Earthquake Events Detected by Continuous Gravimeters in Indonesia

Earthquake monitoring and early warning systems play a critical role in mitigating the impact of seismic events on human lives and infrastructure. Continuous gravimeters, sensitive instruments designed to measure second changes in gravitational forces, have emerged as valuable tools for earthquake detection and monitoring. This paper reports the significance of continuous gravimeters in detecting earthquake events, highlighting their principle of operation and contributions to seismic research. In contrast to traditional seismic sensors, continuous gravimeters offer a unique perspective on seismic activity by detecting mass redistributions resulting from the movement of Earth’s tectonic plates during an earthquake. This capacity enables them to capture the initial moments of an earthquake, contributing to early warning systems that can provide critical seconds to minutes of advance notice, enhancing preparedness and response efforts. Some earthquake events were detected by the gPhoneX gravimeters (numbers #123 and #126) placed in Institut Teknologi Bandung, Ganesha Campus. Four possible location events matched with the timeframe of the seismometer recording data on June 15, 2023. This result shows that continuous gravimeters have the potential capability to advance earthquake monitoring and early warning capabilities. By offering a complementary approach to seismic sensors, these instruments broaden our understanding of earthquake dynamics and contribute to developing more robust and effective strategies for mitigating seismic hazards.


Introduction
Geophysics explores subsurface conditions by examining various characteristics of the Earth, such as density, susceptibility, resistivity, and the behavior of surface waves.Specific techniques are employed to assess these properties, such as gravity measurements to determine density or magnetic methods to evaluate susceptibility.
The gravity method uses a gravimeter to measure the gravity acceleration on each point due to subsurface density variations.Most gravimeters use spring to measure gravity by observing the change in spring length (s).The length of the spring is capricious following the gravity acceleration (g) due to the mass (M) of the surrounding instrument (Figure 1).Since the spring has a high sensitivity, it is also susceptible to seismic vibrations.Therefore, the Earth's surface's vibrations affect the gravimeter's reading, including vibrations caused by earthquakes.
This paper highlights the significance of continuous gravimeters in earthquake detection, emphasizing their operational principles and contributions to seismic research.Unlike traditional seismic sensors, continuous gravimeters detect mass redistributions resulting from tectonic plate movements during earthquakes, enabling early warning systems to provide critical seconds to minutes of advance notice, enhancing preparedness and response efforts.

Method
Two continuous gravimeters (i.e., gPhoneX numbers #123 and #126) [2] were installed in the BSC-B Building on the 2nd floor of ITB Ganesha Campus for instrument testing and to monitor the gravity change around the station (Figure 2).The main parts of the instrument are the gravimeter sensor, timing module, and gMonitor.These gravimeters have 0.1 microgal reading sensitivity, recording one pulse per second of the data.Multiple corrections are essential to ensure the accuracy of gravity readings, accounting for factors such as tides, sea level variations, sensor drift, pressure fluctuations, barometric effects, polar motion, and the influence of oceanic loads.These corrections refine the obtained gravity data, ultimately yielding a more precise measurement.When no external disturbances are present, the corrected gravity measurements should remain relatively stable over a short timeframe.However, sudden spikes in the data indicate specific detected events, which could be linked to instrument-related activities or seismic events (as depicted in the flowchart in Figure 3).
The corrected gravity data will reveal these interferences in earthquake detection using gravimeter instruments.Gravity signals will exhibit spikes akin to those seen on a seismograph.It's important to note that these spikes are not genuine signals from a mass anomaly but are caused by vibrations that disrupt the gravimeter sensor's equilibrium.Distinguishing between different types of events becomes possible by analyzing the gravity sensor's velocity, position, and/or long-level data.This differentiation is crucial for accurately identifying and characterizing seismic events and separating them from other forms of interference.

Results
Over 17 days, we utilized two gPhoneX gravimeters to capture continuous gravity data.Throughout our observation, we identified multiple anomalies in the data.We conducted thorough processing and analysis to differentiate between seismic and non-seismic occurrences.On June 13, 2023, recorded data, we detected a spike, but after examining the velocity, position, and long-level sensor, we found no evidence of seismic activity.The spike has occurred in a very short period, not followed by a weakening signal.The velocity and position sensor data also showed no vibrations due to an earthquake.Long-level readings strengthen this argument, where the data shows that the spike occurred due to a shift in the leveling axis, as indicated by the blue dashed line in Figure 4.This discovery contrasts with the June 15, 2023, recorded events, as exemplified by the red dashed line in Figure 4.The seismic event shows a signal similar to a seismometer reading, started by a high amplitude signal followed by a decaying signal over time.
Despite our efforts to focus on seismic events, we could not pinpoint the precise location of the earthquake directly from the gravity data.Since the recorded data came from a single station, it is impossible to predict the source location.To gain more comprehensive insights, we cross-checked the preliminary seismic events with seismometer data from Indonesia's Meteorology, Climatology, and Geophysics Agency (BMKG) [3].The results confirmed that some earthquakes did indeed occur during the timeframe near the gPhoneX measurement station (Figure 5).However, we could not determine the earthquake's exact location due to insufficient data.The earthquake data service database [3] only provided limited information during that time, as many individuals reported experiencing more earthquakes [4,5,6,7] than what was available in the provided data.
This research suggests that ongoing gravity data can prove valuable in identifying and tracking earthquakes.With the ability to sense changes in mass due to the movement of Earth's tectonic plates during an earthquake, continuous gravimeters offer a distinct perspective on seismic activity.This feature enables them to capture the initial moments of an earthquake, which can significantly contribute to early warning systems and provide critical seconds to minutes of advance notice, as reported in another study [8].This, in turn, can strengthen preparedness and response efforts.Furthermore, continuous gravimeters have the potential to advance earthquake monitoring and early warning capabilities.By complementing seismic sensors, these instruments broaden our comprehension of earthquake dynamics and aid in developing more reliable and effective strategies for mitigating seismic hazards.

Conclusion
Continuous gravimeters are good tools for detecting and monitoring seismic events such as earthquakes.These advanced devices can detect changes in mass caused by movements of tectonic plates, providing valuable early warning before an earthquake occurs.Integrating continuous gravimeters into earthquake monitoring systems can significantly improve preparedness and response efforts, enhancing our ability to respond to and recover from seismic activity in vulnerable areas.Furthermore, this study emphasizes the importance of using advanced data analysis techniques to distinguish genuine seismic signals from other types of interference, such as shifting leveling axes or instrument-related artifacts.It is crucial to carefully scrutinize these potential sources of error to ensure accurate earthquake detection.Through meticulous data analysis, continuous gravimeters can contribute to scholarly knowledge, increase the precision and reliability of identifying seismic events, and advance our understanding of mitigating seismic hazards.Ultimately, this promotes greater safety and resilience in our communities.

Figure 3 .
Figure 3.The non-steady value of corrected gravity data indicates there are interference sources.

Figure 4 .Figure 5 .
Figure 4.The signal of continuous gravity measurement (top).The spikes on the top image indicate certain events that were detected.These events can be linked to the activities of an instrument (shown in blue dashes on the middle image) or seismic events (shown in red dashes on the bottom image).Using the velocity, position, and/or long level of the gravity sensor makes it possible to distinguish between different types of events.The GUI is presented by Tsoft software version 2.2.15, provided by the Royal Observatory of Belgium[9].