Gravity signatures at the upper plate above subducting asperities along Sunda Arc

The subducting Indo-Australian plate along the Sunda Arc has various degrees of roughness near the trench. The variation in the subducted plate’s seafloor morphology should affect the upper plate’s dynamic at the forearc region and further in land, which would appear in the form of variation in seismicity along the arc. In this paper, we explored the pattern in topography and gravity along the forearc at two regions. First is northern Sumatra, where some ridges have been subducted. The second is east Java, where part of the oceanic plateau of Roo Rise has been subducted. In addition, we also examined the seismicity in those two regions. The elongated ridges pattern of the subducted ridges was observed in the forearc region of Sumatra as the higher anomalies. It is probably due to the continuous nature of the ridge. The broader ruggedness, such as in Roo Rise, does not reflect directly at the Java forearc. However, the anomalies are chaotic and might represent the condition of the rugged subducted plate. The seismicity events at the area of the subducted ridge formed a straight line at the same position where the ridges are assumed to be. Meanwhile, the events on the opposite side of the plateau showed more sporadic distribution, which might indicate the condition of the irregular morphology of the subducted plate.


Introduction
The effect of rough subducted plates in the form of a single or a complex of seamounts, aseismic ridge and plateau has been speculated by many studies.They might influence the morphology, structure, direction of movement of the plate, and earthquake properties [1][2][3][4].The rough subducting plate is also related to the variation of the uplifted and subsided features at the forearc regions [5][6].
The uplifted process would occur with some consequences.A model of a single seamount indicated the development of a pair of faults due to the base of the seamount: a forethrust (landward dipping) due to the top, and backthrust (seaward dipping) [7].Further along the subducting slab, a numerical modeling studied the effect of the seamount on the slab break-off.They found that the slab break-off occurred earlier for the subduction with seamount than without by about 2 Myr difference [8].The slab break-off feature is the reason for the variation of the deepest depths of earthquakes of the same-age seafloor.The most significant issue to explore is the effect of these processes of rough subduction on the earthquake distribution.Several studies indicated that subducting seamounts do not relate to large earthquakes [9,10], but instead, they relate to the distribution of the slip [9,[11][12][13].Furthermore, those studies suggested that a continuous seamount chain across the seismogenic layer might prevent ruptures due to the restriction of the spatial extent of the slip plane.A study by van Rijsingen [10] quantified the roughness of the seafloor adjacent to the trench.They thoroughly studied the effect of the seafloor roughness at several subducted slabs and correlated the roughness to the megathrust that occurred in the past.Their main conclusion is that the less rough the seafloor near the trench, the higher the probability of a large magnitude of earthquake. Figure 1 shows their result along the Sunda Arc, with two boxes of our study locations.In this study, we analyzed the gravity signatures of the uplifted upper plate and geological structures that the rough subducted seafloors might cause in two areas of the Sunda Arc.First is northern Sumatra, where several ridges are on the subducting plate.The second is at the east of Java, where there are plateaus on the subducting plate.With two different types of roughness, we attempted to identify different properties in the forearc/upper plate as the response to the roughness of the subducted plate.

Methods
Satellite altimetry-based gravity data is valuable for tectonic regional study.The global free-air anomaly is available at the Scripps Institution of Oceanography (https://topex.ucsd.edu)[14].The free air anomaly (FAA) maps provide the long wavelengths of the gravity anomalies.We applied several derivative-based filters, as the edge enhancement methods, to obtain the shorter wavelength, which might give local variations in structural pattern.
The edge enhancement method calculates of gradients of spatial distribution value of anomalies.The gradients correlate to the changing physical properties of the subsurface.Since the data is spatial, we have the horizontal direction of values.Therefore, we started with the horizontal derivative, which is the gradient of the potential field anomaly in x and y directions.The second vertical derivative (SVD) is calculated based on the Laplace equation, where the sum of all directions (x, y, and z) second derivatives is zero [15].The tilt derivative or tilt angle (TDR) is the ratio of the vertical and total absolute amplitude of the horizontal gradient of the field [16,17].
As complementary in analysis, we also used the topography data from Topex catalog [18], the centroid moment tensor data from the Global CMT database [19,20] and generated the maps using GMT 5 [21].

Northern Sumatra
The earthquake distribution shown in Figure 2 shows that the thrusts dominate the occurrences offshore of Sumatra.As also mentioned by a seismicity study from that showed at least three linear seismicity tracks at the continuation of the Investigator Fracture Zone (IFZ), with depths from 80 to 200 km [22].
Several ridges of the Investigator Fracture Zone are on the seafloor of the subducting slab in this area.The free-air anomaly map (Figure 2 top right) generally shows north-south lineations of high anomaly at the seafloor of the subducting plate.The accretionary ridge and forearc basin features appear on the other side of the northwest-southeast trend trench.However, those features are not continuously parallel to the trench.Several NE-SW patterns can be observed in broad clusters at the forearc.One of the most prominent features is the high anomaly at Batu Islands, which is supposed to be a part of the basin with low anomaly.This area is one example of the uplifted crust that developed at the continuation of the IFZ ridge.
The vertical derivative map (Figure 2 bottom left) displays the general NW-SE pattern of the subducting plate, except for some variations between the Nias and Siberut Islands, where the Batu Islands are.In this area, there are some high and low north-south patterns.They are precisely at the continuation of the ridge with a high anomaly value.Similar patterns can be seen in the tilt derivative map (Figure 2 bottom right) with more definite lineation.Along this line of anomaly, we also see the earthquake events form a continuous line from the trench to the land.This northeast-southwest lineation of events happens at the area where the IFZ has been subducting.The uplifted part of the Batu Islands is comparable to the Fitzcarrald Arch, which was supposed to be developed due to the subducting Nazca Ridge in the south of America [23].

East Java
The topography map of the offshore of the south of East Java displays the broad elevated morphology of the seafloor of the subducting plate, known as the Roo Rise plateau.The accretionary (frontal) prism developed along the forearc region in various highs.The earthquake CMT distribution indicates a normal solution of events along the trench (Figure 3 top left) due to the plate bending.In the forearc area, thrust/reverse events with trench parallel directions are related to the activity of the seismogenic zone.In the same area, a few normal events are supposed to be related to the uplift progression caused by the subducting seamount [24].
The Free-air anomaly map shows two long clusters of low anomalies indicating the position of the trench and the forearc basin.The vertical derivative and the tilt derivative maps (Figure 3) also show almost uniform patterns of west by north -east by south direction.Compared to the result from Sumatra, the derivative gravity anomaly at Java's forearc region appears more chaotic.However, we can recognize several circular patterns that might be caused by subducted rougher seafloor beneath that area of the upper plate.The edges of those circular patterns coincide with the location of earthquake events.Some small clusters of high anomalies appear at the southern trench, indicating the location of seamounts and plateaus.There is also a smaller circular high anomaly at the east of the forearc region, which might indicate an uplifted part due to the subducted rough plate.In general, the derivative pattern at the Java forearc has more complexity that was hard to define, which might indicate the sporadical roughness of the subducting seafloor of the continuation of the Roo Rise Plateau.The inhomogeneous pattern of the forearc might also reflect the eroded frontal prism due to the subducted seamounts [24,25].

Conclusions
Gravity signatures of the Sunda forearc were developed through the edge detection method.Some uplifted parts of the upper plate are directly identified from the free-air anomaly data, but more structures can be defined from the derivatives edge enhancement methods.Northern Sumatra, where the IFZ ridge has been subducting, has a low roughness, but the effect is directly on the continuation of the ridge.South of East Java, where the Roo Rise Plateau has been subducting, has a relatively higher roughness, and the impact on the upper plate is more dispersed.Information on the effect of the subducting rough seafloor is essential since the seamounts could be the barrier of large earthquakes and influence the rupture distribution.In addition, the uplifted process due to subducting rugged might cause the extensional fault.

Acknowledgement
This study is a part of our studies on the effect of the subducted ridge under the upper plate of Sunda Arc subduction system, funded by "Rumah Program Kebencanaan" from the Research Organization for Earth Sciences and Maritime BRIN No B-2531/III.4/PR.07.00/11/2022.

Figure 1 .
Figure 1.The long wavelength roughness from van Rijsingen et al (2018) [10].The two red boxes are the study locations.

Figure 2 .
Figure 2. Topography and regional centroid moment tensor (CMT) solutions (top left), free-air anomaly (top right), vertical derivative convolution (bottom left) and tilt derivative (bottom right) of of northern Sumatra.

Figure 3 .
Figure 3. Topography and regional centroid moment tensor (CMT) solutions (top left), free-air anomaly (top right), vertical derivative (bottom left) and tilt derivative (bottom right) of eastern Java.