Continual Advance in Earth Physics Research Group at Physics Study Program, Unesa: What’s new and the next step

Earth Physics Research Group (EPRG) is one of three groups of research running at Physics Study Program, the State University of Surabaya, Indonesia, where a number of research projects with corresponding topics have been conducted (and some are in progress) by the group members and associated students having final projects in the field of earth physics since 2018. Whereas the research roadmap of the group has been presented in association with definitive research projects for 25 years long starting from 2011, the specific goal of this paper is to shortly summarise all academic achievement in terms of research performance made by the group members during the last five years. The majority of the recent works was mainly based on computational work, where some were completed in collaboration with researchers from other universities and a national agency and others were performed by the group members and selected students. The topics were spread across disciplines in earth physics that included tectonic earthquakes, tsunami generation and propagation of seismic and non-seismic origin, volcanic eruptions and an integrated disaster mitigation study. A small portion of the projects were performed using a chosen method of applied geophysics. These studies have ended up with publications in recent years, where the saline points of the key findings are here presented. Future studies focusing on vulnerability to earthquake hazards in the northern areas of Java and on volcanic and meteo-tsunamis are also discussed in the context of possible tsunamis induced by seismic sources or volcanic processes.


Introduction
The boundary of Indonesian territory is made up with a border line, stretching over 18,000 km long from three major tectonic plate boundaries, namely the Indo-Australian, Eurasian and Pacific Plates that encompass one of the most active tectonic regions on Earth [1].This characterises Indonesia with highcurvature, seismically active subduction zones, as well as numerous inland faulting zones [2,3].Indonesia is also geographically located at the perimeter of the so-called Pacific Ring of Fire for which it is home to 129 active volcanos [4].Thus, the Indonesian highly complex, volcano-tectonic setting comprises convergent plate boundaries, extending from off the west coasts of the north Sumatera to the Banda Arc in the eastern provinces, making both ground-based and coastal regions in Indonesia vulnerable to geological disasters that include tectonic earthquakes, tsunamis and volcanic eruptions.While the primary cause for tsunami generation and its corresponding propagation is attributable to earthquake of tectonic origin for global observations [5], in the Indonesian context, however, previous studies on possible tsunami sources also pointed out volcanic processes [6,7] and landslides [8][9][10] as potential sources of such a disaster, in addition to seismic sources [11,12].
As the potency for geological hazards in Indonesia is large enough and in the light of increasingly widespread dense population in domestic regions [13] prone to these hazards, research focusing on this subject is of significance.Recent research progress has been made by the EPRG members during the last five years along with international publications sourced from three disciplines of earth physics, as depicted in Fig. 1.This supports for the development of disaster mitigation study and risk reduction.It is then the primary purpose of the present paper is to report and discuss some of this progress.

Projects towards Seismological and Volcanological Topics
One of challenging problems in seismic studies is determination of a reliable scaling relationship, outlining a functional relation between earthquake source parameters, such as seismic moment Mo or moment magnitude Mw and rupture area A. Using waveform inversion from the inland shallow crustal occurrences around Japan, widely known as a seismically active region, Miyakoshi et al. [14] reported that the source scaling is much affected by the source regionality.For magnitudes of greater than Mw 7.4 they found scaling saturation but did not relate it to specific asymptotic values of either magnitude or rupture area.Hence, the earthquake size estimate based on the empirical scaling of Mw and A remains challenging for events with large magnitudes.Using the global earthquake datasets of varying magnitudes from moderate to large sizes during 1960-2015, Prastowo et al. [15] derived the scaling for four distinct types of faulting mechanism that included strike-slip, normal, reverse and subduction events.This work claimed that the Mw scale for the subduction megathrust events asymptotes to a value of Mw ≈ 9.3, close to Mw 9.2 found for the 1960 Chilean event [16,17] and possible to achieve at rupture areas much larger than those for strike-and dip-slips (normal and reverse) faulting mechanisms.The magnitude-area scalings were proved to be consistent with those reported by earlier work [18] and considered well suited for tsunami-earthquake hazard analysis and risk assessment in Indonesia.
In another investigation into global seismicity using earthquake datasets during 1906-2015 events from interplate and intraplate processes (becoming a final project completed by a final year student), Mw was then proved to be proportional to 0.3 ln Mo for all the source depths, including shallow, intermediate and deep sources with major earthquakes were found to locate themselves at 0-20 km deep beneath the surface for shallow events, at 100-200 km for intermediate events and at 550-600 km for deep events.The results provide insight into a better understanding of two measures of earthquake strength, namely Mw and Mo in terms of a mathematical Mw-Mo relation found to be independent of the source depth and location.
Other projects for both the EPRG members and final year students include kinematic examination of earthquake rupture processes using the so-called multiple signal classification of back-projection imaging (MUSIC BPI) [19,20], referred to seismic signal processing with high resolution that relies on the signal coherence from P-seismic waveforms recorded by a dense seismic array and filtered at high frequency of 0.25-1.0Hz.Applied to the case of the 2018 Palu-Donggala earthquake that in turn generated a locally propagating tsunami inside Palu Bay, this technique was successful to demonstrate a NNW-SSE trend in the aftershock data of the event [21].Similarity found in the rupture direction to the lineament of the strike-slip Palu-Koro fault suggested that the 2018 Palu tsunami-earthquake event is partly associated with the active fault, consistent with previous work [22,23].Following this study, a project was completed this year by a final year student (but the work has not yet published), further utilising MUSIC BPI to resolve rupture propagation during the 2004 Sumatera-Andaman earthquake.Kinematics of the propagation was well resolved by about the same values of rupture duration and its corresponding extent, speed and directivity as those found in reference work [24,25].Currently, another MUSIC BPI project for examining rupture processes of the 2006 Yogyakarta earthquake is being performed.The current study focuses on whether energy radiation is sourced from Opak Fault or a combination of shallow seismo-volcano processes in the presence of Merapi Volcano located nearby to the north-west direction of the hypocentre [26,27].
Research associated with a volcano was also finished last year when the EPRG members examined the flow of lahars observed from the 4 December 2021 Semeru eruption.The eruption was initiated by the extreme rainfall a few days earlier, triggering the collapse of the dome on the top, which led to pyroclastic flows made up of hot lavas and lahar deposits, running quickly downstream of the slope.Collecting gravity data from TOPEX satellite measurements and using some specific software, Realita et al. [28] analysed the data and prompted that the flow descends downstream in the direction of the southeast, extending to a distance of approximately 20 km away, in good agreement with field observations from past eruptions [29][30][31].In a project currently performed by a final year student, the gravity method is being utilised to identify geothermal potential in the Arjuno-Welirang volcanic system.In addition, during a year of 2022, the group members finished writing a student handbook entitled The Physics of Volcanos [4] for teaching purposes at Physics Study Program, Unesa.
In the last year's study [32], the group worked with an alumnus from Physics Study Program, exploring the best appropriate technique suitable for characterising seismicity through determination of a reliable declustering process and completeness magnitude for eastern Indonesian provinces, including West Nusa Tenggara (NTB) and East Nusa Tenggara (NTT).The final results revealed that the Reasenberg and Best Combination (BC) techniques were both reliable for accurate calculations of magnitude of completeness Mc and the other two seismic parameters, namely a and b.However, these parameters were affected by the specific declustering algorithm chosen and accurate calculation of Mc.A further study working on detailed seismicity in these regions of interest is just completed, leading to a submission to a reputable, peer-reviewed journal.

Projects towards Tsunami Research
Tsunami research has been increasingly important in Indonesia since the deadliest 'boxing day' tsunami on 26 December 2004.In response to such a devastating disaster and its resulting fatalities, Indonesian Agency for Geophysics, Climatology and Meteorology (BMKG) has set up a tsunami early warning system (Ina-TEWS), running routinely tsunami monitoring mainly based on seismic signals.This is because most tsunamis are initiated by earthquakes of tectonic origin.Since the early stage, Ina-TEWS has developed over time.For example, the warning system is now able to run effectively by taking earthquake-tsunami parameters into account for enhancing performance, where it takes only about 4 minutes for tsunami alert to be issued after a submarine earthquake occurs [33][34][35].
An alternative monitoring technique using secondary magnetic signals induced by tsunami passage has also been investigated by the EPRG member to detect tsunami propagation in the open ocean [36].The shortcomings of this technique include the fact that tsunami-magnetic observations are rare to use compared to seismic measurements.Theoretical considerations from previous work [37][38][39] stated that the movement of electrically conducting ocean water across the background magnetic field lines induces weak magnetic signals.These signals were, however, observed by in situ sensitive instruments during large trans-Pacific tsunamis [40][41][42][43] as magnetic anomalies of ~ tens nanotesla, compared with the ambient main field of 30−6 ×104 nT (http://www.ngdc.noaa.gov/IAGA/vmod/).
Motivated by the above studies, Prastowo et al. [44] and Prastowo et al. [45] considered tsunami cases in Indonesia either generated by an earthquake of tectonic origin in the 2004 Indian Ocean tsunami, a submarine landslide in the 2018 Palu tsunami or a flank failure of Anak Rakata volcano in the 2018 Sunda Strait volcanic tsunami.They observed that the vertical component bz of the secondary magnetic field is well predicted by theory, consistent with recorded magnetograms available for use of tsunami detection and accessible at sites https://intermagnet.org/ and/or http://www.bcmt.fr/.The main point to note is that early detection of tsunami excitation and its corresponding propagation via tsunami magnetic signals is possible to do by theory, possibly validated by available magnetogram.Going further, a new project is then offered for a final-year student this year, examining magnetic signatures from the case of the 15 January 2022 Hunga Tonga-Hunga Ha'apai (HTHH) tsunami.A crucial question raised for the 2022 HTHH case is whether a meteo-tsunami induces magnetic signals [46] and if it does, whether the signals can be both predicted by theory and observed by magnetogram.
Another important issue associated with the 2022 HTHH case is the presence of a dual mechanism that controls tsunami propagation, following an explosive eruption of the submarine volcano [46,47], for which the wave travels at different speeds, namely a volcanic tsunami with the long wave speed in the near-field and a fast-travelling meteo-tsunami in the far-field [48][49][50].The EPRG put attention to this issue by examining complete travel time data, a technique similar to earlier work [51], and then by estimating the speed of the meteo-tsunami before in turn investigating amplitude attenuation during the propagation.Using tsunami waveforms provided by Deep-ocean Assessment Reports of Tsunamis (DARTs) and tide gauges operated by National Oceanic and Atmospheric Administration (NOAA) at https://www.ngdc.noaa.gov/hazard/dart/2022tonga.html,we found that the wave travels at 220 ms-1 for about 2,700 km from the volcano in the near-field before further advancing at the Lamb wave speed of approximately 304 ms-1 in the far-field, in good agreement with previous work [52][53][54].In addition, the maximum amplitude-distance analysis revealed that the observed amplitude is a power function of travel distance, within which the amplitude drops rapidly in the early stage before travelling at nearly constant amplitude [55].During this final stage, the wave decays with time with the decay time is estimated to be 9.5 h, consistent with previous work [48,50,53].
When studying tsunamis generated by undersea earthquakes, of particular interest is a relationship between earthquake magnitude and maximum tsunami amplitude from which we can parameterise the sea surface elevation due to tsunami passage with the source magnitude or estimate the magnitude by measuring the resulting sea surface elevation during tsunami propagation.Driven by this idea, the head of the EPRG along with a number of physics students examined seven trans-Pacific and six Indonesian tsunamis [56], using data from monitoring instruments (DART buoys and tide gauges) accessed at http://ngdc.noaa.gov.The results revealed that the earthquake magnitude Mw is better parameterised with the mean amplitude η.For the trans-Pacific tsunamis, Mw = 0.77 log η + 8.84, in good agreement with previous work [57].For the Indonesian events, Mw = 1.92 log η + 10.36, indicating different dynamics of tsunami wave propagation across both the Pacific and Indian Oceans.The apparent difference is thus attributable to differences in ocean irregular bottom topography and tsunami directivity.

Projects towards Vulnerability and Disaster Mitigation Study
Good background in physics relevant to geological disasters is necessary for university students.The physics of such disasters can be delivered to the students via lecturing in a classroom setting or for their final projects in the university.Two EPRG members were in collaboration with lecturers from Physics Education Study Program as well as a researcher from Charles Darwin University (CDU), Australia, to conduct joint-research on the implementation of Science, Technology, Engineering and Mathematics-Disaster Risk Reduction (STEM-DRR) education in building potential strategy required for increasing awareness through improving knowledge of disaster preparedness to minimise disaster risk at all cost [58].A key finding from this research is a better perception of integrated STEM-DRR education with two critical points to note; the strategy depends on the type and location of a disaster and a follow-up study with multidisciplinary approaches to solve the difficulties in practice needs to be performed by considering a broader range of research area coverage.
In a further study, Anggaryani et al. [59] explored the involvement of university students as experiential learners in practising virtual reality (VR) as a pedagogical tool for technology-assisted learning.The specific goal was to promote STEM-DRR at higher education.Ten students in their final year were selected to use modern equipment of MilleaLab VR media available for examining students' responses to a set of VR learning associated with a disaster.The study reported that VR media provides insight into the use of digital technology, helpful for educational purposes in a wider sense, including the introduction of disaster preparedness to university students.The VR use was found to widen the students' ideas about disasters by bringing interactive, engaging and meaningful learning into a real journey of life.Finally, the work suggested that technology-enhanced learning with modern equipment supplied, in support of distance learning, is proven economical and efficient.
In another study but with no direct relation to educational aspects, we examined vulnerability to earthquake hazards in the Java northern regions [60] that poses increasing danger to local community in the northern coasts of Java.In addition to persistent threats from the active subduction zone located off the Java south coasts [1][2][3]11,12] potential earthquake sources from shallow crustal seismicity in the northern regions of Java induced by inland fault activity have been in search [61][62][63][64][65][66][67].The primary goal of this study was to assess vulnerability in the presence of seemingly connecting crustal faults along the northern regions.The methods included calculations of both a-and b-values inferred from seismic zones made available for the study area and determination of source mechanisms derived from seismic inversions obtained for two recent events.The results showed that the highest seismicity rates are found in the northern regions of West Java and Banten (corresponding to the highest a-value of 8.55) and the relatively high-stress tectonic regimes in the northern areas of Central Java and East Java (corresponding to the lowest b-value of 0.8).The high seismicity rate in the West Java northern side is supported by [68].The high-stress tectonic regimes in the north of Central Java and East Java were in line with the results of time-domain moment tensor (TDMT) solutions, giving magnitudes of Mw 6.7 and Mw 7.0 with a resulting major double-couple component for the normal faulting types found for the events in examination, consistent with USGS.Of particular interest, the results reported claimed for the significance of awareness of seismic threats possible to occur in the Java northern regions thereby requiring investigations into detailed seismicity in the regions of interest for future work.

Conclusion
Vulnerability and disaster mitigation studies in the Indonesian context require a comprehensive, multidisciplinary approaches.These studies are associated with anthropogenic (hydrometeorological) and non-anthropogenic (geological) hazards.However, the focus of the present review is only on those classified into geological hazards that involve tectonic earthquakes, tsunamis and volcanic eruptions.The EPRG at Physics Study Program, Unesa has facilitated its members along with final year students to conduct research in these topics and it is the specific purpose of the review is to briefly summarise all research activities performed in the group during the last five years.Some projects are completed in collaboration with researchers from other universities and/or a relevant agency with the topics chosen are diverse but limited to earth physics or applied geophysics.These have ended up with publications in recent years.The key points to note from the recent publication are provided as follows.Firstly, tsunami magnetic signals could be considered as an alternative solution to early tsunami detection.Given that these signals are relatively weak compared to the main field, sensitive magnetic sensors are required to better detect a propagating tsunami.In this sense, future research in instrumentation and magnetic sensors is thus challenging.Secondly, a comprehensive seismic study examining seismicity rate and vulnerability in a region of interest with a more detailed look at unidentified faults is urgently needed to properly assess seismic hazard potential in the region.Thirdly, the existing Ina-TEWS is only designed for detection of tsunami excitation and propagation generated by tectonic earthquake.As the 2018 Sunda Strait tsunami is initiated by volcanic processes of Anak Rakata volcano as well as the 2022 complex HTHH tsunami propagation is controlled by a dual physics mechanism, future work on a volcanic tsunami and/or a meteo-tsunami is necessary.For Indonesian context, this is performed to better suit the problem of a possible disaster associated with tsunamis induced by seismic sources or volcanic processes, including the one driven by atmospheric forcings.

Figure 1 .
Figure 1.Research roadmap run by Earth Physics Research Group, Physics Study Program, the State University of Surabaya, outlining research topics or projects that have been conducted, as well as those are in progress and for future work from 2011 to 2035.