- about the subsurface structure and strength char- verges rapidly, saving computational time and can acteristics (e.g., bulk modulus, shear modulus and accommodate a model with as many layers as density) can be useful for seismic hazard assess- required for traveltimes computation at regional, ments. This can provide useful information in the local or Beld work subsurface investigations. The construction of infrastructural projects (e.g., tomographic inversion scheme incorporates the bridges, tunnels, dams and sky-scrapers). The LSQR algorithm with enhancement by damping ultimate goal of seismic tomography is to show 3D and smoothing (Zhao 2004). The scheme can read variation in seismic velocity within a model. direct arrival time downloads from the bulletin of Whether the variations observed is due to International Seismological Center (BISC), sum- (a) temperature, (b) composition or (c) physical mary Ble from the Incorporated Research Insti- properties is an open debate (Rawlinson et al. tutions for Seismology (IRIS) and s-Ble from 2014). SEISAN software (Havskov and Ottemoller 1999). The aim of this study is to image the sub- Apart from information about the regional p wave surface of the region using Brst-arrival p wave velocity distribution, the result of this study will data from local and regional earthquakes that provide a platform for near-surface tomographic have occurred within and around the SSZ. studies in the surrounding regions. However, this Compared to later phases, Brst-arrival p waves study is limited in scope to local and regional are easier to identify and are often the best seismic tomography. Traveltime accuracy of the dataset available for local and regional tomo- ray tracing algorithm is limited to epicentral dis- graphic studies. More so, the use of Brst-arrival tances of about 1200±200 km. This is due to the p waves diminishes concerns about signal-to- shooting method of ray tracing used for the for- noise ratio associated with the use of later ward calculation. At epicentral distances beyond arrivals with amplitudes comparable to the 1400 km, the estimated takeoA angle for crustal background noise. To achieve the aim of this events are very close to 90°, at which takeoA study, a seismic tomographic inversion angle, rays cannot computationally get to scheme is developed. In general, seismic stations. J. Earth Syst. Sci. (2021) 130:23 Page 3 of 13 23 2. Earthquake data about 7 cm. The SFS is among the world’s major strike-slip faults with different convergence rate for The data used in this work is obtained from the different segments of the fault (Molnar and Dayem IRIS and the BISC with the bulk of the data being 2010). The Sumatra shear (dextral strike slip) fault arrival time picks from the BISC. The earthquakes is approximately 1500 km, cutting through the (magnitude C 3.0 mb) are conBned to the top 100 entire Sumatra Island. The mechanism for the km in focal depths, latitude ranges from 10°Nto Sumatra shear zone is an open debate. Shuib 10°S and longitude spans 93°–115°E. In 2018, this (2009) proposed extrusion resulting from collision study obtained all available arrival time data from between India and Eurasia, while Hutchison (2010) the BISC between 1964 and 2016. The BISC data believes Oroclines mechanism is responsible. The ended in 2016 as at the time of data collection for fundamental difference in their proposed mecha- this study. Review of the BISC data is usually nism is in relative plate motion and velocity of about 24 months behind time because arrival times subduction. The subduction velocity is less than are manually checked by analysts at the BISC. the plate velocity, resulting in accretion and Later in 2019, the revised BISC dataset until mid- shearing. 2017 was added to the dataset. Waveform data According to the report of the Malaysian from the IRIS free online data archive between Meteorological Department (MMD and ASM 2006 and 2018 are retrieved with a software 2009), the tectonic framework for the whole of (JWEED) developed by seismology department at Malaysia covers between longitudes 90°–140°E the University of South Carolina (USC 2012). This and latitudes of 20°–12°S.