Anomalous propagation of long-period sPL waves from an intermediate-depth earthquake in the Mariana region

Anomalous propagation of long-period sPL waves from an intermediate-depth earthquake in the Mariana region | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Anomalous propagation of long-period sPL waves from an intermediate-depth earthquake in the Mariana region Ukawa Motoo, Minoru Kobayashi, Yoshio Fukao This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7823286/v1 This work is licensed under a CC BY 4.0 License Status: Under Revision Version 1 posted 5 You are reading this latest preprint version Abstract Seismic waveforms are sensitive to the Earth’s structure of seismic velocity and attenuation. Unusually long-period sP waves from an intermediate-depth earthquake (Mw 7.5, Depth = 260 km) in the northern Mariana region were observed by the broadband seismic network in Japan. The dominant period ranges from 20 to 50 s, which is significantly longer than the source time function of the earthquake. On the basis of the epicentral distance range of their appearance and the prograde particle motions in the vertical‒radial plane, we identified them as sPL waves, i.e., PL waves converted from the upgoing S waves at the surface or the ocean bottom. Wavelet analysis revealed two series, the island arc type and the oceanic type, in which the dominant period of sPL waves increases linearly with epicentral distance. The propagation paths of the former type are mainly through the Izu‒Mariana arc and the Japan islands, and those of the latter are mainly through the Philippine Sea Basin. Synthetic seismograms of spherically symmetric Earth structures indicate that the difference in the linear relationships between them is explained primarily by the difference in crustal thickness. Two important results concerning the upper mantle structure are obtained: 1) a small velocity reduction (0 ~ 1%) in P waves is observed in the upper mantle beneath the Philippine Sea plate, and 2) high attenuation of S waves, \(\:{Q}_{\mu\:}=20\sim30,\) is observed in the mantle wedge above the hypocenter. The sPL waves provide useful information about the crust–upper mantle structure, independent of direct P waves and S waves. sPL waves Philippine Sea plate Seismic velocity Seismic attenuation Upper mantle Mantle wedge Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 Figure 15 Figure 16 Full Text Supplementary Files GraphicAbs.jpg Cite Share Download PDF Status: Under Revision Version 1 posted Reviewers agreed at journal 30 Oct, 2025 Reviewers invited by journal 29 Oct, 2025 Editor assigned by journal 26 Oct, 2025 First submitted to journal 20 Oct, 2025 Editorial decision: Minor Revision 19 Oct, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7823286","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":536692510,"identity":"ab5616f2-aac0-4e0c-840f-79edc7eea202","order_by":0,"name":"Ukawa Motoo","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA+UlEQVRIiWNgGAWjYDACHgST8QGDwQE4j5mgFiDNbECyFjYJBoYDOBXCgcGZw88kfjBsk7Nnbz5WXVBwR86cgceA4UcNA7s5Li1n28wkexhuG/PwHEu7PcPgmbFlA48BY88xBmbLBhxazjOY3eBhuJ3YI5FjdpvH4HDihvtvDBh4G4A+w+FIg/Ps327+gWopBms5ALTlLz4tZ3uAhkO1MMO0MOOzRfLMmfLfMgZAv5w5liwN1GJscICt4LDMMQmcfuE7k77Z8E3FbTn29uaDn3n+HJYzOMC88eGbGptkXCEGdR4aH+gkiWR0QcLAjnQto2AUjIJRMEwBANIDVbvnGGNKAAAAAElFTkSuQmCC","orcid":"https://orcid.org/0009-0003-6438-3194","institution":"Nihon University: Nihon Daigaku","correspondingAuthor":true,"prefix":"","firstName":"Ukawa","middleName":"","lastName":"Motoo","suffix":""},{"id":536692511,"identity":"57dd13e9-acb6-4ece-adcc-cbc90a667dd2","order_by":1,"name":"Minoru Kobayashi","email":"","orcid":"","institution":"Fujitsu Kabusiki 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05:43:14","extension":"xml","order_by":54,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":163944,"visible":true,"origin":"","legend":"","description":"","filename":"EPSPD25003261structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-7823286/v1/fec900a1f9415b69589f63a0.xml"},{"id":95528292,"identity":"f688daa1-d453-45f3-b241-b6f33e15fc7d","added_by":"auto","created_at":"2025-11-10 10:15:50","extension":"html","order_by":55,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":180701,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-7823286/v1/248da23ecd2831fa951c1957.html"},{"id":95504294,"identity":"6adcec8b-1954-4e87-9458-08e7c4b64c25","added_by":"auto","created_at":"2025-11-10 05:43:10","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":3171067,"visible":true,"origin":"","legend":"\u003cp\u003eMap showing the epicenter and the F-net stations. A star indicates the epicenter of the earthquake that we investigated. Plus symbols represent the locations of the F-net stations. The great circle paths from the epicenter to some stations used in the present study are shown by red solid lines. Red triangles are Holocene volcanoes provided by the Global Volcanism Program (2025). Tectonic units are labeled: KPR, Kyushu Palau Ridge; AP, Amami Plateau; DR, Daito Ridge; ODR, Oki Daito Ridge; MR, Mariana Trough; WMR, West Mariana Ridge.\u003c/p\u003e","description":"","filename":"Figure1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7823286/v1/4c2de5c635ebdb7e982fb971.jpg"},{"id":95504293,"identity":"ea1da2b4-701c-4a99-87ec-61105fd4a411","added_by":"auto","created_at":"2025-11-10 05:43:10","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":3214630,"visible":true,"origin":"","legend":"\u003cp\u003eExamples of three-component seismograms and particle motions of the sPL phase. For JIZ (epicentral distance: 13.3°), \u003cstrong\u003ea\u003c/strong\u003ebandpass-filtered seismograms (blue) with the original ones (red), \u003cstrong\u003eb\u003c/strong\u003eparticle motions of the sPL phase on a transverse‒radial plane and \u003cstrong\u003ec\u003c/strong\u003eparticle motions of the sPL phase on a radial‒vertical plane. For IGK (epicentral distance: 17.2°), \u003cstrong\u003ed\u003c/strong\u003e bandpass-filtered seismograms (blue) with the original ones (red), \u003cstrong\u003ee\u003c/strong\u003e particle motions of the sPL phase on a transverse‒radial plane, and \u003cstrong\u003ef \u003c/strong\u003eparticle motions of the sPL phase on a radial‒vertical plane. The travel times calculated for PREM are indicated by arrows and purple lines for the P, S and sP phases. Green boxes indicate the seismic phase targeted. In \u003cstrong\u003eb\u003c/strong\u003e, \u003cstrong\u003ec\u003c/strong\u003e, \u003cstrong\u003ee\u003c/strong\u003e and \u003cstrong\u003ef\u003c/strong\u003e, blue lines with triangles show the particle motions for the filtered seismograms. The particle motions of the original seismograms are depicted by red lines. The origin of the time axes is the origin time of the earthquake determined by the USGS.\u003c/p\u003e","description":"","filename":"Figure2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7823286/v1/b012f4679fa1ea03e1927faa.jpg"},{"id":95504315,"identity":"9762f0cd-a397-4a83-9933-77f4e8186d44","added_by":"auto","created_at":"2025-11-10 05:43:12","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":3059967,"visible":true,"origin":"","legend":"\u003cp\u003eSeismic records of the F-net stations aligned in order of epicentral distances. \u003cstrong\u003ea\u003c/strong\u003e Vertical component. \u003cstrong\u003eb\u003c/strong\u003eRadial component. \u003cstrong\u003ec\u003c/strong\u003e Transverse component. Travel time curves of P, sP and S phases calculated for PREM are superimposed on the seismograms.\u003c/p\u003e","description":"","filename":"Figure3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7823286/v1/d969756aa80298055d4e8e49.jpg"},{"id":95504290,"identity":"f52c7a4c-051f-454c-b5a6-087d1c75a824","added_by":"auto","created_at":"2025-11-10 05:43:10","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":701324,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic illustration of sPL waves. Propagation of sPL waves is schematically illustrated by thick green arrows. Some of the seismic rays of the sP waves that contribute to the formation of sPL waves are indicated by thin green lines. Seismic rays of the direct P and S phases are plotted as thin lines in blue and red, respectively. The seismic rays are calculated for PREM.\u003c/p\u003e","description":"","filename":"Figure4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7823286/v1/20d76f904d28f8ce7a8e401c.jpg"},{"id":95529872,"identity":"1ec991ad-1419-4f3e-ac6b-6180b0b0dd1a","added_by":"auto","created_at":"2025-11-10 10:17:34","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":1683512,"visible":true,"origin":"","legend":"\u003cp\u003eFourier spectral amplitudes of the P, S and sPL phases. \u003cstrong\u003ea\u003c/strong\u003e JIZ and \u003cstrong\u003eb\u003c/strong\u003e IGK. The Fourier spectral amplitudes are plotted in blue, red and green for the P, S and sPL phases, respectively. The time window for each phase is indicated by a box in the same color for each phase on the seismogram. The spectral peaks of each phase are indicated by solid circles.\u003c/p\u003e","description":"","filename":"Figure5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7823286/v1/2e29c01a496440b42bc541b8.jpg"},{"id":95504347,"identity":"a206331e-aac6-4e4c-abd6-745775633ba0","added_by":"auto","created_at":"2025-11-10 05:43:13","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":3571878,"visible":true,"origin":"","legend":"\u003cp\u003eResults of the wavelet transform. \u003cstrong\u003ea\u003c/strong\u003e JIZ, \u003cstrong\u003eb\u003c/strong\u003e ASI, \u003cstrong\u003ec\u003c/strong\u003eWJM, \u003cstrong\u003ed\u003c/strong\u003e TSA, \u003cstrong\u003ee\u003c/strong\u003e INN, and \u003cstrong\u003ef\u003c/strong\u003e IGK. The top of each figure shows the normalized seismogram of the radial component, and the middle and bottom show the wavelet transformation of the seismogram. The color scale indicates the wavelet amplitude (Torrence and Compo 1998). Stars indicate the maximum amplitude of the wavelet around the sPL waves in frequency–time space.\u003c/p\u003e","description":"","filename":"Figure6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7823286/v1/48ecfb4f302083f71c0fd49b.jpg"},{"id":95504296,"identity":"ed7d946d-c321-4fd4-b6b1-d5abd8cdd54d","added_by":"auto","created_at":"2025-11-10 05:43:11","extension":"jpg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":3663111,"visible":true,"origin":"","legend":"\u003cp\u003eDistribution of the dominant periods and types of the sPL waves. \u003cstrong\u003ea\u003c/strong\u003e Map showing the distribution of the dominant periods of the sPL phase observed at the F-net stations. \u003cstrong\u003eb\u003c/strong\u003e Map showing the locations of the oceanic-type (solid symbols) and island arc-type (open symbols) F-net stations. Circles indicate the F-net stations located west of line N353°E (the red line) from the epicenter, and triangles indicate those located east of the red line.\u003c/p\u003e","description":"","filename":"Figure7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7823286/v1/0db8492391cb2cd870b52b3d.jpg"},{"id":95529047,"identity":"c7595f7f-7848-45ee-a65b-a9260bd63818","added_by":"auto","created_at":"2025-11-10 10:16:43","extension":"jpg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":237575,"visible":true,"origin":"","legend":"\u003cp\u003eA graph of the variation in the dominant period versus the epicentral distance. The straight line divides the graph into two groups: the island arc type (upper) and the oceanic type (lower). The symbols are the same as those in Fig. 7b.\u003c/p\u003e","description":"","filename":"Figure8.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7823286/v1/3506d10660dcc0afdd4ba94c.jpg"},{"id":95504308,"identity":"5f315376-f3bc-4716-adaa-1019c871392c","added_by":"auto","created_at":"2025-11-10 05:43:11","extension":"jpg","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":1868704,"visible":true,"origin":"","legend":"\u003cp\u003eSynthetic velocity seismograms calculated for PREM. Red and blue seismograms show the vertical and radial components, respectively. The travel time curves of P (blue), S (red) and sP (green) calculated for PREM are indicated.\u003c/p\u003e","description":"","filename":"Figure9.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7823286/v1/e2a95639e79f6a9daae3a32e.jpg"},{"id":95528861,"identity":"16ce48a7-7edc-489a-8318-08da947ee4bb","added_by":"auto","created_at":"2025-11-10 10:16:32","extension":"jpg","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":1516976,"visible":true,"origin":"","legend":"\u003cp\u003eThree-component synthetic seismograms (top) and particle motions (bottom). \u003cstrong\u003ea\u003c/strong\u003e Epicentral distance of 11°, and \u003cstrong\u003eb\u003c/strong\u003e epicentral distance of 15°. Particle motion on the transverse‒radial plane is bottom left and that on the radial‒vertical plane is bottom right. The particle motions are plotted for the time windows indicated by the green boxes on the seismograms.\u003c/p\u003e","description":"","filename":"Figure10.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7823286/v1/7086e6fa79fe9d42c9432995.jpg"},{"id":95529645,"identity":"334ac6b7-9458-477d-8af8-7521021a1cef","added_by":"auto","created_at":"2025-11-10 10:17:20","extension":"jpg","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":4555078,"visible":true,"origin":"","legend":"\u003cp\u003eWavelet transformation of the synthetic radial component seismograms in Fig. 10. The amplitudes of the seismograms are normalized in the wavelet transformation. The color scale indicates the wavelet amplitude (Torrence and Compo 1998). Stars indicate the maximum amplitude of the wavelet around the sPL waves in frequency–time space. \u003cstrong\u003ea\u003c/strong\u003e Epicentral distance of 11° and \u003cstrong\u003eb\u003c/strong\u003e epicentral distance of 15°.\u003c/p\u003e","description":"","filename":"Figure11.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7823286/v1/e808d30f35711729fc0602b6.jpg"},{"id":95504323,"identity":"77b3fd3a-faea-422e-8bab-8b183fac090f","added_by":"auto","created_at":"2025-11-10 05:43:12","extension":"jpg","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":1068922,"visible":true,"origin":"","legend":"\u003cp\u003eDP‒ED relationship for tested crustal models. \u003cstrong\u003ea\u003c/strong\u003e P wave velocities and \u003cstrong\u003eb\u003c/strong\u003e S wave velocities. Dotted lines represent the horizontal component of velocity in the upper mantle, which is based on PREM (Dziewonski and Anderson 1981). \u003cstrong\u003ec\u003c/strong\u003e Synthetic DP‒ED relationships calculated for PREM, IASP91 and different crustal models (Crust 10, Crust 15 and Crust 30). The observed dominant periods are plotted in a similar manner to that in Fig. 8.\u003c/p\u003e","description":"","filename":"Figure12.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7823286/v1/dca841762dc9ebe4c3bd51bb.jpg"},{"id":95528324,"identity":"d3b75cc8-26cf-484b-8e23-fdaa955d8a85","added_by":"auto","created_at":"2025-11-10 10:15:56","extension":"jpg","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":1222646,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of the upper mantle velocity structure on the synthetic DP‒ED relationship. \u003cstrong\u003ea\u003c/strong\u003e Examples of the P wave velocity profiles (blue lines) showing the lid and low-velocity structure of the test models, Crust10-S5-L100, Crust10-S5-L99 and Crust10-S5-L97. The S wave velocity profiles are also indicated by red lines.\u003cstrong\u003e b\u003c/strong\u003e Synthetic DP‒ED relationships for PREM and the oceanic type models. \u003cstrong\u003ec\u003c/strong\u003e Synthetic DP‒ED relationships for PREM, IASP91 and the island arc-type models. The observed dominant periods in \u003cstrong\u003eb\u003c/strong\u003e and \u003cstrong\u003ec\u003c/strong\u003e are plotted in a similar manner to that in Fig. 8.\u003c/p\u003e","description":"","filename":"Figure13.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7823286/v1/ef86847eee435e3022b6daca.jpg"},{"id":95504342,"identity":"fe82afa4-d87f-463e-bcfd-b8465fabaeaf","added_by":"auto","created_at":"2025-11-10 05:43:13","extension":"jpg","order_by":14,"title":"Figure 14","display":"","copyAsset":false,"role":"figure","size":2216510,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of the observed and synthetic seismograms for the oceanic type sPL waves. The observed and synthetic seismograms are shown as black and red lines, respectively. The synthetic seismograms were calculated for the Crust 10-S5-L100 model with various values of \u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;and with the PREM \u003cem\u003eQ\u003c/em\u003e from the Moho to a depth of 260 km. \u003cstrong\u003ea\u003c/strong\u003e PREM, \u003cstrong\u003eb\u003c/strong\u003e \u0026nbsp;\u0026nbsp;=30, \u003cstrong\u003ec\u003c/strong\u003e \u0026nbsp;\u0026nbsp;=20 and \u003cstrong\u003ed\u003c/strong\u003e \u0026nbsp;\u0026nbsp;=10. The seismograms are filtered in the frequency band from 0.02 to 0.03 Hz. Solid circles on the seismograms represent the maximum amplitude points.\u003c/p\u003e","description":"","filename":"Figure14.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7823286/v1/1701f58f26ca831f80d55b90.jpg"},{"id":95504316,"identity":"5e2671b6-3eda-4412-873a-3c3e26a3c330","added_by":"auto","created_at":"2025-11-10 05:43:12","extension":"jpg","order_by":15,"title":"Figure 15","display":"","copyAsset":false,"role":"figure","size":1915688,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of the observed and synthetic seismograms for the island arc type sPL waves. The observed and synthetic seismograms are shown as black and red lines, respectively. The synthetic seismograms were calculated for the Crust 30-S1-L100 model with various values of \u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;and with the PREM \u003cem\u003eQ\u003c/em\u003e from the Moho to a depth of 260 km. \u003cstrong\u003ea\u003c/strong\u003e PREM, \u003cstrong\u003eb\u003c/strong\u003e \u0026nbsp;\u0026nbsp;=30, \u003cstrong\u003ec\u003c/strong\u003e \u0026nbsp;\u0026nbsp;=20 and \u003cstrong\u003ed\u003c/strong\u003e \u0026nbsp;\u0026nbsp;=10. The seismograms are filtered in the frequency band from 0.02 to 0.03 Hz. Solid circles on the seismograms represent the maximum amplitude points.\u003c/p\u003e","description":"","filename":"Figure15.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7823286/v1/299362d964faa62846d7a728.jpg"},{"id":95504304,"identity":"0d82c767-996d-4ef8-8106-7e0447a1e338","added_by":"auto","created_at":"2025-11-10 05:43:11","extension":"jpg","order_by":16,"title":"Figure 16","display":"","copyAsset":false,"role":"figure","size":2013569,"visible":true,"origin":"","legend":"\u003cp\u003eHypocenter distribution in the Mariana area. The top figure shows the distribution of epicenters in the PDE catalog from 2000 to 2023, and the bottom figure shows a 100 km wide vertical cross section along the blue line in the top figure. The stars indicate the event analyzed in this study. Black triangles at the top of the figure represent Holocene volcanoes. The high attenuation region is indicated by the pink shaded area in the bottom figure.\u003c/p\u003e","description":"","filename":"Figure16.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7823286/v1/7b6cd624f95957e31c738875.jpg"},{"id":95654051,"identity":"e0e78691-7d2e-4404-9f65-2ede738266c1","added_by":"auto","created_at":"2025-11-11 16:09:25","extension":"pdf","order_by":1,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":32370009,"visible":true,"origin":"","legend":"","description":"","filename":"EPSUkawaKobayashiFukao.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7823286/v1_covered_567242b5-3e57-4411-bdf4-17964003d1f6.pdf"},{"id":95504321,"identity":"e056582d-a704-4fe4-a157-f05861569dad","added_by":"auto","created_at":"2025-11-10 05:43:12","extension":"jpg","order_by":20,"title":"","display":"","copyAsset":false,"role":"supplement","size":49302,"visible":true,"origin":"","legend":"","description":"","filename":"GraphicAbs.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7823286/v1/75c45022476c06ec83c94038.jpg"}],"financialInterests":"","formattedTitle":"Anomalous propagation of long-period sPL waves from an intermediate-depth earthquake in the Mariana region","fulltext":[],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":false,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":true,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":true,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"earth-planets-and-space","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"epsp","sideBox":"Learn more about [Earth, Planets and Space](http://earth-planets-space.springeropen.com)","snPcode":"","submissionUrl":"https://www.editorialmanager.com/epsp/default.aspx","title":"Earth, Planets and Space","twitterHandle":"@SpringerOpen","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"BMC/SO AJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"sPL waves, Philippine Sea plate, Seismic velocity, Seismic attenuation, Upper mantle, Mantle wedge","lastPublishedDoi":"10.21203/rs.3.rs-7823286/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7823286/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eSeismic waveforms are sensitive to the Earth\u0026rsquo;s structure of seismic velocity and attenuation. Unusually long-period sP waves from an intermediate-depth earthquake (Mw 7.5, Depth\u0026thinsp;=\u0026thinsp;260 km) in the northern Mariana region were observed by the broadband seismic network in Japan. The dominant period ranges from 20 to 50 s, which is significantly longer than the source time function of the earthquake. On the basis of the epicentral distance range of their appearance and the prograde particle motions in the vertical‒radial plane, we identified them as sPL waves, i.e., PL waves converted from the upgoing S waves at the surface or the ocean bottom. Wavelet analysis revealed two series, the island arc type and the oceanic type, in which the dominant period of sPL waves increases linearly with epicentral distance. The propagation paths of the former type are mainly through the Izu‒Mariana arc and the Japan islands, and those of the latter are mainly through the Philippine Sea Basin. Synthetic seismograms of spherically symmetric Earth structures indicate that the difference in the linear relationships between them is explained primarily by the difference in crustal thickness. Two important results concerning the upper mantle structure are obtained: 1) a small velocity reduction (0\u0026thinsp;~\u0026thinsp;1%) in P waves is observed in the upper mantle beneath the Philippine Sea plate, and 2) high attenuation of S waves, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{Q}_{\\mu\\:}=20\\sim30,\\)\u003c/span\u003e\u003c/span\u003e is observed in the mantle wedge above the hypocenter. The sPL waves provide useful information about the crust\u0026ndash;upper mantle structure, independent of direct P waves and S waves.\u003c/p\u003e","manuscriptTitle":"Anomalous propagation of long-period sPL waves from an intermediate-depth earthquake in the Mariana region","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-11-10 05:43:05","doi":"10.21203/rs.3.rs-7823286/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"","date":"2025-10-30T21:09:58+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-10-29T06:53:50+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-10-26T13:50:32+00:00","index":"","fulltext":""},{"type":"submitted","content":"Earth, Planets and Space","date":"2025-10-21T03:39:17+00:00","index":"","fulltext":""},{"type":"decision","content":"Minor Revision","date":"2025-10-20T02:32:53+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"earth-planets-and-space","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"epsp","sideBox":"Learn more about [Earth, Planets and Space](http://earth-planets-space.springeropen.com)","snPcode":"","submissionUrl":"https://www.editorialmanager.com/epsp/default.aspx","title":"Earth, Planets and Space","twitterHandle":"@SpringerOpen","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"BMC/SO AJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"0789e989-ef12-4247-827d-51bf0d1796e9","owner":[],"postedDate":"November 10th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"in-revision","subjectAreas":[],"tags":[],"updatedAt":"2025-12-15T13:51:30+00:00","versionOfRecord":[],"versionCreatedAt":"2025-11-10 05:43:05","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7823286","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7823286","identity":"rs-7823286","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}