Reduced Mass Flux due to Mixing in Controlled Exchange Flows with Possible Applications to The Indonesian Through-Flow

The Indonesian Through-Flow (ITF) is a system of surface ocean currents flowing from the western Pacific Ocean through numerous straits and sills within the eastern Indonesian seas into the Indian Ocean. The ITF carries a large amount of heat and water affecting climate on regional and global scales. A more detailed, comprehensive study by either direct measurements, numerical simulations or laboratory experiments is then needed to fully understand the flow. In this context, laboratory experiments are used to study the characteristics of mixing in density-driven exchange flows, so as to examine similar situations occurring in the flow. Theexperiments have also particular relevance to improving predictions of mixing in deep overflows, as well as to understanding the dynamics of exchange flows between water bodies such as estuaries, marginal seas and the open ocean. Vigorous turbulence which leads to irreversible mixing is observed in the experiments and the resulting reduction in exchange flux is measured to be 82% of the maximal exchange predicted by hydraulic theory. The possible application of the experimental results to Indonesian sea settings is also discussed as the results are relevant to physical oceanographic problems in the Indonesian waters

ESTIMASI PARAMETER a-VALUE DAN b-VALUE UNTUK ANALISIS STUDI SEISMISITAS DAN POTENSI BAHAYA BENCANA GEMPA TEKTONIK DI WILAYAH MALUKU UTARA

Abstrak Studi seismisitas dan analisis potensi bahaya bencana seismik di wilayah Maluku Utara dapat dilakukan dengan menentukan parameter a-value dan b-value di wilayah tersebut. Kedua parameter mendiskripsikan level seismisitas dan akumulasi stres mekanik yang disimpan oleh batuan geologi bawah permukaan di wilayah tersebut. Secara prinsip, parameter a-value dan b-value ditentukan dari distribusi frekuensi-magnitudo gempa melalui hukum Gutenberg-Richter. Dalam penelitian ini, hukum Gutenberg-Richter diterapkan pada sumber gempa tektonik dari katalog gempa USGS (http://earthquake.usgs.gov/earthquakes/) selama 2009-2019 dengan kedalaman mencapai 551 km dan variasi magnitudo . Kedua parameter dihitung dengan metode least-squares dan maximum likelihood, di mana perbedaan signifikan a-value dan b-value menurut kedua metode tersebut merefleksikan level akurasi kedua metode tersebut. Metode maximum likelihood memberikan a-value dan b-value yang lebih akurat karena melibatkan penapisan data sebelum proses pengolahan data. Persamaan empiris Gutenberg-Richter yang diperoleh dari metode maximum likelihood adalah , di mana a = 9,73 dan b = 1,39 dengan  adalah jumlah kejadian gempa dan  adalah gempa dengan magnitudo lebih besar dari  (batas bawah magnitudo di mana hukum Gutenberg-Richter berlaku valid). Analisis variasi spasial dan temporal b-value serta variasi spasial a-value berhasil merekonstruksi 3 kejadian gempa relatif besar antara 2009-2019. Kombinasi temuan b-value ≈ 1,4, a-value ≈ 9,7 dan a-value (annual) ≈ 8,7 dengan bantuan ZMAP6,0 menunjukkan bahwa seluruh wilayah Maluku Utara merupakan wilayah yang rentan terhadap bencana gempa dengan frekuensi gempa tinggi yang dipicu seismisitas relatif tinggi di wilayah tersebut. Temuan ini memicu peningkatan kesadaran dan kesiagaan terhadap potensi bahaya bencana gempa tektonik di Maluku Utara.   Kata Kunci: seismisitas, bencana seismik, a-value, b-value, hukum Gutenberg-Richter Abstract Seismic studies and corresponding seismic hazard analysis in North Maluku can be performed using determination of a-value and b-value parameters. These parameters describe seismicity level and mechanical stress accumulated in subsurface structure in the region of interest. In principle, a-value and b-value were obtained from frequency-magnitude distribution provided by Gutenberg-Richter law. In this study, this law was generated using earthquake datasets from USGS at http://earthquake.usgs.gov/earthquakes/, where events occurred between 2009-2019 with varying magnitudes  and depths to 551 km. The methods included the least-squares and maximum likelihood, where significant differences in the parameters acquired reflect levels of accuracy. The maximum likelihood method yielded accurate results for a-value and b-value due to data declustering prior to data processing. The Gutenberg-Richter law in a log-linear expression  was obtained, where a = 9.73 and b = 1.39 with  is the cumulative number of occurence and  denotes events with magnitudes greater than  (defined as the magnitude at which the lower end of the distribution starts to deviate from the Gutenberg-Richter law). Analysis of spatial and temporal variations of b-value and spatial variation of a-value successfully reconstructed 3 occurrences of large magnitudes during 2009-2019. A combined finding of b-value ≈ 1,4, a-value ≈ 9,7 dan a-value (annual) ≈ 8,7 by ZMAP6.0 found for North Maluku reveals that the whole parts of the region are vulnerable to tectonic earthquakes with high frequency owing to relatively high seismicity. This calls for increased awareness of and preparedness for possible seismic threats in North Maluku.   Keywords: seismicity, seismic hazard, a-value, b-value, Gutenberg-Richter la

PENENTUAN PARAMETER SEISMIK a-VALUE DAN b-VALUE UNTUK ANALISIS POTENSI BENCANA GEMPA DI WILAYAH MALUKU

Abstrak Kerentanan wilayah Maluku terhadap bencana gempa dipelajari melalui analisis potensi bahaya bencana gempa dengan menentukan parameter seismik a-value dan b-value yang diperoleh dari data statistik gempa tektonik yang terjadi di wilayah tersebut selama kurun waktu tertentu. Kedua parameter seismik mendiskripsikan level seismisitas dan stress mekanik yang disimpan dalam lipatan batuan bawah permukaan. Secara prinsip, kedua parameter ditentukan dari distribusi frekuensi-magnitudo gempa dengan bantuan hukum Gutenberg-Richter. Pada penelitian ini, data sekunder distribusi frekuensi-magnitudo gempa diperoleh dari katalog USGS (http://earthquake.usgs.gov/earthquakes/search/) dengan variasi kedalaman sumber sampai 625 km di bawah permukaan dan variasi magnitudo gempa  selama kurun waktu 2009-2019. Perhitungan parameter a-value dan b-value dilakukan dengan metode kuadrat terkecil dan maximum likelihood untuk uji konsistensi dan reliabilitas estimasi kedua parameter tersebut. Estimasi berdasarkan metode maximum likelihood memberikan hasil yang lebih akurat dan stabil karena melibatkan penapisan data sebelum proses pengolahan data dilakukan. Berdasarkan hasil analisis diperoleh persamaan empiris Gutenberg-Richter dengan metode maximum likelihood adalah , di mana  dan  dengan  adalah frekuensi kejadian gempa dan  adalah magnitudo yang lebih besar dari , batas bawah magnitudo di mana hukum Gutenberg-Richter masih berlaku. Akurasi hasil estimasi a-value dan b-value dijamin melalui penentuan  yang akurat, di mana  = 5,0 diperoleh dari plot distribusi frekuensi-magnitudo gempa. Berdasarkan nilai parameter a-value dan b-value, dapat disimpulkan bahwa wilayah Maluku memiliki level seismisitas yang relatif tinggi dan rentan terhadap potensi bencana gempa tektonik yang dipicu oleh aktivitas seismo-tektonik patahan lokal Sorong dan dua zona mikro-subduksi Busur Sangihe dan Busur Halmahera.   Kata Kunci: seismisitas Maluku, gempa tektonik, a-value, b-value, hukum Gutenberg-Richter Abstract The vulnerability of Molucca to seismic hazards can be examined through analysis of earthquake-event potential in the region. This analysis can be performed by determining parameters a-value and b-value acquired from datasets of events during a time period, describing seismicity rate and mechanical stress accumulated within local crustal rocks. These parameters were estimated using the Gutenberg-Richter law in this study using frequency-magnitude distribution obtained from the USGS catalog at http://earthquake.usgs.gov/earthquakes/search/ with varying depths to 625 km below the surface and earthquake sizes  during 2009-2019. The calculations were carried out using two separate methods, the least squares and the maximum likelihood to search for consistency and reliability of the results. Estimates using the maximum likelihood method provides results that are more accurate and stable due to data filtering prior to data processing. Using the maximum likelihood method selected, the Gutenberg-Richter empirical equation was found to be , where  and  with  denotes the event frequency and  represent magnitudes greater than , defined as the lower end of magnitudes above which the Gutenberg-Richter law applies. The accuracy in a-value and b-value estimates was provided by accurate determination of = 5,0 directly obtained from the plot of the frequency-magnitude distribution. Based on the calculated parameters, it can be concluded that Molucca is in the relatively high level of seismicity and prone to seismic threats owing to a combined effect of seismo-tectonic activities from both a local active Sorong fault and subduction processes of Sangihe arc and Halmahera arc.   Keywords: Molucca seismicity, tectonic earthquake, a-value, b-value, Gutenberg-Richter la

ANALISIS RELASI MOMEN SEISMIK DAN MAGNITUDO MOMEN UNTUK VARIASI KEDALAMAN SUMBER GEMPA TEKTONIK (SHALLOW, INTERMEDIATE, AND DEEP SOURCES)

Abstrak Karakteristik gempa dapat dipelajari melalui relasi antara parameter-parameter sumber. Dua parameter sumber yang berfungsi sebagai ukuran kekuatan gempa adalah magnitudo momen  dan momen seismik . Penelitian ini fokus pada analisis relasi empiris    untuk variasi kedalaman: shallow (0-100 km), intermediate (100-300 km), dan deep (  300 km) sources. Data penelitian adalah 242 gempa tektonik yang diperoleh dari berbagai sumber reliabel (publikasi internasional) baik intraplate maupun interplate events yang terjadi di seluruh dunia, termasuk Indonesia antara 1905-2016 dengan magnitudo antara 4,45  9,20. Hasil penelitian berupa persamaan empiris untuk setiap kedalaman sumber,  = 0,287 ln  + 5,952 (shallow sources),  = 0,290 ln  + 6,047 (intermediate sources) dan  = 0,285 ln  + 5,816 (deep sources). Ketiga persamaan tersebut bersifat self-consistent karena merepresentasikan relasi empiris    dengan bentuk grafik yang sama. Nilai koefisien ln  untuk ketiga persamaan tidak berbeda sampai dengan desimal kedua sedangkan variasi kecil nilai konstanta tidak memberikan informasi yang jelas tentang perbedaan antara ketiga sumber gempa. Relasi empiris    yang diperoleh dari penelitian ini konsisten dengan temuan penelitian terdahulu, yaitu  merupakan fungsi logaritmik . Selain temuan tersebut, penelitian ini juga menemukan distribusi mayor kedalaman sumber adalah kurang dari 20 km (shallow sources), antara 100-200 km (intermediate sources), dan 550-600 km (deep sources). Temuan ini konsisten dengan karakteristik seismisitas global. Hasil-hasil penelitian ini memberikan pemahaman tentang relasi    yang bersifat universal (tidak bergantung pada kedalaman sumber gempa) dan global (tidak bergantung pada lokasi kejadian gempa).   Kata Kunci: magnitudo momen, momen seismik, relasi empiris   , variasi kedalaman sumber Abstract Characteristics of earthquakes can be examined using analysis of source parameters. Two source parameters used as a measure of earthquake strength are momen magnitude  and seismic moment . This study focuses on analysis of a    empirical relation for varying depths: shallow (0-100 km), intermediate (100-300 km), dan deep (  300 km). The data were 242 occurrences (intraplate and interplate events worldwide between 1905-2016) obtained from reliable published studies with varying magnitudes of 4.45  9.20. The results were in the form of an empirical equation for each source depth,  = 0.287 ln  + 5.952 (shallow sources),  = 0.290 ln  + 6.047 (intermediate sources) dan  = 0.285 ln  + 5.816 (deep sources). These equations are self-consistent as they represent the    relation in the same form of graphs. Coefficients of ln  are similar for all the three equations to two decimal places while small variations of constants give no clear information on differences in the source depth. The    relation obtained is consistent with previous work in the sense that  is a logarithmic function of . In addition, the current study found major earthquakes were distributed at depths below 20 km for shallow sources, between 100-200 km for intermediate sources, 550-600 km for deep sources, in good agreement with characteristics of global seismicity. The results of this study provide a better understanding of the    relation, which is universal and global, independent of source depth and location.   Keywords: moment magnitude, seismic moment,    empirical relation, varying depth

SEISMISITAS JAWA TIMUR DAN POTENSI BAHAYA BENCANA SEISMIK TERKAIT

Abstrak  Seismisitas Jawa Timur dan potensi bencana gempa tektonik terkait dapat dipelajari melalui analisis parameter seismik: -value, -value, dan anomali -value. Nilai  merefleksikan level seismisitas, nilai  menunjukkan level stres batuan bawah permukaan, dan anomali nilai  menjadi prekursor gempa besar. Ketiga parameter tersebut secara akurat diperoleh dengan memanfaatkan data frekuensi-magnitudo (FMD) dari katalog USGS antara 1973-2020. Kurva FMD menjadi basis penerapan hukum Gutenberg-Richter melalui pendekatan maximum likelihood untuk data gempa tektonik dengan magnitudo antara 3,1  7,8 dan kedalaman mencapai 574 km di bawah permukaan. Perhitungan ketiga parameter dilakukan dengan membagi wilayah Jawa Timur menjadi Zona Utara, Zona Selatan, dan Zona Utuh dengan hasil-hasil untuk Zona Utara:  = 5,77 dan  = 0,96; Zona Selatan:  = 6,49 dan  = 0,81; dan Zona Utuh:  = 6,80 dan  = 0,87. Berbeda dengan nilai  dan , anomali nilai  tidak dinyatakan dalam bentuk angka melainkan dalam bentuk peta variasi spasio-temporal -value. Analisis hasil-hasil perhitungan  dan  untuk ketiga zona seismik menunjukkan bahwa seismisitas wilayah selatan Jawa Timur lebih tinggi daripada seismisitas wilayah utara karena stres tinggi batuan yang dipicu aktivitas seismo-tektonik sepanjang zona subduksi dekat Palung Jawa. Dengan demikian, wilayah selatan Jawa Timur lebih rentan terhadap potensi bencana gempa seismik. Analisis variasi spasio-temporal -value menemukan wilayah selatan Jawa Timur dengan nilai  yang rendah, bertepatan dengan saat gempa besar (tsunami earthquake) terjadi pada bulan Juni 1994. Hasil-hasil penelitian ini sama dengan temuan penelitian terdahulu yang relevan dengan studi seismisitas Pulau Jawa.   Kata Kunci: seismisitas Jawa Timur, -value, -value, anomali -value, hukum Gutenberg-Richter Abstract Seismicity in East Java and its potency for earthquakes can be examined using parameters: -value, -value, and -value anomaly. Parameter -value reflects seismicity level, -value indicates stress level of subsurface structure, and -value anomaly is a precursor for a large event. The parameters were accurately obtained from frequency-magnitude distribution (FMD) in the region of interest between 1973-2020 based on USGS catalogue. The FMD serves as a basis for the Gutenberg-Richter (GR) law through maximum likelihood for data with varying magnitudes of 3.1  7.8 and depths reaching 574 km below the surface. Determination of the parameters was performed by dividing East Java into Northern Zone, Southern Zone, the whole Zone. The results were for Northern Zone:  = 5,77 and  = 0,96; Southern Zone:  = 6,49 and  = 0,81; and the whole Zone:  = 6,80 and  = 0,87, respectively. Different from -value and -value, the anomaly in -value was not represented in numbers but it was given in the spasio-temporal -value. Analysis of the results for each zone showed that seismicity in the southern region of East Java is relatively higher than in the north, due to a high-stress region induced by tectonic activities along the subduction zone near the Java Trench. Therefore, the southern region is vulnerable to geohazards. Analysis of spasio-temporal -value found a low -value off the south coast, associated with a location of tsunami earthquake in June 1994. The current results are consistent with previous findings for Java seismicity.   Keywords: East Java seismicity, -value, -value, -value anomaly, Gutenberg-Richter la

ANALISIS ATENUASI MAXIMUM TSUNAMI AMPLITUDE DAN ENERGY DECAY TIME UNTUK TSUNAMI LINTAS SAMUDERA PASIFIK

Abstrak Saat merambat di laut lepas, tsunami mengalami pelemahan energi yang terukur sebagai pelemahan amplitudo maksimum. Dalam hal ini, evolusi amplitudo maksimum bisa digunakan untuk menentukan energy decay time yang didefinisikan sebagai waktu dari saat pembangkitan tsunami sampai saat tsunami tidak lagi mendapat pasokan energi dari sumber. Grafik atenuasi amplitudo maksimum untuk ketujuh kasus tsunami dalam penelitian ini (Samoa 2009; Maule, Chili 2010; Tohoku, Jepang 2011; Iquique, Chili 2014; Illapel, Chili 2015; Chiapas, Mexico 2017 dan Hunga Tonga–Hunga Haʻapai (HTHH), Tonga 2022) menunjukkan pelemahan amplitudo maksimum melalui dua fase. Fase pertama ditandai pelemahan amplitudo maksimum secara cepat terhadap jarak dan waktu tempuh dalam pengamatan medan dekat (3200 km). Fase kedua ditandai amplitudo maksimum yang relatif konstan dalam pengamatan medan jauh (> 3200 km). Penentuan energy decay time ketujuh kasus telah mempertimbangkan sebaran energi gelombang tsunami dalam perimeter Samudera Pasifik. Untuk kasus Maule, Chili 2010; Tohoku, Jepang 2011; Iquique, Chili 2014 dan Illapel, Chili 2015, energy decay time adalah 22,5 jam tidak berbeda jauh dari temuan terdahulu. Untuk kasus Samoa 2009, energy decay time adalah 16,36 jam tidak berbeda jauh dari temuan terdahulu pada kasus yang sama dan untuk volcanic tsunami (HTHH, Tonga 2022), energy decay time adalah 14,46 jam, indikasi serapan internal yang dipicu oleh distribusi frekuensi tsunami. Untuk kasus Chiapas, Mexico 2017, energy decay time hanya 9,23 jam, indikasi large power dissipation karena serapan eksternal oleh continental shelves dekat episenter gempa. Kata Kunci: maximum tsunami amplitude, jarak dan waktu tempuh, energy decay time Abstract When propagating at sea, a tsunami experiences energy attenuation measured as maximum amplitude reduction used to determine energy decay time defined as the time taken from tsunami generation to a time when the wave no longer gets energy supply from the source. Graphs of the amplitude reduction for seven cases in this study (2009 Samoa; 2010 Maule, Chile; 2011 Tohoku, Japan; 2014 Iquique, Chile; 2015 Illapel, Chile; 2017 Chiapas, Mexico and 2022 Hunga Tonga–Hunga Haʻapai (HTHH, Tonga) showed systematically attenuated maximum amplitude through two phases. The first phase was characterized by rapid attenuation of the amplitude with respect to travel distance and time in near-field observations (3200 km). The second one was characterized by a relatively constant maximum amplitude in far-field observations (> 3200 km). Energy decay time determination has considered tsunami energy distribution in the Pacific perimeter. For the case of the 2010 Maule, Chile; 2011 Tohoku, Japan; 2014 Iquique, Chile and 2015 Illapel, Chile, mean decay time was 22.5 hours, slightly different from previous finding. For the 2009 Samoa, the decay time was 16.36 hours, slightly different from previous finding for the same case while for a volcanic tsunami (HTHH, Tonga 2022), the decay time was 14.46 hours, indicating internal absorption due to frequency distribution. For the 2017 Chiapas, Mexico, the decay time was only 9.23 hours, showing large power dissipation due to external factors, such as continental shelves near the epicenter. Keywords: maximum tsunami amplitude, travel distance, travel time, energy decay tim

A simple parameterization for tsunami run-up prediction

The linear shallow-water approximation is commonly used to describe tsunami propagation, where the wave is assumed as a long surface gravity wave. The evolution of wave height during its propagation from offshore to onshore is a classic problem. When arriving at a shoreline, the increased wave height causes severe destruction on infrastructures and fatalities. This problem has then been an important issue within the context of disaster risk reduction as it gives rise to the importance of tsunami run-up prediction. Using maximum run-up data from past events, we tested the applicability of the Green’s law based on shoaling only to calculate run-ups and found that the basic Green’s law was in doubt. Then, we examined energy density conservation involving refraction effect but no dissipation and derived a simple formula for parameterizing run-up height. Detailed descriptions on factors affecting run-ups, such as complex bathymetry and topography are not yet considered in the current study. The aim of this study is therefore to determine whether the modified Green’s law is applicable for tsunami run-up prediction using local water depths as external parameters and ray spacing widths in the normal direction of wave fronts related to refraction. The results are consistent with the measured run-ups, where approximately 70% of total points of observations confirm the modified Green’s law with a reasonable accuracy